GZA is performing site assessments at two separate former dry cleaning spaces with similar conditions and challenges in New York City. The first location is part of a slab-on-grade construction strip mall on Coney Island and is an active professional space. The second location is also part of a strip mall connected to a series of store fronts in Brownsville and occupies an entire city block of Brooklyn. In the past such limited concentrations of tetrachloroethylene (PCE) in groundwater may have led to the conclusion that the contamination was de minimis. However, in light of the recent focus on vapor intrusion (VI) issues, GZA conducted sub-slab vapor investigations which resulted in the identification of high level exceedences of PCE above acceptable background concentrations, requiring mitigation. Phase II investigation at the Coney Island identified a slight exceedence of PCE above the New York State Department of Environmental Conservation (NYSDEC) cleanup level of 5 ppb in groundwater. A monitoring well installed at the Brownsville location indicated 12 ppb of PCE in a well immediately downgradient. Subsequent sub-slab vapor samples collected at both locations resulted in PCE concentrations of 12,000 to 23,000 ug/m3, well above the NYS Board of Health criteria requiring mitigation. A zone of soil contamination has not been identified at either site. However, sub slab vapors of this order of magnitude can result from off-gassing of groundwater contamination even at concentrations near the drinking water standard. Additionally, “vapor clouds” can develop in drainage pipes and release into the subsurface and accumulate beneath slabs. This presentation will show that a VI condition can result from low levels of PCE in groundwater and limited evidence of soil contamination, and will review the ASTM Vapor Intrusion Standard and the importance of assessing VI issues during due diligence.
