Biodegradation of chlorinated solvents in a water unsaturated topsoil

In order to investigate topsoils as potential sinks for chlorinated solvents from the atmosphere, the degradation of trichloromethane (CHCl3), 1,1,1-trichloroethane (CH3CCl3), tetrachloromethane (CCl4), trichloroethene (C2HCl3) and tetrachloroethene (C2Cl4) was studied in anoxic laboratory experiments designed to simulate denitrifying conditions in water unsaturated topsoil. Active denitrification was demonstrated by measuring the release of 15N in N2 to the headspace from added 15N labeled nitrate. The degradation of chlorinated aliphatic compounds was followed by measuring their concentrations in the headspace above the soil. The headspace concentrations of all the chlorinated solvents except CH3CCl3 were significantly (Pless-than-or-equals, slant0.05) lower after 41 days in biologically active batches as compared to sterile batches. For the compounds with significantly decreasing headspace concentrations, the decline was the least for CHCl3 within the 41 days of incubation. The headspace concentrations of trichloro- and tetrachloroethene decreased more than 50% during the first 20 days with no considerable indication of abiotic transformation. While slow abiotic removal was observed, tetrachloromethane was completely biotransformed after 16 days. Based on the results in this study, we conclude that anaerobic topsoils are potential sinks for these contaminants, and that a natural attenuation potential exists, even in water unsaturated topsoils.