COMPARISON BETWEEN MODEL SIMULATIONS AND FIELD RESULTS FOR INSITU BIORESTORATION OF CHLORINATED ALIPHATICS .2. COMETABOLIC TRANSFORMATIONS

A nonsteady-state model is presented and used in simulating the results of field experiments where chlorinated ethenes were cometabolically degraded by methanotrophic bacteria. In cometabolism, contaminants are degraded fortuitously by microbes growing on a primary substrate, which in this case is methane. The model includes microbial processes of microbial growth, utilization of the electron donor (methane) and the electron acceptor (oxygen), and the cometabolic transformation of the chlorinated ethenes coupled with the transport processes of advection, dispersion, and sorption onto the aquifer solids. Model simulations of the chlorinated ethenes biotransformation resulting from the biostimulation of methanotrophic bacteria agreed well with the field observations. A kinetic model for the cometabolic transformation that included the competitive inhibition by methane was required to match the field observations. The simulations duplicated the cyclic oscillations in concentration of the chlorinated ethenes that resulted from the pulse feeding of methane. Rate-limited sorption and desorption were also required in order to match the extended tailing in the concentration response that was observed along with the pulsed oscillations in the concentration due to competitive inhibition. The kinetic parameters for the cometabolic transformation, derived from model fits to the field observations, differed for the different chlorinated ethenes tested. Vinyl chloride (VC) and trans-dichloroethylene (t-DCE) had the highest biotransformation rate coefficients (k/K(s)), which were in the range of methane itself, the primary substrate for methanotrophic growth. Cis-dichloroethylene (c-DCE) and trichloroethylene (TCE) were degraded at rates one and two orders of magnitude lower than methane, respectively. The transformation rate for TCE was consistent with laboratory-derived rates from microbes enriched and isolated from the test zone and grown under conditions that most represented those in the field. The kinetic model also permitted the activation and the deactivation of microbial mass towards the cometabolic transformation, which was based on cell growth and decay, and first-order deactivation.