Hydrologic Assessment of Pigeon Lake - TownLife
PIGEON LAKE WATERSHED ASSOCIATION HYDROLOGIC ASSESSMENT OF PIGEON LAKE HYDROGEOLOGY, WATER QUALITY, AND WATER BALANCE
Hydrologic Assessment of Pigeon Lake Hydrogeology, Water Quality, and Water Balance
E00041301 5 March 2010 WorleyParsons Canada Ltd. Infrastructure & Environment Division Suite 705, 10240 - 124 Street Edmonton, AB T5N 3W6 CANADA Phone: +1 780 496 9055 Facsimile: +1 780 496 9575 www.worleyparsons.com © Copyright 2010 WorleyParsons
PIGEON LAKE WATERSHED ASSOCIATION HYDROLOGIC ASSESSMENT OF PIGEON LAKE HYDROGEOLOGY, WATER QUALITY, AND WATER BALANCE
PROJECT E00041301 - HYDROLOGIC ASSESSMENT OF PIGEON LAKE FILE LOC.: EDMONTON REV
DESCRIPTION
0
Final
1
Final
ORIG
REVIEW
WORLEYPARSONS APPROVAL
H.Anaya
M.Shome
M.Maccagno
H.Anaya
M. Shome
M.Maccagno
j:\e000s\e00041301 (pigeon lake)\report\final report\pigeon lake report_rev 1.doc Page i
DATE
CLIENT APPROVAL
DATE
18-Feb-10
3-Mar-10
E00041301 : Rev 1 : 5 March 2010
PIGEON LAKE WATERSHED ASSOCIATION HYDROLOGIC ASSESSMENT OF PIGEON LAKE HYDROGEOLOGY, WATER QUALITY, AND WATER BALANCE
Disclaimer The information presented in this document was compiled and interpreted exclusively for the purposes stated in Section 1.2 of the document. WorleyParsons provided this report for Pigeon Lake Watershed Association solely for the purpose noted above. WorleyParsons has exercised reasonable skill, care, and diligence to assess the information acquired during the preparation of this report, but makes no guarantees or warranties as to the accuracy or completeness of this information. The information contained in this report is based upon, and limited by, the circumstances and conditions acknowledged herein, and upon information available at the time of its preparation. The information provided by others is believed to be accurate but cannot be guaranteed. WorleyParsons does not accept any responsibility for the use of this report for any purpose other than that stated in Section 1.2 and does not accept responsibility to any third party for the use in whole or in part of the contents of this report. Any alternative use, including that by a third party, or any reliance on, or decisions based on this document, is the responsibility of the alternative user or third party. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of WorleyParsons. Any questions concerning the information or its interpretation should be directed to H.Anaya or M. Maccagno.
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EXECUTIVE SUMMARY A hydrologic assessment of Pigeon Lake was completed in October 2009 for the Pigeon Lake Watershed Association. Lake levels have generally declined from an historical high of approximately 850.63 metres above sea level (masl) in July 1981 to a low of approximately 849.4 masl in September 2009. At the same time, agricultural land use intensity in the Pigeon Lake watershed has increased, both in terms of livestock populations and cropland area, resulting in an increased nutrient input to the lake. When the lake is below the weir crest, nutrients (especially phosphorus) are no longer flushed from the lake. If the lake levels continue to decline, eutrophication in the form of algal blooms will continue, and there will be serious and perhaps irreversible effects on the health of the lake’s aquatic ecosystems. The statistical analysis of the annual precipitation covering the period from 1961 to 2009 resulted in a long-term average annual precipitation of 454 mm. The average annual precipitation over the last 10 years is 389 mm. It is obvious that during the last 10 years the region has experienced a dry period. Furthermore, the annual precipitation has been generally declining since 1996. The average annual lake evaporation is 662 mm. Unless the net evaporation (i.e. deficit of precipitation to evaporation) is compensated by an adequate volume of surface water runoff and/or groundwater contribution, water levels of the lake will decline. A water balance model was developed using daily time steps covering the period from 1993 to 2009, since the required information for all the modelling parameters was available for this period. The model was developed and calibrated against existing flow conditions. Over the modelled period, the total 3 mean annual inflow to the lake was 246.1 mm (23,970 dam /yr) which includes surface runoff of 197.5 3 3 mm (19,240 dam /yr) and groundwater from the upper bedrock of 48.6 mm (4,740 dam /yr). Groundwater from the upper bedrock (Paskapoo Formation) accounts for approximately 20% of the average annual inflow to Pigeon Lake. The average annual outflow including water withdrawals from 3 the lake is 44.8 mm (4,370 dam /yr). The net inflow (difference of total inflow and total outflow) is then 3 3 201.3 mm (19,600 dam /yr). The net evaporation over the lake surface is 219 mm (21,330 dam /yr) over the modelled period. Since the net inflow to the lake is less than the net evaporation, Pigeon Lake 3 is subject to a net annual deficit of 17.7 mm (1,730 dam /yr). Lake water residence time exceeds 100 years. The longer the residence time, the greater the tendency toward eutrophication. Lake water residence time could be decreased by maintaining the lake level above the weir crest elevation so that more frequent outflow occurs. The water balance model indicated that raising the September 2009 lake level (849.4 masl) to the weir level (849.95 masl) would 3 3 take approximately 3.8 years if an additional inflow of 0.5 m /s (43,200 m /day) is added to the lake in 3 3 Scenario 1 and 1.8 years at an inflow rate of 1 m /s (86,400 m /day) in Scenario 2. The retention time could be decreased from >100 years in the current condition to 58 and 49 years by adding flow according to Scenarios 1 and 2, respectively. These hypothetical scenarios assumed that similar
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hydrologic conditions will prevail in the future. With water levels above the weir elevation, nutrients from the lake would flush and water quality would improve, but nutrient input issues would still need to be addressed. Potential sources of water input include the North Saskatchewan River and the lower Paskapoo 3 6 3 Formation. Average flow rate of the North Saskatchewan River is 175 m /s (15.1 x 10 m /day), so that 3 3 the diversion required to supply 1 m /s (86,400 m /day) represents approximately 0.6% of the North Saskatchewan River flow. The cost of such a diversion may appear high but should be weighed against the cost of not intervening in the health of the lake, i.e. quantify the total social, economic, and environmental value of a healthy Pigeon Lake in terms of recreational, resource, and ecosystem use. The approach to groundwater supply and its sustainability would need to be examined closely, from the perspectives both of current domestic/agricultural groundwater use and ecosystem functioning. Water quality in either the North Saskatchewan River and lower Paskapoo Formation would need to be characterized in detail with respect to compatibility with the lake. Timely and tangible actions are required or a continued downward trend in lake levels will lead to the ongoing and perhaps irreversible deterioration of the lake’s health.
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CONTENTS 1.
INTRODUCTION...................................................................................................................1 1.1
General.....................................................................................................................1
1.2
Objectives ................................................................................................................1
2.
METHODS AND SCOPE OF WORK....................................................................................2
3.
SUMMARY OF PIGEON LAKE HYDROLOGY ....................................................................4 3.1
Pigeon Lake Drainage Basin ...................................................................................4
3.2
Physical Characteristics of Pigeon Lake..................................................................4
3.3
Surface Water Hydrology .........................................................................................5 3.3.1
Surface Runoff and Outflows .................................................................................5
3.3.2
Precipitation and Lake Evaporation .......................................................................6
3.3.3
Historical Variation of Pigeon Lake Water Levels..................................................6
3.3.4
Residence Time .....................................................................................................7
3.4
Hydrogeology ...........................................................................................................7 3.4.1
Hydrostratigraphy...................................................................................................7
3.4.2
Results of Water Well Search ................................................................................9
3.4.3
Hydrogeologic Cross-Section ................................................................................9
3.4.4
Groundwater Flow, Groundwater Levels, and Pigeon Lake Water Levels ............9
3.4.5
Water Well Yields vs. Well Depths ......................................................................11
3.4.6
Water Well Chemistry ..........................................................................................11
3.4.7
Conceptual Model of Groundwater Flow .............................................................11
3.5
3.6
Water Quality .........................................................................................................13 3.5.1
Summary of Pigeon Lake Water Quality..............................................................13
3.5.2
Comparison to Other Lakes .................................................................................18 Land Use Changes ................................................................................................19
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3.7
Declining Lake Water Levels and Effects on Habitat.............................................26
3.8
Licensed and Unlicensed Water Use.....................................................................28
4.
WATER BALANCE MODELLING .......................................................................................30 4.1
General...................................................................................................................30
4.2
Calibration ..............................................................................................................31
4.3
Relative Contributions of Hydrologic Variables to the Water Balance...................31
4.4
Water Level Maintenance ......................................................................................34
4.5
Application of Water Balance Model ......................................................................34
4.6
Limitations of the Water Balance Model ................................................................35
5.
DISCUSSION AND SYNTHESIS........................................................................................36
6.
CONCLUSIONS AND RECOMMENDATIONS...................................................................38
7.
CLOSURE ...........................................................................................................................41
8.
REFERENCES....................................................................................................................43
Figures FIGURE 1
SITE LOCATION MAP
FIGURE 2
REGIONAL TOPOGRAPHY AND DRAINAGE
FIGURE 3
PIGEON LAKE BATHYMETRY
FIGURE 4
PIGEON LAKE STAGE-STORAGE AND STAGE-AREA RELATIONSHIPS
FIGURE 5
GENERATED INFLOW HYDROGRAPH
FIGURE 6
HISTORICAL LAKE LEVELS
FIGURE 7
ESTIMATED AND PUBLISHED LAKE EVAPORATION DATA
FIGURE 8
LAKE EVAPORATION AND PRECIPITATION 1993-2009
FIGURE 9
BEDROCK GEOLOGY
FIGURE 10
DRIFT THICKNESS
FIGURE 11
DRIFT GEOLOGY
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FIGURE 12
HYDROGEOLOGIC CROSS-SECTION A-A’
FIGURE 13
WELL YIELD VS. WELL DEPTH
FIGURE 14
MODELLED AND MEASURED LAKE WATER LEVEL
FIGURE 15
PIGEON LAKE WATERSHED AVERAGE ANNUAL WATER BALANCE
FIGURE 16
RELATIVE CONTRIBUTIONS TO PIGEON LAKE WATER BALANCE
FIGURE 17
MODEL APPLICATION – SCENARIO 1
FIGURE 18
MODEL APPLICATION – SCENARIO 2
Tables within Text TABLE A PHYSICAL CHARACTERISTICS OF PIGEON LAKE ..................................................... 4 TABLE B PRECIPITATION STATISTICS EDMONTON INTERNATIONAL AIRPORT................... 6 TABLE C FECAL COLIFORM COUNTS (AS CFU) PER 100 ML IN PIGEON LAKE (2006) ....... 18 TABLE D COMPARISON OF SELECT WATER QUALITY PARAMETERS ................................ 18 TABLE E THEORETICAL TOTAL EXTERNAL PHOSPHORUS LOADING TO PIGEON LAKE .. 25 TABLE F YEARLY WATER BALANCE ........................................................................................ 32 TABLE G AVERAGE ANNUAL WATER BALANCE - PIGEON LAKE .......................................... 33
Figures within Text FIGURE A TOTAL PHOSPHORUS AND WATER LEVELS IN PIGEON LAKE ........................... 13 FIGURE B TOTAL NITROGEN AND WATER LEVELS IN PIGEON LAKE .................................. 14 FIGURE C TOTAL NITROGEN TO TOTAL PHOSPHORUS RATIO AND WATER LEVELS IN PIGEON LAKE ..................................................................................................... 15 FIGURE D CHLOROPHYLL A AND WATER LEVELS IN PIGEON LAKE ................................... 16 FIGURE E TEMPERATURE AND DISSOLVED OXYGEN CONCENTRATIONS IN PIGEON LAKE .................................................................................................................... 17 FIGURE F TOTAL CATTLE AND SWINE IN THE PIGEON LAKE WATERSHED ....................... 20 FIGURE G ESTIMATED MANURE PRODUCTION IN THE PIGEON LAKE WATERSHED........ 21
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FIGURE H MANURE PHOSPHORUS LEVELS IN THE PIGEON LAKE WATERSHED.............. 22 FIGURE I MANURE NITROGEN LEVELS IN THE PIGEON LAKE WATERSHED...................... 23 FIGURE J AREA OF CROPLAND WITHIN THE PIGEON LAKE WATERSHED ......................... 24 FIGURE K AREA OF FERTILIZED CROPLAND WITHIN THE PIGEON LAKE WATERSHED ... 25 FIGURE L PERCENTAGE OF NORMAL LAKE AREA LOST AS LAKE LEVELS DROP ............ 27 FIGURE M PERCENTAGE OF LAKE VOLUME LOST AS WATER LEVELS DROP................... 28
Appendices APPENDIX 1
ESTIMATION OF LAKE EVAPORATION
APPENDIX 2
WATER WELL DRILLING REPORTS
APPENDIX 3
BOREHOLE ECC-2008-03
APPENDIX 4
GROUNDWATER LEVEL DATA
APPENDIX 5
WATER WELL YIELD AND CHEMISTRY DATA
APPENDIX 6
SURFACE WATER AND GROUNDWATER USE LICENSES
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1. 1.1
INTRODUCTION General
Pigeon Lake is one of the most popular recreational lakes in Alberta. The goals of the Pigeon Lake Watershed Association (PLWA) are to enhance, preserve, and protect the Pigeon Lake watershed. Concerned by lowered lake water levels and deteriorating lake water quality, the PLWA retained WorleyParsons to conduct a hydrological assessment of Pigeon Lake. The purposes of the hydrological assessment were to evaluate the hydrologic controls on lake levels, and identify possible scientifically-based solutions to the problems of lake level and water quality decline.
1.2
Objectives
The objectives of the hydrologic assessment were to: a)
develop a water balance model of the lake;
b)
assess the relative importance of the contributing factors that are responsible, or potentially responsible, for affecting lake levels, including groundwater/surface water interactions;
c)
evaluate lake water quality; and,
d)
if possible, develop recommendations for “next steps” that will contribute to the sustainability of the lake.
The assessment used available information (e.g. climate, hydrology, hydrogeology, water well, and water quality data) to develop an integrated conceptual model of the hydrology of the Pigeon Lake watershed.
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2.
METHODS AND SCOPE OF WORK
The following activities were carried out to meet the study objectives. Site Reconnaissance –a field visit was conducted on October 22, 2009 to confirm lake outlet geometry. Data Sources – publically-available topographic maps, land cover, land use change; bedrock and drift geology maps, geologic cross-sections, water well records, regional groundwater levels, groundwater and surface water chemistry data, previous and current hydrogeologic studies; lake bathymetry, historical water levels of Pigeon Lake, stream flow, and climate data; surface water and groundwater allocation and usage data, previous studies, and any other available information relevant to water balance and water quality. The data specifically included: •
recorded water level data for Pigeon Lake;
•
recorded climate information (i.e. precipitation and temperatures) at the Edmonton International Airport;
•
available recorded flow data for the following streams in the vicinity of the lake: −
Lloyd Creek Water Survey of Canada (WSC) station Number 05CCC00.
−
Muskeg Creek WSC station number 05FA912.
−
Maskwa Creek WSC station number 05FA014.
•
available information on physical characteristics, such as stage-storage, stage-discharge, and stage-area relationships for Pigeon Lake;
•
recorded water well drilling reports and active groundwater licenses (Alberta Environment [AENV] Groundwater Information Centre [GIC] database);
•
AENV groundwater observation wells network (GOWN); and
•
drilling logs from the Alberta Geological Survey.
Water quality data have been collected for Pigeon Lake since 1972. The University of Alberta conducted water quality testing from 1973 to 1975, as did AENV in 1971 to 1972, and sporadically from 1983 to 2008. In 1982, a water quality study was commissioned by the Battle River Regional Planning Commission, and in 1988 a volunteer citizen monitoring program was initiated by AENV. Lilley Environmental Consulting and Earle (1998) conducted a water quality assessment and drafted a “Pigeon Lake Watershed Management Plan”, which was subsequently adopted by the Counties of
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Leduc and Wetaskiwin and several summer villages on Pigeon Lake in 2000. The Alberta Lake Management Society (ALMS) sampled Pigeon Lake in 2001, and Aquality Environmental Consulting Ltd. sampled the lake for caffeine in 2006 and for nutrients following an algal bloom in the summer of 2007. A State of the Watershed Report was completed in 2008 (Aquality 2008). Data from the above sources are synthesized in Section 3.5. Data Analysis – this task included the following steps: •
climate trend analysis from regional stations (temperature, precipitation, and evapotranspiration);
•
site-specific and regional hydrological trend analysis (annual yield, peak flows, base flows, and lake levels);
•
hydrogeological mapping and cross-sections with respect to the lake and watershed, including topography, bathymetry, the presence and distribution of the major hydrogeologic units, groundwater recharge, groundwater levels, groundwater flow, and groundwater chemistry;
•
water quality analysis, including possible temporal trends, comparison of data to water quality guidelines, examination of surrounding land use;
•
licensed and estimated unlicensed water usage;
•
development of a conceptual, quantitative water balance; and
•
a description of model uncertainty.
Synthesis – integrate the data analysis to determine the major drivers for the observed changes in lake levels and lake water quality.
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3.
SUMMARY OF PIGEON LAKE HYDROLOGY
3.1
Pigeon Lake Drainage Basin
Pigeon Lake is located 60 km southwest of the City of Edmonton (Figure 1). Relief in the area ranges from level to undulating. Regionally, ground elevations range from approximately 1,200 m above sea level (masl) to the southwest to 600 masl in the northeast (Figure 2). The Pigeon Lake watershed is located within the Battle River Basin, which is a part of the North Saskatchewan River Basin. The North Saskatchewan River is about 50 km to the northwest. Approximately 50 percent of the watershed is forest-covered, 46 percent is cleared for agriculture, and 4 percent is developed for cottage and residential use (Mitchell and Prepas 1990).
3.2
Physical Characteristics of Pigeon Lake
Table A below presents a summary of the physical characteristics of Pigeon Lake.
