Groundwater abstraction and contamination studies at Thiruvidanthai ...
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JOURNAL OF COASTAL SCIENCES Journal homepage: www.jcsonline.co.nr ISSN: 2348 – 6740
Volume 2 Issue No. 1 - 2015
Pages 12-18
Groundwater abstraction and contamination studies at Thiruvidanthai Village, along East Coast Road in Chennai using electrical resistivity method with geochemical analysis * M. Mathiazhagan, T. Selvakumar, S. Mahenthiran, Madhavi Ganesan Centre for Water Resources, Anna University, Chennai 600 025, India
ABSTRACT
ARTICLE INFO
Electrical resistivity methods have been widely used to study groundwater contamination. The decrease in resistivity caused by salinization of groundwater helps to identify the contaminant zones. Resistivity sounding determines the thickness and resistivity of different horizontal or low dipping subsurface layers including the aquifer zone. Surface geophysical surveys provide an effective way to image the subsurface and the groundwater zone without a larger number of observation wells. Resistivity sounding generally identifies the subsurface formations, the aquifer zone as well as the formations saturated with fresh or saline/brackish water. The protection of groundwater resources from pollution has been a high priority topic in recent years. The paradox of the modern society is that, some of our efforts towards economic prosperity and increased standard of living could be detrimental to the overall quality of life due to encroachment upon the nature beyond its sustenance level or rejection of pollutants to environment exceeding its assimilative capabilities. With the increasing concern for groundwater protection, problem of predicting the movement by pollutants has gained greater attention. This project aims to detect groundwater contamination of coastal aquifer of Thiruvidanthai village near by Chennai. In the recent years, geophysical techniques applications are used for groundwater exploration. The Schlumberger configuration was used to create one dimensional apparent resistivity model. The apparent resistivity ranges from 12.15 Ohm-m to 76.3 Ohm-m in the shallow aquifer (0 to 40 m). The low apparent resistivity indicated that the groundwater has been contaminated by sea water.
Received 2 November 2014 Accepted 19 March 2015 Available online 23 March 2015 Keywords Electrical resistivity Apparent resistivity Groundwater Aquifer Saline intrusion Chennai
*Corresponding author, E-mail address: [email protected] Phone: +91 9952463867 © 2015 – Journal of Coastal Sciences. All rights reserved
1. Introduction Groundwater is the main source of drinking water and it has a vital importance in developed and developing countries. Nearly 80% of all diseases arises as a result of using unsafe and contaminated water. Geophysical techniques of investigating the composition, structure and nature of the subsurface have reached a high degree of sophistication with the convergence of the need to investigate the earth for scientific and societal problems. The electrical resistivity technique is particularly suitable for identifying the subsurface formations–the aquifer zone as well as the formations saturated with saline/brackish water. Geophysics provides spatially integrated information, which may be superior for some purposes to the point data provided by drilling. The electrical resistivity method provides a veritable tool for mapping the degree and the immediate subsurface vicinity. It is a fast, economic, and non-invasive method of studying groundwater contamination, as well as other environmental issues and it has proved to be promising and useful as predicted. The method is not used to directly detect the contaminants, rather it is used in the investigation of the geological environment through which the contaminants move, and in the determination of the distribution of pollutant in space and time through monitoring. However, there are some serious limitations in such investigations as they fail to distinguish between formations of similar resistivities such as saline clay and saline sand, and the 12
causes of low resistivity due to water quality. Ambiguity regarding low resistivity also arises from the enhanced mobility of ions in areas of high geothermal activity. Scale limitations involving electrode spacing, depth of investigation and required resolution is also a drawback for resistivity soundings. Again, some combination of resistivity and thickness of subsurface formations can produce an identical anomaly and hence give rise to ambiguity. An integration of geophysical methods with data interpretation largely resolves the uncertainty. The chemical analyses of groundwater samples are helpful in studying the hydrogeological conditions and saline contamination of aquifer zones. This also discriminates between the lithology and water quality effects when the two cannot be differentiated by a resistivity survey alone. The objective of the present research was to examine the utility of integration of geophysical data and geochemical analyses of water samples for groundwater and saline contamination studies. Todd (1959) indicated that the ratio of chloride and bicarbonate ions in groundwater is directly related to the extent of sea water intrusion in coastal aquifers. Adeoti et al. (2010) integrated geophysical survey involving electrical resistivity and induced polarization methods to found saline water plumes where they occur in different part of the area investigated and the results shows the effectiveness and usefulness of electrical resistivity and induced polarization method in mapping the saline water intrusion problem. ORIGINAL
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Nowroozi et al. (1999) opined that the resistivity sounding method is a powerful tool for delineating the fresh water/salt water interface in the eastern shore of Virginia and mapped the subsurface zones intruded by saline water. Yechieli (2000) studied the interface between fresh and saline water in the Dead Sea area using in situ profiles of electrical conductivity (EC) of water. Albouy et al. (2001) described the utility of both electrical resistivity and electromagnetic methods for coastal groundwater studies because of the large contrast in resistivity between fresh water-bearing and saline waterbearing formations. Groundwater abstraction intensifies migration of contaminants to the subsurface, activates salt water encroachment into pumped aquifers from neighboring ones, and sea water intrusion into coastal wells (Kalmias and Gregorauskas 2002). Gnanasundar and Elango (1999) studied the investigation of alluvial aquifers with large resistivity contacts. Identification of the presence and location of a leachate plume by (Roe et al. 2010), delineation of the groundwater potential aquifers (Coker, 2012). Assessing the zone of mixing between seawater and groundwater in the coastal aquifer in south of Chennai by Sathish et al. (2011). Ginsberg and Levanton (1976); Frohlich et al. (1994) indicated that sea water intrusion is a common problem in coastal region. Geo-electrical methods are widely used to identify zone of contamination with salt water. However their effectiveness is lessened in sandy aquifers with clay and peat layers as it is difficult to discriminate between salt freshwater under the conditions. Geophysical resistivity surveys are regularly used for studies related to groundwater investigations. Resistivity profiling delineates the lateral changes in resistivity that can be correlated with steeply dipping interfaces between two geological formations in the contamination of groundwater and consequently environmental problems. The main objective of this project is to detect the groundwater contamination in the coastal aquifer due do abstraction of groundwater and polluting effect at Thiruvidanthai along the East Coast Road, Chennai. It is aimed to assist in the reduction of groundwater contamination practices and suggest appropriate
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remedial measures which will be useful for betterment of people living near the coastal zone.
