Wet and Dry Season Vapor Flux Measurements and Health Risk Assessment at an Office Building Above a Groundwater Contaminant Plume

摘要

Groundwater sampling conducted in 2002 at an industrial site in Oregon indicated that dissolved vinyl chloride concentrations in shallow groundwater adjacent to a large office building exceeded site-specific screening levels for indoor air inhalation risks. The groundwater screening levels were derived during earlier studies based on the Johnson & Ettinger vapor intrusion model, using site-specific and default input parameters and state-recommended exposure factors. To further evaluate risks to the office building occupants, a Tier 3 vapor intrusion and health risk assessment was performed in 2003. The study consisted of measuring vapor flux and ambient air concentrations of vinyl chloride and 14 other chlorinated volatile organic compounds (VOCs) at several locations inside and outside of the building. Two sampling rounds were conducted: one in the wet season (March) and one in the dry season (October). Vapor flux was measured using a surface emission flux chamber; ambient air samples were collected in Summa canisters. The samples were collected at locations considered most likely to be affected by vapor intrusion. The results for wet and dry seasons were similar, indicating that vapor flux at the site is relatively insensitive to seasonal factors (e.g., soil moisture content). Vinyl chloride was not detected in the flux or ambient air samples. Several VOCs were detected in the outdoor flux, indoor flux, and indoor air samples, suggesting that a complete infiltration pathway exists for these compounds. However, comparison of the sampling data to indoor air screening levels derived from Oregon and Environmental Protection Agency risk criteria indicates that the health risks from vapor intrusion are below regulatory limits. The Tier 3 assessment performed at this site was critical in demonstrating to the public and the state regulatory agency that under current conditions, VOC vapors from the groundwater contaminant plume do not pose a threat to human health.