Radionuclide Dating of Hyporheic Fine Particles From ...
Radionuclide Dating of Hyporheic Fine Particles From the Bed Material of the South River at Relative River Mile 2.95-3.0 Jim Pizzuto, Department of Geological Sciences (GS) Kyungsoo Yoo, Department of Plant and Soil Sciences (PSS) With graduate students: Chunmei Chen (PSS), Pramenath Narinesingh (GS) Katherine Skalak (GS)
Outline • • • • • •
Motivation Study Area Methods Results Discussion Conclusions
Motivation - Significance • Sand, silt, clay, and fine organic particulate matter stored in the matrix of gravel river beds is important because it influences: – Ecological processes in stream beds • Benthic organisms • Fish spawning • Etc.
– Cycling of contaminants (MERCURY) and nutrients
Key Processes • Deposition and resuspension of fine particles into and out of the gravel matrix – During periods when the gravel bed is not necessarily in motion
• Scour and deposition of coarse bed material during high flows • Net deposition or erosion through time
Initial Hypotheses To Be Tested • Silt, clay, and organic material near the surface of the bed are affected by high discharges several times a year • The bed should be scoured and filled on a scale of decimeters annually • Both processes should be more important than long term bed elevation changes caused by net sedimentation or erosion
10 cm
Bed Elevation or Cum. Mass fine sediment/area (both above arbitrary datum)
Graphical Illustration of Hypothesis 1 year
Time (years)
Implied Sediment Age Distribution With Depth 1 10
Depth (cm)
25
50
100
Age (years) 10
100
Study Area • South River, Virginia • Relative River Mile 2.95-3.0
Location of the Study Area
Here….
Coring Sites
Geomorphic Map of The Study Area
Coring Sites
Typical Cross-Section
Slow Lateral Migration, 1930-2005
Lateral migration
Methods • Overview of Approach • Coring methods • Methods used to sample suspended sediment • Adjusting radionuclide profiles for changes in grain size and organic content
Approach • Use Pb-210, Cs-137, and Be7 to estimates “ages” of sediment in the river bed • Analyze suspended sediment in contemporary events to estimate radionuclide activities at time of deposition • Adjust all results as necessary for differences in texture and organic content • Also measure Hg concentrations – Known history of Hg contamination provides independent check on the ages determined using radionuclides
The Radionuclides • Pb-210 – Half-life of 22.26 years
• Be-7 – Half-life of 53.3 days
• Cs-137 – Generated by atmospheric nuclear weapons testing – Peak atmospheric concentrations - 1963
Radionuclide Analysis • Activities determined by gamma detection – Dr. Kyungsoo Yoo’s laboratory • UD Plant and Soil Science
– All the hard work: • Chunmei Chen, PhD graduate student, Plant and Soil Science
From Dated Core Samples
Skalak and Pizzuto, in review
Hg Concentration on Suspended Sediment in Transport
South River Hg Chronologies From Modeling Hg on Suspended Sediment
19301950
1950-present Skalak and Pizzuto, in prep
Coring…..
Sampling Suspended Sediment • Installed “passive” sediment collection devices at downstream coring location at RRM 3 • To be sampled more or less immediately after a “high discharge” event
Passive Suspended Sediment Sampler Location RRM 3.0
4 Passive Suspended Sediment Samplers
Suspended Sediment Collectors In Place….
