During the completion of the Remedial Investigation (RI), the hydrogeologic and groundwater quality conditions at the site were characterized using an extensive network of monitoring wells, extraction wells, wetland drive points, and piezometers. Contaminant plumes with dissolved VOC concentrations in excess of drinking water standards (“regulatory VOC plumes”) in groundwater were delineated 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.”

The total VOC mass at the site was estimated to be 546,700 kg in 1996, and was thought to be distributed approximately as follows:

• Unsaturated Soil: 2,200 kg sorbed and dissolved, and 1,300 kg as NAPL; for a total of 3,500 kg, or 0.64% of the estimated total VOC mass;
• Overburden NAPL Area: 460,000 kg as NAPL, or 84% of the estimated total VOC mass;
• Overburden Groundwater: 1,900 kg dissolved, 9,300 kg sorbed; for a total of 11,200 kg, or 2.1% of the estimated total VOC mass; and
• Bedrock: 39,000 kg dissolved and sorbed (combined), 33,000 kg NAPL, or 7.1% and 6.0% of the estimated total VOC mass, respectively.
• Bedrock NAPL Area: NAPL is known to be present in the bedrock, but its extent has not been defined.

In summary, the majority of the VOC mass is in the form of NAPL in the Overburden NAPL Area.
In 2015, the Groundwater Site Conceptual Model for the site was updated. This updated presented a revised VOC mass estimate that took into account remediation completed since 1996. The most significant effort was in-situ thermal remediation, which removed approximately 220,000 kg of VOCs between May 2014 and February 2015. Ongoing natural degradation is another significant contributor to the reduction in VOC mass. A summary of the changes since 1996 is provided below:

Soil

The distribution of VOCs in soil covers much of the Operations Area, suggesting that solvent VOCs likely entered the subsurface in varying quantities at many locations within the Operations Area. Likely known entry points include the two former unlined lagoons, drum storage areas, and truck loading/unloading areas.

Overburden and Bedrock Groundwater

Overburden Groundwater Aquifer

The shallow overburden groundwater VOC plume associated with the SRSNE Site is located within approximately 300 feet of the Operations Area and the NTCRA 1 Containment Area (FS Figure 1-10). The middle overburden groundwater VOC plume associated with the SRSNE Site extends into the center of the Town Well Field (FS Figure 1-11). The southern extension of the middle overburden VOC plume attenuated to below regulatory standards following the startup of the NTCRA 2 groundwater containment system. The deep overburden groundwater VOC plume associated with the SRSNE Site extends into the northern portion of the Town Well Field (FS Figure 1-12). A second unrelated VOC source is interpreted to exist near the southwestern portion of the Town Well Field.

Bedrock Groundwater Aquifer

The shallow and deep bedrock groundwater VOC plumes associated with the SRSNE Site extend into the central portion of the Town Well Field (FS Figure 1-13 and 1-14).

NAPL Zones

NAPL thickness measurements at wells and piezometers indicated dense NAPL (DNAPL) at four overburden and three bedrock wells and piezometers in the Operations Area and the (downgradient) former Cianci Property. Measurable light NAPL (LNAPL) was observed at one overburden well, indicating a limited distribution of potentially recoverable LNAPL. As part of the RI, “probable” and “potential” NAPL zone boundaries in both overburden and bedrock were delineated. A subsequent field-based NAPL delineation study further refined the area in the overburden where most of the NAPL appears to be located.

Overburden Probable and Potential NAPL Zones

The overburden probable NAPL zone defined in the RI Report covers an area of approximately 214,000 square feet (4.9 acres), and extends east from the Operations Area to the vicinity of the Quinnipiac River, and southeast to the northern edge of the Town Well Field. The overburden potential NAPL zone defined in the RI Report covers an area of approximately 540,000 square feet (12.4 acres).

Additional studies were performed subsequent to the completion of the RI to further refine the area and volume of overburden within which the majority of NAPL is located. The resulting 1.5-acre NAPL source area was targeted for the evaluation of mass reduction remedial alternatives in the FS.

Bedrock Probable and Potential NAPL Zones

As defined in the RI Report, the bedrock probable NAPL zone covers an area of approximately 260,000 square feet (6.0 acres), and extends from the Operations Area eastward to the vicinity of the Quinnipiac River, and north (up gradient based on non-pumping head data) to the location of the former Cianci Water Supply Well. The bedrock potential NAPL zone defined in the RI Report covers an area of approximately 618,000 square feet (14.2 acres). The depth of the bedrock potential NAPL zone was not investigated directly during the RI. The depth of the NAPL zone may be inferred indirectly, based on the three-dimensional distribution of dissolved VOCs and groundwater flow directions. Based on the interpreted depth of the VOC plume in bedrock and the vertical hydraulic gradients, it appears that the NAPL zone may potentially extend to a depth on the order of 200 feet below grade within the footprint of the bedrock probable NAPL zone.

Surface Water and Sediment

Previous investigations at the site indicated that the drainage culvert that conveys Operations Area runoff from the ditch east of the Operations Area across the former Cianci Property to the Quinnipiac River had historically collected contaminated surface water and groundwater. Supplemental surface water sampling performed during the RI confirmed that contaminated groundwater drains into the culvert and impacts surface water at the culvert discharge. Although EPA removed the surficial VOC‑ and PCB‑impacted soil/sediment from the Operations Area catch basin outfall and railroad grade ditches upstream of the culvert in 1992, some VOC‑ and PCB‑ contaminated soil/sediment likely remains at depth, in the culvert, and/or in the area where the culvert discharges at the Quinnipiac River.