Based on known site conditions, it is theorized that the elevated levels of petroleum related compounds associated with this disposal site are attributed to the former use as gas station (i.e. former USTs) and were encountered at a depth of 10-15 feet below grade surface (bgs) and confined to a fine to medium grained sand strata.
The gasoline plume migrated to the east-southeast impacting the easterly abutting asphalt-paved parking lot, the northwesterly portion of the easterly abutting residential property, and the southeasterly corner of the residential property. The elevated levels of heavy metals, specifically Cadmium, Lead and Zinc; and the elevated levels of PAH contamination are attributed to the presence of “historical fill” material observed in the upper soils at the site and is attributed anthropogenic background. The depth of the “historic fill” varies across the site from 0 to 6 feet below surface grade (bsg).
Based on the implementation of a remedial injection program, removal of approximately 400 cubic tons of mostly heavy metal, PAH and TCLP Lead filled Historical Fill; the installation of a passive vapor barrier beneath the newly constructed building, as part of Release Abatement Measure (RAM) Plan implemented concurrently with the redevelopment of the 508 Somerville Avenue portions of the site; and including the results of multiple soil, groundwater, indoor air and sub-slab soil gas testing in conjunction with the findings of a Method 3 Characterization, it is the opinion of IES that a condition of No Significant Risk exist under current and future site conditions. Furthermore, RAM activities also included implementing exposure controls, including Best Management Practices for Non-commercial Gardening, to support this Permanent Solution With Conditions submittal. All utility corridors were backfilled with clean material brought to the site. In addition, all unpaved surfaces have a three-foot clean cover and the reminder of the site is covered with asphalt, concrete and/or the site building.
Finally, a sub-slab depressurization system (SSDS) was installed in a nearby residential building at to mitigate the IRA/vapor intrusion associated with RTN 3-32389. However, based on subsequent indoor air testing and groundwater test results the Critical Exposure Pathway (CEP) has been eliminated and vapor intrusion does not exist.
Based on known site conditions there have been no impacts to any sensitive receptors, no break out to nearby surface water bodies, or known impacts to nearby residents, and it has been demonstrated that a Permanent Solution with Conditions has been achieved for this Disposal Site, and an Activity Use and Limitation (AUL) is not required to maintain a condition of No Significant Risk.