Adjusting Radionuclide Counts for Differences in Texture • Suspended sediment has very different texture from bed sediment • Samples of the bed sediment differ from each other
“Modeling” Concept •
Partition measured activities into 4 components of particle composition: – – – –
•
Organic matter Silt-clay Fine-sand Medium-coarse sand
Activity of a sample composed of 100% of each textural component at the time of deposition, Aitot0: – Treated as a constant unique for each textural component
•
Measured activities then depend on: – Amount of each component in the sample (known) – Initial activities of each component (unknown) – Sample ages • Unknown for cores • 0 for suspended sediment
Computing the Activity of Each Sample 4
4
1
1
At Ai Aitot 0 f i e At-total activity measured in a core sample Ai-activity of textural component I Aitot0-activity of textural component i at time 0 fi-proportion of textural component i in a sample – decay constant (0.03114/yr for Pb210) t – sample age
t
Calibration: vary parameters until computed At = measured At 4
4
1
1
At Ai Aitot 0 f i e
t
At-total activity measured in a core sample Ai-activity of textural component I Aitot0-activity of textural component i at time 0 (unknown) fi-proportion of textural component i in a sample – decay constant (0.03114/yr for Pb210) t – sample age (unknown)
Known values, Unknown values, Measured Activities, Computed Results 13 observed activities values, 14 unknowns to be determined…….. A single slice from a core
Core SR-2-3-09-3
Core SR-2-3-09-5
Suspended Sediment Samples
Observed Activity 0.00555 0.00271 0.00186 0.00076 0.01648 0.00475 0.00231 0.00197 0.00101 0.00057 0.10400 0.10400 0.06300 Decay constant yr
-1
Organic Total initial activity 0.08 Age (years) 20 0.014 45 0.022 75 0.031 95 0.017 2 0.024 18 0.021 30 0.008 40 0.011 75 0.025 110 0.009 0 0.031 0 0.079 0 0.053 0.03115
mud fine sand coarse sand 0.09 0.02 0.005 fractions for each sample 0.059 0.067 0.860 0.067 0.043 0.867 0.148 0.078 0.743 0.095 0.164 0.724 0.032 0.204 0.740 0.007 0.033 0.939 0.004 0.026 0.962 0.012 0.004 0.974 0.038 0.007 0.931 0.141 0.042 0.807 0.533 0.436 0.000 0.580 0.342 0.000 0.469 0.477 0.000
Computed total activity 0.00648 0.00321 0.00204 0.00087 0.01182 0.00437 0.00248 0.00196 0.00098 0.00060 0.05917 0.06529 0.05606
Results
Suspended Sediment From 3 “High Flows” Sampled Date
Peak Discharge at Waynesboro, VA
April 3, 2009
910
May 7, 2009
330
May 27, 2009
392
Note: 1 year recurrence interval discharge at Waynesboro = 2513 cfs
Particle Size and Organic Material, Suspended Sediment and Bed Material (typical values) Sample
Loss On Ignition (%)
% Silt-Clay
% vf-f Sand
% med.coarse Sand
% Gravel
Suspended Sediment
5
53
47
0
0
Bed Material
0.5
0.4
2
19
78.6
Note: gravel removed from bed material before measuring radionuclide activity but huge textural differences between suspended sediment and bed material remain (% Silt-Clay and % med-coarse sand).
Be7 Activities, Suspended Sediment and Bed Material Sample SR-6-9-09-Qss SR-5-7-09-Qss SR-4-3-09 Split 1 SR-4-3-09 Split 2 SR-4-3-09 Split 3 All Bed Samples
Be7 Activity (Bq/gm) 0.085 0.05 0.025 0.017 0.015 0
- All suspended sediment samples were exposed to atmospheric sources of Be7 within months… - All bed samples have …..no measeable Be7 activity. Bed material is “old” compared to suspended sediment
Unadjusted Cs-137 Profiles
Core SR-2-3-09-3 SR-2-3-09-3 Cumulative Grain Size Fractions and Loss-On-Ignifition (%)
Cs-137 Activity (Bq/gm) 0.00050
0.00100
0.00150
0.00200
0.1
0.00250
1
10
Silt-Clay
100
Fine Sand
0
0
5
5 10
Med-Coarse Sand
15 20
LOI
Depth (cm)
Depth (cm)
0.00000
suspended sediment
10 1963 ?
15 20
Gravel
25
25
Core SR-2-3-09-3 SR-2-3-09-5 Cumulative Grain Size Fractions and Loss-On-Ignifition (%)
Cs-137 Activity (Bq/gm) 0.00050
0.00100
0.00150
0.00200
0.1
0.00250
1
10
100
Silt-Clay
0
0
Fine Sand
5
5
Med-Coarse Sand
10 15 20 25
1963 ?
suspended sediment
Depth (cm)
Depth (cm)
0.00000
10
Gravel
15
LOI
20 25
Are peak concentrations dated at 1963 real, or related to textural variations??
