VAPOR INTRUSION ASSESSMENT AT A CHLORINATED SOLVENT SPILL SITE

At sites where volatile organic compounds (VOCs) have contaminated soil or groundwater, there is the potential for chemical vapors to volatilize and accumulate beneath or inside nearby buildings. Vapor intrusion is controlled by soil type; bedrock lithology and fabric; and the depth and concentration of the contaminant source. Vapor intrusion can also be affected by building construction and ventilation characteristics.

In the 1970s and 1980s, perchloroethylene (PCE) was spilled onto the ground surface at two dry cleaning shops at a shopping center in eastern New York, contaminating groundwater. Since the mid-1990s, a groundwater remediation system has been used to control and shrink the dissolved solvent plume, which still contains PCE at concentrations up to 2.5 mg/L. In 2005, state regulatory agencies directed that a vapor intrusion assessment be conducted to determine whether PCE was accumulating beneath the buildings or in indoor air.

During the 2005 heating season, vapor samples were collected from seven locations at the shopping center: Four locations inside the main grocery store, and three locations inside smaller, individual shops.

To collect sub-slab vapor samples, sampling ports were installed by drilling small holes through the floor slab and installing vapor-tight, stainless steel fittings, which extend to one inch below the base of the slab. Sub-slab vapor samples were collected using depressurized summa canisters, with the flow controller on each canister set to collect a 6-liter vapor sample. Simultaneous indoor air samples were collected using identical canisters. A simultaneous ambient outdoor air sample was also collected. Samples were analyzed for PCE and associated compounds trichloroethylene; cis-1,2-dichloroethylene; and vinyl chloride via EPA Method TO-15.

All sub-slab vapor samples contained detectable PCE, which ranged in concentration from 8.7 to 7,200 µg/m³. Sub-slab PCE beneath the smaller shops ranged from 14 to 220 µg/m³. The highest indoor air reading for PCE was 8.6 µg/m³.

Based on these vapor measurements, we conclude that sub-slab vapor concentrations alone are not necessarily predictive of indoor vapor concentrations or potential human exposure, nor are indoor air concentrations predictive of sub-slab vapor accumulation.