2. Study area The study area is located in Thiruporur Taluk, Kanchipuram District, in Tamil Nadu State with latitudes 12o80’ and longitude 80o20’. The study area generally experiences hot and humid climatic conditions and receives rainfall under the influence of both southeast and northeast monsoons. Most of the precipitation occurs in the form of cyclonic storm caused due to the depressions in Bay of Bengal during northeast monsoon period. The southwest monsoon rainfall is highly erratic and summer rains are negligible. The normal annual rainfall over the district varies from 1105 mm to 1214mm. High relative humidity between 58 and 84% prevail throughout the year. The minimum and maximum temperature are 20°C and 37°C respectively. The daytime heat is oppressive and the temperature is as high as 43°C. The coastal plain displays a fairly low level or gently rolling surface and only slightly elevated above the local water surfaces. The straight trend of the coastline is a result of development of a vast alluvial plain. There are a number of sand dunes in the coastal tract. Sandy coastal alluvial (arenacious soil) occurs along the seacoast as a narrow belt.
3. Methodology The resistivity technique examines horizontal and vertical discontinuities in the electrical properties of the ground. It measures earth resistivity by passing an electrical current into the ground and measuring the resulting potentials created (Zhody et al. 1974). This method involves the supply of direct current or low-frequency alternating current into the ground through a pair of electrodes and the measurement of the resulting potential through another pair of electrodes (potential electrodes). Because the current is known and the potential can be measured an apparent resistivity can be
Fig. 1 Location map of the study area
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calculated. The apparent resistivity of the subsurface material is a function of the magnitude of the current, the recorded potential difference and the geometry of the electrode array is used. The current electrodes spacing (AB) increases after each reading while the potential electrodes spacing (MN) increases only when deemed necessary and controlled by the relation AB/2 ≥ 5MN/2 as required by the Schlumberger array (Figure 2). For Schlumberger soundings, we used ABEM SAS 1000 Terrameter. The sounding curves were interpreted to determine the apparent resistivity and thicknesses of the subsurface layers. The data interpretation was performed with IX1Dv2 software.
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4. Results and Discussion 4.1. Qualitative interpretation of VES curves
For qualitative interpretation method, the shape of the field curve is observed to assess the number of layers and their resistivity. It gives information about the number of layers, their continuity through the area and reflects the degree of homogeneity or heterogeneity of an individual layer. Figure 3 shows examples of the resistivity sounding curves in the study area. These soundings are characterized by relatively high resistivity values in the first layer for the top soil dry zone (Silt). The second and third layer was reflecting the effect of groundwater on the geophysical parameters (resistivities) as well as the variation in the depths of the layers and their nature. The descending branch of these curves indicates a resistive topsoil underlain by a conductive materials of weathered rock or unconfined/ shallow aquifer and hard rock or confined or deeper aquifer. The depth of the intrusion is increased as distance increased from the coastline (Oyedele and Momoh 2009). The form of the geoelectrical sounding curves throughout the study area is H – type only except VES D, E and G locations with two layers curve. The VES A and VES B were conducted with 100 m penetration, for other Fig. 2 Schlumberger Sounding or Vertical Electrical Sounding (VES) remaining stations it was 30 m penetration. The interpretive models for each VES station are based on apparent resistivity, thickness and Total of 8 Vertical Electrical Sounding (VES) was carried out in depth which provides quantitative interpretation (Table 1). the Thiruvidanthai village in Chennai. The maximum of outer current Generally all curves were similar type indicating the uniform single electrode spacing is 200 m and minimum spacing is 60 m. The inner lithology in the study area. potential electrode maximum spacing is 10 m and minimum spacing is 4 m. Out of 8 VES locations, 4 VES locations were south to north Location Layers ρa (Ohm-m) Thickness (m) Depth direction measuring the readings namely A, B, G and H. These 4 VES (m) locations except G, have the outer current electrode spacing of 200 m A 1 12462.3 0.61 0.61 and inner potential electrode spacing of 10 m. Remaining 4 VES 2 1065.3 2.28 2.89 locations were east to west direction measuring the readings namely 3 70.9* C, D, E and F. These 4 VES locations including G location, have the B 1 2392.4 1.69 1.69 outer current electrode spacing of 60m and inner