A Multimedia, Multiple-Exposure Pathway Methodology for Deriving Risk-Based Standards for Tetrachloroethylene (PCE) in Soil

For contaminants of concern at hazardous-waste sites, the State of California develops exposure criteria referred to as Applied Action Levels (AALs). An AAL is the concentration of a contaminant in a source medium that, when exceeded, is expected to pose an unacceptable risk to human health. Typically, AALs are calculated separately for individual environmental media (e.g., air, water, or soil) and only for a limited number of routes of exposure. This approach fails to recognize the multimedia character of many soil contaminants, and by excluding significant exposure routes, could underestimate risk. Using tetrachloroethylene (perchloroethylene, PCE) in soil as an example, we describe an alternative methodology for driving risk-based AALs from estimates of intake that account for multimedia, multiple-pathway exposure.