The concentrations of chlorinated VOC contamination present in groundwater at the JJT and BP sites (up to 420 mg/l TCE in deep overburden deposits) is indicative of the possible presence of a dense non-aqueous phase liquid (i.e. pure TCE solvent) in the subsurface. This material, in the form of pure product, will sink within the aquifer due to its density being greater than that of water. Several bedrock wells installed at these sites found that TCE is present in the upper bedrock aquifer at concentrations ranging to 30 mg/l. The limited data collected by DEP during our subsurface overburden investigation downgradient of these sites found that there does not appear to be a significant dissolved chlorinated VOC plume migrating within the Mill Brook sub-basin. This may indicate that a high percentage of the chlorinated VOC mass is migrating and/or is immobilized within the bedrock.

The assessment work completed to date to evaluate the extent, magnitude, and migration potential of the contaminant plume has found that there does not appear to be any direct concerns relative to vapor impacts to buildings or contamination of private wells (other than problems noted at Kelly Hill that are most likely not related). Results of the groundwater investigation completed by DEP downgradient of the JJT TCE plume also found that the residences in the Middlesex Avenue and Clark Street area are not at risk from vapor impacts due to the high concentrations of dissolved chlorinated VOCs emanating from the JJT/BP sites.

Recommendations, Lower Main Street Sites

The Phase II investigation being performed by the Potentially Responsible Parties for the Lower Main Street sites, as directed by the 21E regulations, should include the installation of deep bedrock wells to determine the extent of contamination migrating from these sites. The installation of bedrock wells should also determine whether groundwater contamination is migrating in a westerly direction towards the private wells of Kelly Hill. Selected sites located in this area may be recommended for audit by the Bureau of Waste Site Cleanup.

Discussion, Silver Lake Area

Review of the files for the 21E sites located in proximity to Silver Lake found that residual gasoline contamination is present in the soil and groundwater at the two former gasoline station sites. The general direction of surface and groundwater flow for this area is in an easterly direction along Lubbers Brook. Residual groundwater contamination from these sites will migrate easterly, discharging to Lubbers Brook and likely presenting no threat to surface waters of Silver Lake.

Recommendations, Silver Lake Area

An additional round of groundwater samples from the Default 1B sites located in the northern section of the study area should be collected to determine the potential impacts to the surrounding residential area and Silver Lake.