potential electrode 2 1590.1 2.06 2.6 spacing of 4 m. Due to the presence of residential houses and 3 54* highway road, further current extension of electrode spacing could not be possible. C 1 5977.9 0.88 0.88 The apparent resistivity data are associated with varying depths 2 591.6 1.17 1.7 relative to the distance between the current and potential electrodes 3 42.15* and can be interpreted qualitatively and quantitatively in terms of D 1 500.2 1.9 1.9 lithologic and/or geohydrologic model. In the qualitative 2 54* interpretation method, the shape of the field curve is observed to E 1 2092.5 1.8 1.8 assess the number of layers and their resistivity. In the quantitative 2 23.15* interpretation method true resistivity ‘ρ’ and layer thickness ‘h’ as F 1 467.4 1.9 1.9 the fundamental characteristics of a geoelectric layer are obtained. 2 33.47* 20.76 22.68 The results of this method are represented in the form of the 3 196.3 resistivity values that can be used for preparing an iso-apparent 1 2589 1.8 1.8 electric resistivity map (Mathiazhagan, et al. 2012; 2013). The G quantitative interpretation of VES curves in this study was done by 2 46.8* the well–known method of curve matching. In curve matching H 1 462 1.5 1.5 technique, the field VES curves are compared with set of theoretical 2 35.4* 17.5 18.98 curves to obtain ‘ρ’ and ‘h’. 3 268 Groundwater samples from 5 wells were taken for chemical Table 1 Interpretation Results of Resistivity curve in the study Area. 70.9* analysis using standard analytical techniques (APHA, 1985). The first Aquifer Apparent Resistivity well is located at 150 m east of temple tank, second well is located at 60 m south of temple tank, third well at 60 m north of temple tank 4.2. Quantitative Interpretation and fourth well at 15 m away from the 3rd well, fifth well is located at 100 m perpendicular to the 2nd well west of temple tank. Surface The VES curves were also interpreted quantitatively from a purely water sample from the temple tank and another tank located at 200 theoretical view point to delineate the subsurface succession of the m away from the west of 3rd well was also taken for chemical geoelectric layers in the study area. The results are shown the Table analysis. 1. The data analyses show that the area under investigation can 14
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Fig. 3 Typical sounding curve of the study area
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generally be interpreted as a three layered region down to the depth of investigation. The resistivity of the surface layer (first layer) ranges between 462 Ωm – 12462.3 Ωm and are typically indicative of dry layer (Silty) with topsoil variation. This layer reaches its maximum thickness at VES location D and F 1.9 m and about 0.61 m at VES location A. The depth variation ranges from 0.61 m to 1.9 m. The second layer is characterized by relatively low resistivity values 23.21 Ωm - 1065.3 Ωm. This layer is shallow and unconfined aquifer with weathered rock. The thickness of this layer varies greatly from one locality to another where its maximum thickness 20.76 m at VES location F and its minimum thickness 1.17 at VES location C. The depth variation ranges from 1.7 m to 22.68 m. The resistivity of the third layer ranges from 12.15 Ωm to 70.9 Ωm and it was interpreted as charnockite rock. The thickness and depth varies from ∞. Eight sets of reading (From VES A to VES H) were taken for Vertical Electrical Sounding (VES) and with these readings the value of apparent resistivity (ρa) was found out. The ρa value indicates the geological aspects that is type of lithology and type of water present in the aquifer.
4.3. Geochemical Analysis
Fig. 4 Rho – A Vs Electricl Conductivity
Fig. 6 Rho-A Vs Cl-
Fig. 5 Rho-A Vs TDS
Fig. 7 Rho-A Vs Total Hardness
Location
EC (ms/ppt)
Well 1 & F
3.2
TDS (ms/ppt) 13.6
Well 2 & D
0.63
Well 3 & C
pH 7.3
Cl(mg/l) 318
2.9
7.2
1.13
5.01
Well 4 & H
4.41
Well 5 & A
0.17
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The results of the geochemical analysis are shown in Table 2. The electrical condictivity values were high at well 4 (4.41 ms/ppt) and low values are observed in well 5 (0.17 ms/ppt) followed by TDS values with maximium in well 4 (18.7 ms/ppt) and minimum in well 5 (0.8 ms/ppt). The pH values is found to be low in well 5, when compared with others wells. The chloride values were high at well 4 (344.9 mg/l) and low values at well 5 (15 mg/l). Total Hardness values were maximum at well 4 (476 mg/l) and minimum at well 5 (55 mg/l). Resisitivity profiling coupled with resistivity sounding, periodic chemical analysis of groundwater samples, and data integration was found to be a highly effective method for determining the fresh water areas and the saline water contaminated zones, as well as the mode and cause of saline water intrusion. Such integrated research also evolved a new concept of minimum resistivity of a subsurface formation in an area below which groundwater contained in it is brackish/saline and unsuitable for drinking.