Unadjusted Pb-210 Profiles SR-2-3-09-3 Activity (Bq/gm) 0.0050
0.0150
0.0250
0.0350
Depth (cm)
0 5 10
Pb210 "excess" Pb210 activity from deposited particles
15 20
Ra226
"in situ"Pb210
25
SR-2-3-09-5 Activity (Bq/gm) 0.0050
0.0150
0.0250
0.0350
0 Depth (cm)
-“In situ” Pb210 ~ constant with depth and in both cores -“excess” Pb210 decays through several 22.26 yr ½ lives..?? -Implies sediment at depth is …”old”
5 Pb210
10 "excess Pb210 activity from depositedparticles
15 20 25
"in situ" Pb210
Ra226
Unadjusted “Excess” Pb210 Only SR-2-3-09-3 Pb-210 Activity (Bq/gm) 0.00010
0.00100
0.01000
0.10000
1.00000
Depth (cm)
0 5 10 suspended sediment
15 20 25
SR-2-3-09-5 Pb-210 Activity (Bq/gm) 0.00010
0.00100
0.01000
0.10000
0 Depth (cm)
-Looks like several ½ lives of decay - though profiles need to be adjusted for changes in particle composition between layers
5 10 15 20 25
suspended sediment
1.00000
“Excess Pb210” Activities, Adjusted for Texture and Age Computed Pb210 Activity
1
Root mean square error = 20% 0.1 Core 3 Core 5
0.01
Qss 1:1
0.001
0.0001 0.0001
0.001
0.01
0.1
Observed Pb210 Activity
1
Initial Activities by Particle Classification
Particle Classification Total Initial Activity (Bq/gm)
Organic 0.08
mud 0.09
fine sand 0.02
coarse sand 0.005
Initial Activities May Be Explained By Changes in Particle Surface Area with Grain Size…
6
Med-coarse sand
11
Fine-med sand
16
Silt-clay
Relative Activity & Grain Surface Area
Relative Initial Activity vs Relative Nominal Surface Area for Spherical Sediment
Initial Activity Surface Area
1 0
0.1
0.2
Grain Diameter (mm)
0.3
0.4
Fraction of Total Activity, Medium-Coarse Sand Core SR-2-3-09-3 % Excess Pb210 Activity on Medium-Coarse Sand 0
5
10
15
20
25
30
35
40
Depth (cm)
0 5 10 15 20 25
Core SR-2-3-09-5 % Excess Pb210 Activity on Medium-Coarse Sand 0
Depth (cm)
0 5 10 15 20 25
10
20
30
40
50
60
70
80
~ half of the activity is associated with particles likely transported at least intermittently as bedload!
Normalize Cs137 Profiles By Texture • Correct profiles for any vertical structure related to particle size • Normalize to “mean suspended sediment” textural composition – Based on initial activities determined from “modeling” Pb210 – i.e., assume partitioning of Cs137 and Pb210 are similar
Cs137 Normalized to Suspended Sediment Core SR-2-3-09-3
Interpretations
Normalized Cs137 Activity (Bq/gm) 0
0.0005
0.001
0.0015
0.002
0.0025
Depth (cm)
0 5
suspended sediment
10 1963
15 20 25
Core SR-2-3-09-5 Normalized Cs137 Activity (Bq/gm) 0
0.0005 0.001 0.0015 0.002
0.0025 0.003 0.0035 0.004
Depth (cm)
0 5 10 15 20 25
suspended sediment 1963
1. Peak concentrations (1963) are not related to texture) 1. Implied dates might be correct! 2. Suspended sediment activities are higher than they should be for “present” conditions.
Final Chronology Age (years) 0
20
40
60
80
100
120
0
Depth (cm)
5 Core 3 Pb 210 10
Core 5 Pb 210 Core 3 Cs 137
15
Core 5 Cs137 20 25
Note that Pb210 and Cs137 Chronologies agree well!!