Table A Physical Characteristics of Pigeon Lake Drainage Area
Lake Area
(km )
2
(km )
187
96.7
2
Ratio of Drainage Area to Lake Area
Maximum Volume
Mean Depth
Maximum Depth
(10 m )
(m)
(m)
2:1
600
6.2
9.1
6
3
Highest Water Level (masl)
Lowest Water Level (masl)
850.63
849.42
Source: Mitchell and Prepas (1990)
The ratio of drainage basin area to lake area (2:1) is small. Drainage area to lake surface area ratio indicates how a lake may respond to climatic fluctuation. Lakes with a small ratio of drainage area to lake area are generally more sensitive to climatic variation, since area of evaporation is large while area of land drainage is small. Water flows into the lake through several intermittent streams that drain the western and northwestern portions of the watershed. The outlet is Pigeon Lake creek, which flows in a southwesterly direction into the Battle River (Figure 2). The outflow from the lake has been controlled by an engineered weir structure since 1914. The outlet structure has been rebuilt twice: once in 1940 and the latest in 1986 (Mitchell and Prepas 1990). From 1986 to 1992, recommendations to Alberta Environment from the Association of Pigeon Lake Municipalities lead to weir operational procedures, adopted in November 1992, which are in place today. The weir crest is at an elevation of 849.95 masl. The width of the weir structure is approximately 5 m.
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Lake bathymetry is shown on Figure 3. The stage-storage and storage-area relationships are from Mitchell and Prepas (1990) and presented on Figure 4. As shown on Figure 4, the maximum depth is 6 3 approximately 9 m and the storage volume is 600 x 10 m . The surface area of the lake was 2 2 101.5 km at the highest recorded water level of 850.63 masl and 95.2 km at the lowest recorded water level of 849.4 masl.
3.3
Surface Water Hydrology
Several factors play a role in the water balance of a lake. In addition to surface runoff from within a drainage basin and outflow from the lake, two additional key hydrologic factors, namely, the volume of precipitation (gain of water) falling on the lake area and the volume of evaporation (loss of water) leaving the lake area, can play a major role in the water balance, particularly for the lakes with a small ratio of drainage area to lake area. Key variables with respect to the hydrology of Pigeon Lake are described below.
3.3.1
Surface Runoff and Outflow s
For this study, runoff contribution to Pigeon Lake was estimated using stream flow data from nearby Water Survey of Canada (WSC) gauging stations with similar hydrologic characteristics. The measured stream flows at the WSC stations consist of direct runoff components and base flow components. The base flow components include the contribution of shallow groundwater to the streams. The daily contributions of local surface runoff into the lake were computed using the recorded flow data of Lloyd Creek WSC Station No. 05CC009, Muskeg Creek WSC station No. 05FA912, and Maskwa Creek WSC Station No. 05FA014. Station locations are shown on Figure 2. Runoff contribution to Pigeon Lake was estimated by transposing stream flow data from these nearby WSC stations using a drainage area ratio adjustment for each time step. The generated data are shown on Figure 5, along with the historic precipitation data for the same period. The analysis of the generated inflow data series covering the period from 1993 to 2009 indicates an average annual surface runoff of 197.5 mm. The 1993 to 2009 period of record was selected since the required information for the modelling parameters and the outlet structure operation is complete for this time period. Data for the year 2009 were received as advance information subject to correction by the WSC. Surface water outflow from Pigeon Lake occurs when the water level exceeds the weir crest elevation of 849.95 masl. Since no hydrometric gauging station is located to record outflow from the lake (when it occurs) a general weir flow equation was used to estimate the outflow from the lake for the water balance model (Section 4). Utilizing the historical recorded water levels in the lake (Figure 6), the 3 surface outflow from the lake was estimated to vary between 0 and 2.0 m /s. Figure 6 also shows historical precipitation data.
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3.3.2
Precipitation and Lake Evaporation
Since site-specific precipitation data are unavailable, historical precipitation data recorded at the Edmonton International Airport were taken as representative data Pigeon Lake. The estimated monthly lake evaporation covering the period from 1969 to 1996 is available in Bothe (1999). However, an estimate of the daily lake evaporation covering the period to 2009 was a key requirement of the present study. Hence, the Thornthwaite water balance method was used to estimate lake evaporation. Measured temperature data from the Edmonton International Airport were used to estimate the missing period. Figure 7 compares the published and estimated daily evaporation. The procedure for estimating lake evaporation is provided in detail in Appendix 1. The summary of the statistical analysis of the annual precipitation covering the period from 1961 to 2009 is presented in Table B. The long term average annual precipitation is 454 mm and the average annual precipitation over the last 10 years is 389 mm. It is obvious that during the last 10 years the region has experienced a dry period. The long term average annual lake evaporation is 662.0 mm.
Table B Precipitation Statistics Edmonton International Airport Total Annual (mm)
Long Term (1961-2009)
Last Ten Years (2000-2009)
Minimum, mm
267.4
267.4
Maximum, mm
651.1
473.5
Average, mm
454.0
389.0
Median, mm
448.8
398.1
The annual precipitation and lake evaporation data covering the period from 1961 to 2009 are graphically shown on Figure 8. Data for the year 2009 were received as advance information subject to correction by the WSC. It is seen that the Pigeon Lake area is generally subject to more lake evaporation than the average annual precipitation. Furthermore, Figure 8 shows that the annual precipitation has been generally declining since 1996. Unless the net evaporation (i.e. deficit of precipitation to evaporation) is compensated by an adequate volume of surface water runoff and/or groundwater contribution, water levels of the lake will decline.
3.3.3
Historical Variation of Pigeon Lake Water Levels
The water levels of Pigeon Lake have been monitored since 1972 at WSC station No. 05FA013. The historical recorded water levels are shown on Figure 6. Recorded data for 2009 have been obtained as advance information subject to correction. Lake levels have generally declined from an historical high
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of approximately 850.63 masl in July 1981 to a low of approximately 849.4 masl in September 2009. Time of surface water outflow from the lake has also decreased (Figure 6).
3.3.4
Residence Time
Residence time (or lake retention time) is calculated as the total volume of water in a lake divided by the mean outflow rate, which gives the time that water (or a dissolved substance) remains in a lake. Residence time is an important indicator of lake water quality, since the longer the residence time the greater the tendency of a lake toward eutrophication (i.e. enrichment in dissolved nutrients that stimulate the growth of aquatic plant life, usually resulting in the depletion of dissolved oxygen). In the case of Pigeon Lake the estimated residence time (over 100 years, Mitchell and Prepas 1990) is considered relatively long. By contrast, the average lake-water residence time of Sylvan Lake is 20 to 35 years (Baker 2009).
3.4 3.4.1
Hydrogeology Hydr ostratigraphy
Available regional hydrogeologic maps and reports, and water well drilling records were reviewed to build a conceptual view of the hydrogeology of Pigeon Lake. East of Pigeon Lake, the near-surface bedrock unit is the Paleocene/Upper Cretaceous Scollard Formation (Figure 9). The Scollard Formation is an interbedded, interfingering sequence of argillaceous sandstone, siltstone, mudstone, and shale. The unit as a whole shows considerable lateral and vertical heterogeneity. The Scollard Formation contains a number of thick coal seams, including the Nevis Coal Seam (~0.5 m thick) and the Ardley Coal Seam (~2.5 m thick), mainly in the upper part of the unit. The Ardley Coal Seam is the most-extracted coal unit in the west-central Alberta Plains (Glass 1990). The Ardley Seam is a target of coal bed methane development. The total thickness of the Scollard Formation in the area is approximately 135 m (Hydrogeological Consultants Ltd. 2008). The base of groundwater protection (BGWP) or depth to “non-saline” groundwater (i.e. mineralization less than 4,000 mg/L total dissolved solids or TDS) is at approximately 620 masl beneath the Pigeon Lake area (EUB 2007), corresponding to a depth of approximately 240 mbgs. The BGWP in this area is considered to include the Scollard Formation as a member of the Paskapoo Formation as per Irish (1970). Below the Scollard Formation is the Battle Formation, a bentonitic shale that generally has poor potential as a groundwater resource. The near-surface bedrock unit below Pigeon Lake is the Paskapoo Formation (Figure 9), which consists of interbedded mudstone, siltstone, and sandstone, with some fossiliferous limestone, coal, and bentonitic beds. Prominent are 15 to 20 m thick medium- to coarse-grained sandstone units. Thickness of the Paskapoo Formation in the Pigeon Lake area is approximately 80 m. The regional dip
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of the Paskapoo Formation is to the southwest at approximately 3 to 4 m/km (Ceroici 1979). The base of the Paskapoo Formation subcrops approximately 10 km northeast of Pigeon Lake (Figure 9). Surface expressions of groundwater discharge, such as lakes, springs, and fens, could be expected to occur near the subcrop. The Paskapoo Formation is divided in ascending stratigraphic order into the Haynes, Lacombe, and Dalehurst members. Hydrogeological Consultants Ltd. (2008) interpreted that the Dalehurst Member occurs immediately west of Pigeon Lake (Figure 9). On the other hand, data collected by the Alberta Geological Survey (Energy Resources Conservation Board [ERCB] 2009a) suggest that the Dalehurst Member is actually located further west. In the Pigeon Lake area, the best aquifers are the sandstone units of the Paskapoo Formation, from 3 which yields of 150 to 650 m /day can be obtained (Ozoray 1972). According to Hydrogeological Consultants Ltd. (2008), bedrock deposits have estimated expected yields of approximately 10 to over 3 3 100 m /day with some wells reporting over 650 m /day. Ceroici (1979) reported yields up to 3 -4 3,300 m /day. The Alberta Geological Survey (ERCB 2009b) reported hydraulic conductivities of 10 to -6 10 m/s. Groundwater quality in the Paskapoo Formation is expected to be good with mineralization usually below 1,000 mg/L and often below 500 mg/L TDS. The chemical character is generally calciummagnesium-bicarbonate, but in the discharge areas of longer flow systems, sodium is the predominant cation and mineralization can be 1,500 mg/L or higher. Naturally-elevated sulphate, chloride, or iron concentrations are limited to small areas in isolated locations (Ozoray 1972). Although the Paskapoo Formation is one of the single largest sources of potable groundwater in the Canadian Prairies, little information is available regarding the sustainable yield and regional distribution of groundwater supply within the aquifer system (Grasby et al. 2008). In general, an approximately 3 to 40 m thickness of Quaternary drift deposits overlay the bedrock in the Pigeon Lake area (Figure 10). The overburden deposits generally consist of till, along with glaciofluvial and glaciolacustrine deposits, with some eolian and fluvial deposits (Figure 11). No buried preglacial valleys of significance appear present in the Pigeon Lake area. Shallow, local aquifers are possible in glaciofluvial deposits (i.e. sand and gravel meltwater channels within clayey till), as well as eolian sands. Recent fluvial deposits in the Battle Lake area and elsewhere could contain significant proportions of sand and gravel and, where saturated, could function as local aquifers. Yields of 3 between and 10 and 100 m /day can be expected for the drift aquifers (Hydrogeological Consultants Ltd. 2008). The chemical composition of groundwater in the drift is more variable than in bedrock, reflecting the more local nature of groundwater flow systems and lithologic variation within the drift. Groundwater in areas covered by lower permeability clayey till are typified by elevated sulphate and higher mineralization (Bibby 1974). Mineralization can vary naturally from less than 500 mg/L TDS in eolian
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deposits to more than 5,000 mg/L in clayey till. Drift waters can also be characterized by high iron content (i.e. up to 5 mg/L).
3.4.2
Results of Water Well Search
A search of the AENV water well database identified 2,768 wells within the Pigeon Lake watershed (Appendix 2). Of the 2,768 reported wells, more than 90% were classified as used for domestic purposes, and less than 9% were classified for stock, industrial, dewatering, municipal, irrigation, and investigation or monitoring use, and for coal test holes (Appendix 2). Reported total depths of the wells ranged from 6 to approximately 2,200 m with an average depth of 37 mbgs. Most of the shallow wells in the area (i.e. < 40 m deep) appear completed within the drift or upper bedrock deposits.
3.4.3
Hydr ogeologic Cr oss-Section
Records from the AENV water well database and ERCB/AGS borehole ECC-2008-03 (Appendix 3) were used to create a southwest to northeast hydrogeological cross-section across Pigeon Lake (Figure 12). Borehole ECC-2008-03 is located near the southeast end of Pigeon Lake (Figure 11). While the AENV database is extensive, the spatial accuracy of many of the wells is typically not greater than the centre of the quarter section (± 800 m; see Appendix 2, where some of the wells appear to be located in the lake itself). Moreover, drilling reports are not necessarily consistent in description of lithology. The most reliable lithological data were considered to be from research borehole ECC-200803. The cross-section (Figure 12) indicates approximately 6 to 45 m of unconsolidated drift above the bedrock. The drift deposits generally consist of sandy units and clayey till. The drift is underlain by approximately 60 to 70 m of fine- to coarse-grained sandstone units of the Paskapoo Formation. The ECC-2008-03 borehole indicates a 7 m thick mudstone unit separating the upper and lower Paskapoo (Appendix 3). Below the mudstone unit, coarse sandstone units appear thicker and more prevalent in that borehole. The upper Scollard Formation at borehole ECC-2008-03 borehole mainly comprises mudstone and siltstone with occasional thin coal and fine-grained sandstone units.
3.4.4
Groundw ater Flow , Groundw ater Level s, and Pigeon Lake Water Levels
Direction and gradient of groundwater flow laterally and vertically within the Pigeon Lake watershed, and with respect to the lake, could not be determined with the available AENV data. Nevertheless, groundwater flow is generally controlled by differences in ground surface topography. The water table configuration mirrors the ground surface topography. Differences in the energy of the groundwater,
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owing to differences in topography (and hence water table elevation, and hence position of the water in the earth’s gravitational field), continuously drive the flow of groundwater. The groundwater system as a whole is a hydraulically-continuous, three-dimensional flow field that crosses all manner of geologic boundaries. In drift aquifers of the Pigeon Lake area, groundwater flow is expected to flow from local topographic highs to local topographic lows (i.e. local flow systems on the order of 1 km). In the bedrock, groundwater flow systems on the order of tens of kilometres are expected (Figure 12). Due to the relatively thin drift (Figure 12), a hydraulic connection between Pigeon Lake and the Paskapoo Formation is possible. Regional AENV groundwater monitoring wells are located northwest and south of the Pigeon Lake watershed at distances of 2 km to approximately 60 km. Groundwater levels were plotted against Pigeon Lake water levels for comparison of hydrographic trends (Appendix 4). Evaluating the data from these wells (Appendix 4), a possible hydraulic connection between Pigeon Lake levels and relatively shallow aquifers (up to approximately 30 mbgs) is apparent. On the other hand, monitoring wells with screened intervals deeper than approximately 40 mbgs show little water level correlation. In particular, the following trends were observed. •
The groundwater levels in monitoring wells from approximately 5 to 30 mbgs show a good match with Pigeon Lake levels, suggesting either hydraulic continuity with upper drift and bedrock aquifers, or that the water levels in both the lake and upper aquifers are influenced by similar forcing functions (e.g. shorter-term precipitation events and climate fluctuation).
•
Deeper wells show a poorer match with lake levels, suggesting either poorer hydraulic connection between the lake and lower bedrock aquifers, or that the water levels in the lower aquifers are less sensitive to shorter-term forcing functions. Well ECC-2008-03 (Appendix 3) reported a 7 m thick mudstone/claystone zone located at approximately 25 m from the top of the -8 Paskapoo Formation (or 65 mbgs). Estimated hydraulic conductivity of the layer ranged from 10 -9 to 10 m/s (Appendix 3). Depending on the thickness and lateral continuity of this layer, the lower Paskapoo Formation could have limited connection with the upper Paskapoo Formation, hence limiting natural groundwater interactions with Pigeon Lake largely to the upper Paskapoo Formation.
Examining the trend of groundwater levels reported in drilling logs, Grasby et al. (2008) found a general drop in static groundwater level of approximately 3 m over approximately 50 years for the Paskapoo Formation. The drop was attributed either to an exponential increase in the number of drilled water wells (and hence groundwater use) or to increased drilling efficiency, which has made deeper drilling more feasible with time (and deeper wells tend to record a lower static groundwater level).
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3.4.5
Water Well Yields vs. Well Depths
The total number of water well records in the Pigeon Lake watershed listed in the AENV database is 2,768. Data on well yield (pumping test rate), well depth, test date, and test duration are available for 1,989 of those wells (Appendix 5). Figure 13 summarizes well yield versus well depth for wells completed in the Pigeon Lake watershed. Most of the wells in the watershed are completed at depth of less than 100 mbgs and have estimated 3 yields (pumping rates) less than 150 m /day, which may be due more to intended use (i.e. domestic) and testing limitations than hydrogeologic capability. Only ten wells showed yields greater than 3 500 m /day. Most of these wells were completed at a depth of approximately 50 mbgs, which would suggest completion in the upper Paskapoo Formation. Although the available data show no correlation between increased well depth and yield (Figure 13), greater yield would be expected in the lower Paskapoo because of greater available hydraulic head and higher hydraulic conductivity. If groundwater was to be considered as a possible source of supplementary water supply to Pigeon Lake, more detailed hydrogeological investigation would likely focus on sustainable yield of the lower Paskapoo Formation in the Pigeon Lake area.
3.4.6
Water Well Chemist r y
Geochemical data available in the AENV water well database (Appendix 5) were reviewed. Two main groundwater types were recognized: a calcium and/or magnesium bicarbonate water and a sodium bicarbonate water. Reported depths of these wells ranged from approximately 20 to 180 mbgs. Calcium-magnesium-bicarbonate water is expected to be associated more with the drift and sodium bicarbonate water is associated with the bedrock. Mineralization ranged from approximately 300 to 1,000 mg/L TDS, with a mean of approximately 600 mg/L TDS. Chloride concentration was generally less than 10 mg/L and sulphate concentration less than 200 mg/L. Median sodium concentration was approximately 200 mg/L. The data were collected in the 1970s and 1980s (Appendix 5).
3.4.7
Conceptual Model of Groundw ater Flow
Conceptually, groundwater resources of relevance to Pigeon Lake occur within three groundwater flow systems, namely: (1) a local system located within the drift (from a few metres up to 40 m in thickness), (2) an upper bedrock system located within the upper 30 m of the Paskapoo Formation, and (3) a deeper bedrock system located within the lower 30 m of the Paskapoo Formation and possibly the more permeable, upper portions of the Scollard Formation. The shallow local system is influenced by local topography and shorter-term precipitation events. It recharges at local hills and discharges as base flow largely to local streams and Pigeon Lake. With increased depth, the bedrock flow systems are influenced by larger scale topography and longer term climatic influences. Below the Scollard
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Formation, the groundwater likely becomes saline (i.e. approaches 4,000 mg/L TDS or greater). Expected quality of the Paskapoo Formation groundwater is good, with mineralization generally less than 1,000 mg/L TDS, but recent hydrochemical data appear unavailable. The upper bedrock system may be hydraulically connected to Pigeon Lake, but no specific data are available to characterize the connection. The topography and hydrostratigraphy (Figure 12) suggest that groundwater discharge from the upper Paskapoo Formation to Pigeon Lake is possible. The regional topographic gradient in Figure 12 is approximately 0.01. Assuming the lateral hydraulic gradient in the Paskapoo Formation is a subdued replica of the topography, a lateral hydraulic gradient -5 of 0.007 is reasonable. Average hydraulic conductivity of the Paskapoo Formation is 1 x 10 m/s. The upper Paskapoo Formation thickness is approximately 30 m and the length of Pigeon Lake normal to the presumed direction of groundwater flow (i.e. northeast) is approximately 16 km. Assuming the hydraulic “capture zone” of the lake is one-third wider than the lake (i.e. 20.8 km), the estimated deeper 3 3 groundwater contribution to the lake is approximately 0.05 m /s (i.e. 4,320 m /day), which is reasonably consistent with the upper Paskapoo groundwater input used for the water balance model (Section 4).