Hardness (mg/l)
Rho-A (ohm-m)
Remarks
451
33.4
60 m South of Temple Tank
137.5
146
54
60 m North of Temple Tank
7.3
299.9
305
42.15
15 m away from the 2nd well
18.7
7.3
344.9
476
35.4
15 m away from the 1st well
0.8
6.9
15
55
70.9
100 m East of Temple Tank
Table 2 Results of Geophysical and Geochemical parameters in the study area
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Fig. 8 Spatial distribution of Chloride (mg/l)in the study area
Fig. 10 Spatial distribution of TDS (ms/ppt)in the study area
Fig. 9 Spatial distribution of EC (ms/ppt)in the study area
Fig. 11 Spatial distribution of Total Hardness (mg/l) in the study area
Fig. 12 Spatial distribution of apparent resistivity in the study area
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Such data integration was successfully applied in a coastal region in India to identify one narrow saline water zone/channel, which caused high TDS and high chloride in the groundwater. Further, the integrated study delineated subsurface saline contaminated zone close to a sea water canal and potable groundwater zones at different depth levels (Choudhury and Saha 2004). Figure 4-7 shows the double Y–axis graph. From figure 4-7 it can be interpreted that when EC, TDS, Cl- and hardness values are high, apparent resistivity is low, and when EC, TDS, Cl- and hardness values are low, apparent resistivity values is found to be high. Figure 8–11 clearly shows that the groundwater is contaminated through over pumping. The western side water quality is very poor comparatively to other side. Figure 12 shows a high apparent resistivity in the eastern side of the aquifer due to less contamination, whereas the western side is contaminated due to over pumping having very low resistivity.
5. Conclusion Resistivity sounding along with chemical analysis of groundwater samples and data integration was found to be a highly effective method for determining the fresh water areas and the saline water contaminated zones as well as the mode and cause of sea water intrusion. Two major factors contribute saline water intrusion observed in the study area. Excessive pumping of groundwater has disturbed the hydrodynamic equilibrium in the aquifer and thereby the reduction of groundwater gradients allows saline-water to displace fresh water in the aquifer. It infers that the saline water intrusion has already occurred in the Thiruvidanthai village. So the groundwater aquifer is under critical stage except well no.5. The Schlumberger sounding resistivity method is confirmed as an efficient tool for investigating the saltwater-freshwater interface in a coastal environment. This method has helped to delineate the areas for groundwater development and the vulnerable zones. Hence, artificial recharge have to be carried out in the zone of aquifer vulnerable area by rain water harvesting in all the households, temple tanks, natural tanks etc.
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Mathiazhagan, M., Selvakumar, T., Madhavi, G. 2012. Detection of solid waste dumpsite-induced groundwater contamination leachate using electrical resistivity method, Proceedings of the Sixth IAHR International groundwater Symposium, Kuwait, 19-21 November, 2012, 307-312 CRC Press. Mathiazhagan, M., Selvakumar, T., Madhavi, G. 2013. Geophysical Technique for Sensing of Solid Waste Dump Site-Induced Groundwater Contamination Leachate, Pollution Research, 32(3), 509-514. Nowroozi, A. A., Stephen, B. H., Henderson, P. 1999. Saltwater intrusion into the fresh water aquifer in the eastern shore of Virginia: A reconnaissance electrical resistivity survey. Journal of Applied Geophysics, 42(1), 1–22. Oyedele, K. F., Momoh, E. I., 2009. Evaluation of sea water intrusion in freshwater aquifers in a lagoon coast: A case study of the University of Lagos Lagoon, Akoka, Nigeria. New York Science Journal, 2(3), 32-42. Roe, J., Triantafilis, J., Santos, F. M. 2010. Detecting a landfill leachate plume using a DUALEM-421 and a laterally constrained inversion model, 19th World Congress of Soil Science, Soil Solutions for a Changing World 1-6 August 2010, Brisbane, Australia. Sathish, S., Elango, L., Rajesh, R., Sarma, V. S. 2011. Assessment of seawater mixing in a coastal aquifer by high resolution electrical resistivity tomography, International Journal of Environmental Science and Technology, 8(3), 483- 492. Todd, D. K. 1959. Ground Water Hydrology. New York: John Wiley & Sons. Yechieli, Y. 2000. Fresh-saline ground water interface in the western Dead Sea area. Ground Water, 38(4), 615–623. Zhody, A. A. A., Eaton, G. P., Mabey, D. R., 1974. Application of Surface Geophysics to Groundwater Investigations. US Geology Survey.
References Adeoti, L., Alile, O. M., Uchegbulam, O. 2010. Geophysical investigation of saline water intrusion into freshwater aquifers: A case study of Oniru, Lagos State, Scientific Research and Essays, 5(3), 248-259. Albouy, Y., Andrieux, P., Rakotondrasoa, G., Ritz, M., Descloitres, M., Join, J. L., Rasolomanana, E. 2001. Mapping coastal aquifers by joint inversion of DC and TEM soundings— Three cases histories. Ground Water, 39(1), 87–97. APHA, 1985. Standard methods for the examination of water and wastewater 16th Edn, Washington, D.C.American Public Health Association. Choudhury, K., Saha, D. K. 2004. Integrated geophysical and chemical study of saline water intrusion, Groundwater, 42(5), 671-677. Coker, J. O., 2012. Vertical electrical sounding (VES) methods to delineate potential groundwater aquifers in Akobo area, Ibadan, South-western Nigeria. Journal of Geology and Mining Research, 4(2), 35-42. Frohlich, R. K., Urish, D. W., Fuller, J., Reilly, M. 1994. Use of geoelectrical method in ground water pollution surveys in a coastal environment. Journal of Applied Geophysics, 32(2), 139–154. Ginsberg, A., Levanton, A. 1976. Determination of saltwater interface by electrical resistivity sounding. Hydrological Science Bulletin, 21(6), 561– 568. Gnanasundar, D., Elango, L. 1999. Groundwater Quality assessment of a coastal aquifer using Geolectrical Techniques, Journal of Environmental Hydrology, 7(2). Kalimas, A., Gregorauskas, M. 2002. Ground water abstraction and contamination in Lithuania as geoindicators of environmental change. Environmental Geology, 42(7), 767–772.