Final vs “Expected” Chronology Age (years) 0
20
40
60
80
100
120
0
Depth (cm)
5 Core 3 Pb 210 10
Core 5 Pb 210 Core 3 Cs 137
15
Core 5 Cs137 20 25
Expected…
Interpretations?? • Very limited scour and fill with a linear distribution of depth through time • Or: – Slow aggradation of the bed through time
• Or: – Both of the above
“Apparent” Net Aggradation Rate
Age (years) 0
20
40
60
80
100
120
0
Depth (cm)
5 Core 3 Pb 210 10
Core 5 Pb 210 Core 3 Cs 137
15
Core 5 Cs137 20 25
2.2 mm/yr
Lateral Migration Rate 1930-2005 ~ 4 cm/year Implies Point Bar Platform Aggradation Rate of ~ 2 mm/yr Lateral migration
Net aggradation
Aggradation Related To Slow Bend Migration?? Age (years) 0
20
40
60
80
100
120
0 Aggradation caused by lateral migration
Depth (cm)
5
Core 3 Pb 210
10
Core 5 Pb 210
15
Core 3 Cs 137 Core 5 Cs137
20 25
2.2 mm/yr
Do Hg Data Support These Ages?? From Dated Core Samples
Skalak and Pizzuto, in review
Hg Concentration on Suspended Sediment in Transport
(Hg > ~ 80 ppm from fine-grained deposits from 1930-1950) From Modeling Hg on Suspended Sediment
19301950
1950-present Skalak and Pizzuto, in prep
Hg Analyses (from a different set of cores) Concentrations - Sand (~98%) and Mud (~2%)
Concentrations Normalized to % Mud Core 6-9-09-1
Core 6-9-09-1
Hg (mg/kg silt-clay)
Hg (mg/kg) 1.00
2.00
3.00
4.00
0.00
5.00
Depth (cm)
Depth (cm)
0.00 0 2 4 6 8 10 12 14 16 18
50.00
100.00
30 35
6.00
8.00
10.00
12.00
14.00
0.00
16.00
Depth (cm)
Depth (cm)
0 5 10 15 20 25
4.00
250.00
300.00
Hg (mg/kg silt-clay)
Hg (mg/kg) 2.00
200.00
Core 6-9-09-2
Core 6-9-09-2
0.00
150.00
0 2 4 6 8 10 12 14 16 18
50.00
100.00
150.00
200.00
250.00
300.00
350.00
0 5 10 15 20 25 30 35
Hg concentrations in both cores MAY be high enough to date from release period, but difficult to compare analyses of bed material with analyses from finer sediments
Core 6-9-09-1 Hg (mg/kg silt-clay) 0.00
50.00
100.00
150.00
200.00
250.00
300.00
0
Depth (cm)
2 4 6 8 10 12 14 16
1950
18
1930
Core 6-9-09-2 Hg (mg/kg silt-clay) 0.00
50.00
100.00
150.00
200.00
250.00
300.00
0 5 Depth (cm)
Silt-Clay Normalized Concentrations with Release Period Estimate
10 15 20 25 30 35
1950 1930
350.00
Summary • We have successfully sampled and dated finegrained sediments from 2 sites on the gravel bed of the South River • Bed material from the two areas sampled is as much as 100 years old at depths of 25 cm! • Hg in these deposits is likely to persist indefinitely • Sediment ages suggest an apparent long term sediment accumulation rate of ~ 2 mm/yr – Possibly related to slow lateral migration and point bar accumulation
Outline • • • • • •
Motivation Study Area Methods Results Discussion Conclusions
Motivation - Significance • Sand, silt, clay, and fine organic particulate matter stored in the matrix of gravel river beds is important because it influences: – Ecological processes in stream beds • Benthic organisms • Fish spawning • Etc.
– Cycling of contaminants (MERCURY) and nutrients
Key Processes • Deposition and resuspension of fine particles into and out of the gravel matrix – During periods when the gravel bed is not necessarily in motion
• Scour and deposition of coarse bed material during high flows • Net deposition or erosion through time
Initial Hypotheses To Be Tested • Silt, clay, and organic material near the surface of the bed are affected by high discharges several times a year • The bed should be scoured and filled on a scale of decimeters annually • Both processes should be more important than long term bed elevation changes caused by net sedimentation or erosion
10 cm
Bed Elevation or Cum. Mass fine sediment/area (both above arbitrary datum)
Graphical Illustration of Hypothesis 1 year
Time (years)
Implied Sediment Age Distribution With Depth 1 10
Depth (cm)
25
50
100
Age (years) 10
100
Study Area • South River, Virginia • Relative River Mile 2.95-3.0
Location of the Study Area
Here….
Coring Sites
Geomorphic Map of The Study Area
Coring Sites
Typical Cross-Section
Slow Lateral Migration, 1930-2005
Lateral migration
Methods • Overview of Approach • Coring methods • Methods used to sample suspended sediment • Adjusting radionuclide profiles for changes in grain size and organic content
Approach • Use Pb-210, Cs-137, and Be7 to estimates “ages” of sediment in the river bed • Analyze suspended sediment in contemporary events to estimate radionuclide activities at time of deposition • Adjust all results as necessary for differences in texture and organic content • Also measure Hg concentrations – Known history of Hg contamination provides independent check on the ages determined using radionuclides
The Radionuclides • Pb-210 – Half-life of 22.26 years
• Be-7 – Half-life of 53.3 days
• Cs-137 – Generated by atmospheric nuclear weapons testing – Peak atmospheric concentrations - 1963
Radionuclide Analysis • Activities determined by gamma detection – Dr. Kyungsoo Yoo’s laboratory • UD Plant and Soil Science
– All the hard work: • Chunmei Chen, PhD graduate student, Plant and Soil Science
From Dated Core Samples
Skalak and Pizzuto, in review
Hg Concentration on Suspended Sediment in Transport
South River Hg Chronologies From Modeling Hg on Suspended Sediment
19301950
1950-present Skalak and Pizzuto, in prep
Coring…..
