Estimating DNAPL composition and VOC dilution from extraction well data

Field data from a large-scale ground-water extraction system at the Savannah River Site in South Carolina indicate that considerable dilution of volatile organic compounds (VOCs) occurs at extraction wells when compared with the effective solubilities of the VOC components (TCE and PCE) in the dense, nonaqueous phase liquid (DNAPL) source zone. This dilution is believed to be due to the convergence of both contaminated and uncontaminated streamtubes on the extraction wells studied. Thus, the measured concentrations of dissolved VOCs in extraction wells appear to be principally functions of (1) the effective solubilities of the DNAPL components, and (2) the dilution due to streamtube convergence on the extraction wells. A simple method for computing DNAPL composition and the dilution factor from extraction well data is presented for the case when dilution is the dominant mechanism that causes the measured ground-water concentrations of dissolved VOC components to be significantly less than their respective effect solubilities relative to the DNAPL mixture. This method successfully computes the molar composition of a PCE/TCE DNAPL, bailed from a pair of monitor wells at the Savannah River Site, using the measured concentrations of PCE and TCE in ground waters from the same pair of wells. The method is then used to predict the variation of DNAPL composition within the aquifer from VOC data sampled at all 11 extraction wells. Such information is of considerable use in choosing solvent-specific surfactants for enhanced remediation operations