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JOURNAL OF COASTAL SCIENCES Journal homepage: www.jcsonline.co.nr ISSN: 2348 – 6740
Volume 2 Issue No. 1 - 2015
Pages 12-18
Groundwater abstraction and contamination studies at Thiruvidanthai Village, along East Coast Road in Chennai using electrical resistivity method with geochemical analysis * M. Mathiazhagan, T. Selvakumar, S. Mahenthiran, Madhavi Ganesan Centre for Water Resources, Anna University, Chennai 600 025, India
ABSTRACT
ARTICLE INFO
Electrical resistivity methods have been widely used to study groundwater contamination. The decrease in resistivity caused by salinization of groundwater helps to identify the contaminant zones. Resistivity sounding determines the thickness and resistivity of different horizontal or low dipping subsurface layers including the aquifer zone. Surface geophysical surveys provide an effective way to image the subsurface and the groundwater zone without a larger number of observation wells. Resistivity sounding generally identifies the subsurface formations, the aquifer zone as well as the formations saturated with fresh or saline/brackish water. The protection of groundwater resources from pollution has been a high priority topic in recent years. The paradox of the modern society is that, some of our efforts towards economic prosperity and increased standard of living could be detrimental to the overall quality of life due to encroachment upon the nature beyond its sustenance level or rejection of pollutants to environment exceeding its assimilative capabilities. With the increasing concern for groundwater protection, problem of predicting the movement by pollutants has gained greater attention. This project aims to detect groundwater contamination of coastal aquifer of Thiruvidanthai village near by Chennai. In the recent years, geophysical techniques applications are used for groundwater exploration. The Schlumberger configuration was used to create one dimensional apparent resistivity model. The apparent resistivity ranges from 12.15 Ohm-m to 76.3 Ohm-m in the shallow aquifer (0 to 40 m). The low apparent resistivity indicated that the groundwater has been contaminated by sea water.
Received 2 November 2014 Accepted 19 March 2015 Available online 23 March 2015 Keywords Electrical resistivity Apparent resistivity Groundwater Aquifer Saline intrusion Chennai
*Corresponding author, E-mail address: [email protected] Phone: +91 9952463867 © 2015 – Journal of Coastal Sciences. All rights reserved
1. Introduction Groundwater is the main source of drinking water and it has a vital importance in developed and developing countries. Nearly 80% of all diseases arises as a result of using unsafe and contaminated water. Geophysical techniques of investigating the composition, structure and nature of the subsurface have reached a high degree of sophistication with the convergence of the need to investigate the earth for scientific and societal problems. The electrical resistivity technique is particularly suitable for identifying the subsurface formations–the aquifer zone as well as the formations saturated with saline/brackish water. Geophysics provides spatially integrated information, which may be superior for some purposes to the point data provided by drilling. The electrical resistivity method provides a veritable tool for mapping the degree and the immediate subsurface vicinity. It is a fast, economic, and non-invasive method of studying groundwater contamination, as well as other environmental issues and it has proved to be promising and useful as predicted. The method is not used to directly detect the contaminants, rather it is used in the investigation of the geological environment through which the contaminants move, and in the determination of the distribution of pollutant in space and time through monitoring. However, there are some serious limitations in such investigations as they fail to distinguish between formations of similar resistivities such as saline clay and saline sand, and the 12
causes of low resistivity due to water quality. Ambiguity regarding low resistivity also arises from the enhanced mobility of ions in areas of high geothermal activity. Scale limitations involving electrode spacing, depth of investigation and required resolution is also a drawback for resistivity soundings. Again, some combination of resistivity and thickness of subsurface formations can produce an identical anomaly and hence give rise to ambiguity. An integration of geophysical methods with data interpretation largely resolves the uncertainty. The chemical analyses of groundwater samples are helpful in studying the hydrogeological conditions and saline contamination of aquifer zones. This also discriminates between the lithology and water quality effects when the two cannot be differentiated by a resistivity survey alone. The objective of the present research was to examine the utility of integration of geophysical data and geochemical analyses of water samples for groundwater and saline contamination studies. Todd (1959) indicated that the ratio of chloride and bicarbonate ions in groundwater is directly related to the extent of sea water intrusion in coastal aquifers. Adeoti et al. (2010) integrated geophysical survey involving electrical resistivity and induced polarization methods to found saline water plumes where they occur in different part of the area investigated and the results shows the effectiveness and usefulness of electrical resistivity and induced polarization method in mapping the saline water intrusion problem. ORIGINAL
ARTICLE
J O U R N A L
Nowroozi et al. (1999) opined that the resistivity sounding method is a powerful tool for delineating the fresh water/salt water interface in the eastern shore of Virginia and mapped the subsurface zones intruded by saline water. Yechieli (2000) studied the interface between fresh and saline water in the Dead Sea area using in situ profiles of electrical conductivity (EC) of water. Albouy et al. (2001) described the utility of both electrical resistivity and electromagnetic methods for coastal groundwater studies because of the large contrast in resistivity between fresh water-bearing and saline waterbearing formations. Groundwater abstraction intensifies migration of contaminants to the subsurface, activates salt water encroachment into pumped aquifers from neighboring ones, and sea water intrusion into coastal wells (Kalmias and Gregorauskas 2002). Gnanasundar and Elango (1999) studied the investigation of alluvial aquifers with large resistivity contacts. Identification of the presence and location of a leachate plume by (Roe et al. 2010), delineation of the groundwater potential aquifers (Coker, 2012). Assessing the zone of mixing between seawater and groundwater in the coastal aquifer in south of Chennai by Sathish et al. (2011). Ginsberg and Levanton (1976); Frohlich et al. (1994) indicated that sea water intrusion is a common problem in coastal region. Geo-electrical methods are widely used to identify zone of contamination with salt water. However their effectiveness is lessened in sandy aquifers with clay and peat layers as it is difficult to discriminate between salt freshwater under the conditions. Geophysical resistivity surveys are regularly used for studies related to groundwater investigations. Resistivity profiling delineates the lateral changes in resistivity that can be correlated with steeply dipping interfaces between two geological formations in the contamination of groundwater and consequently environmental problems. The main objective of this project is to detect the groundwater contamination in the coastal aquifer due do abstraction of groundwater and polluting effect at Thiruvidanthai along the East Coast Road, Chennai. It is aimed to assist in the reduction of groundwater contamination practices and suggest appropriate
O F
C O A S T A L
S C I E N C E S
remedial measures which will be useful for betterment of people living near the coastal zone.
