1. COGENT HYDROLOGIC
In-Situ Measurement of Seepage Velocity and
Chlorobenzene Degradation Rates in Freshwater
Sediment
Andrew J.B. Cohen, Ph.D.
Principal Hydrogeologist
Cogent HydroLogic, LLC
Westfield, New Jersey

2. Acknowledgements
 URS Corporation, Newark, DE

3. General Outline
1. Background
3. Conceptual Model
2. Sampling
4. Pore Water Measurements and
Modeling
0
20
40
60
80
100
120
0.0 0.5 1.0
Depth[cm]
C/Co
Analytical Solution
SCD78-Chlorobenzene

4. Freshwater Canal
 Non-tidal
 200 wide
 4 deep
 Sediment: 3.5 thick

5. Sediment Characteristics
5
Top of Sediment 
Sediment/Aquifer Contact 
Organic-rich silt

6. Primary VOCs
 Monochlorobenzene
 Dichlorobenzenes
 Benzene
 Aniline
 N-nitrosodiphenylamine
 O-toluidine
6

7. Sediment
Cores
On-shore soil boringsInclined Wells
Sediment Piezometers
Example Sampling

8. Site Characterization

9. Sampling
Aquifer
0
1
2
3
4
5
6
Depth (ft)
Peepers Centrifuging
Bulk
chemistry
Sediment
Canal
Surface Water
Groundwater
Piezometers

10. Sampling
 147 Sediment samples
 76 Sediment borings
 70 Soil borings
 43 GW sampling locations
 14 Surface water samples
 13 Sediment peepers for pore water analysis
 40 Centrifuge samples
 8 Temporary Piezometers
10

11. Chlorobenzene Distribution
11

12. Groundwater
 Bird’s Eye View
12

13. >24”
6-12”
18-24”
Sediment
13
0-6”Sediment
Sed. Conc.
(mg/Kg)

14. DNAPL
Vert. Exagg. = 10x
SN
Canal
Sediment
N
S
Cross-Section
14
Confining Unit
Aquifer
Residual DNAPL

15. Conceptual Model of
Groundwater—Surface Water Interaction
15

16. Conceptual Model
 Cross-section
16
Canal
Residual
DNAPL
SEEP
15

17. Conceptual Model
 Cross-section
17
Canal
Residual
DNAPL
15
Also:
Very High
Chloride
Concentrations
Associated with
On-Site Brine Use

18. Pore Water Analysis

19. Methods
 Use sediment peepers to measure pore water
concentration profiles
 Fit data with an analytical solution to estimate fate &
transport characteristics (inverse modeling)

20.  5” X 14”
 1” sample depth interval
 Incubation time: 3-8 wks
Sediment “Peepers”
Source: http://ead.univ-angers.fr/~geologie/pages_membres/EM_english.html

21. Sediment Peeper
21
This study:
 5” x 14”
 Teflon housing
 Teflon membrane
 28 rows spaced 0.5 in
 5 cm3 and 10 cm3
samples:
– VOCs
– Chloride
– Iron, Nitrate, Sulfate,
Manganese
– pH, ORP, Spec. Cond.

22. Peeper Locations
22
Sediment Chlorobenzene
Conc. (mg/Kg) in the 6”-
12” depth interval
200 ft
PLAN VIEW

23. 0 1,000 2,000 3,000
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Concentration (micrograms/liter)
DepthBelowTopofSediment(cm)
SCD79
0 1,000 2,000 3,000
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Concentration (micrograms/liter)
DepthBelowTopofSediment(cm)
SCD77
0 1,000 2,000 3,000
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Concentration (micrograms/liter)
DepthBelowTopofSediment(cm)
SCD78
0 1,000 2,000 3,000
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Concentration (micrograms/liter)
DepthBelowTopofSediment(cm)
SCD82
0 1,000 2,000 3,000
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Concentration (micrograms/liter)
DepthBelowTopofSediment(cm)
SCD83
0 1,000 2,000 3,000
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Concentration (micrograms/liter)
DepthBelowTopofSediment(cm)
SCD84
ND (4 ug/L)
Chlorobenzene Pore Water Profiles
84
83
80
81
82
79
77
78
20 ug/L

24. Redox
 Anaerobic sediment
 Very thin oxic/anoxic transition zone
-300-200-100 0 100 200 300
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
ORP (mV)
DepthBelowTopofSediment(cm)
-300-200-100 0 100 200 300
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
ORP (mV)
DepthBelowTopofSediment(cm)
-300-200-100 0 100 200 300
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
ORP (mV)
DepthBelowTopofSediment(cm)
-300-200-100 0 100 200 300
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
ORP (mV)
DepthBelowTopofSediment(cm)
SCD77 SCD78 SCD81 SCD83

25. Chloride
Chlorobenzene
Chlorobenzene (micrograms/Liter)
Chloride (milligrams/Liter)
• VOC trends ≠ Cl trend
• Evidence of
Chlorobenzene
Degradation?
Chloride and Chlorobenzene Profiles
Flow

26. Pore Water Modeling

27. Quantitative Analysis Tool
 Steady-state solute transport model

28. Contaminated Sediment
Surface Water
Cap Analytical Model
Aquifer
Pore
Water in
Sand Cap
Upward
flow
Boundary Condition
Bioturbation zone

29. Surface Water
Analytical Model – can be used w/out a “cap”
Aquifer
Pore Water in
Natural Sediment
Boundary Condition
Upward
flow
Bioturbation zone

30. Quantitative Analysis: Step-wise Approach
1. Use conservative species (chloride) to quantify
mechanical transport processes
• Advection
• Diffusion
• Bioturbation
2. Analyze VOC data using model to quantify
degradation rate

31. Chloride Profile 0 500,000 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Depth(in)
Chloride Porewater Conc (ug/L) at Location SCD81
sediment
surface water
aquifer
Peeper

32. Chloride Data + Analytical Solution
 Data match yields pore water velocity
32
v=30 cm/yr v=22 cm/yr
CC

33. Chlorobenzene + Analytical Solution
 Consistent with literature and other site data
0
20
40
60
80
100
120
0.0 0.5 1.0
Depth[cm]
C/Co
Analytical Solution
SCD78-Chlorobenzene
t1/2=504 d
v=27 cm/yr
0
20
40
60
80
100
120
0.0 0.5 1.0
Depth[cm]
C/Co
Analytical Solution
SCD78-Chlorobenzene
t1/2=280 d
v=48 cm/yr

34. Summary
 Unusual cause of sediment contamination at this site:
– Groundwater plume discharge
 Used analytical model of pore water flux to estimate fate and
transport parameters
 Chloride plume provides unique condition to indirectly measure
seepage velocity
 The estimated chlorobenzene anaerobic degradation half-life is
consistent with literature values (280-504 days).
 Findings consistent with other site data:
– Presence of dehalococcoides in sediment
– Compound-Specific Isotope Analysis using sediment pore water samples
34

35. Questions?
Andrew Cohen, Ph.D.
Cogent HydroLogic, LLC
acohen@cogenthydrologic.com
www.cogenthydrologic.com

36. END