U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Cometabolic Biotransformation of Trichloroethylene and CIS-1,2-Dichloroethylene in Unsaturated-Zone Soil at Picatinny Arsenal, New Jersey

Abstract

This study determined the feasibility of in situ cometabolic remediation of gas-phase chlorinated ethenes at Picatinny Arsenal, New Jersey. Cometabolic biotransformation of trichloroethylene (TCE) and cis-1,2-dichloroethylene (cis-DCE) was studied in a laboratory experiment with unsaturated-zone soil cores collected within a contaminant plume near the source. Concentrations of TCE and cis-DCE in soil, soil gas, and ground water within the plume also were quantified. At the highest concentrations studied, TCE (3.2 mM) mixed with cis-DCE (4.3 mM) was degraded most rapidly (0.05 mmol TCE/L/d, 0.09 mmol cis-DCE/L/d) in acclimated soils with a 3-percent methane headspace. Degradation of TCE at lesser concentrations (1.3 mM) as the only chloroethene present in fertilized, acclimated soil was much faster (0.12 mmol TCE/L/d) in the presence of 1.2 percent methane. Most of the unsaturated-zone TCE (>99.9 percent) near the contaminant source was sorbed to soil. Results of soil-gas analyses indicate that the concentration of unsaturated-zone TCE near the contaminant source was highest (0.32 mM) in a shallow clay layer, and decreased with depth to the water table. This indicates that the original contamination was mostly from condensed TCE which drained from a degreasing tank to a nearby dry well. The concentration of TCE in unsaturated-zone soil elsewhere throughout the plume increased with depth, indicating that most unsaturated-zone TCE contamination resulted from volatilization losses of TCE from contaminated shallow ground water.