2. Study area The study area is located in Thiruporur Taluk, Kanchipuram District, in Tamil Nadu State with latitudes 12o80’ and longitude 80o20’. The study area generally experiences hot and humid climatic conditions and receives rainfall under the influence of both southeast and northeast monsoons. Most of the precipitation occurs in the form of cyclonic storm caused due to the depressions in Bay of Bengal during northeast monsoon period. The southwest monsoon rainfall is highly erratic and summer rains are negligible. The normal annual rainfall over the district varies from 1105 mm to 1214mm. High relative humidity between 58 and 84% prevail throughout the year. The minimum and maximum temperature are 20°C and 37°C respectively. The daytime heat is oppressive and the temperature is as high as 43°C. The coastal plain displays a fairly low level or gently rolling surface and only slightly elevated above the local water surfaces. The straight trend of the coastline is a result of development of a vast alluvial plain. There are a number of sand dunes in the coastal tract. Sandy coastal alluvial (arenacious soil) occurs along the seacoast as a narrow belt.
3. Methodology The resistivity technique examines horizontal and vertical discontinuities in the electrical properties of the ground. It measures earth resistivity by passing an electrical current into the ground and measuring the resulting potentials created (Zhody et al. 1974). This method involves the supply of direct current or low-frequency alternating current into the ground through a pair of electrodes and the measurement of the resulting potential through another pair of electrodes (potential electrodes). Because the current is known and the potential can be measured an apparent resistivity can be
Fig. 1 Location map of the study area
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calculated. The apparent resistivity of the subsurface material is a function of the magnitude of the current, the recorded potential difference and the geometry of the electrode array is used. The current electrodes spacing (AB) increases after each reading while the potential electrodes spacing (MN) increases only when deemed necessary and controlled by the relation AB/2 ≥ 5MN/2 as required by the Schlumberger array (Figure 2). For Schlumberger soundings, we used ABEM SAS 1000 Terrameter. The sounding curves were interpreted to determine the apparent resistivity and thicknesses of the subsurface layers. The data interpretation was performed with IX1Dv2 software.
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4. Results and Discussion 4.1. Qualitative interpretation of VES curves
For qualitative interpretation method, the shape of the field curve is observed to assess the number of layers and their resistivity. It gives information about the number of layers, their continuity through the area and reflects the degree of homogeneity or heterogeneity of an individual layer. Figure 3 shows examples of the resistivity sounding curves in the study area. These soundings are characterized by relatively high resistivity values in the first layer for the top soil dry zone (Silt). The second and third layer was reflecting the effect of groundwater on the geophysical parameters (resistivities) as well as the variation in the depths of the layers and their nature. The descending branch of these curves indicates a resistive topsoil underlain by a conductive materials of weathered rock or unconfined/ shallow aquifer and hard rock or confined or deeper aquifer. The depth of the intrusion is increased as distance increased from the coastline (Oyedele and Momoh 2009). The form of the geoelectrical sounding curves throughout the study area is H – type only except VES D, E and G locations with two layers curve. The VES A and VES B were conducted with 100 m penetration, for other Fig. 2 Schlumberger Sounding or Vertical Electrical Sounding (VES) remaining stations it was 30 m penetration. The interpretive models for each VES station are based on apparent resistivity, thickness and Total of 8 Vertical Electrical Sounding (VES) was carried out in depth which provides quantitative interpretation (Table 1). the Thiruvidanthai village in Chennai. The maximum of outer current Generally all curves were similar type indicating the uniform single electrode spacing is 200 m and minimum spacing is 60 m. The inner lithology in the study area. potential electrode maximum spacing is 10 m and minimum spacing is 4 m. Out of 8 VES locations, 4 VES locations were south to north Location Layers ρa (Ohm-m) Thickness (m) Depth direction measuring the readings namely A, B, G and H. These 4 VES (m) locations except G, have the outer current electrode spacing of 200 m A 1 12462.3 0.61 0.61 and inner potential electrode spacing of 10 m. Remaining 4 VES 2 1065.3 2.28 2.89 locations were east to west direction measuring the readings namely 3 70.9* C, D, E and F. These 4 VES locations including G location, have the B 1 2392.4 1.69 1.69 outer current electrode spacing of 60m and inner