Based on known site conditions, it is theorized that the elevated levels of petroleum related compounds and heavy metals (Arsenic, Cadmium, Total Chromium, Lead and Nickel) associated with RTN 3-31687 are limited to the soil, and a reportable release to groundwater was not identified. The release is attributed to the former use of the site a gasoline filling station (former USTs) and “Historic Fill” material containing coal ash observed at the site. The depth of the “historic fill” varies across the site from 5 to 10 feet below surface grade (bsg). The petroleum contamination is mostly at a depth of 10-12 feet below grade, confined to a fine to medium grained sand strata at the soil and groundwater interface just above the clay, with varying thicknesses in the vicinity of test borings B-11 and B-16 at a depth of 8-12 feet bsg. The historical gasoline plume appears to have slightly impacted the southerly abutting residential property (57 Webster Avenue). However, due to the absence of a reportable release to groundwater and the results of a soil gas survey conducted at the southerly downgradient portion of the Disposal Site at the property boundary between 70 Prospect Street and Webster Avenue vapor instruction has been addressed.
Based on the removal of the two soil heavy metal hot spots, including the removal of approximately 866.82 cubic yards (1,300.24 tons) of mostly heavy metal and PAH impacted fill material; the installation of a passive vapor barrier beneath all portions of the site building except beneath the vented parking garage, as part of Release Abatement Measure (RAM) Plan implemented concurrently with the redevelopment of the site with a five-story mixed commercial and residential building; and based on the results of multiple soil, groundwater, 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 exists under current 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.
To reduce soil disposal cost, IES treated and stabilized approximately 761 cubic yards of soil containing elevated concentrations of TCLP for Lead that met hazardous waste standards. Stabilizing the material on-site re-classified it as Non-Hazardous Waste/Remediation Waste and was disposed of at a significantly lower cost as Part of the RAM.
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.