Sampling Suspended Sediment • Installed “passive” sediment collection devices at downstream coring location at RRM 3 • To be sampled more or less immediately after a “high discharge” event
Passive Suspended Sediment Sampler Location RRM 3.0
4 Passive Suspended Sediment Samplers
Suspended Sediment Collectors In Place….
Adjusting Radionuclide Counts for Differences in Texture • Suspended sediment has very different texture from bed sediment • Samples of the bed sediment differ from each other
“Modeling” Concept •
Partition measured activities into 4 components of particle composition: – – – –
•
Organic matter Silt-clay Fine-sand Medium-coarse sand
Activity of a sample composed of 100% of each textural component at the time of deposition, Aitot0: – Treated as a constant unique for each textural component
•
Measured activities then depend on: – Amount of each component in the sample (known) – Initial activities of each component (unknown) – Sample ages • Unknown for cores • 0 for suspended sediment
Computing the Activity of Each Sample 4
4
1
1
At Ai Aitot 0 f i e At-total activity measured in a core sample Ai-activity of textural component I Aitot0-activity of textural component i at time 0 fi-proportion of textural component i in a sample – decay constant (0.03114/yr for Pb210) t – sample age
t
Calibration: vary parameters until computed At = measured At 4
4
1
1
At Ai Aitot 0 f i e
t
At-total activity measured in a core sample Ai-activity of textural component I Aitot0-activity of textural component i at time 0 (unknown) fi-proportion of textural component i in a sample – decay constant (0.03114/yr for Pb210) t – sample age (unknown)
Known values, Unknown values, Measured Activities, Computed Results 13 observed activities values, 14 unknowns to be determined…….. A single slice from a core
Core SR-2-3-09-3
Core SR-2-3-09-5
Suspended Sediment Samples
Observed Activity 0.00555 0.00271 0.00186 0.00076 0.01648 0.00475 0.00231 0.00197 0.00101 0.00057 0.10400 0.10400 0.06300 Decay constant yr
-1
Organic Total initial activity 0.08 Age (years) 20 0.014 45 0.022 75 0.031 95 0.017 2 0.024 18 0.021 30 0.008 40 0.011 75 0.025 110 0.009 0 0.031 0 0.079 0 0.053 0.03115
mud fine sand coarse sand 0.09 0.02 0.005 fractions for each sample 0.059 0.067 0.860 0.067 0.043 0.867 0.148 0.078 0.743 0.095 0.164 0.724 0.032 0.204 0.740 0.007 0.033 0.939 0.004 0.026 0.962 0.012 0.004 0.974 0.038 0.007 0.931 0.141 0.042 0.807 0.533 0.436 0.000 0.580 0.342 0.000 0.469 0.477 0.000
Computed total activity 0.00648 0.00321 0.00204 0.00087 0.01182 0.00437 0.00248 0.00196 0.00098 0.00060 0.05917 0.06529 0.05606
Results
Suspended Sediment From 3 “High Flows” Sampled Date
Peak Discharge at Waynesboro, VA
April 3, 2009
910
May 7, 2009
330
May 27, 2009
392
Note: 1 year recurrence interval discharge at Waynesboro = 2513 cfs
Particle Size and Organic Material, Suspended Sediment and Bed Material (typical values) Sample
Loss On Ignition (%)
% Silt-Clay
% vf-f Sand
% med.coarse Sand
% Gravel
Suspended Sediment
5
53
47
0
0
Bed Material
0.5
0.4
2
19
78.6
Note: gravel removed from bed material before measuring radionuclide activity but huge textural differences between suspended sediment and bed material remain (% Silt-Clay and % med-coarse sand).
