Characterizing exposure to chemicals from soil vapor intrusion using a two-compartment model

Though several different models have been developed for sub-surface migration, little attention has been given to the effect of subsurface transport on the indoor environment. Existing methods generally assume that a house is one well-mixed compartment. A two-compartment model was developed to better characterize this exposure pathway; the model treats the house as two well-mixed compartments, one for the basement and one for the remainder of the house. A field study was completed to quantify parameters associated with the two-compartment model, such as soil gas intrusion rates and basement to ground floor air exchange rates. Two residential test houses in Paulsboro, New Jersey were selected for this study. All experiments were completed using sulfur hexafluoride (SF6) as a tracer gas. Soil gas intrusion rates were found to be highly dependent on the soil gas to basement pressure difference, varying from 0.001 m3 m−2 h−1 for a pressure drop of –0.2 Pa to 0.011 m3 m−2 h−1 for a pressure drop of –6.0 Pa. Basement ventilation rates ranged from 0.17 to 0.75 air changes per hour (ACH) for basement to ambient pressure differences ranging from –1.1 to –7.6 Pa (relative to ambient). Application of experimental results in conjunction with the two-compartment model indicate that exposures are highly dependent on gas intrusion rates, basement ventilation rate, and fraction of time spent in the basement. These results can also be significantly different when compared with the simple well-mixed house assumption.