Calculation of soil cleanup criteria for carcinogenic volatile organic compounds as controlled by the soil-to-indoor air exposure pathway

Two deterministic models that calculate volatilization of contaminants from soil were compared and used to calculate soil cleanup criteria for carcinogenic volatile organic compounds so that acceptable total doses would not be exceeded through the inhalation of indoor air. A hypothetical scenario was used which assumed that subsurface contaminant diffuses through a layer of clean soil and then is swept into the building interior via advection, where long-term inhalation of contaminant was assumed to occur. The first model was one of three recently described by Little et al. It assumed a constant source concentration and infinite thickness for the contaminated soil zone, and resulted in a steady-state interior air concentration. The model was modified to allow for first-order decay of the contaminant source. The second model was adapted from the behavior assessment model of Jury et al. It calculated time-dependent volatilization, simulated the depletion of source contaminant via both volatilization and degradation, and could be used with a contaminated zone of finite thickness. For a given chemical, the predicted total exposure (and the resulting cleanup criteria) varied by up to seven orders of magnitude, depending on the model, whether a finite or infinite contaminated soil layer thickness was used and whether degradation was considered. When calculating soil cleanup standards for volatile organic compounds in the part-per-million range or lower, it appears to be important to consider contaminant source depletion with time and the actual thickness of the contaminated soil layer.