COGENT HYDROLOGIC

In-Situ Measurement of Seepage Velocity andChlorobenzene Degradation Rates in Freshwater

Sediment

Andrew J.B. Cohen, Ph.D.Principal HydrogeologistCogent HydroLogic, LLCWestfield, New Jersey

Acknowledgements

URS Corporation, Newark, DE

General Outline1. Background

3. Conceptual Model

2. Sampling

4. Pore Water Measurements andModeling 0

20

40

60

80

100

120

0.0 0.5 1.0

Dept

h [c

m]

C/Co

Analytical Solution

SCD78-Chlorobenzene

Freshwater Canal

Non-tidal 200 wide 4 deep Sediment: 3.5 thick

Sediment Characteristics

5

Top of Sediment

Sediment/Aquifer Contact

Organic-rich silt

Primary VOCs

Monochlorobenzene Dichlorobenzenes Benzene Aniline N-nitrosodiphenylamine O-toluidine

6

SedimentCores

On-shore soil boringsInclined Wells

Sediment Piezometers

Example Sampling

Site Characterization

Sampling

Aquifer

0

1

2

3

4

5

6

Depth (ft)

Peepers CentrifugingBulkchemistry

Sediment

CanalSurface Water

Groundwater

Piezometers

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

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Chlorobenzene Distribution

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Groundwater Bird’s Eye View

12

>24”

6-12”

18-24”

Sediment13

0-6”Sediment

Sed. Conc.(mg/Kg)

DNAPL

Vert. Exagg. = 10x

SN

Canal

Sediment

N

S

Cross-Section14

Confining Unit

Aquifer

Residual DNAPL

Conceptual Model ofGroundwater—Surface Water Interaction

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Conceptual Model

Cross-section

16

Canal

ResidualDNAPL

SEEP

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Conceptual Model

Cross-section

17

Canal

ResidualDNAPL

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Also:Very HighChlorideConcentrationsAssociated withOn-Site Brine Use

Pore Water Analysis

Methods

Use sediment peepers to measure pore waterconcentration profiles

Fit data with an analytical solution to estimate fate &transport characteristics (inverse modeling)

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

Sediment Peeper21

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.

Peeper Locations22

Sediment ChlorobenzeneConc. (mg/Kg) in the 6”-

12” depth interval

200 ftPLAN VIEW

0 1,000 2,000 3,000-10

-505

101520253035404550556065707580859095

100

Concentration (micrograms/liter)

Depth Below Top of Sedim

ent (cm)

SCD79

0 1,000 2,000 3,000-10

-505

101520253035404550556065707580859095

100

Concentration (micrograms/liter)

Depth Below Top of Sedim

ent (cm)

SCD77

0 1,000 2,000 3,000-10

-505

101520253035404550556065707580859095

100

Concentration (micrograms/liter)

Depth Below Top of Sedim

ent (cm)

SCD78

0 1,000 2,000 3,000-10

-505

101520253035404550556065707580859095

100

Concentration (micrograms/liter)

Depth Below Top of Sedim

ent (cm)

SCD82

0 1,000 2,000 3,000-10

-505

101520253035404550556065707580859095

100

Concentration (micrograms/liter)

Depth Below Top of Sedim

ent (cm)

SCD83

0 1,000 2,000 3,000-10

-505

101520253035404550556065707580859095

100

Concentration (micrograms/liter)

Depth Below Top of Sedim

ent (cm)

SCD84

ND (4 ug/L)

Chlorobenzene Pore Water Profiles

84

83

80

81

82

7977

78

20 ug/L

Redox

Anaerobic sediment Very thin oxic/anoxic transition zone

-300-200-100 0 100 200 300-20-15-10

-505

1015202530354045505560657075808590

ORP (mV)

Depth Below Top of Sedim

ent (cm)

-300-200-100 0 100 200 300-20-15-10

-505

1015202530354045505560657075808590

ORP (mV)

Depth Below Top of Sedim

ent (cm)

-300-200-100 0 100 200 300-20-15-10

-505

1015202530354045505560657075808590

ORP (mV)

Depth Below Top of Sedim

ent (cm)

-300-200-100 0 100 200 300-20-15-10

-505

1015202530354045505560657075808590

ORP (mV)

Depth Below Top of Sedim

ent (cm)

SCD77 SCD78 SCD81 SCD83

Chloride

Chlorobenzene

Chlorobenzene (micrograms/Liter)

Chloride (milligrams/Liter)

• VOC trends ≠ Cl trend• Evidence of

ChlorobenzeneDegradation?

Chloride and Chlorobenzene Profiles

Flow

Pore Water Modeling

Quantitative Analysis Tool

Steady-state solute transport model

Contaminated Sediment

Surface Water

Cap Analytical Model

Aquifer

PoreWater in

Sand CapUpward

flow

Boundary Condition

Bioturbation zone

Surface Water

Analytical Model – can be used w/out a “cap”

Aquifer

Pore Water inNatural Sediment

Boundary Condition

Upwardflow

Bioturbation zone

Quantitative Analysis: Step-wise Approach

1. Use conservative species (chloride) to quantifymechanical transport processes• Advection• Diffusion• Bioturbation

2. Analyze VOC data using model to quantifydegradation rate

Chloride Profile0 500,000 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000

-5

0

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30

35

40

45

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55

60

65

70

75

80

Depth (in)

Chloride Porewater Conc (ug/L) at Location SCD81

sediment

surface water

aquifer

Peeper

Chloride Data + Analytical Solution

Data match yields pore water velocity

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v=30 cm/yr v=22 cm/yr

CC

Chlorobenzene + Analytical Solution

Consistent with literature and other site data

0

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Dept

h [c

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C/Co

Analytical Solution

SCD78-Chlorobenzene

t1/2=504 dv=27 cm/yr

0

20

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0.0 0.5 1.0

Dept

h [c

m]

C/Co

Analytical Solution

SCD78-Chlorobenzene

t1/2=280 dv=48 cm/yr

Summary

Unusual cause of sediment contamination at this site:– Groundwater plume discharge

Used analytical model of pore water flux to estimate fate andtransport parameters

Chloride plume provides unique condition to indirectly measureseepage velocity

The estimated chlorobenzene anaerobic degradation half-life isconsistent 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

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Questions?

Andrew Cohen, Ph.D.Cogent HydroLogic, LLCacohen@cogenthydrologic.comwww.cogenthydrologic.com

END