Probabilistic approach to residential vapor intrusion exposure screening for Chlorinated Volatile Organic Compounds: a case study in San Antonio,Texas

The potential for subsurface volatile chemicals to migrate through the soil and impact indoor air quality is an increasingly important exposure pathway at contaminated sites. The characterization of this pathway is highly uncertain and dependent upon many site and building-specific parameters. A probability house-by-house model, based on the Johnson-Ettinger algorithm, is developed based on the current and historic conditions of groundwater contamination of tricholorethylene and tetrachlorethylene from activities at the former Kelly Air Force Base in San Antonio, Texas. The analysis suggests that historically 5.5% of houses exceed PCE screening levels (0.41 ug/m3) at the mean level and 85.3% at the 95th percentile; for TCE (at 0.25 ug/m3) it is 49% and 99% respectively. The current EPA model is slightly less conservative than the new parameterization by Johnson (2005). Comparison with measured samples suggests the probabilistic model underestimates exposure. Soil properties and air exchange rates are the most input critical parameters