Wildlife Refuge Dam Decommissioning and Sediment Management

The United States Fish and Wildlife Service (USFWS) leveraged the expertise of GFT to design a plan to breach the Lower Peverly Dam, prevent the downstream migration of contaminated sediments, and ecologically restore the area within the Great Bay National Wildlife Refuge in Newington, New Hampshire.

Potential dam failure was imminent given that the aging dam was in dire condition, and its spillway exhibited signs of critical failure. The refuge is part of the Pease Air Force Base Superfund Site, so breaching the dam had to address the potential downstream migration of contaminated sediments.

GFT conducted topographic and bathymetric surveys to establish baseline conditions and identify natural resources within the area. Coordination with refuge staff, regional experts, and regulatory personnel assisted in identifying key aspects and concerns for designing the dam breach and treatment of the dewatered reservoir.

Our team also developed the sediment sampling plan, which was reviewed and approved by the New Hampshire Department of Environmental Services (NHDES). To predict contaminant mobility and identify the flow conditions needed to prevent contaminant migration, hydrologic and hydraulic models developed by GFT evaluated storm flow conditions and identified high velocity and shear stress areas. GFT developed a diversion of water plan to safely pass concentrated flows around the work area, preventing erosion and downstream movement of contaminated sediment during construction when the dewatered reservoir was denuded and in its most vulnerable state.

GFT prepared the field investigation plans and performed the investigations under a Site-Specific Health and Safety Plan. Reservoir sediments were collected and analyzed for:

• Inorganic compounds.
• Volatile and semi-volatile organic compounds.
• Polychlorinated biphenyls (PCB).
• Pesticides, herbicides, and perfluoroalkyl and polyfluoroalkyl substances (PFAS) constituents.
• General chemistry.

Sediment results were compared to the New Hampshire Freshwater and Marine Threshold Values and determined to present a moderate- to high-risk level consistent with prior Environmental Protection Agency (EPA) sediment analyses. Further analyses determined that arsenic contamination was the primary constituent of concern, as 8,200 cubic yards of reservoir sediments would be converted to riparian zone and floodplain soils with the decommissioned dam. NHDES accepted the sediment sampling characterization report.

To fully authorize the project at the federal and state levels, GFT led presentations and coordination efforts with the U.S. Army Corps of Engineers and NHDES. The team coordinated with NHDES’s Coastal Zone Management Program, Shorelands Protection Program, Wetlands Bureau, the New Hampshire Natural Heritage Bureau, and New Hampshire Fish and Game Department.

These agencies contributed to the review and authorization of GFT’s permit applications and final permit approvals to construct the project. GFT authored the project’s long-term site monitoring plans, which were approved by NHDES for implementation by the USFWS.

The final design option consisted of breaching the dam, installing grade controls, and re-establishing a native riparian zone with wetlands, shallow pools, and riparian habitats. By design, the grade control structures create shallow pools and naturally conceal themselves with new vegetative growth and riparian vegetation as the final condition of the restoration. Contaminated sediments will remain indefinitely locked in place by the grade controls, along with subsequent years of organic matter accumulation and continuous root development to bind sediments in perpetuity.

This innovative ecological restoration restored 6.2 acres of open water pond into a sustainable wetland/riparian habitat, with GFT collaborating with the contractor throughout the construction process. The project used timber matting and controlled equipment access into contaminated areas to minimize sediment disturbances and tree clearing. Engineering and institutional controls remain in place as the final use condition of the refuge.

The USFWS’ decision to decommission the Lower Peverly Dam prevented the uncontrolled release of contaminated reservoir sediments from the dam’s inevitable failure. Using GFT’s clever decommissioning design solution that prevented head cutting and obstructed the migration of the contaminated sediments into the Great Bay, a naturally beautiful wetland/ and riparian habitat area in the Great Bay National Wildlife Refuge was restored.