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Site 3 - Southern Plume

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Site 3 - Chlorinated Solvent Groundwater Plume, Southern Plume (SFTA)

sfta

Site Description

Chlorinated VOCs are present in groundwater in the southern portion of NWIRP Bedford south of Hartwell Road in the area known as the Southern Flight Test Area (SFTA). The SFTA site is completely unrelated to Site 3 and is handled as a separate site even though it is referred to as Site 3. The source of groundwater contamination has not been determined; however, the source is not likely to the north because chlorinated VOCs were not detected in groundwater wells upgradient of the highest detected concentrations. The highest concentrations of chlorinated VOCs (predominantly trichloroethylene [TCE]), were detected in two bedrock wells (MW-8B and MW-24R) that are 250 feet southeast of the Hanger Building 5 at depths of 26 to 37 feet below the concrete slab. No VOCs were found in surface soil just beneath the concrete slab. There are no known solvent spills or solvent storage areas in the southern parcel. Therefore, it is the Navy's belief that the original source of the groundwater VOCs in the southern plume is the adjacent Air Force Superfund site (Hascom Site 1) where groundwater is contaminated with TCE. Groundwater flow in the SFTA is predominantly to the south and southeast in both the surficial and bedrock aquifers.

Remedial Actions

An RI/RA found no unacceptable risks associated with analytes in soil for current and likely future site use. Groundwater monitoring for monitored natural attenuation evaluation began in 1993. Semi-annual groundwater monitoring was initiated in 2002. In 2008, the Navy and the Air Force signed a Memorandum of Understanding (MOU) to ensure that groundwater monitoring and information sharing would continue for the SFTA and Air Force sites. An ESD was finalized in 2014 to incorporate SFTA into the monitored natural attenuation (MNA) program, and to implement the LUCs established in the Site 3 ROD for the SFTA. This ESD will ensure the continued monitoring, control, and oversight of the SFTA cleanup under CERCLA. Groundwater monitoring data and TCE concentration trends indicate that the residual contamination in SFTA groundwater is attenuating at a rate such that target groundwater cleanup goals will be met within an acceptable timeframe of 9 to 16 years. The Navy estimates that by 2030, this site will be clean and the SFTA LUCs can be eliminated. Decreases in TCE concentrations are likely attributable to natural attenuation processes occurring in SFTA groundwater, and to the operation of an Air Force groundwater extraction system located at Hanscom Site 1 to the east of the SFTA to address a separate area of groundwater contamination.

The SFTA property is currently being sold. To support the sale, the Navy prepared these documents: an Operating Properly and Successfully (OPS) Demonstration, an Interim Remedial Action Completion Report (IRACR), Activity and Use Limitations (AUL), and an Environmental Condition of Property (ECP).

The FYR required additional/supplemental sampling because of the SFTA's proximity to the nearby fire training area at Hanscom Air Force Base (AFB). There was a high probability that the emergent contaminants, perfluoroalkyl substances (PFAS) (including perfluorooctanoic acid [PFOA] and perfluorooctanesulfonic acid [PFOS]) could have migrated onto Navy property. Initially, the PFAS that were detected in Site 3 groundwater were at concentrations below the public health advisory levels; however, in 2016, EPA significantly reduced the health advisory values for PFOA and PFOS to 70 parts per trillion. Because PFAS concentrations exceeded these criteria, PFAS analysis was added to the LTM sampling events to obtain additional data, but it is not part of the long-term monitoring (LTM) program for the SFTA. Once regulatory guidance is available and sufficient data have been collected, the Navy will coordinate with the U.S. Air Force to determine if additional investigations and evaluation of risk of PFAS in groundwater are warranted based on the concentrations and availability of Tier 1, 2, or 3 toxicity values.

Previous Investigations

Previous investigations, plans and documentation at this site are: RI/RA; FS; and a ROD.

Current Status

LUCs have been implemented; Semi-annual Groundwater Monitoring is being conducted; Remedial Action is in process.

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