Degradation of carbon tetrachloride in a reducing groundwater environment: implications for natural attenuation

Abstract

Several laboratory experiments have demonstrated degradation of carbon tetrachloride (CT) in groundwater, but there appear to have been no corroborating long-term field studies. Investigations conducted in 1989 and 1999 at an industrial site constructed on an infilled estuarine environment in France provide data over a decade for which CT degradation could be evaluated. A Dense Non-Aqueous Phase Liquid (DNAPL) containing oil and >90% CT that was present in 1989 was absent in the extremely reducing site groundwater in both 1999 and 2000 (average Eh=−170 mV at pH 7, sulfide up to 21 mg l −1, and Fe +2 up to 3.2 mg l −1). These conditions facilitated dechlorination of CT to chloroform (CF) present at up to 46 mg l −1, and methylene chloride (up to 75 mg l −1). Carbon disulfide (CS 2), a terminal degradation product in reducing environments in laboratory experiments, was present at a mass ratio averaging 2.4:1 CF:CS 2, indicative of abiotic degradation. The lack of detection of the separate phase CT, the ratio of CF:CS 2, the presence of low molecular weight organic acids (i.e., acetate ∼900 mg l −1; citrate 360 mg l −1; and propionate, up to 111 mg l −1) and pyrite in conjunction with excess inorganic Cl in groundwater are all indicators of ongoing degradation of the chlorinated compounds. However, while natural attenuation of chloromethanes may be a viable adjunct to strategies designed to remediate CT in reducing groundwater, its efficacy is hard to quantify in complex field environments where upgradient sources are still present.