Vapour Intrusion RA Methodology: A Comparison of Empirical vs Modeled (J&E) Soil Vapour and Indoor Air Concentrations
The Johnson and Ettinger (J&E) model for vapour intrusion is heavily depended on by consultants in Ontario for the generation of risk-based Property-Specific Standards (PSS) intended for the filing of Records of Site Condition (RSCs) under Ontario Regulation 153/04. The model was also used by Ontario regulators to derive the current soil and ground water Standards for volatile organic compounds (VOCs) and is an integral part of the Tier 2 Modified Generic Risk Assessment (MGRA) model. However, the model is widely viewed as being highly conservative with respect to predictions of vapour intrusion risks. Therefore, we endeavored to validate the model and input parameters that are considered suitable for risk assessment by the MOE, using empirical data collected from several risk assessment sites in Ontario. We compared the predicted source vapour and indoor air concentrations of several VOCs, as modeled for a number of risk assessments, to actual measured soil vapour and/ or indoor air concentrations. Our results suggest that the model over-predicts indoor air concentrations by at least an order of magnitude, when used with soil data. Data for three chlorinated VOCs (tetrachloroethylene, trichloroethylene and vinyl chloride), measured at three sites, were evaluated. Modeled indoor air concentrations based on maximum ground water concentrations of the same VOCs, at the same sites, were less conservative but generally still overestimated the indoor air exposure concentrations. Over-predictions of vapour infiltration obtained using soil data are probably due to the MOE-mandated assumption that a single point source exceedance extends beneath the entire footprint of the building in question. Thus, estimates of the spacial extent of impacted areas and revised modeling results, for soil impacts, were also prepared for evaluation. In most cases, the presence of ground water contaminants was accompanied by concurrent soil impacts, which likely accounts for under-predictions of indoor air concentrations using ground water data alone. The results of our assessment also suggest that source vapour concentrations predicted by the J&E model are generally higher than actual soil gas measurements. Modeled source vapour concentrations of chlorinated VOCs (1,1-dichloroethylene, trichloroethylene, tetrachloroethylene) on one site, or petroleum hydrocarbons (benzene, toluene, xylenes) measured on a second site, in soil and/ or ground water, were consistently higher than measured soil vapour concentrations of the same parameters. It is our hope that comparisons such as these, of predicted values versus empirical data, will aid in future development of more appropriate models for vapour intrusion, to be used in the risk assessment of brownfield sites.
