{In Situ} Remediation of Contaminated Groundwater via Enhanced Reductive Dehalogenation and Dual-Screened Wells

Groundwater contaminated by chlorinated solvents, principally cis-dichloroethene (cis-DCE), was cleaned {in situ} by a technology that combines enhanced reductive dechlorination with dual-screened treatment wells. The prolonged historic presence of cis-DCE at the contaminated site suggested that natural attenuation rates were limited by the supply of electron donors. Therefore, propionate was added to the contaminated groundwater to serve as an electron donor and to accelerate the reductive dechlorination process. Propionate was added from the ground surface via a pair of dual-screened wells emplaced in the contaminated portion of the aquifer. The wells were screened at two depths, from 3.0--7.6 m below ground surface (bgs) and from 9.1--12.2 m bgs. These wells functioned to intercept the contaminant plume, augment the contaminated water with propionate, recirculate a portion of the contaminated water, and release treated water for continued downgradient migration. Treatment occurred wholly {in situ}. Within the recirculation zone of the well pair, cis-DCE was effectively removed during a two-month period of operation. In the lower aquifer zone, 800 μ g/L cis-DCE was converted stoichiometrically to ethene. In the upper aquifer zone, the concentration of cis-DCE was reduced from over 400 μ g/L to less than 40 μ g/L. Dechlorination was accompanied by significant sulfate reduction, but not by methanogenesis. The hydraulics of the groundwater flow are described with a relatively simple analytical mathematical model. Measured concentrations of a bromide tracer agree very well with model predictions, suggesting that the model is valid for this contaminated site. At this site, it appears sufficient to model the aquifer as consisting of two homogeneous layers separated by an impermeable aquitard; smaller-scale heterogeneity in the hydraulic conductivity can apparently be ignored.