Be7 Activities, Suspended Sediment and Bed Material Sample SR-6-9-09-Qss SR-5-7-09-Qss SR-4-3-09 Split 1 SR-4-3-09 Split 2 SR-4-3-09 Split 3 All Bed Samples
Be7 Activity (Bq/gm) 0.085 0.05 0.025 0.017 0.015 0
- All suspended sediment samples were exposed to atmospheric sources of Be7 within months… - All bed samples have …..no measeable Be7 activity. Bed material is “old” compared to suspended sediment
Unadjusted Cs-137 Profiles
Core SR-2-3-09-3 SR-2-3-09-3 Cumulative Grain Size Fractions and Loss-On-Ignifition (%)
Cs-137 Activity (Bq/gm) 0.00050
0.00100
0.00150
0.00200
0.1
0.00250
1
10
Silt-Clay
100
Fine Sand
0
0
5
5 10
Med-Coarse Sand
15 20
LOI
Depth (cm)
Depth (cm)
0.00000
suspended sediment
10 1963 ?
15 20
Gravel
25
25
Core SR-2-3-09-3 SR-2-3-09-5 Cumulative Grain Size Fractions and Loss-On-Ignifition (%)
Cs-137 Activity (Bq/gm) 0.00050
0.00100
0.00150
0.00200
0.1
0.00250
1
10
100
Silt-Clay
0
0
Fine Sand
5
5
Med-Coarse Sand
10 15 20 25
1963 ?
suspended sediment
Depth (cm)
Depth (cm)
0.00000
10
Gravel
15
LOI
20 25
Are peak concentrations dated at 1963 real, or related to textural variations??
Unadjusted Pb-210 Profiles SR-2-3-09-3 Activity (Bq/gm) 0.0050
0.0150
0.0250
0.0350
Depth (cm)
0 5 10
Pb210 "excess" Pb210 activity from deposited particles
15 20
Ra226
"in situ"Pb210
25
SR-2-3-09-5 Activity (Bq/gm) 0.0050
0.0150
0.0250
0.0350
0 Depth (cm)
-“In situ” Pb210 ~ constant with depth and in both cores -“excess” Pb210 decays through several 22.26 yr ½ lives..?? -Implies sediment at depth is …”old”
5 Pb210
10 "excess Pb210 activity from depositedparticles
15 20 25
"in situ" Pb210
Ra226
Unadjusted “Excess” Pb210 Only SR-2-3-09-3 Pb-210 Activity (Bq/gm) 0.00010
0.00100
0.01000
0.10000
1.00000
Depth (cm)
0 5 10 suspended sediment
15 20 25
SR-2-3-09-5 Pb-210 Activity (Bq/gm) 0.00010
0.00100
0.01000
0.10000
0 Depth (cm)
-Looks like several ½ lives of decay - though profiles need to be adjusted for changes in particle composition between layers
5 10 15 20 25
suspended sediment
1.00000
“Excess Pb210” Activities, Adjusted for Texture and Age Computed Pb210 Activity
1
Root mean square error = 20% 0.1 Core 3 Core 5
0.01
Qss 1:1
0.001
0.0001 0.0001
0.001
0.01
0.1
Observed Pb210 Activity
1
Initial Activities by Particle Classification
Particle Classification Total Initial Activity (Bq/gm)
Organic 0.08
mud 0.09
fine sand 0.02
coarse sand 0.005
Initial Activities May Be Explained By Changes in Particle Surface Area with Grain Size…
6
Med-coarse sand
11
Fine-med sand
16
Silt-clay
Relative Activity & Grain Surface Area
Relative Initial Activity vs Relative Nominal Surface Area for Spherical Sediment
Initial Activity Surface Area
1 0
0.1
0.2
Grain Diameter (mm)
0.3
0.4
Fraction of Total Activity, Medium-Coarse Sand Core SR-2-3-09-3 % Excess Pb210 Activity on Medium-Coarse Sand 0
5
10
15
20
25
30
35
40
Depth (cm)
0 5 10 15 20 25
Core SR-2-3-09-5 % Excess Pb210 Activity on Medium-Coarse Sand 0
Depth (cm)
0 5 10 15 20 25
10
20
30
40
50
60
70
80
~ half of the activity is associated with particles likely transported at least intermittently as bedload!
