The Raritan Arsenal was used by the U.S. Army from World War I until the 1960s as an ammunition storage and renovation facility. It has since been redeveloped and reoccupied for multi-unit commercial use. There are chlorinated solvents in groundwater at the Site, which are being monitored and remediated. The New Jersey Department of Environmental Protection (NJDEP) requested an assessment of subsurface vapor intrusion into indoor air and required the U.S. Army Corps of Engineers (Corps), now responsible for Site cleanup actions, to conduct extensive sub-slab and indoor air monitoring. Three years of monitoring data repeatedly showed detectable concentrations of volatile organic compounds (VOCs) in groundwater, soil gas and indoor air samples, interpreted by the NJDEP as a complete exposure pathway despite indications of interior sources for both tetrachloroethene (PCE) and trichloroethene (TCE). Repeat sampling of groundwater, sub-slab soil gas and indoor air by the previous consultant simply confirmed detections of PCE and TCE in all three media. Prior to the NJDEP issuing an Order for expensive mitigation, the Corps retained Geosyntec through Shaw Environmental (Shaw) to review historic data, provide an independent interpretation, and conduct a single phase of data collection to resolve once and for all the relative contributions of vapors from sources inside the building versus any contribution of vapors from the subsurface, and refine the mitigation program to an appropriate level.
Geosyntec's Scope of Services
Geosyntec prepared a presentation with a forensic analysis of the historic data, demonstrating to the NJDEP that interior sources were likely to be contributing a significant background level of PCE and TCE. NJDEP agreed to allow us to design and implement an additional data collection event that included an innovative, quantitative passive diffusive sampling method. We utilized a polydimethylsiloxane (PDMS) membrane for long-term average indoor air sampling with low-level reporting limits at about half the cost of conventional sampling. Geosyntec and Dr. Tadeusz Gorecki of the University of Waterloo, in a cooperative effort, developed this method to provide benefits distinct from the benefits of conventional Summa canister sampling. They allow monitoring over a time-period of sufficient length to represent average conditions; they are easily deployed; have a low degree of visibility; and the costs are significantly less than current TO-15 sampling and analysis. Our assessment also included a detailed review of the building ventilation system and cross-slab pressure differential measurements, as well as real-time indoor air screening using the EPA Trace Atmospheric Gas Analysis (TAGA) unit. We presented a detailed analysis of the data to the NJDEP, clearly identifying the presence of interior sources of solvents associated with electronics equipment in one of the current building occupant's warehouse areas. As a result, NJDEP concurred that three of the four buildings in question did not require vapor intrusion mitigation. Geosyntec proposed an innovative passive mitigation system for the remaining building powered by wind turbines above the roof level, a low-cost, sustainable system that provides ventilation of vapors from beneath the building, appropriate for situations where indoor air concentrations are below regulatory guidelines but sub-slab concentrations are moderately higher than guideline values. This system is now in a 5-year monitoring phase. Geosyntec is using PDMS samplers to monitor the performance of the mitigation system with the expectation of gaining acceptance of this innovative sampling method by the NJDEP.
Geosyntec's detailed analysis of the existing data, strategic and innovative sampling program and applied research for passive sub-slab venting demonstrated to the satisfaction of the NJDEP that there is no complete pathway for vapor intrusion into indoor air. This reduced the Corps' long-term liability from approximately $1 million to approximately $120,000. Our negotiation with NJDEP for acceptance of passive diffusive samplers for monitoring the sub-slab soil gas and indoor air could result in at least a 50 percent reduction in the annual monitoring costs. The low-maintenance, wind-driven venting system has already been suggested by the NJDEP as an option for another building.