Concord oversight committee questions EPA draft on holding basin, sampling and backfill materials

Speaker 1 convened the Main Street Oversight Committee to review the EPA 60% remedial design for the STARMet holding basin and related documents. Committee members focused on sampling density, backfill materials, trench stability and whether the containment wall should be keyed into bedrock or the glacial till.

Speaker 3 described differences in sampling grids used during investigation, noting the committee prefers denser composite sampling rather than single samples on a 30-foot grid to better characterize contamination. The committee had submitted comments on predesign investigations and confirmed those documents were in the meeting packet and posted where allowed.

On cleanup standards, Speaker 3 urged that the project “use the site cleanup levels rather than RALs,” and added, “The RALs is always gonna be higher than the site cleanup level.” Members agreed to include a comment to EPA contact Cara Nuremberg that the committee strongly prefers the conservative site cleanup levels when appropriate.

Technical concerns centered on backfill choices. Speaker 3, citing field experience, warned that bentonite seals can desiccate and crack above the water table and asked how the design will address long-term desiccation. Members said they did not know what the documents described as “plastic cement” and requested evidence of the material’s long-term durability before endorsing it.

The committee also discussed construction logistics. Speaker 3 flagged that narrow trenches (described in the design as roughly 3 feet wide and possibly deep) will face substantial hydrostatic and overburden pressure and asked how contractors will keep trenches open during installation. Multiple members said those questions are primarily contractor-driven and may be answered in the RFP and the 95% design; the committee asked for clearer specifications and monitoring plans in that next stage.

A major change in the draft design drew particular scrutiny: earlier designs showed the containment wall keyed into bedrock, but the 60% design suggests roughly two-thirds of the wall would be keyed only into the surrounding till. Speaker 2 reported EPA has signaled comfort with keying into till based on their tests, but members said they wanted documentation characterizing the till’s hydraulic properties before accepting the change.

Members also discussed monitoring and access. The design would leave a set of monitoring wells active inside the holding basin to allow sampling after iron injection; the committee asked whether those wells would pierce the final cap and how they would be accessed without compromising the cap integrity. Speaker 4 noted the plan for a number of flush-mounted wells and urged explicit detail in the 95% design.

On contaminants and remedies, Speaker 2 summarized that historical nitrate plumes had reached the river, while uranium moves more slowly through the subsurface. The revised approach includes injecting divalent iron into formation soils to sequester uranium and render it less soluble; Speaker 2 said the iron injections would occur before wall construction because injection operations could damage the wall. The project team also expects to operate pump-and-treat systems for organics (including 1,4-dioxane) with long-term institutional controls and escrow funds to prevent well use and to fund required monitoring.

Action items and next steps: the committee agreed to circulate concise comments focused on three topics — bentonite/desiccation and alternative materials, clarification of plastic-cement properties and longevity, and documentation supporting keying the wall into till versus bedrock — and to ask the project team for the 95% design details on trenches, contractor methods, monitoring-well access and cap penetrations. Ray (name referenced in the meeting) will draft the committee’s letter to Cara Nuremberg for quick committee review ahead of the stated comment deadline.

The committee did not reach a final technical conclusion on the wall-depth change or backfill materials; members asked for the 95% design data and clearer evidence before endorsing those aspects.