An empirical analysis of the groundwater-to-indoor-air exposure pathway: The role of background concentrations in indoor air

To further our understanding of the groundwater-to-indoor-air exposure pathway, a database of 270 paired groundwater and indoor air measurements of volatile organic compounds (VOCs) from 31 sites has been compiled and analyzed. Using regression analyses, these data have been analyzed to (1) detect evidence of indoor air impacts from dissolved petroleum hydrocarbons or chlorinated solvents in underlying groundwater, (2) estimate the true attenuation factor for volatilization from groundwater to indoor air, and (3) assess the utility of popular groundwater-to-indoor-air transport models for evaluating this exposure pathway. Key findings include the following: (1) >95% of the indoor air concentrations fall within or below the range of national background indoor air values; (2) there is no correlation between petroleum constituent concentrations measured in groundwater and the concentrations of these VOCs in indoor air of overlying structures; and (3) for chlorinated solvent cases, a correlation between groundwater and overlying indoor air is observed; however, the average true attenuation factor, after correcting for background, is approximately 6.9 x 10(-5), well below the default I x 10(-3) value used by USEPA Vapor Intrusion Guidance to determine groundwater screening levels.