Hydrostratigraphic Mapping of the Milford-Souhegan Glacial Drift Aquifer, and Effects of Hydrostratigraphy on Transport of PCE, Operable Unit 1, Savage Superfund Site, Milford, New Hampshire

摘要

The Savage Municipal Well Superfund site in the Town of Milford, New Hampshire, was underlain by a 0.5-square mile plume (as mapped in 1994) of volatile organic compounds (VOCs), most of which consisted of tetrachloroethylene (PCE). The plume occurs mostly within highly transmissive stratified-drift deposits but also extends into underlying till and bedrock. The plume has been divided into two areas called Operable Unit 1 (OU1), which contains the primary source area, and Operable Unit 2 (OU2), which is defined as the extended plume area outside of OU1. The OU1 remedial system includes a low-permeability barrier wall that encircles the highest detected concentrations of PCE and a series of injection and extraction wells to contain and remove contaminants. The barrier wall likely penetrates the full thickness of the sand and gravel; in many places, it also penetrates the full thickness of the underlying basal till and sits atop bedrock. From 1998 to 2004, PCE concentrations decreased by an average of 80 percent at most wells outside the barrier wall. However, inside the barrier, PCE concentrations greater than 10,000 micrograms per liter (ig/L) still exist (2008). The remediation of these areas of recalcitrant PCE presents challenges to successful remediation. The U.S. Geological Survey (USGS), in cooperation with the New Hampshire Department of Environmental Services (NHDES) and the U.S. Environmental Protection Agency (USEPA), Region 1, is studying the solute transport of VOCs (primarily PCE) in contaminated groundwater in the unconsolidated sediments (overburden) of the Savage site and specifically assisting in the evaluation of the effectiveness of remedial operations in the OU1 area. As part of this effort, the USGS analyzed the subsurface stratigraphy to help understand hydrostratigraphic controls on remediation.