| An improved multimedia model for forecasting the exposure concentration of volatile organic chemicals (VOC) from subsurface leaks is developed. The model is based on ability of the VOC to diffuse from ground water in the soil gas and uses easily obtainable physical data from the contaminated site. The improvements to the model include: (1) the calculation of the total reaction rate for the site (k;When the forecasted exposure concentrations from the improved diffusivity model are compared to the forecasted exposure concentrations from an existing fugacity model, there is a large difference in the forecasts, up to three orders of magnitude (1000 times more). This difference is likely to be come from the simplifying assumptions used in the fugacity model. When a sensitivity analysis is performed on the improved diffusivity model, the forecasted exposure concentrations vary primarily with the physical rather than the subjective parameters. The physical parameters are those assumption variables which are easily or accurately measured, e.g., the water-filled and total porosity of the unsaturated zone, and the concentration of the contaminant in the ground water. The subjective model variables are those assumptions variables which have to be estimated or are not easily measured, e.g., transport rates, fugacity, etc. Uncertainty analyses of the improved diffusivity and fugacity models show there is less uncertainty associated with the corecasts of the improved diffusivity model than the fugacity model. This implies that the improved diffusivity model is more accurate than the fugacity model.;The calculation of the reaction rate for the site has the potential to include a quantification of remediation efforts, such as pump-and-treat or vacuum extraction treatments. This may prove to a useful guide for regulators or project managers trying to evaluate the best method for remdiating a site with subsurface contamination. |