potential electrode 2 1590.1 2.06 2.6 spacing of 4 m. Due to the presence of residential houses and 3 54* highway road, further current extension of electrode spacing could not be possible. C 1 5977.9 0.88 0.88 The apparent resistivity data are associated with varying depths 2 591.6 1.17 1.7 relative to the distance between the current and potential electrodes 3 42.15* and can be interpreted qualitatively and quantitatively in terms of D 1 500.2 1.9 1.9 lithologic and/or geohydrologic model. In the qualitative 2 54* interpretation method, the shape of the field curve is observed to E 1 2092.5 1.8 1.8 assess the number of layers and their resistivity. In the quantitative 2 23.15* interpretation method true resistivity ‘ρ’ and layer thickness ‘h’ as F 1 467.4 1.9 1.9 the fundamental characteristics of a geoelectric layer are obtained. 2 33.47* 20.76 22.68 The results of this method are represented in the form of the 3 196.3 resistivity values that can be used for preparing an iso-apparent 1 2589 1.8 1.8 electric resistivity map (Mathiazhagan, et al. 2012; 2013). The G quantitative interpretation of VES curves in this study was done by 2 46.8* the well–known method of curve matching. In curve matching H 1 462 1.5 1.5 technique, the field VES curves are compared with set of theoretical 2 35.4* 17.5 18.98 curves to obtain ‘ρ’ and ‘h’. 3 268 Groundwater samples from 5 wells were taken for chemical Table 1 Interpretation Results of Resistivity curve in the study Area. 70.9* analysis using standard analytical techniques (APHA, 1985). The first Aquifer Apparent Resistivity well is located at 150 m east of temple tank, second well is located at 60 m south of temple tank, third well at 60 m north of temple tank 4.2. Quantitative Interpretation and fourth well at 15 m away from the 3rd well, fifth well is located at 100 m perpendicular to the 2nd well west of temple tank. Surface The VES curves were also interpreted quantitatively from a purely water sample from the temple tank and another tank located at 200 theoretical view point to delineate the subsurface succession of the m away from the west of 3rd well was also taken for chemical geoelectric layers in the study area. The results are shown the Table analysis. 1. The data analyses show that the area under investigation can 14
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Fig. 3 Typical sounding curve of the study area
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generally be interpreted as a three layered region down to the depth of investigation. The resistivity of the surface layer (first layer) ranges between 462 Ωm – 12462.3 Ωm and are typically indicative of dry layer (Silty) with topsoil variation. This layer reaches its maximum thickness at VES location D and F 1.9 m and about 0.61 m at VES location A. The depth variation ranges from 0.61 m to 1.9 m. The second layer is characterized by relatively low resistivity values 23.21 Ωm - 1065.3 Ωm. This layer is shallow and unconfined aquifer with weathered rock. The thickness of this layer varies greatly from one locality to another where its maximum thickness 20.76 m at VES location F and its minimum thickness 1.17 at VES location C. The depth variation ranges from 1.7 m to 22.68 m. The resistivity of the third layer ranges from 12.15 Ωm to 70.9 Ωm and it was interpreted as charnockite rock. The thickness and depth varies from ∞. Eight sets of reading (From VES A to VES H) were taken for Vertical Electrical Sounding (VES) and with these readings the value of apparent resistivity (ρa) was found out. The ρa value indicates the geological aspects that is type of lithology and type of water present in the aquifer.
4.3. Geochemical Analysis
Fig. 4 Rho – A Vs Electricl Conductivity
Fig. 6 Rho-A Vs Cl-
Fig. 5 Rho-A Vs TDS
Fig. 7 Rho-A Vs Total Hardness
Location
EC (ms/ppt)
Well 1 & F
3.2
TDS (ms/ppt) 13.6
Well 2 & D
0.63
Well 3 & C
pH 7.3
Cl(mg/l) 318
2.9
7.2
1.13
5.01
Well 4 & H
4.41
Well 5 & A
0.17
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The results of the geochemical analysis are shown in Table 2. The electrical condictivity values were high at well 4 (4.41 ms/ppt) and low values are observed in well 5 (0.17 ms/ppt) followed by TDS values with maximium in well 4 (18.7 ms/ppt) and minimum in well 5 (0.8 ms/ppt). The pH values is found to be low in well 5, when compared with others wells. The chloride values were high at well 4 (344.9 mg/l) and low values at well 5 (15 mg/l). Total Hardness values were maximum at well 4 (476 mg/l) and minimum at well 5 (55 mg/l). Resisitivity profiling coupled with resistivity sounding, periodic chemical analysis of groundwater samples, and data integration was found to be a highly effective method for determining the fresh water areas and the saline water contaminated zones, as well as the mode and cause of saline water intrusion. Such integrated research also evolved a new concept of minimum resistivity of a subsurface formation in an area below which groundwater contained in it is brackish/saline and unsuitable for drinking.