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3.5
Water Quality
3.5.1
Summar y of Pi geon Lake Water Qualit y
Total phosphorus concentrations in Pigeon Lake and lake water levels are summarized in Figure A below. Figure A Total Phosphorus and Water Levels in Pigeon Lake
0.08
850.400
0.07
850.300 850.200
Total Phosphorus (mg/L)
0.06
Total Phosphorus 850.100
0.05 850.000 0.04 849.900
WATER_LEVEL Linear (Total Phosphorus)
0.03 849.800 0.02
849.700
0.01
849.600
0 Feb-82
849.500 Aug-87
Jan-93
Jul-98
Jan-04
Jul-09
The green line in Figure A indicates the Alberta and Canadian Council of Ministers of the Environment (CCME 2007) Surface Water Guidelines for the Protection of Aquatic Life (PAL) value for phosphorus. Total phosphorus levels appear to have exceeded the CCME-PAL guideline of 0.05 mg/L more regularly since 2003, with all but one sample exceeding the guideline, although recent data are sparse. Decreases in lake water levels can lead to the increasing concentration of phosphorus levels, which can lead to greater plant and algal productivity within a lake. External sources of phosphorus to a lake include manure, fertilizer, sewage, and dustfall. If water levels continue to decline, and land use practices remain constant, increased phosphorus levels in the lake can be expected. Pigeon Lake is mesotrophic to eutrophic, with total phosphorus concentrations ranging from 0.01 to 0.07 mg/L. Seasonal variations in concentrations of total phosphorus and chlorophyll a, an indicator of algal productivity, can be considerable. Total phosphorus and chlorophyll a concentrations typically
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increase during summer in shallow lakes of Alberta, with peak levels occurring in late August. Increasing concentrations of phosphorus in the water column can lead to algal blooms. The relatively long retention time of water in Pigeon Lake (over 100 years) exacerbates the problem because little flushing allows the phosphorus to accumulates over time. Figure B below compares total nitrogen levels with lake levels over time.
Figure B Total Nitrogen and Water Levels in Pigeon Lake
1.4
850.400 850.300
1.2
850.200 Total Nitrogen
Total Nitrogen (mg/L)
1 850.100 0.8
850.000
0.6
849.900
WATER_LEVEL Linear (Total Nitrogen)
849.800 0.4 849.700 0.2
849.600
0 Feb-82
849.500 Aug-87
Jan-93
Jul-98
Jan-04
Jul-09
The green line indicates the CCME (2007) surface water guideline for nitrogen (1 mg/L). Total nitrogen levels show a temporal pattern similar to total phosphorus, i.e. increasing nitrogen concentrations as water levels have declined, and recently exceeding the CCME-PAL water quality guideline. A decrease in water levels can lead to a concentrating effect on nitrogen levels, similar to phosphorus. Sources of nitrogen to a lake include manure, fertilizer, sewage, dustfall, and biological fixation by certain types of algae. Nitrogen is a nutrient that can encourage plant and algal growth within a lake. The ratio of total nitrogen to total phosphorus (the TN:TP ratio) is shown below in Figure C.
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Figure C Total Nitrogen to Total Phosphorus Ratio and Water Levels in Pigeon Lake
70
850.400 850.300
Total Nitrogen : Total Phosphorus
60 850.200 TN:TP Ratio
50 850.100
WATER_LEVEL 40
850.000
30
849.900
Linear (TN:TP Ratio)
849.800 20 849.700 10 849.600 849.500
0 Feb-82
Aug-87
Jan-93
Jul-98
Jan-04
Jul-09
The TN:TP ratio affects nutrient availability for phytoplankton growth. When nutrient levels are balanced and no single nutrient is limiting, the TN:TP ratio is almost always 16:1 (commonly referred to as the Redfield ratio). Deviations from this ratio will indicate either nitrogen or phosphorus limitation. In the early 1980s, Pigeon Lake was highly phosphorus-limited (TN:TP ratio of 40:1) and over time the ratio has decreased, but the lake still remains phosphorus limited (TN:TP of approximately 20:1 in the most recent samples). The changing ratio suggests that, although nitrogen input to the lake is increasing, total phosphorus input is increasing at a greater rate. Numerous studies have been done on the TN:TP ratio and its effects on phytoplankton communities within lakes. A switch to cyanobacterial (blue-green algae) dominance, as seen in Pigeon Lake over the past few years, can result from a number of factors, including changes in the TN:TP ratio (Dokulil and Teubner 2000). Low TN:TP ratios benefit cyanobacteria (Smith 1983). Elevated lake temperatures are also preferred. Since cyanobacteria are not a good food source for many aquatic organisms, their populations can flourish unchecked (Elser 1999). In general, the depth of the euphotic (or productive) zone is approximately 2 to 3 times the Secchi depth of the lake (Kalff 2002). The Secchi depth of Pigeon Lake is about 2 m, which means that plants and algae can grow at depths up to 4 to 6 m (see also Haag and Noton 1981). The abundance of shallow, well-lit underwater habitat yields conditions that can further encourage the growth and dominance of cyanobacteria.
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Chlorophyll a levels are shown below in Figure D.
Figure D Chlorophyll a and Water Levels in Pigeon Lake
50
850.400
45
850.300
Chlorophyll-a (µg/L)
40
850.200 Chlorophyll-a
35 850.100
WATER_LEVEL
30 850.000
Linear (Chlorophyll-a)
25 849.900 20 849.800 15 849.700
10
849.600
5 0 Feb-82
849.500 Aug-87
Jan-93
Jul-98
Jan-04
Jul-09
Chlorophyll a is a photosensitive pigment present in most algae and plants. Its density in surface water samples provides a proxy for total algal biomass within a water body. In Figure D, the relationship between chlorophyll a level and lake level appears weak, which suggests that Pigeon Lake is generally productive no matter the water level (i.e. the dominant species of algae may change but the overall productivity remains the same). Due to the large size and shallow depth of Pigeon Lake, water mixes from the lake surface to the bottom on windy days during most of the open-water period. As a result, the water temperature is generally uniform (Bidgood 1972), with dissolved oxygen concentrations remaining relatively stable throughout the water column to a depth of 8 m. Only at depths greater than 8 m does the water column become devoid of oxygen (Figure E below). Dissolved oxygen may also be depleted near the lake bottom by late winter; however, winterkill of fish is unlikely, because there is sufficient dissolved oxygen in the upper portions of the water column. Similarly, water temperature is relatively constant throughout the water column (Figure E), and the lake does not stratify (i.e. no thermocline forms to separate warmer water near the surface from colder water at greater depths).
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Figure E Temperature and Dissolved Oxygen Concentrations in Pigeon Lake
Source: McEachern et al. (2001)
The CCME recreational water quality guideline for fecal coliforms is 200 colony forming units (CFU) of E. coli per 100 mL of sample (CCME 2007). Ma-Me-O Beach coliform levels have approached the guideline level (a 180 CFU value was measured in September 2006, Table C below) at which time a health advisory was posted at the beach. Fecal coliform bacteria are of concern from a human health perspective. When counts of these organisms are elevated, the risk of waterborne gastroenteritis increases for swimmers and other recreational lake users. The presence of these bacteria in aquatic environments may indicate that the water has been contaminated with the fecal material from humans or animals (e.g. cattle, swine).
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Table C Fecal Coliform Counts (as CFU) per 100 mL in Pigeon Lake (2006) Locations
Mean
Low
High
Crystal Springs
50
---
---
Grandview
15
10
20
Ma-Me-O Beach
104
10
180
Mulhurst Bay
60
---
---
Poplar Bay
20
---
---
Silver Beach
13
10
20
Source: David Thompson Regional Health Authority (2006)
3.5.2
Comparison to Other Lakes
Select water quality parameters (Table D below) were compared between Pigeon Lake and four other lakes in the area: Wabamun, Buck, Gull, and Sylvan Lake. Buck Lake and Pigeon Lake were somewhat hydrochemically similar, only displaying variance in total phosphorus concentrations. With respect to total phosphorus concentration, Wabamun and Pigeon Lake were similar, and Sylvan Lake was lower. Similarly for total nitrogen, Pigeon Lake was similar to Wabamun Lake, and less than Gull Lake but greater than Sylvan Lake.
Table D Comparison of Select Water Quality Parameters Lake
Buck
Wabamun
Total Phosphorus (mg/L)
0.058
Total Nitrogen
1
Gull
Sylvan
Pigeon
0.032
0.044
0.021
0.035
0.87
0.99
1.61
0.718
0.92
TN:TP
24
34
39
37
24
pH
8.3
8.5
9.1
8.8
8.4
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Lake
Buck
Wabamun
TDS (mg/L)
125
Electrical Conductivity Chlorophyll a
1
Gull
Sylvan
Pigeon
261
768
336
157
230
453
1,207
589
289
31.5
9.7
8.3
4.5
17.5
1
includes west and east bays, main basin and Moonlight Bay Source: Alberta Environment (2009)
All of the lakes appear phosphorus-limited, with TN:TP ratio ranging from 24:1 to 39:1, and are mesotrophic to eutrophic. Chlorophyll a varies widely between the lakes, with Buck Lake showing a high of 31.5, and Sylvan Lake a low of 4.5. Sylvan Lake has a relatively clear water column with Secchi depths often greater than 4 m, likely due to the significant groundwater contribution to the lake (27 to 35% of total annual lake water input, Baker 2009). TDS (and hence electrical conductivity) are highest in Gull Lake.
3.6
Land Use Changes
To identify changes in land use practices that may have affected water quality in Pigeon Lake, data from the Canada Census of Agriculture from 1971 to 2006 (Agriculture and Agri-Food Canada and Statistics Canada 2008) were used to estimate changes in cattle population and agricultural land use within the Pigeon Lake watershed. Changes in land use that may be affecting nutrient levels in Pigeon Lake are described below. From 1971 to 2006, cattle population within the Pigeon Lake watershed has increased from approximately 3,000 to 7,000 (Figure F below), based on the agricultural census data. Over the same time period, the total swine population in the watershed increased from approximately 1,000 to nearly 3,000.
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Figure F Total Cattle and Swine in the Pigeon Lake Watershed
9000
3500
8000
TCATTL
3000
7000
TPIGS 2500
2000
5000 4000
1500
Total Pigs
Total Cattle
6000 Linear (TCATTL) R² = 0.6559 Linear (TPIGS) R² = 0.7755
3000 1000 2000 500
1000 0 1960
0 1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
As a result of livestock population increases (primarily cattle and swine) total manure production has increased 2- to 3-fold from 1971 to 2006 (Figure G below).
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Figure G Estimated Manure Production in the Pigeon Lake Watershed
90000000
Total Manure Production (kg)
80000000 70000000 60000000 50000000
LVKGMAN
40000000
Linear (LVKGMAN)
30000000 R² = 0.8028 20000000 10000000 0 1960
1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
Figure H below indicates a three-fold increase in manure phosphorus levels within the Pigeon Lake watershed, based on the agricultural census data.
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Figure H Manure Phosphorus Levels in the Pigeon Lake Watershed
160000
Phosphorus from Manure (kg)
140000 120000 100000 LVKGP
80000
Linear (LVKGP) 60000 R² = 0.8269 40000 20000 0 1960
1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
Similarly, Figure I below indicates a three-fold increase in manure nitrogen levels within the Pigeon Lake watershed.
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Figure I Manure Nitrogen Levels in the Pigeon Lake Watershed
600000
Nitrogen from Manure (kg)
500000
400000 LVKGNI 300000
Linear (LVKGNI)
200000
R² = 0.8176
100000
0 1960
1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
Cropland within the Pigeon Lake watershed has increased in area by approximately 700 ha since 1971 (Figure J below) based on the agricultural census data.
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Figure J Area of Cropland within the Pigeon Lake Watershed 5600 5500
Total Cropland Area (ha)
5400 5300 5200 CROPLND Linear (CROPLND)
5100
R² = 0.3386 5000 4900 4800 4700 1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
Year
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
Fertilized cropland area has increased two-fold within the Pigeon Lake watershed (Figure K). Fertilized cropland can be a source of readily-available phosphorus and nitrogen both from dustfall and from the runoff of excess fertilizer application.
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Figure K Area of Fertilized Cropland within the Pigeon Lake Watershed
3500
Fe rtilized Cropland Area (ha)
3000
2500
2000 FERTIL Linear (FERTIL)
1500
R² = 0.4446 1000
500
0 1960
1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
As noted in the nutrient budget for Pigeon Lake (Mitchell and Prepas 1990), dust fall can contribute a large proportion of the theoretical external phosphorus load to the lake (Table E below). Agricultural practices (30% of the total external phosphorus supply to Pigeon Lake) have been shown to increase nutrient concentrations in aquatic ecosystems and may have contributed to the algal bloom in Pigeon Lake in 2007 (Pigeon Lake Watershed Association 2007). Phosphorus can come from point and diffuse agricultural sources. Point sources include runoff from farms and dairies and seepage from manure storage. Diffuse sources include fields, where manure spreading, soil erosion, surface runoff, and drainage represent the major pathways of phosphorus transport into aquatic ecosystems (Daniel et al. 1998).
Table E Theoretical Total External Phosphorus Loading to Pigeon Lake Sources Watershed
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Phosphorus (kg/yr)
Percent of total
Forest/brush
900
16
Agricultural/cleared
1,702
30
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Sources Residential/cottage Sewage
1
2
Precipitation/dust fall Total
Phosphorus (kg/yr)
Percent of total
770
14
133
2
2,127
38
5632
100
2
Annual areal loading (g/m of lake surface) 0.06 1“
Residential/cottage” sources include runoff from these land cover/use types, excluding point sources such as sewage effluent from leaking septic systems
2
Sewage sources unmeasured; assumed that 4% of all sewage effluent from residences and camps entered the lake (see Mitchell 1982). The largest sources of phosphorus are indicated by bold text. Source: Alberta Environment (1989)
The transport of soil particles by wind can also be an environmental issue (e.g. Niemeyer et al. 1999), particularly in regions where clay and silt form a major component of soil deposits, such as central Alberta. Apart from naturally-occurring wind erosion, agricultural tillage practices can also redistribute soil particles and nutrients; however, unlike wind erosion events, dust production due to tillage extends over much longer periods, normally on the order of weeks. Thus, the amounts of soil released from fields by tillage can be considerable. Dust storms redistribute fine soil particles along with nutrients, pesticides, and microorganisms (e.g. fungi, bacteria, viruses). Moreover, the concentrations of nutrients and pesticides in airborne dust can be up to ten times greater than in topsoil (Fritz 1993). This dust can be deposited into aquatic ecosystems.
3.7
Declining Lake Water Levels and Effects on Habitat
Lake bathymetry was examined to quantify the effects of decreasing lake level on physical characteristics of the lake. The bathymetric maps and analyses are based on digital elevation model bathymetric data from the Alberta Geological Survey (2008). These data are a 50 m grid of lake bed elevations, based on digitized contour maps previously produced for the lake, updated and corrected using satellite imagery. Lake levels were defined based on the sill established by the weir on Pigeon Lake (849.95 masl). This was the lake level against which all other contours were calculated and compared. Lake area and lake volume under reduced water levels were calculated by excluding all grid points that lay above a given elevation. Lake areas were determined by a count of all such points multiplied by the area of a single 2 grid cell (2,500 m ); volumes were calculated by multiplying the depth at each grid point by the grid cell area, and summing across all points that fell below the specified elevation.
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Declining lake levels lead to littoral zone area loss (Figure L below), water volume loss (Figure M below), and loss of deep, colder water refugia for fish. Bathymetric analysis indicates that a water level decrease by 0.5 m below the sill level results in a loss of about 10% of the area below 9 m depth (Figure L). If lake levels continue to drop, the deep water habitat is lost at a rate of approximately 17% per half meter drop in water level.
Figure L Percentage of Normal Lake Area Lost as Lake Levels Drop
40%
50%
60%
70%
80%
90%
100%
0 -1 -2
Sill Level 0.25 m Drop
-3
De pth (m)
0.5 m Drop -4
0.75 m Drop
-5
1.0 m Drop 1.25 m Drop
-6
1.5 m Drop
-7 -8 -9
A drop in water levels of 1.5 m from the sill level results in a loss of lake volume on the order of 20% (Figure M below).
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Figure M Percentage of Lake Volume Lost as Water Levels Drop
40%
50%
60%
70%
80%
90%
100%
0 -1 -2
Sill Level 0.25 m Drop
-3
De pth (m)
0.5 m Drop -4
0.75 m Drop
-5
1.0 m Drop 1.25 m Drop
-6
1.5 m Drop
-7 -8 -9
With respect to the loss of littoral area, the greatest loss would occur along the southeast shores of the lake, in the Ma-Me-O and Norris Beach areas. Littoral area is important fish habitat and spawning ground and the loss of this area could lead to a decline in the success of local fish populations. As noted in Mitchell and Prepas (1990), one of the most important lake whitefish spawning areas in Pigeon Lake is the southeast shore. The euphotic zone of Pigeon Lake extends to a depth of approximately 4 m (Haag and Noton 1981). Loss of area or volume above this depth will result in a loss of littoral habitat. If water levels are sustained at lower levels, the littoral habitat will eventually re-establish as vegetation takes root in formerly deep-water habitats. However, if levels drop too rapidly for re-establishment to keep pace with the loss of littoral habitats, it may lead to the collapse of species that depend on these areas, including important food- and sport fish species such as lake whitefish and northern pike.
