Summary of aerobic TCE degradation rates: Enzyme activity probes and microcosms

Enzyme activity probes (EAP) have been developed to directly assess chlorinated solvent degradation in aerobic aquifers; EAP focus on the cometabolic degradation of trichloroethene (TCE) by aromatic and methane oxygenase enzymes. Initial screening of several field sites including: Test Area North of the Idaho National Laboratory; Technical Area V of Sandia National Laboratories; T-Area of the Savannah River National Laboratory; Operable Unit 10 of Hill AFB; and the Building 3001 Site of Tinker AFB, found that cometabolic activity and associated TCE metabolism were significant at all of these contaminated sites. Based on significant and broadly distributed EAP responses, microcosm studies to provide estimates of cometabolic TCE degradation rates were performed using groundwater from the contaminant plumes. All of these sites share geochemical similarities (aerobic conditions, low iron and sulfate, moderate amounts of phosphate, nitrogen, and total carbon) but vary with respect to (a) TCE concentrations (range <10 μg L^-1 - 20,000 μg L^-1), and (b) geohydrologic conditions. Microcosm experiments were designed to: (1) confirm by direct observation that indigenous microbial populations at sites are capable of degrading TCE; (2) determine normalized TCE degradation rates for in-situ populations based on EAP analysis; and (3) apply the normalized rates and field EAP data to estimate TCE degradation half-lives. The EAP measurements for all of the subject sites indicated significant levels of organisms capable of cometabolism in the majority of monitoring wells sampled. These data provided direct evidence that microbial populations present at the various sites are capable of degrading TCE via cometabolism, but the field EAP data do not provide information on the rate or significance of cometabolism in controlling plume expansion. The microcosm studies coupled with microcosm and laboratory EAP measurements confirmed cometabolic TCE degradation and resulted in measured TCE degradation rates in multiple and geochemically distinct monitoring wells. The first order rate calculations for these sites indicated that TCE half-lives for in situ populations under field conditions ranged from approximately 2 to 35 years for these sites. In general, wells which showed significant activity with the probes also showed degradation capacity during the microcosm studies, resulting in complete degradation of the TCE. Additionally, control studies using molecular tools (EAP, PCR/qPCR, FISH) verified that all or most of the degradation of the contaminant could be attributed to the aerobic cometabolic activity. Collectively, these studies provide clear evidence that aerobic cometabolic processes can contribute significantly to the attenuation of the contaminant in situ.