Distribution and Mass of COCs in Groundwater

During completion of the RI, the hydro-geologic and groundwater quality conditions at the Site were characterized using an extensive network of monitoring wells, extraction wells, wetland drive points, and piezometers. Based on the results of these evaluations, the extent of NAPL and dissolved phase COCs in overburden and bedrock groundwater were delineated or estimated (BBL 1998).

NAPL Zones

As part of the RI, "probable" and "potential" NAPL zone boundaries in both overburden and bedrock were delineated (BBL 1998). A subsequent field-based NAPL delineation study in support of the FS further refined the area in the overburden where most of the NAPL appears to be located (BBL and USEPA 2005). The Overburden NAPL area is shown on Figure L-14. The estimated extent of NAPL in bedrock is shown on Figure L-15.

Dissolved Phase Groundwater Plume

The shallow overburden groundwater VOC plume associated with the Site extends approximately 300 feet east of the Operations Area and the NTCRA 1 Containment Area (Figure L-9). The middle overburden groundwater VOC plume associated with the Site extends into the center of the Town Well Field (Figure L-10). The southern extension of the middle overburden VOC plume attenuated to below regulatory standards following the startup of the NTCRA 2 groundwater capture system. The deep overburden groundwater VOC plume associated with the SRSNE Site extends into the northern portion of the Town Well Field (Figure L-11). A second unrelated VOC source is interpreted near the southwestern portion of the Town Well Field. The shallow and deep bedrock groundwater VOC plumes associated with the Site extend into the central portion of the Town Well Field (Figures L-12 and L-13).

Groundwater COC plumes with dissolved VOC concentrations in excess of drinking water standards ("regulatory VOC plumes") were delineated during the RI based on fundamental groundwater hydraulics and solute-transport principles, as well as exceedances of regulatory criteria such as Federal Maximum Contaminant Levels (MCLs) and State of Connecticut Class GA/GAA Groundwater Protection Criteria (BBL 1998). The regulatory VOC plumes were re-evaluated based on groundwater monitoring results from the April 2005 (final) sampling event of the Interim Monitoring and Sampling (IMS) program (BBL 2005). The IMS program was performed to monitor the status of the plume between the completion of the RI and the issuance of the ROD, and covered 14 semi-annual sampling events at 25 monitoring wells. Based on a comparison of the April 2005 plume extent (Figures L-9 through L-13) with plume extent shown in the RI (BBL 1998), the southward extent of SRSNE-related COCs with concentrations above MCLs or GWPCs has decreased in middle and deep overburden and shallow bedrock groundwater since the completion of the RI and startup of the NTCRA 2 groundwater capture system. Overburden and bedrock groundwater capture area boundaries are shown on Figures L-16 and L-17.