3.8
Licensed and Unlicensed Water Use
“Water allocation” refers to the amount of water that licensees are entitled to withdraw from surface or groundwater sources. “Licensed water use” is the amount of water that licensees are expected to consume, including any losses that occur. “Consumption” means that the water is taken out of the basin and unavailable for re-use (e.g. oilfield injection or export crop production). The difference between water withdrawal and water consumption is called return flow, which is put back in the basin
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and available for re-use, although water quality may have changed during use (e.g. treated sewage, irrigation run-off, industrial cooling water). Actual water use is the actual amount of water consumed (or lost to re-use), and is the difference between the volume actually withdrawn and the volume actually returned. Data supplied by AENV indicated 45 active surface water licenses and 61 active groundwater licenses within the Pigeon Lake watershed (Appendix 6). The total surface water allocation was 3 3 361,820 m /year (990 m /day). Most of the surface water allocations were for agricultural use. The 3 largest allocation (333,330 m /year) was for Imperial Oil Resources Ltd., although that specific allocation appears no longer used. Most of the groundwater allocations were also for agricultural use, 3 but the larger allocations (greater than 10,000 m /year) were for residential developments. Total 3 3 groundwater allocation was 118,609 m /year (325 m /day). Information on actual water use appears unavailable. Licenses are not required for rural residential groundwater use. The AENV database indicated 3 2,768 wells within the Pigeon Lake Watershed (Appendix 6). Assuming an annual diversion of 76 m 3 per well (i.e. 208 L/day) gives a total groundwater withdrawal of approximately 210,000 m /year 3 (575 m /day). In the water balance model, average annual surface water and groundwater withdrawal 3 was estimated at 1,000 m /day (Section 4.3).
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4. 4.1
WATER BAL ANCE MODELLING General
A water balance model for Pigeon Lake covering the period from 1993 to 2009 was developed and calibrated (Figure 14) to simulate the existing flow conditions. The water balance model was developed on a daily basis covering the period from 1993 to September 2009, since the required information for all the modelling parameters was available for this period. Input data for 2009 were received as advance information subject to correction. A water balance model quantitatively accounts for the factors that influence lake levels. These factors include precipitation, lake evaporation, surface runoff, lake inflows/outflows, groundwater inflows/outflows, and water usage (surface water and groundwater). A hydrological water balance provides an understanding of the relative importance of the factors that influence lake levels. It can also be used as a management tool to estimate the amount of water required from another source to maintain a lake at a set level. A water balance model for Pigeon Lake was developed for this study using the following model input and output parameters: •
historical time series of estimated inflows to Pigeon Lake;
•
historical time series data of daily temperature and precipitation;
•
historical time series data of estimated lake evaporation;
•
long term average deep groundwater flow;
•
historical time series data of daily water levels;
•
historical water withdrawal data; and
•
stage-storage, stage-area, and stage-discharge relationships.
In order to develop a water balance model using these hydrologic variables, mathematical relations between stage (lake water level) and lake surface area and stage and lake storage volumes are required. Utilizing the stage-storage and stage-area relations from Mitchell and Prepas (1990), the following linear relations were developed for modelling purposes. Stage-Area relation: Area = (88.125 + 5076 x (Stage – 848)) km
2
Stage-Storage relation:
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6
Volume = (91.174 x (Stage – 848) + 465.065) x 10 m
3
Based on the outlet structure operating with one stop log and a fish ladder, the following relations were developed for modelling the outflow based on the lake level (i.e. Stage-Discharge). Outflow
= 0; if Stage < 849.65 masl
(7)
3
= 1.43 (Stage - 849.65) m /s; if 849.65 masl < Stage < 849.95 masl. 2
3
= 12.852(Stage - 849.8) + 2.083 (Stage - 849.8) + 0.041 m /s; if Stage > 849.95 masl. Model calibration is described below.
4.2
Calibration
The developed model was calibrated against the historical recorded water level data. Since actual measurements are unavailable of the runoff volume entering into the lake or lake evaporation, scaling factors on these parameters were used for the purpose of calibration. The difference of the median values between computed and recorded lake levels and the maximum lake level difference were used as the criteria to calibrate the model. Figure 14 shows the comparison between the modelled and recorded water levels at Pigeon Lake. The water levels simulated by the model match the actual water levels reasonably well, i.e. the difference of the median values between the computed and recorded water levels was 0.001 m and the maximum lake level difference was 0.2 m.
4.3
Relative Contributions of Hydrologic Variables to the Water Balance
Based on the calibrated water balance model for Pigeon Lake covering the period from 1993 to 2009, average values for all the key hydrologic variables and relative contribution of the variables were determined. Annual averages can be expressed in terms of millimetres (mm) taking into account the 3 3 area of the lake or volumetrically (dam or m ), as follows (Table F below and Figure 15).
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Table F Yearly Water Balance Year
Total Annual Volume Gain Due to Direct Precipitation 3 (dam )
Water Volume Loss Due to Lake Evaporation 3 (dam )
Annual Surface Water Inflow Volume 3 (dam )
Annual Ground water Inflow Volume 3 (dam )
Total Annual Outflow Volume 3 (dam )
Annual Withdrawals 3 Volume (dam )
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
35266 56508 42286 64080 50482 50238 43199 43227 39391 24873 38972 45762 42434 45789 39131 28429 28145
61757 66743 63085 59411 65850 70785 65240 63739 63859 57119 64362 59848 64280 66466 64174 62501 77654
13850 15360 8600 32140 26330 14040 40210 32100 12290 6980 19790 8040 25380 9330 45880 10660 4060
4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740
202 232 184 1521 17763 6363 16202 15803 3718 1532 643 97 345 181 2202 1176 163
350 350 350 350 350 350 350 350 350 350 350 350 350 350 350 350 350
Note: 1 dam3 = 1,000 m3
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Table G Average Annual Water Balance - Pigeon Lake mm
m /year
3
m /day
3
dam /year
3
Inputs Precipitation Deep Groundwater Inflow Surface Water Inflow Total Inflow Total Inputs
438.0 48.6 197.5 246.1 684.10
42,653,000 4,733,000 19,233,000 23,966,000 66,618,000
116,860 12,970 52,700 65,670 182,520
42,660 4,740 19,240 23,970 66,620
Outputs Lake Evaporation Withdrawals Outflow Total Outflow Total Outputs
657.0 3.6 41.3 44.8 701.8
63,979,000 347,000 4,020,000 4,367,000 68,345,000
175,290 960 11,020 11,970 187,250
63,980 350 4,020 4,370 68,350
Net Evaporation Net Inflow Net Deficit
219.0 201.3 17.7
21,327,000 19,599,000 1,728,000
58,440 53,700 4,740
21,330 19,600 1,730
Notes: m3/year, m3/day, and dam3/year values are rounded. 3
Over the modelled period, the total mean annual inflow to the lake is 246.1 mm (23,970 dam /yr) which 3 includes surface runoff of 197.5 mm (19,240 dam /yr) and groundwater from the upper bedrock of 3 48.6 mm (4,740 dam /yr). Groundwater from the upper bedrock accounts for approximately 20% and surface water inflow accounts for 80% of the average annual inflow to Pigeon Lake. The average 3 annual outflow including water withdrawals from the lake is 44.8 mm (4,370 dam /yr). The net inflow 3 (difference of total inflow and total outflow) is then 201.3 mm (19,600 dam /yr). The net evaporation 3 (deficit of precipitation to evaporation) over the lake surface is 219 mm (21,330 dam /yr) over the modelled period. Since the net inflow to the lake is less than the net evaporation, Pigeon Lake is 3 subject to a net annual deficit of 17.7 mm (1,730 dam /yr). Relative contribution is also expressed as a pie chart in Figure 16. 3
In other words, the average annual water input to the lake is 684.1 mm (182,520 m /day) compared to 3 an average annual water output of 701.8 mm (187,250 m /day). Direct precipitation onto the lake accounts for approximately 64% of the lake level response, surface water inflow from the watershed accounts for about 29% of the response and groundwater from upper bedrock (Paskapoo Formation) accounts for 7% of the response. Lake evaporation contributes to 93% of water loss from the lake
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surface, whereas the outflow over the weir through the lake outlet channel appears to be 6% and the water withdrawals from the lake accounts for 1% of total water loss on an annual basis averaged over the modelling period. Due to the small drainage area of the watershed, and the low groundwater contribution, relative volumes of precipitation and evaporation have an appreciable effect on lake levels.
4.4
Water Level Maintenance
The health of the lake, in terms of eutrophication and habitat loss, is at risk if water levels are not increased, either by natural processes or preventive measures. Natural processes include greater precipitation, less evaporation, and greater surface water runoff to the lake. Preventive measures include supply of water to the lake from another source to bring the lake level to a predetermined elevation. A predetermined elevation can be established on the basis of water quantity, water quality, and ecology. The water balance model can estimate the volume of water needed to achieve a predetermined lake level. The study did not address the quantity of water needed for ecological maintenance of the lake, nor did it evaluate in detail any sources for water diversion. These issues were beyond the scope of the present study. Nevertheless, the water balance model can indicate the volume of water that may be required to establish a sustainable lake based practically on weir crest elevation, assuming that regular overflow of the weir crest is good for water quality and, hence, the health of the lake.
4.5
Application of Water Balance Model
The water balance model represents existing conditions. It can also be used to evaluate various scenarios, including the effect of additional input (or output). Two hypothetical input scenarios to achieve a water level of 849.95 masl (existing weir crest elevation) were modelled: 3
3
•
(Scenario 1) adding a constant inflow rate of 0.5 m /s (43,200 m /day);
•
(Scenario 2) adding a constant inflow rate of 1 m /s (86,400 m /day).
3
3
For both scenarios, water was added only when the lake level was below the weir elevation. Figures 17 3 3 and 18, respectively, show the effects of adding 0.5 m /s and 1 m /s on lake water levels. To bring the lake level from the September 2009 level of 849.4 masl to 849.95 masl would take 3 3 approximately 3.8 years if the diverted rate was 0.5 m /s, and 1.8 years if the diverted rate was 1 m /s. From the modelling, it can be further estimated that the retention time could be decreased from >100 years in the current condition to 58 and 49 years by adding flow according to Scenarios 1 and 2, respectively. These hypothetical scenarios assumed that the similar hydrologic conditions will prevail in the future. In comparison to the above diversion rates, the average flow rate of the North
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3
6
3
Saskatchewan River is approximately 175 m /s (15.1 x 10 m /day), and the best Paskapoo wells have 3 3 reported yields up to approximately 0.04 m /s (3,500 m /day). The water balance model indicated that raising the September 2009 lake level (849.4 masl) to the weir 3 level (849.95 masl) would take approximately 3.8 years if an additional inflow of 0.5 m /s 3 3 (43,200 m /day) is added to the lake in Scenario 1 and 1.8 years at an inflow rate of 1 m /s 3 (86,400 m /day) in Scenario 2. The retention time could be decreased from >100 years in the current condition to 58 and 49 years by adding flow according to Scenarios 1 and 2, respectively.
4.6
Limitations of the Water Balance Model
The model was developed based on available historical data. Results of the water balance model compared well with the available historical data. Errors in the estimation of evaporation data can have significant effects on modelled lake levels for shallow water bodies such as Pigeon Lake. Since precipitation and evaporation data are spatially variable, adjustment factors in utilizing these hydrologic variables were used to match the measured and simulated lake levels. Limited data were available for the stations in the vicinity of the lake that were used to represent the generated historical surface runoff, therefore adjustment factors were applied in order to match the measured and simulated lake levels. The application of this model for future management scenarios is carried out with the assumption that historical hydrological response is representative of the response that can be expected in the future. The limitations of this assumption should be kept in mind when considering the modelling results.
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5.
DISCUSSION AND SYNTHESIS
Water levels in Pigeon Lake are inherently sensitive to precipitation and evaporation. Precipitation accounts for approximately 64% of lake level response and surface water inflow accounts for about 29%. Upper bedrock groundwater flow contribution is only about 7%. Lake water residence time exceeds 100 years. Lake evaporation accounts for approximately 93% of the water output from the lake. Water balance modelling indicates a net water deficit of 17.7 mm, with an average annual water 3 input to the lake is 684.1 mm (182,520 m /day) compared to an average annual water output of 701.8 3 mm (187,250 m /day). For the past 30 years, agricultural land use intensity in the Pigeon Lake watershed has increased, both in terms of livestock population and cropland area, resulting in an increased nutrient input to the watershed. When the lake level is below the weir elevation, nutrient concentrations (especially phosphorus) increase because nutrients can no longer be flushed from the lake. To bring the lake level from the September 2009 level of 849.4 masl to the weir level would take 3 3 approximately 3.8 years at a diversion rate of 0.5 m /s (43,200 m /day) under modelled Scenario 1, 3 3 and 1.8 years at a rate of 1 m /s (86,400 m /day) under modelled Scenario 2. The required pipe size to convey such flows into the lake from a water source would vary between 450 mm (18 inch) to 600 mm (24 inch) depending on the pumped flow rate. The retention time could be decreased from >100 years in the current condition to 58 and 49 years in Scenarios 1 and 2, respectively. These hypothetical scenarios assumed that the similar hydrologic conditions will prevail in the future. With water levels above the weir elevation, phosphorus in the lake would flush and water quality would be expected to improve, but nutrient input issues would still need to be addressed. The time it would take for measurable lake water quality improvement could be estimated by nutrient budget modelling, which was beyond the scope of this study. The implications of flushing on water quality downstream of the lake in Pigeon Lake creek would need to be assessed. Potential sources of supplementary water input (when the lake level is below the weir) include the North Saskatchewan River and/or the lower Paskapoo Formation. Average flow rate of the North 3 6 3 3 Saskatchewan River is 175 m /s (15.1 x 10 m /day), so the diversion required to supply 1 m /s 3 (86,400 m /day) represents approximately 0.6% of the river’s flow. Such a diversion would result in no loss to the North Saskatchewan River system since Pigeon Lake drains to the Battle River system, which drains back to the North Saskatchewan River. The costs of such a diversion would appear high but should be weighed against the net benefit of keeping the lake healthy, i.e. the total social, economic, and environmental value of Pigeon Lake to Albertans in terms of recreational, resource, and ecosystem use (e.g. Olewiler 2004). For the lower Paskapoo Formation, the study suggested that the yield of individual vertical wells would perhaps provide less than a tenth of the needed diversion volume. The approach to groundwater
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supply and its sustainability would need to be examined closely, from the perspectives both of current domestic/agricultural groundwater use and ecosystem functioning. For example, would the pumping affect groundwater in shallow residential- or agricultural-use wells? Would any valued spring- or fendependent ecosystems inside or beyond the Pigeon Lake watershed be affected by pumping? What would be the advantages and feasibility of horizontal collector wells? Water quality in the North Saskatchewan River and lower Paskapoo Formation would also need to be characterized with respect to compatibility with the lake.
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6.
CONCLUSIONS AND RECOMMENDATIONS
The hydrology of the Pigeon Lake watershed was studied. Results are summarized as follows. •
The ratio of drainage basin area to lake area is 2:1, which is relatively small. Lakes with a small drainage area to lake area ratio are generally more sensitive to climatic variation.
•
A water balance model was developed using daily time step covering the period from 1993 to 2009, since the required information for all the modelling parameters were available for this period. The model was developed and calibrated against existing flow conditions.
•
Over the modelled period, the total mean annual inflow to the lake is 246.1 mm (23,970 dam /yr) 3 which includes surface runoff of 197.5 mm (19,240 dam /yr) and groundwater from the upper 3 bedrock formation of 48.6 mm (4,740 dam /yr). Groundwater from the upper bedrock formation accounts for approximately 20% and surface water runoff accounts for 80% of the average annual inflow to Pigeon Lake. The average annual outflow including water withdrawals from the 3 lake is 44.8 mm (4,370 dam /yr). The net inflow (difference of total inflow and total outflow) is 3 201.3 mm (19,600 dam /yr).
•
The average annual precipitation in the watershed over the modelling period (1993 to 2009) is 3 3 438 mm (42,600 dam /yr) and the average annual lake evaporation is 657 mm (63,980 dam /yr). The net evaporation (deficit of precipitation to evaporation) over the lake surface is 219 mm 3 (21,330 dam /yr). Since the net inflow to the lake is less than the net evaporation, Pigeon Lake is subject to a net annual deficit.
•
Water balance modelling indicated a net water deficit of 17.7 mm (1,730 dam /yr), with average 3 annual water input to the lake of 684.1 mm (66,620 dam /yr) compared to an average annual 3 water output of 701.8 mm (68,350 dam /yr). Pigeon Lake levels have been generally declining since 1981.
•
In the Pigeon Lake area, the best aquifers are the sandstone units of the Paskapoo Formation, 3 from which yields of 150 to approximately 3,300 m /day from vertical wells are expected. Anticipated groundwater quality is good with mineralization below 1,500 mg/L TDS.
•
For the past 30 years, agricultural land use intensity in the Lake watershed has increased, both in terms of livestock population and cropland area, resulting in increased nutrient input to the watershed. When the lake is below the weir elevation, nutrients in the lake will tend to be more concentrated and lake water quality deterioration will accelerate.
•
A daily water balance model for Pigeon Lake was developed. According to the water balance model, relative annual contribution to lake water levels is as follows:
3
3
−
precipitation contributes 64% to the lake;
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−
surface water inflow to the lake (including shallow groundwater base flow) contributes 29%;
−
deeper groundwater inflow (upper Paskapoo Formation) contributes 7% of the total input to the lake;
−
lake evaporation accounts for 93% of the water output from the lake;
−
outflows through the lake outlet channel and withdrawals from the lake account for only 7%.
•
Due to the small drainage area of the watershed and the low groundwater contribution, relative volumes of precipitation and evaporation have a substantial effect on lake levels.
•
Eutrophication in the form of algal blooms will continue if the lake levels continue to decline, and there will be serious and perhaps irreversible effects on the health of the lake’s aquatic ecosystems.
•
Declining lake levels can lead to littoral zone area loss, which in turn can lead to the collapse of fish species populations that depend on these areas, including important food- and sport fish such as lake whitefish and northern pike.
•
Bringing the lake level from the September 2009 level to the weir level would take approximately 3 3 3.8 years at a diversion rate of 0.5 m /s (43,200 m /day) in Scenario 1 and 1.8 years at a rate of 3 3 1 m /s (86,400 m /day) in Scenario 2. The retention time could be decreased from >100 years in the current condition to 58 and 49 years in Scenarios 1 and 2, respectively. These hypothetical scenarios assumed that the similar hydrologic conditions will prevail in the future.
•
The required pipe size to convey such flows into the lake from a water source would vary between 450 mm (18 inch) to 600 mm (24 inch) depending on the pumped flow rate.
•
The time it would take for measurable lake water quality improvement at the above input rates could be estimated by nutrient budget modelling, which was beyond the scope of this study.
•
The implications of flushing on water quality downstream of the lake in Pigeon Lake creek would need to be assessed.