Normalize Cs137 Profiles By Texture • Correct profiles for any vertical structure related to particle size • Normalize to “mean suspended sediment” textural composition – Based on initial activities determined from “modeling” Pb210 – i.e., assume partitioning of Cs137 and Pb210 are similar
Cs137 Normalized to Suspended Sediment Core SR-2-3-09-3
Interpretations
Normalized Cs137 Activity (Bq/gm) 0
0.0005
0.001
0.0015
0.002
0.0025
Depth (cm)
0 5
suspended sediment
10 1963
15 20 25
Core SR-2-3-09-5 Normalized Cs137 Activity (Bq/gm) 0
0.0005 0.001 0.0015 0.002
0.0025 0.003 0.0035 0.004
Depth (cm)
0 5 10 15 20 25
suspended sediment 1963
1. Peak concentrations (1963) are not related to texture) 1. Implied dates might be correct! 2. Suspended sediment activities are higher than they should be for “present” conditions.
Final Chronology Age (years) 0
20
40
60
80
100
120
0
Depth (cm)
5 Core 3 Pb 210 10
Core 5 Pb 210 Core 3 Cs 137
15
Core 5 Cs137 20 25
Note that Pb210 and Cs137 Chronologies agree well!!
Final vs “Expected” Chronology Age (years) 0
20
40
60
80
100
120
0
Depth (cm)
5 Core 3 Pb 210 10
Core 5 Pb 210 Core 3 Cs 137
15
Core 5 Cs137 20 25
Expected…
Interpretations?? • Very limited scour and fill with a linear distribution of depth through time • Or: – Slow aggradation of the bed through time
• Or: – Both of the above
“Apparent” Net Aggradation Rate
Age (years) 0
20
40
60
80
100
120
0
Depth (cm)
5 Core 3 Pb 210 10
Core 5 Pb 210 Core 3 Cs 137
15
Core 5 Cs137 20 25
2.2 mm/yr
Lateral Migration Rate 1930-2005 ~ 4 cm/year Implies Point Bar Platform Aggradation Rate of ~ 2 mm/yr Lateral migration
Net aggradation
Aggradation Related To Slow Bend Migration?? Age (years) 0
20
40
60
80
100
120
0 Aggradation caused by lateral migration
Depth (cm)
5
Core 3 Pb 210
10
Core 5 Pb 210
15
Core 3 Cs 137 Core 5 Cs137
20 25
2.2 mm/yr
Do Hg Data Support These Ages?? From Dated Core Samples
Skalak and Pizzuto, in review
Hg Concentration on Suspended Sediment in Transport
(Hg > ~ 80 ppm from fine-grained deposits from 1930-1950) From Modeling Hg on Suspended Sediment
19301950
1950-present Skalak and Pizzuto, in prep
Hg Analyses (from a different set of cores) Concentrations - Sand (~98%) and Mud (~2%)
Concentrations Normalized to % Mud Core 6-9-09-1
Core 6-9-09-1
Hg (mg/kg silt-clay)
Hg (mg/kg) 1.00
2.00
3.00
4.00
0.00
5.00
Depth (cm)
Depth (cm)
0.00 0 2 4 6 8 10 12 14 16 18
50.00
100.00
30 35
6.00
8.00
10.00
12.00
14.00
0.00
16.00
Depth (cm)
Depth (cm)
0 5 10 15 20 25
4.00
250.00
300.00
Hg (mg/kg silt-clay)
Hg (mg/kg) 2.00
200.00
Core 6-9-09-2
Core 6-9-09-2
0.00
150.00
0 2 4 6 8 10 12 14 16 18
50.00
100.00
150.00
200.00
250.00
300.00
350.00
0 5 10 15 20 25 30 35
Hg concentrations in both cores MAY be high enough to date from release period, but difficult to compare analyses of bed material with analyses from finer sediments
Core 6-9-09-1 Hg (mg/kg silt-clay) 0.00
50.00
100.00
150.00
200.00
250.00
300.00
0
Depth (cm)
2 4 6 8 10 12 14 16
1950
18
1930
Core 6-9-09-2 Hg (mg/kg silt-clay) 0.00
50.00
100.00
150.00
200.00
250.00
300.00
0 5 Depth (cm)
Silt-Clay Normalized Concentrations with Release Period Estimate
10 15 20 25 30 35
1950 1930
350.00
Summary • We have successfully sampled and dated finegrained sediments from 2 sites on the gravel bed of the South River • Bed material from the two areas sampled is as much as 100 years old at depths of 25 cm! • Hg in these deposits is likely to persist indefinitely • Sediment ages suggest an apparent long term sediment accumulation rate of ~ 2 mm/yr – Possibly related to slow lateral migration and point bar accumulation