Hardness (mg/l)
Rho-A (ohm-m)
Remarks
451
33.4
60 m South of Temple Tank
137.5
146
54
60 m North of Temple Tank
7.3
299.9
305
42.15
15 m away from the 2nd well
18.7
7.3
344.9
476
35.4
15 m away from the 1st well
0.8
6.9
15
55
70.9
100 m East of Temple Tank
Table 2 Results of Geophysical and Geochemical parameters in the study area
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Fig. 8 Spatial distribution of Chloride (mg/l)in the study area
Fig. 10 Spatial distribution of TDS (ms/ppt)in the study area
Fig. 9 Spatial distribution of EC (ms/ppt)in the study area
Fig. 11 Spatial distribution of Total Hardness (mg/l) in the study area
Fig. 12 Spatial distribution of apparent resistivity in the study area
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Such data integration was successfully applied in a coastal region in India to identify one narrow saline water zone/channel, which caused high TDS and high chloride in the groundwater. Further, the integrated study delineated subsurface saline contaminated zone close to a sea water canal and potable groundwater zones at different depth levels (Choudhury and Saha 2004). Figure 4-7 shows the double Y–axis graph. From figure 4-7 it can be interpreted that when EC, TDS, Cl- and hardness values are high, apparent resistivity is low, and when EC, TDS, Cl- and hardness values are low, apparent resistivity values is found to be high. Figure 8–11 clearly shows that the groundwater is contaminated through over pumping. The western side water quality is very poor comparatively to other side. Figure 12 shows a high apparent resistivity in the eastern side of the aquifer due to less contamination, whereas the western side is contaminated due to over pumping having very low resistivity.
5. Conclusion Resistivity sounding along with chemical analysis of groundwater samples and data integration was found to be a highly effective method for determining the fresh water areas and the saline water contaminated zones as well as the mode and cause of sea water intrusion. Two major factors contribute saline water intrusion observed in the study area. Excessive pumping of groundwater has disturbed the hydrodynamic equilibrium in the aquifer and thereby the reduction of groundwater gradients allows saline-water to displace fresh water in the aquifer. It infers that the saline water intrusion has already occurred in the Thiruvidanthai village. So the groundwater aquifer is under critical stage except well no.5. The Schlumberger sounding resistivity method is confirmed as an efficient tool for investigating the saltwater-freshwater interface in a coastal environment. This method has helped to delineate the areas for groundwater development and the vulnerable zones. Hence, artificial recharge have to be carried out in the zone of aquifer vulnerable area by rain water harvesting in all the households, temple tanks, natural tanks etc.
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Mathiazhagan, M., Selvakumar, T., Madhavi, G. 2012. Detection of solid waste dumpsite-induced groundwater contamination leachate using electrical resistivity method, Proceedings of the Sixth IAHR International groundwater Symposium, Kuwait, 19-21 November, 2012, 307-312 CRC Press. Mathiazhagan, M., Selvakumar, T., Madhavi, G. 2013. Geophysical Technique for Sensing of Solid Waste Dump Site-Induced Groundwater Contamination Leachate, Pollution Research, 32(3), 509-514. Nowroozi, A. A., Stephen, B. H., Henderson, P. 1999. Saltwater intrusion into the fresh water aquifer in the eastern shore of Virginia: A reconnaissance electrical resistivity survey. Journal of Applied Geophysics, 42(1), 1–22. Oyedele, K. F., Momoh, E. I., 2009. Evaluation of sea water intrusion in freshwater aquifers in a lagoon coast: A case study of the University of Lagos Lagoon, Akoka, Nigeria. New York Science Journal, 2(3), 32-42. Roe, J., Triantafilis, J., Santos, F. M. 2010. Detecting a landfill leachate plume using a DUALEM-421 and a laterally constrained inversion model, 19th World Congress of Soil Science, Soil Solutions for a Changing World 1-6 August 2010, Brisbane, Australia. Sathish, S., Elango, L., Rajesh, R., Sarma, V. S. 2011. Assessment of seawater mixing in a coastal aquifer by high resolution electrical resistivity tomography, International Journal of Environmental Science and Technology, 8(3), 483- 492. Todd, D. K. 1959. Ground Water Hydrology. New York: John Wiley & Sons. Yechieli, Y. 2000. Fresh-saline ground water interface in the western Dead Sea area. Ground Water, 38(4), 615–623. Zhody, A. A. A., Eaton, G. P., Mabey, D. R., 1974. Application of Surface Geophysics to Groundwater Investigations. US Geology Survey.
References Adeoti, L., Alile, O. M., Uchegbulam, O. 2010. Geophysical investigation of saline water intrusion into freshwater aquifers: A case study of Oniru, Lagos State, Scientific Research and Essays, 5(3), 248-259. Albouy, Y., Andrieux, P., Rakotondrasoa, G., Ritz, M., Descloitres, M., Join, J. L., Rasolomanana, E. 2001. Mapping coastal aquifers by joint inversion of DC and TEM soundings— Three cases histories. Ground Water, 39(1), 87–97. APHA, 1985. Standard methods for the examination of water and wastewater 16th Edn, Washington, D.C.American Public Health Association. Choudhury, K., Saha, D. K. 2004. Integrated geophysical and chemical study of saline water intrusion, Groundwater, 42(5), 671-677. Coker, J. O., 2012. Vertical electrical sounding (VES) methods to delineate potential groundwater aquifers in Akobo area, Ibadan, South-western Nigeria. Journal of Geology and Mining Research, 4(2), 35-42. Frohlich, R. K., Urish, D. W., Fuller, J., Reilly, M. 1994. Use of geoelectrical method in ground water pollution surveys in a coastal environment. Journal of Applied Geophysics, 32(2), 139–154. Ginsberg, A., Levanton, A. 1976. Determination of saltwater interface by electrical resistivity sounding. Hydrological Science Bulletin, 21(6), 561– 568. Gnanasundar, D., Elango, L. 1999. Groundwater Quality assessment of a coastal aquifer using Geolectrical Techniques, Journal of Environmental Hydrology, 7(2). Kalimas, A., Gregorauskas, M. 2002. Ground water abstraction and contamination in Lithuania as geoindicators of environmental change. Environmental Geology, 42(7), 767–772.
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