•
Potential sources of water input include the North Saskatchewan River and the lower Paskapoo 3 6 3 Formation. Average flow rate of the North Saskatchewan River is 175 m /s (15.1 x 10 m /day), 3 3 so that the diversion required to supply 1 m /s (86,400 m /day) represent approximately 0.6% of the North Saskatchewan River flow. The costs of such a diversion would appear high but should be weighed against the net benefit of keeping the lake healthy, i.e. the total social, economic, and environmental value of Pigeon Lake to Albertans in terms of recreational, resource, and ecosystem use.
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E00041301 5 March 2010 WorleyParsons Canada Ltd. Infrastructure & Environment Division Suite 705, 10240 - 124 Street Edmonton, AB T5N 3W6 CANADA Phone: +1 780 496 9055 Facsimile: +1 780 496 9575 www.worleyparsons.com © Copyright 2010 WorleyParsons
PIGEON LAKE WATERSHED ASSOCIATION HYDROLOGIC ASSESSMENT OF PIGEON LAKE HYDROGEOLOGY, WATER QUALITY, AND WATER BALANCE
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Disclaimer The information presented in this document was compiled and interpreted exclusively for the purposes stated in Section 1.2 of the document. WorleyParsons provided this report for Pigeon Lake Watershed Association solely for the purpose noted above. WorleyParsons has exercised reasonable skill, care, and diligence to assess the information acquired during the preparation of this report, but makes no guarantees or warranties as to the accuracy or completeness of this information. The information contained in this report is based upon, and limited by, the circumstances and conditions acknowledged herein, and upon information available at the time of its preparation. The information provided by others is believed to be accurate but cannot be guaranteed. WorleyParsons does not accept any responsibility for the use of this report for any purpose other than that stated in Section 1.2 and does not accept responsibility to any third party for the use in whole or in part of the contents of this report. Any alternative use, including that by a third party, or any reliance on, or decisions based on this document, is the responsibility of the alternative user or third party. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of WorleyParsons. Any questions concerning the information or its interpretation should be directed to H.Anaya or M. Maccagno.
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EXECUTIVE SUMMARY A hydrologic assessment of Pigeon Lake was completed in October 2009 for the Pigeon Lake Watershed Association. Lake levels have generally declined from an historical high of approximately 850.63 metres above sea level (masl) in July 1981 to a low of approximately 849.4 masl in September 2009. At the same time, agricultural land use intensity in the Pigeon Lake watershed has increased, both in terms of livestock populations and cropland area, resulting in an increased nutrient input to the lake. When the lake is below the weir crest, nutrients (especially phosphorus) are no longer flushed from the lake. If the lake levels continue to decline, eutrophication in the form of algal blooms will continue, and there will be serious and perhaps irreversible effects on the health of the lake’s aquatic ecosystems. The statistical analysis of the annual precipitation covering the period from 1961 to 2009 resulted in a long-term average annual precipitation of 454 mm. The average annual precipitation over the last 10 years is 389 mm. It is obvious that during the last 10 years the region has experienced a dry period. Furthermore, the annual precipitation has been generally declining since 1996. The average annual lake evaporation is 662 mm. Unless the net evaporation (i.e. deficit of precipitation to evaporation) is compensated by an adequate volume of surface water runoff and/or groundwater contribution, water levels of the lake will decline. A water balance model was developed using daily time steps covering the period from 1993 to 2009, since the required information for all the modelling parameters was available for this period. The model was developed and calibrated against existing flow conditions. Over the modelled period, the total 3 mean annual inflow to the lake was 246.1 mm (23,970 dam /yr) which includes surface runoff of 197.5 3 3 mm (19,240 dam /yr) and groundwater from the upper bedrock of 48.6 mm (4,740 dam /yr). Groundwater from the upper bedrock (Paskapoo Formation) accounts for approximately 20% of the average annual inflow to Pigeon Lake. The average annual outflow including water withdrawals from 3 the lake is 44.8 mm (4,370 dam /yr). The net inflow (difference of total inflow and total outflow) is then 3 3 201.3 mm (19,600 dam /yr). The net evaporation over the lake surface is 219 mm (21,330 dam /yr) over the modelled period. Since the net inflow to the lake is less than the net evaporation, Pigeon Lake 3 is subject to a net annual deficit of 17.7 mm (1,730 dam /yr). Lake water residence time exceeds 100 years. The longer the residence time, the greater the tendency toward eutrophication. Lake water residence time could be decreased by maintaining the lake level above the weir crest elevation so that more frequent outflow occurs. The water balance model indicated that raising the September 2009 lake level (849.4 masl) to the weir level (849.95 masl) would 3 3 take approximately 3.8 years if an additional inflow of 0.5 m /s (43,200 m /day) is added to the lake in 3 3 Scenario 1 and 1.8 years at an inflow rate of 1 m /s (86,400 m /day) in Scenario 2. The retention time could be decreased from >100 years in the current condition to 58 and 49 years by adding flow according to Scenarios 1 and 2, respectively. These hypothetical scenarios assumed that similar
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hydrologic conditions will prevail in the future. With water levels above the weir elevation, nutrients from the lake would flush and water quality would improve, but nutrient input issues would still need to be addressed. Potential sources of water input include the North Saskatchewan River and the lower Paskapoo 3 6 3 Formation. Average flow rate of the North Saskatchewan River is 175 m /s (15.1 x 10 m /day), so that 3 3 the diversion required to supply 1 m /s (86,400 m /day) represents approximately 0.6% of the North Saskatchewan River flow. The cost of such a diversion may appear high but should be weighed against the cost of not intervening in the health of the lake, i.e. quantify the total social, economic, and environmental value of a healthy Pigeon Lake in terms of recreational, resource, and ecosystem use. The approach to groundwater supply and its sustainability would need to be examined closely, from the perspectives both of current domestic/agricultural groundwater use and ecosystem functioning. Water quality in either the North Saskatchewan River and lower Paskapoo Formation would need to be characterized in detail with respect to compatibility with the lake. Timely and tangible actions are required or a continued downward trend in lake levels will lead to the ongoing and perhaps irreversible deterioration of the lake’s health.
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CONTENTS 1.
INTRODUCTION...................................................................................................................1 1.1
General.....................................................................................................................1
1.2
Objectives ................................................................................................................1
2.
METHODS AND SCOPE OF WORK....................................................................................2
3.
SUMMARY OF PIGEON LAKE HYDROLOGY ....................................................................4 3.1
Pigeon Lake Drainage Basin ...................................................................................4
3.2
Physical Characteristics of Pigeon Lake..................................................................4
3.3
Surface Water Hydrology .........................................................................................5 3.3.1
Surface Runoff and Outflows .................................................................................5
3.3.2
Precipitation and Lake Evaporation .......................................................................6
3.3.3
Historical Variation of Pigeon Lake Water Levels..................................................6
3.3.4
Residence Time .....................................................................................................7
3.4
Hydrogeology ...........................................................................................................7 3.4.1
Hydrostratigraphy...................................................................................................7
3.4.2
Results of Water Well Search ................................................................................9
3.4.3
Hydrogeologic Cross-Section ................................................................................9
3.4.4
Groundwater Flow, Groundwater Levels, and Pigeon Lake Water Levels ............9
3.4.5
Water Well Yields vs. Well Depths ......................................................................11
3.4.6
Water Well Chemistry ..........................................................................................11
3.4.7
Conceptual Model of Groundwater Flow .............................................................11
3.5
3.6
Water Quality .........................................................................................................13 3.5.1
Summary of Pigeon Lake Water Quality..............................................................13
3.5.2
Comparison to Other Lakes .................................................................................18 Land Use Changes ................................................................................................19
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3.7
Declining Lake Water Levels and Effects on Habitat.............................................26
3.8
Licensed and Unlicensed Water Use.....................................................................28
4.
WATER BALANCE MODELLING .......................................................................................30 4.1
General...................................................................................................................30
4.2
Calibration ..............................................................................................................31
4.3
Relative Contributions of Hydrologic Variables to the Water Balance...................31
4.4
Water Level Maintenance ......................................................................................34
4.5
Application of Water Balance Model ......................................................................34
4.6
Limitations of the Water Balance Model ................................................................35
5.
DISCUSSION AND SYNTHESIS........................................................................................36
6.
CONCLUSIONS AND RECOMMENDATIONS...................................................................38
7.
CLOSURE ...........................................................................................................................41
8.
REFERENCES....................................................................................................................43
Figures FIGURE 1
SITE LOCATION MAP
FIGURE 2
REGIONAL TOPOGRAPHY AND DRAINAGE
FIGURE 3
PIGEON LAKE BATHYMETRY
FIGURE 4
PIGEON LAKE STAGE-STORAGE AND STAGE-AREA RELATIONSHIPS
FIGURE 5
GENERATED INFLOW HYDROGRAPH
FIGURE 6
HISTORICAL LAKE LEVELS
FIGURE 7
ESTIMATED AND PUBLISHED LAKE EVAPORATION DATA
FIGURE 8
LAKE EVAPORATION AND PRECIPITATION 1993-2009
FIGURE 9
BEDROCK GEOLOGY
FIGURE 10
DRIFT THICKNESS
FIGURE 11
DRIFT GEOLOGY
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FIGURE 12
HYDROGEOLOGIC CROSS-SECTION A-A’
FIGURE 13
WELL YIELD VS. WELL DEPTH
FIGURE 14
MODELLED AND MEASURED LAKE WATER LEVEL
FIGURE 15
PIGEON LAKE WATERSHED AVERAGE ANNUAL WATER BALANCE
FIGURE 16
RELATIVE CONTRIBUTIONS TO PIGEON LAKE WATER BALANCE
FIGURE 17
MODEL APPLICATION – SCENARIO 1
FIGURE 18
MODEL APPLICATION – SCENARIO 2
Tables within Text TABLE A PHYSICAL CHARACTERISTICS OF PIGEON LAKE ..................................................... 4 TABLE B PRECIPITATION STATISTICS EDMONTON INTERNATIONAL AIRPORT................... 6 TABLE C FECAL COLIFORM COUNTS (AS CFU) PER 100 ML IN PIGEON LAKE (2006) ....... 18 TABLE D COMPARISON OF SELECT WATER QUALITY PARAMETERS ................................ 18 TABLE E THEORETICAL TOTAL EXTERNAL PHOSPHORUS LOADING TO PIGEON LAKE .. 25 TABLE F YEARLY WATER BALANCE ........................................................................................ 32 TABLE G AVERAGE ANNUAL WATER BALANCE - PIGEON LAKE .......................................... 33
Figures within Text FIGURE A TOTAL PHOSPHORUS AND WATER LEVELS IN PIGEON LAKE ........................... 13 FIGURE B TOTAL NITROGEN AND WATER LEVELS IN PIGEON LAKE .................................. 14 FIGURE C TOTAL NITROGEN TO TOTAL PHOSPHORUS RATIO AND WATER LEVELS IN PIGEON LAKE ..................................................................................................... 15 FIGURE D CHLOROPHYLL A AND WATER LEVELS IN PIGEON LAKE ................................... 16 FIGURE E TEMPERATURE AND DISSOLVED OXYGEN CONCENTRATIONS IN PIGEON LAKE .................................................................................................................... 17 FIGURE F TOTAL CATTLE AND SWINE IN THE PIGEON LAKE WATERSHED ....................... 20 FIGURE G ESTIMATED MANURE PRODUCTION IN THE PIGEON LAKE WATERSHED........ 21
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FIGURE H MANURE PHOSPHORUS LEVELS IN THE PIGEON LAKE WATERSHED.............. 22 FIGURE I MANURE NITROGEN LEVELS IN THE PIGEON LAKE WATERSHED...................... 23 FIGURE J AREA OF CROPLAND WITHIN THE PIGEON LAKE WATERSHED ......................... 24 FIGURE K AREA OF FERTILIZED CROPLAND WITHIN THE PIGEON LAKE WATERSHED ... 25 FIGURE L PERCENTAGE OF NORMAL LAKE AREA LOST AS LAKE LEVELS DROP ............ 27 FIGURE M PERCENTAGE OF LAKE VOLUME LOST AS WATER LEVELS DROP................... 28
Appendices APPENDIX 1
ESTIMATION OF LAKE EVAPORATION
APPENDIX 2
WATER WELL DRILLING REPORTS
APPENDIX 3
BOREHOLE ECC-2008-03
APPENDIX 4
GROUNDWATER LEVEL DATA
APPENDIX 5
WATER WELL YIELD AND CHEMISTRY DATA
APPENDIX 6
SURFACE WATER AND GROUNDWATER USE LICENSES
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1. 1.1
INTRODUCTION General
Pigeon Lake is one of the most popular recreational lakes in Alberta. The goals of the Pigeon Lake Watershed Association (PLWA) are to enhance, preserve, and protect the Pigeon Lake watershed. Concerned by lowered lake water levels and deteriorating lake water quality, the PLWA retained WorleyParsons to conduct a hydrological assessment of Pigeon Lake. The purposes of the hydrological assessment were to evaluate the hydrologic controls on lake levels, and identify possible scientifically-based solutions to the problems of lake level and water quality decline.
1.2
Objectives
The objectives of the hydrologic assessment were to: a)
develop a water balance model of the lake;
b)
assess the relative importance of the contributing factors that are responsible, or potentially responsible, for affecting lake levels, including groundwater/surface water interactions;
c)
evaluate lake water quality; and,
d)
if possible, develop recommendations for “next steps” that will contribute to the sustainability of the lake.
The assessment used available information (e.g. climate, hydrology, hydrogeology, water well, and water quality data) to develop an integrated conceptual model of the hydrology of the Pigeon Lake watershed.
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2.
METHODS AND SCOPE OF WORK
The following activities were carried out to meet the study objectives. Site Reconnaissance –a field visit was conducted on October 22, 2009 to confirm lake outlet geometry. Data Sources – publically-available topographic maps, land cover, land use change; bedrock and drift geology maps, geologic cross-sections, water well records, regional groundwater levels, groundwater and surface water chemistry data, previous and current hydrogeologic studies; lake bathymetry, historical water levels of Pigeon Lake, stream flow, and climate data; surface water and groundwater allocation and usage data, previous studies, and any other available information relevant to water balance and water quality. The data specifically included: •
recorded water level data for Pigeon Lake;
•
recorded climate information (i.e. precipitation and temperatures) at the Edmonton International Airport;
•
available recorded flow data for the following streams in the vicinity of the lake: −
Lloyd Creek Water Survey of Canada (WSC) station Number 05CCC00.
−
Muskeg Creek WSC station number 05FA912.
−
Maskwa Creek WSC station number 05FA014.
•
available information on physical characteristics, such as stage-storage, stage-discharge, and stage-area relationships for Pigeon Lake;
•
recorded water well drilling reports and active groundwater licenses (Alberta Environment [AENV] Groundwater Information Centre [GIC] database);
•
AENV groundwater observation wells network (GOWN); and
•
drilling logs from the Alberta Geological Survey.
Water quality data have been collected for Pigeon Lake since 1972. The University of Alberta conducted water quality testing from 1973 to 1975, as did AENV in 1971 to 1972, and sporadically from 1983 to 2008. In 1982, a water quality study was commissioned by the Battle River Regional Planning Commission, and in 1988 a volunteer citizen monitoring program was initiated by AENV. Lilley Environmental Consulting and Earle (1998) conducted a water quality assessment and drafted a “Pigeon Lake Watershed Management Plan”, which was subsequently adopted by the Counties of
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Leduc and Wetaskiwin and several summer villages on Pigeon Lake in 2000. The Alberta Lake Management Society (ALMS) sampled Pigeon Lake in 2001, and Aquality Environmental Consulting Ltd. sampled the lake for caffeine in 2006 and for nutrients following an algal bloom in the summer of 2007. A State of the Watershed Report was completed in 2008 (Aquality 2008). Data from the above sources are synthesized in Section 3.5. Data Analysis – this task included the following steps: •
climate trend analysis from regional stations (temperature, precipitation, and evapotranspiration);
•
site-specific and regional hydrological trend analysis (annual yield, peak flows, base flows, and lake levels);
•
hydrogeological mapping and cross-sections with respect to the lake and watershed, including topography, bathymetry, the presence and distribution of the major hydrogeologic units, groundwater recharge, groundwater levels, groundwater flow, and groundwater chemistry;
•
water quality analysis, including possible temporal trends, comparison of data to water quality guidelines, examination of surrounding land use;
•
licensed and estimated unlicensed water usage;
•
development of a conceptual, quantitative water balance; and
•
a description of model uncertainty.
Synthesis – integrate the data analysis to determine the major drivers for the observed changes in lake levels and lake water quality.
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3.
SUMMARY OF PIGEON LAKE HYDROLOGY
3.1
Pigeon Lake Drainage Basin
Pigeon Lake is located 60 km southwest of the City of Edmonton (Figure 1). Relief in the area ranges from level to undulating. Regionally, ground elevations range from approximately 1,200 m above sea level (masl) to the southwest to 600 masl in the northeast (Figure 2). The Pigeon Lake watershed is located within the Battle River Basin, which is a part of the North Saskatchewan River Basin. The North Saskatchewan River is about 50 km to the northwest. Approximately 50 percent of the watershed is forest-covered, 46 percent is cleared for agriculture, and 4 percent is developed for cottage and residential use (Mitchell and Prepas 1990).
3.2
Physical Characteristics of Pigeon Lake
Table A below presents a summary of the physical characteristics of Pigeon Lake.
Table A Physical Characteristics of Pigeon Lake Drainage Area
Lake Area
(km )
2
(km )
187
96.7
2
Ratio of Drainage Area to Lake Area
Maximum Volume
Mean Depth
Maximum Depth
(10 m )
(m)
(m)
2:1
600
6.2
9.1
6
3
Highest Water Level (masl)
Lowest Water Level (masl)
850.63
849.42
Source: Mitchell and Prepas (1990)
The ratio of drainage basin area to lake area (2:1) is small. Drainage area to lake surface area ratio indicates how a lake may respond to climatic fluctuation. Lakes with a small ratio of drainage area to lake area are generally more sensitive to climatic variation, since area of evaporation is large while area of land drainage is small. Water flows into the lake through several intermittent streams that drain the western and northwestern portions of the watershed. The outlet is Pigeon Lake creek, which flows in a southwesterly direction into the Battle River (Figure 2). The outflow from the lake has been controlled by an engineered weir structure since 1914. The outlet structure has been rebuilt twice: once in 1940 and the latest in 1986 (Mitchell and Prepas 1990). From 1986 to 1992, recommendations to Alberta Environment from the Association of Pigeon Lake Municipalities lead to weir operational procedures, adopted in November 1992, which are in place today. The weir crest is at an elevation of 849.95 masl. The width of the weir structure is approximately 5 m.
