A Comprehensive Soil Gas Model for Assessing Indoor Air Risks. R. A. Perona, ERM/Golder Los Alamos Project Team, 555 Oppenheimer Rd., Ste. 100, Los Alamos, NM 87544; G. P. Brorby, Geomatrix Consultants Inc., 100 Pine St., 10th Floor, San Francisco, CA 94111; and J. Wilson, GeoEngineers, Inc., 7504 SW Bridgeport Rd., Portland, OR 97244
A steady-state, one-dimensional probabilistic spreadsheet model has been developed for use in human health risk assessments to predict the concentration of volatile organic compound (VOC) vapors in indoor air as a result of the presence of subsurface VOC soil contamination. The VOC source is modeled as finite and depletable. The model is a synthesis of previously published work that has been modified and supplemented for application to VOC contamination under one of the following conditions: 1) organic contamination is present below the soil saturation concentration, and the soil has sufficient moisture to coat all soil surfaces, 2) organic contamination is present below the soil saturation concentration, and the soil does not have sufficient moisture to form a monomolecular water layer on all soil surfaces, and 3) organic contamination is present at or above a concentration where soil organic carbon, pore water, and pore air are saturated. (Conditions 2 and 3 are not currently addressed by screening-level models recommended by EPA.) In addition, the effect of multiple organic contaminants in soil on the estimated equilibrium soil gas concentration of individual VOCs is evaluated. Equations have been developed to calculate the fraction of VOCs diffusing to the soil surface that may encounter building slab as a function of the shape and dimensions of the contaminated soil zone, the area of the building footprint (and its proximity to or overlap with the area of contamination), and the thickness of the layer of clean soil or fill overlaying the contaminated zone. Quantitative sensitivity analyses performed on the model indicate that the concentrations of an individual VOC and of total organic contamination are generally the driving variables affecting risk from indoor exposure.