Engineered Cap Evaluation October 1, 2012

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Objectives • Background on capping • Capping evaluations

- Long-term chemical isolation - Cap erosion protection evaluation - Habitat evaluation

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Use of Capping Technology • A cap is generally designed to

reduce risk through the following primary functions - Physical isolation of the

contaminated sediment - Stabilization of contaminated

sediment - Chemical isolation of

contaminated sediment sufficient to reduce exposure from dissolved contaminants that may be transported into the water column

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Sediment Cap Components • Erosion Protection (Armor) Layer

- Prevents the chemical isolation layer from being eroded by forces in the river (such as due to river currents during high flow events)

• Chemical Isolation Layer

- Stabilizes the existing sediments - Limits the vertical movement of

dissolved contaminants by advection (flow of groundwater or porewater) and by molecular diffusion (movement across a concentration gradient) over long periods

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Example Sediment Cap Materials Cobbles

Sands

Gravels

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Detailed Engineered Cap Evaluation • Chemical Isolation Layer Evaluation

- Mixing Depths - Contaminant Transport

• Armor Layer Evaluation

- Erosion Protection - Habitat Substrate

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Mixing Layer Evaluation • When the chemical isolation layer material is placed, a very

small amount mixes with the sediments • Information on cap mixing reviewed from over 20 sites

- Indicates the level of mixing is minimal using contemporary construction techniques

Site Measured Mixing Depth (inches)

University of New Hampshire Contaminated Sediment Center - Pilot Cap, NH 1.5

Gasco Site Removal Action, OR 0.74 average Port of Olympia, WA 0.4 average Lower Fox River Phase 1, WI 0.4 average Anacostia River, DC 1.6 average Stryker Bay, MN 0 to 2 (dredged areas) Grasse River - Capping Pilot Study, NY Less than 2

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Mixing in Silver Lake • Pilot Study data from Silver Lake,

which has very soft sediments, also indicate cap can be placed with minimal mixing - Sediment profile imagery

Mixing appears limited to the first inch of isolation layer

materials in non-geofabric areas

SPI-2

2”

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Mixing in Silver Lake • Data from Silver Lake, which has very soft sediments, also

indicate cap can be placed with minimal mixing - PCB measurements in core samples

0.33

148 2.49 ND ND

0.66 ND

178 0.15 ND ND

ND ND

88.8 0.16 ND ND

NA

TOP

REM

4-6” 2-4” 0-2” SED

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Chemical Isolation Layer Evaluation • Model used to evaluate PCB transport within cap

- Cap design model developed by Dr. Danny Reible at University of Texas

- Consistent with USEPA guidance

- Use model to determine thickness and composition (TOC) of cap required to maintain flux at a level less than 5% of steady-state value for 100 years

- Similar to cap evaluations used for the ½ Mile and Silver Lake designs

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Chemical Isolation Layer Evaluation • Model used to evaluate PCB transport within cap (cont.)

- Inputs developed based on USEPA’s fate and transport model

- Groundwater velocity is uncertain; value of 1 cm/d (365 cm/yr) used for example calculations

- ½ Mile seepage meter data all < 1 cm/yr

- Silver Lake seepage meter data average ~ 5 cm/yr

- 1½ Mile seepage meter data average ~ 800 cm/yr

- Conservative assumptions

- Net deposition conservatively ignored

- Infinite source of PCBs assumed directly beneath the cap

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Cap Model Configuration

Armor/Habitat Layer

Chemical Isolation

Compliance Point (Evaluate flux at surface)

Model domain

Overlying Water Column

Sediment

Bioturbation Zone (4 to 6 inches)

Advection Diffusion Dispersion

Sacrificial Mixing Layer

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Results of Cap Modeling • 3-inch isolation layer amended with TOC (a few %) would be

effective in all Reaches

• Sensitivity Analysis: Effect of deposition (3-inch isolation layer)

Isolation Layer Thickness

fOC Required (%)

3 inches 1% – 2%

6 inches ~1%

Reach No Deposition With Deposition [rate in brackets]

Woods Pond 1.5% fOC 0.5% fOC [0.2 cm/yr]

Rising Pond 1.75% fOC 0.25% fOC [0.5 cm/yr]

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Cap Armor Layer Evaluation • Evaluated erosive forces in the River

- River currents during flood events • Simulated flood events using

USEPA’s hydrodynamic model - Various flood events including the

100-year event • Computed stable particle size (D50)

based on the model results using USEPA design guidance (Maynord 1998)

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Stable Particle Sizes: Reach 5A

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Stable Particle Sizes: Woods Pond

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Evaluated Habitat Substrate • Existing riverbed consists of sands, gravels, and cobbles, with

high percentages of fine particles in impoundment areas • Compared computed armor material size with existing river

substrate • Armor material size is generally consistent with existing

substrate, or one size class higher

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Reach 5A Grain Size Data Calculated Stable

Particle Size

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Reach 5A (cont.) Grain Size Data Calculated Stable

Particle Size

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Woods Pond

Calculated Stable Particle Size Grain Size Data

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Engineered Caps in Reach 5A • Developed cap sections for Reach 5A • Example caps in Reach 5A

- 12-inch thick caps - 6-inch chemical isolation layer consisting of sands and TOC

(and allows for mixing) - 6 inches of gravel armor/habitat layer

- 18- to 24-inch thick caps - 6-inch chemical isolation layer consisting of sands and TOC

(and allows for mixing) - 12 inches of cobble armor/habitat layer - Geotextile or 6 inches of gravel may be used as a filter layer

between sand and cobbles

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Example Reach 5A Engineered Cap

Notes: The isolation layer accounts for mixing with underlying sediments. *A geotextile could be considered based on site specifics.

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Engineered Caps in Woods Pond • Developed cap section for Woods Pond • 6- to 9-inch thick caps

- 6- to 9-inches of chemical isolation layer consisting of sands and TOC (and allows for mixing with underlying sediment and bioturbation at surface)

- Sands are stable under high flow conditions (due to the backwater caused by the dam) so a separate armor layer is not required

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Woods Pond

Approx. 6” to 9”

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Summary • Preliminary cap engineering analysis was performed using

methods consistent with EPA guidance • Various cap types/thicknesses can meet objectives • Mixing is expected to be minimal and could be accounted for

within the chemical isolation layer • The cap armor layer could also provide habitat substrate similar

to existing riverbed conditions