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Lake bathymetry is shown on Figure 3. The stage-storage and storage-area relationships are from Mitchell and Prepas (1990) and presented on Figure 4. As shown on Figure 4, the maximum depth is 6 3 approximately 9 m and the storage volume is 600 x 10 m . The surface area of the lake was 2 2 101.5 km at the highest recorded water level of 850.63 masl and 95.2 km at the lowest recorded water level of 849.4 masl.
3.3
Surface Water Hydrology
Several factors play a role in the water balance of a lake. In addition to surface runoff from within a drainage basin and outflow from the lake, two additional key hydrologic factors, namely, the volume of precipitation (gain of water) falling on the lake area and the volume of evaporation (loss of water) leaving the lake area, can play a major role in the water balance, particularly for the lakes with a small ratio of drainage area to lake area. Key variables with respect to the hydrology of Pigeon Lake are described below.
3.3.1
Surface Runoff and Outflow s
For this study, runoff contribution to Pigeon Lake was estimated using stream flow data from nearby Water Survey of Canada (WSC) gauging stations with similar hydrologic characteristics. The measured stream flows at the WSC stations consist of direct runoff components and base flow components. The base flow components include the contribution of shallow groundwater to the streams. The daily contributions of local surface runoff into the lake were computed using the recorded flow data of Lloyd Creek WSC Station No. 05CC009, Muskeg Creek WSC station No. 05FA912, and Maskwa Creek WSC Station No. 05FA014. Station locations are shown on Figure 2. Runoff contribution to Pigeon Lake was estimated by transposing stream flow data from these nearby WSC stations using a drainage area ratio adjustment for each time step. The generated data are shown on Figure 5, along with the historic precipitation data for the same period. The analysis of the generated inflow data series covering the period from 1993 to 2009 indicates an average annual surface runoff of 197.5 mm. The 1993 to 2009 period of record was selected since the required information for the modelling parameters and the outlet structure operation is complete for this time period. Data for the year 2009 were received as advance information subject to correction by the WSC. Surface water outflow from Pigeon Lake occurs when the water level exceeds the weir crest elevation of 849.95 masl. Since no hydrometric gauging station is located to record outflow from the lake (when it occurs) a general weir flow equation was used to estimate the outflow from the lake for the water balance model (Section 4). Utilizing the historical recorded water levels in the lake (Figure 6), the 3 surface outflow from the lake was estimated to vary between 0 and 2.0 m /s. Figure 6 also shows historical precipitation data.
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3.3.2
Precipitation and Lake Evaporation
Since site-specific precipitation data are unavailable, historical precipitation data recorded at the Edmonton International Airport were taken as representative data Pigeon Lake. The estimated monthly lake evaporation covering the period from 1969 to 1996 is available in Bothe (1999). However, an estimate of the daily lake evaporation covering the period to 2009 was a key requirement of the present study. Hence, the Thornthwaite water balance method was used to estimate lake evaporation. Measured temperature data from the Edmonton International Airport were used to estimate the missing period. Figure 7 compares the published and estimated daily evaporation. The procedure for estimating lake evaporation is provided in detail in Appendix 1. The summary of the statistical analysis of the annual precipitation covering the period from 1961 to 2009 is presented in Table B. The long term average annual precipitation is 454 mm and the average annual precipitation over the last 10 years is 389 mm. It is obvious that during the last 10 years the region has experienced a dry period. The long term average annual lake evaporation is 662.0 mm.
Table B Precipitation Statistics Edmonton International Airport Total Annual (mm)
Long Term (1961-2009)
Last Ten Years (2000-2009)
Minimum, mm
267.4
267.4
Maximum, mm
651.1
473.5
Average, mm
454.0
389.0
Median, mm
448.8
398.1
The annual precipitation and lake evaporation data covering the period from 1961 to 2009 are graphically shown on Figure 8. Data for the year 2009 were received as advance information subject to correction by the WSC. It is seen that the Pigeon Lake area is generally subject to more lake evaporation than the average annual precipitation. Furthermore, Figure 8 shows that the annual precipitation has been generally declining since 1996. Unless the net evaporation (i.e. deficit of precipitation to evaporation) is compensated by an adequate volume of surface water runoff and/or groundwater contribution, water levels of the lake will decline.
3.3.3
Historical Variation of Pigeon Lake Water Levels
The water levels of Pigeon Lake have been monitored since 1972 at WSC station No. 05FA013. The historical recorded water levels are shown on Figure 6. Recorded data for 2009 have been obtained as advance information subject to correction. Lake levels have generally declined from an historical high
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of approximately 850.63 masl in July 1981 to a low of approximately 849.4 masl in September 2009. Time of surface water outflow from the lake has also decreased (Figure 6).
3.3.4
Residence Time
Residence time (or lake retention time) is calculated as the total volume of water in a lake divided by the mean outflow rate, which gives the time that water (or a dissolved substance) remains in a lake. Residence time is an important indicator of lake water quality, since the longer the residence time the greater the tendency of a lake toward eutrophication (i.e. enrichment in dissolved nutrients that stimulate the growth of aquatic plant life, usually resulting in the depletion of dissolved oxygen). In the case of Pigeon Lake the estimated residence time (over 100 years, Mitchell and Prepas 1990) is considered relatively long. By contrast, the average lake-water residence time of Sylvan Lake is 20 to 35 years (Baker 2009).
3.4 3.4.1
Hydrogeology Hydr ostratigraphy
Available regional hydrogeologic maps and reports, and water well drilling records were reviewed to build a conceptual view of the hydrogeology of Pigeon Lake. East of Pigeon Lake, the near-surface bedrock unit is the Paleocene/Upper Cretaceous Scollard Formation (Figure 9). The Scollard Formation is an interbedded, interfingering sequence of argillaceous sandstone, siltstone, mudstone, and shale. The unit as a whole shows considerable lateral and vertical heterogeneity. The Scollard Formation contains a number of thick coal seams, including the Nevis Coal Seam (~0.5 m thick) and the Ardley Coal Seam (~2.5 m thick), mainly in the upper part of the unit. The Ardley Coal Seam is the most-extracted coal unit in the west-central Alberta Plains (Glass 1990). The Ardley Seam is a target of coal bed methane development. The total thickness of the Scollard Formation in the area is approximately 135 m (Hydrogeological Consultants Ltd. 2008). The base of groundwater protection (BGWP) or depth to “non-saline” groundwater (i.e. mineralization less than 4,000 mg/L total dissolved solids or TDS) is at approximately 620 masl beneath the Pigeon Lake area (EUB 2007), corresponding to a depth of approximately 240 mbgs. The BGWP in this area is considered to include the Scollard Formation as a member of the Paskapoo Formation as per Irish (1970). Below the Scollard Formation is the Battle Formation, a bentonitic shale that generally has poor potential as a groundwater resource. The near-surface bedrock unit below Pigeon Lake is the Paskapoo Formation (Figure 9), which consists of interbedded mudstone, siltstone, and sandstone, with some fossiliferous limestone, coal, and bentonitic beds. Prominent are 15 to 20 m thick medium- to coarse-grained sandstone units. Thickness of the Paskapoo Formation in the Pigeon Lake area is approximately 80 m. The regional dip
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of the Paskapoo Formation is to the southwest at approximately 3 to 4 m/km (Ceroici 1979). The base of the Paskapoo Formation subcrops approximately 10 km northeast of Pigeon Lake (Figure 9). Surface expressions of groundwater discharge, such as lakes, springs, and fens, could be expected to occur near the subcrop. The Paskapoo Formation is divided in ascending stratigraphic order into the Haynes, Lacombe, and Dalehurst members. Hydrogeological Consultants Ltd. (2008) interpreted that the Dalehurst Member occurs immediately west of Pigeon Lake (Figure 9). On the other hand, data collected by the Alberta Geological Survey (Energy Resources Conservation Board [ERCB] 2009a) suggest that the Dalehurst Member is actually located further west. In the Pigeon Lake area, the best aquifers are the sandstone units of the Paskapoo Formation, from 3 which yields of 150 to 650 m /day can be obtained (Ozoray 1972). According to Hydrogeological Consultants Ltd. (2008), bedrock deposits have estimated expected yields of approximately 10 to over 3 3 100 m /day with some wells reporting over 650 m /day. Ceroici (1979) reported yields up to 3 -4 3,300 m /day. The Alberta Geological Survey (ERCB 2009b) reported hydraulic conductivities of 10 to -6 10 m/s. Groundwater quality in the Paskapoo Formation is expected to be good with mineralization usually below 1,000 mg/L and often below 500 mg/L TDS. The chemical character is generally calciummagnesium-bicarbonate, but in the discharge areas of longer flow systems, sodium is the predominant cation and mineralization can be 1,500 mg/L or higher. Naturally-elevated sulphate, chloride, or iron concentrations are limited to small areas in isolated locations (Ozoray 1972). Although the Paskapoo Formation is one of the single largest sources of potable groundwater in the Canadian Prairies, little information is available regarding the sustainable yield and regional distribution of groundwater supply within the aquifer system (Grasby et al. 2008). In general, an approximately 3 to 40 m thickness of Quaternary drift deposits overlay the bedrock in the Pigeon Lake area (Figure 10). The overburden deposits generally consist of till, along with glaciofluvial and glaciolacustrine deposits, with some eolian and fluvial deposits (Figure 11). No buried preglacial valleys of significance appear present in the Pigeon Lake area. Shallow, local aquifers are possible in glaciofluvial deposits (i.e. sand and gravel meltwater channels within clayey till), as well as eolian sands. Recent fluvial deposits in the Battle Lake area and elsewhere could contain significant proportions of sand and gravel and, where saturated, could function as local aquifers. Yields of 3 between and 10 and 100 m /day can be expected for the drift aquifers (Hydrogeological Consultants Ltd. 2008). The chemical composition of groundwater in the drift is more variable than in bedrock, reflecting the more local nature of groundwater flow systems and lithologic variation within the drift. Groundwater in areas covered by lower permeability clayey till are typified by elevated sulphate and higher mineralization (Bibby 1974). Mineralization can vary naturally from less than 500 mg/L TDS in eolian
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deposits to more than 5,000 mg/L in clayey till. Drift waters can also be characterized by high iron content (i.e. up to 5 mg/L).
3.4.2
Results of Water Well Search
A search of the AENV water well database identified 2,768 wells within the Pigeon Lake watershed (Appendix 2). Of the 2,768 reported wells, more than 90% were classified as used for domestic purposes, and less than 9% were classified for stock, industrial, dewatering, municipal, irrigation, and investigation or monitoring use, and for coal test holes (Appendix 2). Reported total depths of the wells ranged from 6 to approximately 2,200 m with an average depth of 37 mbgs. Most of the shallow wells in the area (i.e. < 40 m deep) appear completed within the drift or upper bedrock deposits.
3.4.3
Hydr ogeologic Cr oss-Section
Records from the AENV water well database and ERCB/AGS borehole ECC-2008-03 (Appendix 3) were used to create a southwest to northeast hydrogeological cross-section across Pigeon Lake (Figure 12). Borehole ECC-2008-03 is located near the southeast end of Pigeon Lake (Figure 11). While the AENV database is extensive, the spatial accuracy of many of the wells is typically not greater than the centre of the quarter section (± 800 m; see Appendix 2, where some of the wells appear to be located in the lake itself). Moreover, drilling reports are not necessarily consistent in description of lithology. The most reliable lithological data were considered to be from research borehole ECC-200803. The cross-section (Figure 12) indicates approximately 6 to 45 m of unconsolidated drift above the bedrock. The drift deposits generally consist of sandy units and clayey till. The drift is underlain by approximately 60 to 70 m of fine- to coarse-grained sandstone units of the Paskapoo Formation. The ECC-2008-03 borehole indicates a 7 m thick mudstone unit separating the upper and lower Paskapoo (Appendix 3). Below the mudstone unit, coarse sandstone units appear thicker and more prevalent in that borehole. The upper Scollard Formation at borehole ECC-2008-03 borehole mainly comprises mudstone and siltstone with occasional thin coal and fine-grained sandstone units.
3.4.4
Groundw ater Flow , Groundw ater Level s, and Pigeon Lake Water Levels
Direction and gradient of groundwater flow laterally and vertically within the Pigeon Lake watershed, and with respect to the lake, could not be determined with the available AENV data. Nevertheless, groundwater flow is generally controlled by differences in ground surface topography. The water table configuration mirrors the ground surface topography. Differences in the energy of the groundwater,
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owing to differences in topography (and hence water table elevation, and hence position of the water in the earth’s gravitational field), continuously drive the flow of groundwater. The groundwater system as a whole is a hydraulically-continuous, three-dimensional flow field that crosses all manner of geologic boundaries. In drift aquifers of the Pigeon Lake area, groundwater flow is expected to flow from local topographic highs to local topographic lows (i.e. local flow systems on the order of 1 km). In the bedrock, groundwater flow systems on the order of tens of kilometres are expected (Figure 12). Due to the relatively thin drift (Figure 12), a hydraulic connection between Pigeon Lake and the Paskapoo Formation is possible. Regional AENV groundwater monitoring wells are located northwest and south of the Pigeon Lake watershed at distances of 2 km to approximately 60 km. Groundwater levels were plotted against Pigeon Lake water levels for comparison of hydrographic trends (Appendix 4). Evaluating the data from these wells (Appendix 4), a possible hydraulic connection between Pigeon Lake levels and relatively shallow aquifers (up to approximately 30 mbgs) is apparent. On the other hand, monitoring wells with screened intervals deeper than approximately 40 mbgs show little water level correlation. In particular, the following trends were observed. •
The groundwater levels in monitoring wells from approximately 5 to 30 mbgs show a good match with Pigeon Lake levels, suggesting either hydraulic continuity with upper drift and bedrock aquifers, or that the water levels in both the lake and upper aquifers are influenced by similar forcing functions (e.g. shorter-term precipitation events and climate fluctuation).
•
Deeper wells show a poorer match with lake levels, suggesting either poorer hydraulic connection between the lake and lower bedrock aquifers, or that the water levels in the lower aquifers are less sensitive to shorter-term forcing functions. Well ECC-2008-03 (Appendix 3) reported a 7 m thick mudstone/claystone zone located at approximately 25 m from the top of the -8 Paskapoo Formation (or 65 mbgs). Estimated hydraulic conductivity of the layer ranged from 10 -9 to 10 m/s (Appendix 3). Depending on the thickness and lateral continuity of this layer, the lower Paskapoo Formation could have limited connection with the upper Paskapoo Formation, hence limiting natural groundwater interactions with Pigeon Lake largely to the upper Paskapoo Formation.
Examining the trend of groundwater levels reported in drilling logs, Grasby et al. (2008) found a general drop in static groundwater level of approximately 3 m over approximately 50 years for the Paskapoo Formation. The drop was attributed either to an exponential increase in the number of drilled water wells (and hence groundwater use) or to increased drilling efficiency, which has made deeper drilling more feasible with time (and deeper wells tend to record a lower static groundwater level).
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3.4.5
Water Well Yields vs. Well Depths
The total number of water well records in the Pigeon Lake watershed listed in the AENV database is 2,768. Data on well yield (pumping test rate), well depth, test date, and test duration are available for 1,989 of those wells (Appendix 5). Figure 13 summarizes well yield versus well depth for wells completed in the Pigeon Lake watershed. Most of the wells in the watershed are completed at depth of less than 100 mbgs and have estimated 3 yields (pumping rates) less than 150 m /day, which may be due more to intended use (i.e. domestic) and testing limitations than hydrogeologic capability. Only ten wells showed yields greater than 3 500 m /day. Most of these wells were completed at a depth of approximately 50 mbgs, which would suggest completion in the upper Paskapoo Formation. Although the available data show no correlation between increased well depth and yield (Figure 13), greater yield would be expected in the lower Paskapoo because of greater available hydraulic head and higher hydraulic conductivity. If groundwater was to be considered as a possible source of supplementary water supply to Pigeon Lake, more detailed hydrogeological investigation would likely focus on sustainable yield of the lower Paskapoo Formation in the Pigeon Lake area.
3.4.6
Water Well Chemist r y
Geochemical data available in the AENV water well database (Appendix 5) were reviewed. Two main groundwater types were recognized: a calcium and/or magnesium bicarbonate water and a sodium bicarbonate water. Reported depths of these wells ranged from approximately 20 to 180 mbgs. Calcium-magnesium-bicarbonate water is expected to be associated more with the drift and sodium bicarbonate water is associated with the bedrock. Mineralization ranged from approximately 300 to 1,000 mg/L TDS, with a mean of approximately 600 mg/L TDS. Chloride concentration was generally less than 10 mg/L and sulphate concentration less than 200 mg/L. Median sodium concentration was approximately 200 mg/L. The data were collected in the 1970s and 1980s (Appendix 5).
3.4.7
Conceptual Model of Groundw ater Flow
Conceptually, groundwater resources of relevance to Pigeon Lake occur within three groundwater flow systems, namely: (1) a local system located within the drift (from a few metres up to 40 m in thickness), (2) an upper bedrock system located within the upper 30 m of the Paskapoo Formation, and (3) a deeper bedrock system located within the lower 30 m of the Paskapoo Formation and possibly the more permeable, upper portions of the Scollard Formation. The shallow local system is influenced by local topography and shorter-term precipitation events. It recharges at local hills and discharges as base flow largely to local streams and Pigeon Lake. With increased depth, the bedrock flow systems are influenced by larger scale topography and longer term climatic influences. Below the Scollard
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Formation, the groundwater likely becomes saline (i.e. approaches 4,000 mg/L TDS or greater). Expected quality of the Paskapoo Formation groundwater is good, with mineralization generally less than 1,000 mg/L TDS, but recent hydrochemical data appear unavailable. The upper bedrock system may be hydraulically connected to Pigeon Lake, but no specific data are available to characterize the connection. The topography and hydrostratigraphy (Figure 12) suggest that groundwater discharge from the upper Paskapoo Formation to Pigeon Lake is possible. The regional topographic gradient in Figure 12 is approximately 0.01. Assuming the lateral hydraulic gradient in the Paskapoo Formation is a subdued replica of the topography, a lateral hydraulic gradient -5 of 0.007 is reasonable. Average hydraulic conductivity of the Paskapoo Formation is 1 x 10 m/s. The upper Paskapoo Formation thickness is approximately 30 m and the length of Pigeon Lake normal to the presumed direction of groundwater flow (i.e. northeast) is approximately 16 km. Assuming the hydraulic “capture zone” of the lake is one-third wider than the lake (i.e. 20.8 km), the estimated deeper 3 3 groundwater contribution to the lake is approximately 0.05 m /s (i.e. 4,320 m /day), which is reasonably consistent with the upper Paskapoo groundwater input used for the water balance model (Section 4).
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3.5
Water Quality
3.5.1
Summar y of Pi geon Lake Water Qualit y
Total phosphorus concentrations in Pigeon Lake and lake water levels are summarized in Figure A below. Figure A Total Phosphorus and Water Levels in Pigeon Lake
0.08
850.400
0.07
850.300 850.200
Total Phosphorus (mg/L)
0.06
Total Phosphorus 850.100
0.05 850.000 0.04 849.900
WATER_LEVEL Linear (Total Phosphorus)
0.03 849.800 0.02
849.700
0.01
849.600
0 Feb-82
849.500 Aug-87
Jan-93
Jul-98
Jan-04
Jul-09
The green line in Figure A indicates the Alberta and Canadian Council of Ministers of the Environment (CCME 2007) Surface Water Guidelines for the Protection of Aquatic Life (PAL) value for phosphorus. Total phosphorus levels appear to have exceeded the CCME-PAL guideline of 0.05 mg/L more regularly since 2003, with all but one sample exceeding the guideline, although recent data are sparse. Decreases in lake water levels can lead to the increasing concentration of phosphorus levels, which can lead to greater plant and algal productivity within a lake. External sources of phosphorus to a lake include manure, fertilizer, sewage, and dustfall. If water levels continue to decline, and land use practices remain constant, increased phosphorus levels in the lake can be expected. Pigeon Lake is mesotrophic to eutrophic, with total phosphorus concentrations ranging from 0.01 to 0.07 mg/L. Seasonal variations in concentrations of total phosphorus and chlorophyll a, an indicator of algal productivity, can be considerable. Total phosphorus and chlorophyll a concentrations typically
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increase during summer in shallow lakes of Alberta, with peak levels occurring in late August. Increasing concentrations of phosphorus in the water column can lead to algal blooms. The relatively long retention time of water in Pigeon Lake (over 100 years) exacerbates the problem because little flushing allows the phosphorus to accumulates over time. Figure B below compares total nitrogen levels with lake levels over time.
Figure B Total Nitrogen and Water Levels in Pigeon Lake
1.4
850.400 850.300
1.2
850.200 Total Nitrogen
Total Nitrogen (mg/L)
1 850.100 0.8
850.000
0.6
849.900
WATER_LEVEL Linear (Total Nitrogen)
849.800 0.4 849.700 0.2
849.600
0 Feb-82
849.500 Aug-87
Jan-93
Jul-98
Jan-04
Jul-09
The green line indicates the CCME (2007) surface water guideline for nitrogen (1 mg/L). Total nitrogen levels show a temporal pattern similar to total phosphorus, i.e. increasing nitrogen concentrations as water levels have declined, and recently exceeding the CCME-PAL water quality guideline. A decrease in water levels can lead to a concentrating effect on nitrogen levels, similar to phosphorus. Sources of nitrogen to a lake include manure, fertilizer, sewage, dustfall, and biological fixation by certain types of algae. Nitrogen is a nutrient that can encourage plant and algal growth within a lake. The ratio of total nitrogen to total phosphorus (the TN:TP ratio) is shown below in Figure C.
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Figure C Total Nitrogen to Total Phosphorus Ratio and Water Levels in Pigeon Lake
70
850.400 850.300
Total Nitrogen : Total Phosphorus
60 850.200 TN:TP Ratio
50 850.100
WATER_LEVEL 40
850.000
30
849.900
Linear (TN:TP Ratio)
849.800 20 849.700 10 849.600 849.500
0 Feb-82
Aug-87
Jan-93
Jul-98
Jan-04
Jul-09
The TN:TP ratio affects nutrient availability for phytoplankton growth. When nutrient levels are balanced and no single nutrient is limiting, the TN:TP ratio is almost always 16:1 (commonly referred to as the Redfield ratio). Deviations from this ratio will indicate either nitrogen or phosphorus limitation. In the early 1980s, Pigeon Lake was highly phosphorus-limited (TN:TP ratio of 40:1) and over time the ratio has decreased, but the lake still remains phosphorus limited (TN:TP of approximately 20:1 in the most recent samples). The changing ratio suggests that, although nitrogen input to the lake is increasing, total phosphorus input is increasing at a greater rate. Numerous studies have been done on the TN:TP ratio and its effects on phytoplankton communities within lakes. A switch to cyanobacterial (blue-green algae) dominance, as seen in Pigeon Lake over the past few years, can result from a number of factors, including changes in the TN:TP ratio (Dokulil and Teubner 2000). Low TN:TP ratios benefit cyanobacteria (Smith 1983). Elevated lake temperatures are also preferred. Since cyanobacteria are not a good food source for many aquatic organisms, their populations can flourish unchecked (Elser 1999). In general, the depth of the euphotic (or productive) zone is approximately 2 to 3 times the Secchi depth of the lake (Kalff 2002). The Secchi depth of Pigeon Lake is about 2 m, which means that plants and algae can grow at depths up to 4 to 6 m (see also Haag and Noton 1981). The abundance of shallow, well-lit underwater habitat yields conditions that can further encourage the growth and dominance of cyanobacteria.
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Chlorophyll a levels are shown below in Figure D.
Figure D Chlorophyll a and Water Levels in Pigeon Lake
50
850.400
45
850.300
Chlorophyll-a (µg/L)
40
850.200 Chlorophyll-a
35 850.100
WATER_LEVEL
30 850.000
Linear (Chlorophyll-a)
25 849.900 20 849.800 15 849.700
10
849.600
5 0 Feb-82
849.500 Aug-87
Jan-93
Jul-98
Jan-04
Jul-09
Chlorophyll a is a photosensitive pigment present in most algae and plants. Its density in surface water samples provides a proxy for total algal biomass within a water body. In Figure D, the relationship between chlorophyll a level and lake level appears weak, which suggests that Pigeon Lake is generally productive no matter the water level (i.e. the dominant species of algae may change but the overall productivity remains the same). Due to the large size and shallow depth of Pigeon Lake, water mixes from the lake surface to the bottom on windy days during most of the open-water period. As a result, the water temperature is generally uniform (Bidgood 1972), with dissolved oxygen concentrations remaining relatively stable throughout the water column to a depth of 8 m. Only at depths greater than 8 m does the water column become devoid of oxygen (Figure E below). Dissolved oxygen may also be depleted near the lake bottom by late winter; however, winterkill of fish is unlikely, because there is sufficient dissolved oxygen in the upper portions of the water column. Similarly, water temperature is relatively constant throughout the water column (Figure E), and the lake does not stratify (i.e. no thermocline forms to separate warmer water near the surface from colder water at greater depths).
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Figure E Temperature and Dissolved Oxygen Concentrations in Pigeon Lake
Source: McEachern et al. (2001)
The CCME recreational water quality guideline for fecal coliforms is 200 colony forming units (CFU) of E. coli per 100 mL of sample (CCME 2007). Ma-Me-O Beach coliform levels have approached the guideline level (a 180 CFU value was measured in September 2006, Table C below) at which time a health advisory was posted at the beach. Fecal coliform bacteria are of concern from a human health perspective. When counts of these organisms are elevated, the risk of waterborne gastroenteritis increases for swimmers and other recreational lake users. The presence of these bacteria in aquatic environments may indicate that the water has been contaminated with the fecal material from humans or animals (e.g. cattle, swine).
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Table C Fecal Coliform Counts (as CFU) per 100 mL in Pigeon Lake (2006) Locations
Mean
Low
High
Crystal Springs
50
---
---
Grandview
15
10
20
Ma-Me-O Beach
104
10
180
Mulhurst Bay
60
---
---
Poplar Bay
20
---
---
Silver Beach
13
10
20
Source: David Thompson Regional Health Authority (2006)
3.5.2
Comparison to Other Lakes
Select water quality parameters (Table D below) were compared between Pigeon Lake and four other lakes in the area: Wabamun, Buck, Gull, and Sylvan Lake. Buck Lake and Pigeon Lake were somewhat hydrochemically similar, only displaying variance in total phosphorus concentrations. With respect to total phosphorus concentration, Wabamun and Pigeon Lake were similar, and Sylvan Lake was lower. Similarly for total nitrogen, Pigeon Lake was similar to Wabamun Lake, and less than Gull Lake but greater than Sylvan Lake.
Table D Comparison of Select Water Quality Parameters Lake
Buck
Wabamun
Total Phosphorus (mg/L)
0.058
Total Nitrogen
1
Gull
Sylvan
Pigeon
0.032
0.044
0.021
0.035
0.87
0.99
1.61
0.718
0.92
TN:TP
24
34
39
37
24
pH
8.3
8.5
9.1
8.8
8.4
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Lake
Buck
Wabamun
TDS (mg/L)
125
Electrical Conductivity Chlorophyll a
1
Gull
Sylvan
Pigeon
261
768
336
157
230
453
1,207
589
289
31.5
9.7
8.3
4.5
17.5
1
includes west and east bays, main basin and Moonlight Bay Source: Alberta Environment (2009)
All of the lakes appear phosphorus-limited, with TN:TP ratio ranging from 24:1 to 39:1, and are mesotrophic to eutrophic. Chlorophyll a varies widely between the lakes, with Buck Lake showing a high of 31.5, and Sylvan Lake a low of 4.5. Sylvan Lake has a relatively clear water column with Secchi depths often greater than 4 m, likely due to the significant groundwater contribution to the lake (27 to 35% of total annual lake water input, Baker 2009). TDS (and hence electrical conductivity) are highest in Gull Lake.
3.6
Land Use Changes
To identify changes in land use practices that may have affected water quality in Pigeon Lake, data from the Canada Census of Agriculture from 1971 to 2006 (Agriculture and Agri-Food Canada and Statistics Canada 2008) were used to estimate changes in cattle population and agricultural land use within the Pigeon Lake watershed. Changes in land use that may be affecting nutrient levels in Pigeon Lake are described below. From 1971 to 2006, cattle population within the Pigeon Lake watershed has increased from approximately 3,000 to 7,000 (Figure F below), based on the agricultural census data. Over the same time period, the total swine population in the watershed increased from approximately 1,000 to nearly 3,000.
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Figure F Total Cattle and Swine in the Pigeon Lake Watershed
9000
3500
8000
TCATTL
3000
7000
TPIGS 2500
2000
5000 4000
1500
Total Pigs
Total Cattle
6000 Linear (TCATTL) R² = 0.6559 Linear (TPIGS) R² = 0.7755
3000 1000 2000 500
1000 0 1960
0 1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
As a result of livestock population increases (primarily cattle and swine) total manure production has increased 2- to 3-fold from 1971 to 2006 (Figure G below).
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Figure G Estimated Manure Production in the Pigeon Lake Watershed
90000000
Total Manure Production (kg)
80000000 70000000 60000000 50000000
LVKGMAN
40000000
Linear (LVKGMAN)
30000000 R² = 0.8028 20000000 10000000 0 1960
1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
Figure H below indicates a three-fold increase in manure phosphorus levels within the Pigeon Lake watershed, based on the agricultural census data.
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Figure H Manure Phosphorus Levels in the Pigeon Lake Watershed
160000
Phosphorus from Manure (kg)
140000 120000 100000 LVKGP
80000
Linear (LVKGP) 60000 R² = 0.8269 40000 20000 0 1960
1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
Similarly, Figure I below indicates a three-fold increase in manure nitrogen levels within the Pigeon Lake watershed.
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Figure I Manure Nitrogen Levels in the Pigeon Lake Watershed
600000
Nitrogen from Manure (kg)
500000
400000 LVKGNI 300000
Linear (LVKGNI)
200000
R² = 0.8176
100000
0 1960
1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
Cropland within the Pigeon Lake watershed has increased in area by approximately 700 ha since 1971 (Figure J below) based on the agricultural census data.
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Figure J Area of Cropland within the Pigeon Lake Watershed 5600 5500
Total Cropland Area (ha)
5400 5300 5200 CROPLND Linear (CROPLND)
5100
R² = 0.3386 5000 4900 4800 4700 1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
Year
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
Fertilized cropland area has increased two-fold within the Pigeon Lake watershed (Figure K). Fertilized cropland can be a source of readily-available phosphorus and nitrogen both from dustfall and from the runoff of excess fertilizer application.
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Figure K Area of Fertilized Cropland within the Pigeon Lake Watershed
3500
Fe rtilized Cropland Area (ha)
3000
2500
2000 FERTIL Linear (FERTIL)
1500
R² = 0.4446 1000
500
0 1960
1970
1980
1990
2000
2010
Ye ar
Source: Agriculture and Agri-Food Canada and Statistics Canada (2008)
As noted in the nutrient budget for Pigeon Lake (Mitchell and Prepas 1990), dust fall can contribute a large proportion of the theoretical external phosphorus load to the lake (Table E below). Agricultural practices (30% of the total external phosphorus supply to Pigeon Lake) have been shown to increase nutrient concentrations in aquatic ecosystems and may have contributed to the algal bloom in Pigeon Lake in 2007 (Pigeon Lake Watershed Association 2007). Phosphorus can come from point and diffuse agricultural sources. Point sources include runoff from farms and dairies and seepage from manure storage. Diffuse sources include fields, where manure spreading, soil erosion, surface runoff, and drainage represent the major pathways of phosphorus transport into aquatic ecosystems (Daniel et al. 1998).
Table E Theoretical Total External Phosphorus Loading to Pigeon Lake Sources Watershed
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Phosphorus (kg/yr)
Percent of total
Forest/brush
900
16
Agricultural/cleared
1,702
30
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Sources Residential/cottage Sewage
1
2
Precipitation/dust fall Total
Phosphorus (kg/yr)
Percent of total
770
14
133
2
2,127
38
5632
100
2
Annual areal loading (g/m of lake surface) 0.06 1“
Residential/cottage” sources include runoff from these land cover/use types, excluding point sources such as sewage effluent from leaking septic systems
2
Sewage sources unmeasured; assumed that 4% of all sewage effluent from residences and camps entered the lake (see Mitchell 1982). The largest sources of phosphorus are indicated by bold text. Source: Alberta Environment (1989)
The transport of soil particles by wind can also be an environmental issue (e.g. Niemeyer et al. 1999), particularly in regions where clay and silt form a major component of soil deposits, such as central Alberta. Apart from naturally-occurring wind erosion, agricultural tillage practices can also redistribute soil particles and nutrients; however, unlike wind erosion events, dust production due to tillage extends over much longer periods, normally on the order of weeks. Thus, the amounts of soil released from fields by tillage can be considerable. Dust storms redistribute fine soil particles along with nutrients, pesticides, and microorganisms (e.g. fungi, bacteria, viruses). Moreover, the concentrations of nutrients and pesticides in airborne dust can be up to ten times greater than in topsoil (Fritz 1993). This dust can be deposited into aquatic ecosystems.
3.7
Declining Lake Water Levels and Effects on Habitat
Lake bathymetry was examined to quantify the effects of decreasing lake level on physical characteristics of the lake. The bathymetric maps and analyses are based on digital elevation model bathymetric data from the Alberta Geological Survey (2008). These data are a 50 m grid of lake bed elevations, based on digitized contour maps previously produced for the lake, updated and corrected using satellite imagery. Lake levels were defined based on the sill established by the weir on Pigeon Lake (849.95 masl). This was the lake level against which all other contours were calculated and compared. Lake area and lake volume under reduced water levels were calculated by excluding all grid points that lay above a given elevation. Lake areas were determined by a count of all such points multiplied by the area of a single 2 grid cell (2,500 m ); volumes were calculated by multiplying the depth at each grid point by the grid cell area, and summing across all points that fell below the specified elevation.
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Declining lake levels lead to littoral zone area loss (Figure L below), water volume loss (Figure M below), and loss of deep, colder water refugia for fish. Bathymetric analysis indicates that a water level decrease by 0.5 m below the sill level results in a loss of about 10% of the area below 9 m depth (Figure L). If lake levels continue to drop, the deep water habitat is lost at a rate of approximately 17% per half meter drop in water level.
Figure L Percentage of Normal Lake Area Lost as Lake Levels Drop
40%
50%
60%
70%
80%
90%
100%
0 -1 -2
Sill Level 0.25 m Drop
-3
De pth (m)
0.5 m Drop -4
0.75 m Drop
-5
1.0 m Drop 1.25 m Drop
-6
1.5 m Drop
-7 -8 -9
A drop in water levels of 1.5 m from the sill level results in a loss of lake volume on the order of 20% (Figure M below).
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Figure M Percentage of Lake Volume Lost as Water Levels Drop
40%
50%
60%
70%
80%
90%
100%
0 -1 -2
Sill Level 0.25 m Drop
-3
De pth (m)
0.5 m Drop -4
0.75 m Drop
-5
1.0 m Drop 1.25 m Drop
-6
1.5 m Drop
-7 -8 -9
With respect to the loss of littoral area, the greatest loss would occur along the southeast shores of the lake, in the Ma-Me-O and Norris Beach areas. Littoral area is important fish habitat and spawning ground and the loss of this area could lead to a decline in the success of local fish populations. As noted in Mitchell and Prepas (1990), one of the most important lake whitefish spawning areas in Pigeon Lake is the southeast shore. The euphotic zone of Pigeon Lake extends to a depth of approximately 4 m (Haag and Noton 1981). Loss of area or volume above this depth will result in a loss of littoral habitat. If water levels are sustained at lower levels, the littoral habitat will eventually re-establish as vegetation takes root in formerly deep-water habitats. However, if levels drop too rapidly for re-establishment to keep pace with the loss of littoral habitats, it may lead to the collapse of species that depend on these areas, including important food- and sport fish species such as lake whitefish and northern pike.
3.8
Licensed and Unlicensed Water Use
“Water allocation” refers to the amount of water that licensees are entitled to withdraw from surface or groundwater sources. “Licensed water use” is the amount of water that licensees are expected to consume, including any losses that occur. “Consumption” means that the water is taken out of the basin and unavailable for re-use (e.g. oilfield injection or export crop production). The difference between water withdrawal and water consumption is called return flow, which is put back in the basin
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and available for re-use, although water quality may have changed during use (e.g. treated sewage, irrigation run-off, industrial cooling water). Actual water use is the actual amount of water consumed (or lost to re-use), and is the difference between the volume actually withdrawn and the volume actually returned. Data supplied by AENV indicated 45 active surface water licenses and 61 active groundwater licenses within the Pigeon Lake watershed (Appendix 6). The total surface water allocation was 3 3 361,820 m /year (990 m /day). Most of the surface water allocations were for agricultural use. The 3 largest allocation (333,330 m /year) was for Imperial Oil Resources Ltd., although that specific allocation appears no longer used. Most of the groundwater allocations were also for agricultural use, 3 but the larger allocations (greater than 10,000 m /year) were for residential developments. Total 3 3 groundwater allocation was 118,609 m /year (325 m /day). Information on actual water use appears unavailable. Licenses are not required for rural residential groundwater use. The AENV database indicated 3 2,768 wells within the Pigeon Lake Watershed (Appendix 6). Assuming an annual diversion of 76 m 3 per well (i.e. 208 L/day) gives a total groundwater withdrawal of approximately 210,000 m /year 3 (575 m /day). In the water balance model, average annual surface water and groundwater withdrawal 3 was estimated at 1,000 m /day (Section 4.3).
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4. 4.1
WATER BAL ANCE MODELLING General
A water balance model for Pigeon Lake covering the period from 1993 to 2009 was developed and calibrated (Figure 14) to simulate the existing flow conditions. The water balance model was developed on a daily basis covering the period from 1993 to September 2009, since the required information for all the modelling parameters was available for this period. Input data for 2009 were received as advance information subject to correction. A water balance model quantitatively accounts for the factors that influence lake levels. These factors include precipitation, lake evaporation, surface runoff, lake inflows/outflows, groundwater inflows/outflows, and water usage (surface water and groundwater). A hydrological water balance provides an understanding of the relative importance of the factors that influence lake levels. It can also be used as a management tool to estimate the amount of water required from another source to maintain a lake at a set level. A water balance model for Pigeon Lake was developed for this study using the following model input and output parameters: •
historical time series of estimated inflows to Pigeon Lake;
•
historical time series data of daily temperature and precipitation;
•
historical time series data of estimated lake evaporation;
•
long term average deep groundwater flow;
•
historical time series data of daily water levels;
•
historical water withdrawal data; and
•
stage-storage, stage-area, and stage-discharge relationships.
In order to develop a water balance model using these hydrologic variables, mathematical relations between stage (lake water level) and lake surface area and stage and lake storage volumes are required. Utilizing the stage-storage and stage-area relations from Mitchell and Prepas (1990), the following linear relations were developed for modelling purposes. Stage-Area relation: Area = (88.125 + 5076 x (Stage – 848)) km
2
Stage-Storage relation:
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6
Volume = (91.174 x (Stage – 848) + 465.065) x 10 m
3
Based on the outlet structure operating with one stop log and a fish ladder, the following relations were developed for modelling the outflow based on the lake level (i.e. Stage-Discharge). Outflow
= 0; if Stage < 849.65 masl
(7)
3
= 1.43 (Stage - 849.65) m /s; if 849.65 masl < Stage < 849.95 masl. 2
3
= 12.852(Stage - 849.8) + 2.083 (Stage - 849.8) + 0.041 m /s; if Stage > 849.95 masl. Model calibration is described below.
4.2
Calibration
The developed model was calibrated against the historical recorded water level data. Since actual measurements are unavailable of the runoff volume entering into the lake or lake evaporation, scaling factors on these parameters were used for the purpose of calibration. The difference of the median values between computed and recorded lake levels and the maximum lake level difference were used as the criteria to calibrate the model. Figure 14 shows the comparison between the modelled and recorded water levels at Pigeon Lake. The water levels simulated by the model match the actual water levels reasonably well, i.e. the difference of the median values between the computed and recorded water levels was 0.001 m and the maximum lake level difference was 0.2 m.
4.3
Relative Contributions of Hydrologic Variables to the Water Balance
Based on the calibrated water balance model for Pigeon Lake covering the period from 1993 to 2009, average values for all the key hydrologic variables and relative contribution of the variables were determined. Annual averages can be expressed in terms of millimetres (mm) taking into account the 3 3 area of the lake or volumetrically (dam or m ), as follows (Table F below and Figure 15).
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Table F Yearly Water Balance Year
Total Annual Volume Gain Due to Direct Precipitation 3 (dam )
Water Volume Loss Due to Lake Evaporation 3 (dam )
Annual Surface Water Inflow Volume 3 (dam )
Annual Ground water Inflow Volume 3 (dam )
Total Annual Outflow Volume 3 (dam )
Annual Withdrawals 3 Volume (dam )
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
35266 56508 42286 64080 50482 50238 43199 43227 39391 24873 38972 45762 42434 45789 39131 28429 28145
61757 66743 63085 59411 65850 70785 65240 63739 63859 57119 64362 59848 64280 66466 64174 62501 77654
13850 15360 8600 32140 26330 14040 40210 32100 12290 6980 19790 8040 25380 9330 45880 10660 4060
4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740 4740
202 232 184 1521 17763 6363 16202 15803 3718 1532 643 97 345 181 2202 1176 163
350 350 350 350 350 350 350 350 350 350 350 350 350 350 350 350 350
Note: 1 dam3 = 1,000 m3
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Table G Average Annual Water Balance - Pigeon Lake mm
m /year
3
m /day
3
dam /year
3
Inputs Precipitation Deep Groundwater Inflow Surface Water Inflow Total Inflow Total Inputs
438.0 48.6 197.5 246.1 684.10
42,653,000 4,733,000 19,233,000 23,966,000 66,618,000
116,860 12,970 52,700 65,670 182,520
42,660 4,740 19,240 23,970 66,620
Outputs Lake Evaporation Withdrawals Outflow Total Outflow Total Outputs
657.0 3.6 41.3 44.8 701.8
63,979,000 347,000 4,020,000 4,367,000 68,345,000
175,290 960 11,020 11,970 187,250
63,980 350 4,020 4,370 68,350
Net Evaporation Net Inflow Net Deficit
219.0 201.3 17.7
21,327,000 19,599,000 1,728,000
58,440 53,700 4,740
21,330 19,600 1,730
Notes: m3/year, m3/day, and dam3/year values are rounded. 3
Over the modelled period, the total mean annual inflow to the lake is 246.1 mm (23,970 dam /yr) which 3 includes surface runoff of 197.5 mm (19,240 dam /yr) and groundwater from the upper bedrock of 3 48.6 mm (4,740 dam /yr). Groundwater from the upper bedrock accounts for approximately 20% and surface water inflow accounts for 80% of the average annual inflow to Pigeon Lake. The average 3 annual outflow including water withdrawals from the lake is 44.8 mm (4,370 dam /yr). The net inflow 3 (difference of total inflow and total outflow) is then 201.3 mm (19,600 dam /yr). The net evaporation 3 (deficit of precipitation to evaporation) over the lake surface is 219 mm (21,330 dam /yr) over the modelled period. Since the net inflow to the lake is less than the net evaporation, Pigeon Lake is 3 subject to a net annual deficit of 17.7 mm (1,730 dam /yr). Relative contribution is also expressed as a pie chart in Figure 16. 3
In other words, the average annual water input to the lake is 684.1 mm (182,520 m /day) compared to 3 an average annual water output of 701.8 mm (187,250 m /day). Direct precipitation onto the lake accounts for approximately 64% of the lake level response, surface water inflow from the watershed accounts for about 29% of the response and groundwater from upper bedrock (Paskapoo Formation) accounts for 7% of the response. Lake evaporation contributes to 93% of water loss from the lake
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surface, whereas the outflow over the weir through the lake outlet channel appears to be 6% and the water withdrawals from the lake accounts for 1% of total water loss on an annual basis averaged over the modelling period. Due to the small drainage area of the watershed, and the low groundwater contribution, relative volumes of precipitation and evaporation have an appreciable effect on lake levels.
4.4
Water Level Maintenance
The health of the lake, in terms of eutrophication and habitat loss, is at risk if water levels are not increased, either by natural processes or preventive measures. Natural processes include greater precipitation, less evaporation, and greater surface water runoff to the lake. Preventive measures include supply of water to the lake from another source to bring the lake level to a predetermined elevation. A predetermined elevation can be established on the basis of water quantity, water quality, and ecology. The water balance model can estimate the volume of water needed to achieve a predetermined lake level. The study did not address the quantity of water needed for ecological maintenance of the lake, nor did it evaluate in detail any sources for water diversion. These issues were beyond the scope of the present study. Nevertheless, the water balance model can indicate the volume of water that may be required to establish a sustainable lake based practically on weir crest elevation, assuming that regular overflow of the weir crest is good for water quality and, hence, the health of the lake.
4.5
Application of Water Balance Model
The water balance model represents existing conditions. It can also be used to evaluate various scenarios, including the effect of additional input (or output). Two hypothetical input scenarios to achieve a water level of 849.95 masl (existing weir crest elevation) were modelled: 3
3
•
(Scenario 1) adding a constant inflow rate of 0.5 m /s (43,200 m /day);
•
(Scenario 2) adding a constant inflow rate of 1 m /s (86,400 m /day).
3
3
For both scenarios, water was added only when the lake level was below the weir elevation. Figures 17 3 3 and 18, respectively, show the effects of adding 0.5 m /s and 1 m /s on lake water levels. To bring the lake level from the September 2009 level of 849.4 masl to 849.95 masl would take 3 3 approximately 3.8 years if the diverted rate was 0.5 m /s, and 1.8 years if the diverted rate was 1 m /s. From the modelling, it can be further estimated that the retention time could be decreased from >100 years in the current condition to 58 and 49 years by adding flow according to Scenarios 1 and 2, respectively. These hypothetical scenarios assumed that the similar hydrologic conditions will prevail in the future. In comparison to the above diversion rates, the average flow rate of the North
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3
6
3
Saskatchewan River is approximately 175 m /s (15.1 x 10 m /day), and the best Paskapoo wells have 3 3 reported yields up to approximately 0.04 m /s (3,500 m /day). The water balance model indicated that raising the September 2009 lake level (849.4 masl) to the weir 3 level (849.95 masl) would take approximately 3.8 years if an additional inflow of 0.5 m /s 3 3 (43,200 m /day) is added to the lake in Scenario 1 and 1.8 years at an inflow rate of 1 m /s 3 (86,400 m /day) in Scenario 2. The retention time could be decreased from >100 years in the current condition to 58 and 49 years by adding flow according to Scenarios 1 and 2, respectively.
4.6
Limitations of the Water Balance Model
The model was developed based on available historical data. Results of the water balance model compared well with the available historical data. Errors in the estimation of evaporation data can have significant effects on modelled lake levels for shallow water bodies such as Pigeon Lake. Since precipitation and evaporation data are spatially variable, adjustment factors in utilizing these hydrologic variables were used to match the measured and simulated lake levels. Limited data were available for the stations in the vicinity of the lake that were used to represent the generated historical surface runoff, therefore adjustment factors were applied in order to match the measured and simulated lake levels. The application of this model for future management scenarios is carried out with the assumption that historical hydrological response is representative of the response that can be expected in the future. The limitations of this assumption should be kept in mind when considering the modelling results.
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5.
DISCUSSION AND SYNTHESIS
Water levels in Pigeon Lake are inherently sensitive to precipitation and evaporation. Precipitation accounts for approximately 64% of lake level response and surface water inflow accounts for about 29%. Upper bedrock groundwater flow contribution is only about 7%. Lake water residence time exceeds 100 years. Lake evaporation accounts for approximately 93% of the water output from the lake. Water balance modelling indicates a net water deficit of 17.7 mm, with an average annual water 3 input to the lake is 684.1 mm (182,520 m /day) compared to an average annual water output of 701.8 3 mm (187,250 m /day). For the past 30 years, agricultural land use intensity in the Pigeon Lake watershed has increased, both in terms of livestock population and cropland area, resulting in an increased nutrient input to the watershed. When the lake level is below the weir elevation, nutrient concentrations (especially phosphorus) increase because nutrients can no longer be flushed from the lake. To bring the lake level from the September 2009 level of 849.4 masl to the weir level would take 3 3 approximately 3.8 years at a diversion rate of 0.5 m /s (43,200 m /day) under modelled Scenario 1, 3 3 and 1.8 years at a rate of 1 m /s (86,400 m /day) under modelled Scenario 2. The required pipe size to convey such flows into the lake from a water source would vary between 450 mm (18 inch) to 600 mm (24 inch) depending on the pumped flow rate. The retention time could be decreased from >100 years in the current condition to 58 and 49 years in Scenarios 1 and 2, respectively. These hypothetical scenarios assumed that the similar hydrologic conditions will prevail in the future. With water levels above the weir elevation, phosphorus in the lake would flush and water quality would be expected to improve, but nutrient input issues would still need to be addressed. The time it would take for measurable lake water quality improvement could be estimated by nutrient budget modelling, which was beyond the scope of this study. The implications of flushing on water quality downstream of the lake in Pigeon Lake creek would need to be assessed. Potential sources of supplementary water input (when the lake level is below the weir) include the North Saskatchewan River and/or the lower Paskapoo Formation. Average flow rate of the North 3 6 3 3 Saskatchewan River is 175 m /s (15.1 x 10 m /day), so the diversion required to supply 1 m /s 3 (86,400 m /day) represents approximately 0.6% of the river’s flow. Such a diversion would result in no loss to the North Saskatchewan River system since Pigeon Lake drains to the Battle River system, which drains back to the North Saskatchewan River. The costs of such a diversion would appear high but should be weighed against the net benefit of keeping the lake healthy, i.e. the total social, economic, and environmental value of Pigeon Lake to Albertans in terms of recreational, resource, and ecosystem use (e.g. Olewiler 2004). For the lower Paskapoo Formation, the study suggested that the yield of individual vertical wells would perhaps provide less than a tenth of the needed diversion volume. The approach to groundwater
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supply and its sustainability would need to be examined closely, from the perspectives both of current domestic/agricultural groundwater use and ecosystem functioning. For example, would the pumping affect groundwater in shallow residential- or agricultural-use wells? Would any valued spring- or fendependent ecosystems inside or beyond the Pigeon Lake watershed be affected by pumping? What would be the advantages and feasibility of horizontal collector wells? Water quality in the North Saskatchewan River and lower Paskapoo Formation would also need to be characterized with respect to compatibility with the lake.
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6.
CONCLUSIONS AND RECOMMENDATIONS
The hydrology of the Pigeon Lake watershed was studied. Results are summarized as follows. •
The ratio of drainage basin area to lake area is 2:1, which is relatively small. Lakes with a small drainage area to lake area ratio are generally more sensitive to climatic variation.
•
A water balance model was developed using daily time step covering the period from 1993 to 2009, since the required information for all the modelling parameters were available for this period. The model was developed and calibrated against existing flow conditions.
•
Over the modelled period, the total mean annual inflow to the lake is 246.1 mm (23,970 dam /yr) 3 which includes surface runoff of 197.5 mm (19,240 dam /yr) and groundwater from the upper 3 bedrock formation of 48.6 mm (4,740 dam /yr). Groundwater from the upper bedrock formation accounts for approximately 20% and surface water runoff accounts for 80% of the average annual inflow to Pigeon Lake. The average annual outflow including water withdrawals from the 3 lake is 44.8 mm (4,370 dam /yr). The net inflow (difference of total inflow and total outflow) is 3 201.3 mm (19,600 dam /yr).
•
The average annual precipitation in the watershed over the modelling period (1993 to 2009) is 3 3 438 mm (42,600 dam /yr) and the average annual lake evaporation is 657 mm (63,980 dam /yr). The net evaporation (deficit of precipitation to evaporation) over the lake surface is 219 mm 3 (21,330 dam /yr). Since the net inflow to the lake is less than the net evaporation, Pigeon Lake is subject to a net annual deficit.
•
Water balance modelling indicated a net water deficit of 17.7 mm (1,730 dam /yr), with average 3 annual water input to the lake of 684.1 mm (66,620 dam /yr) compared to an average annual 3 water output of 701.8 mm (68,350 dam /yr). Pigeon Lake levels have been generally declining since 1981.
•
In the Pigeon Lake area, the best aquifers are the sandstone units of the Paskapoo Formation, 3 from which yields of 150 to approximately 3,300 m /day from vertical wells are expected. Anticipated groundwater quality is good with mineralization below 1,500 mg/L TDS.
•
For the past 30 years, agricultural land use intensity in the Lake watershed has increased, both in terms of livestock population and cropland area, resulting in increased nutrient input to the watershed. When the lake is below the weir elevation, nutrients in the lake will tend to be more concentrated and lake water quality deterioration will accelerate.
•
A daily water balance model for Pigeon Lake was developed. According to the water balance model, relative annual contribution to lake water levels is as follows:
3
3
−
precipitation contributes 64% to the lake;
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−
surface water inflow to the lake (including shallow groundwater base flow) contributes 29%;
−
deeper groundwater inflow (upper Paskapoo Formation) contributes 7% of the total input to the lake;
−
lake evaporation accounts for 93% of the water output from the lake;
−
outflows through the lake outlet channel and withdrawals from the lake account for only 7%.
•
Due to the small drainage area of the watershed and the low groundwater contribution, relative volumes of precipitation and evaporation have a substantial effect on lake levels.
•
Eutrophication in the form of algal blooms will continue if the lake levels continue to decline, and there will be serious and perhaps irreversible effects on the health of the lake’s aquatic ecosystems.
•
Declining lake levels can lead to littoral zone area loss, which in turn can lead to the collapse of fish species populations that depend on these areas, including important food- and sport fish such as lake whitefish and northern pike.
•
Bringing the lake level from the September 2009 level to the weir level would take approximately 3 3 3.8 years at a diversion rate of 0.5 m /s (43,200 m /day) in Scenario 1 and 1.8 years at a rate of 3 3 1 m /s (86,400 m /day) in Scenario 2. The retention time could be decreased from >100 years in the current condition to 58 and 49 years in Scenarios 1 and 2, respectively. These hypothetical scenarios assumed that the similar hydrologic conditions will prevail in the future.
•
The required pipe size to convey such flows into the lake from a water source would vary between 450 mm (18 inch) to 600 mm (24 inch) depending on the pumped flow rate.
•
The time it would take for measurable lake water quality improvement at the above input rates could be estimated by nutrient budget modelling, which was beyond the scope of this study.
•
The implications of flushing on water quality downstream of the lake in Pigeon Lake creek would need to be assessed.
•
Potential sources of water input include the North Saskatchewan River and the lower Paskapoo 3 6 3 Formation. Average flow rate of the North Saskatchewan River is 175 m /s (15.1 x 10 m /day), 3 3 so that the diversion required to supply 1 m /s (86,400 m /day) represent approximately 0.6% of the North Saskatchewan River flow. The costs of such a diversion would appear high but should be weighed against the net benefit of keeping the lake healthy, i.e. the total social, economic, and environmental value of Pigeon Lake to Albertans in terms of recreational, resource, and ecosystem use.
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