SIMULATING BIOREMEDIATION OF CHLOROETHENES IN A FRACTURED ROCK AQUIFER

Reactive transport simulations were conducted to synthesize the results of a field experiment on the enhanced bioremediation of chloroethenes in a heterogeneous fractured-rock aquifer near West Trenton, NJ. The aquifer consists of a sequence of water conducting fractures separated by low-permeability mudstones where transport is diffusion-limited. The enhanced bioremediation experiment was conducted by injecting an electron donor (EOS™) and a microbial consortium (KB1™) that contained Dehalococcoides ethenogenes into a fracture zone that had maximum trichloroethene (TCE) concentrations of 84mM. TCE was significantly biodegraded to dichloroethene, chloroethene and presumably CO₂ at the injection well and at a downgradient well.. The experimental results were used to calibrate a reactive transport model that was used quantify the dominant processes affecting chloroethene fate. A nonreactive transport model was developed to simulate the advection, dispersion and mass transfer of bromide and deuterium tracers. Observed concentrations of both tracers were well described by an advection and dispersion model in the fracture zone coupled with diffusion in the rock matrix. The reactive transport model was developed to simulate the microbially mediated sequential dechlorination reactions as well as iron reduction, sulfate reduction and methanogenesis. The reactive transport model was calibrated to concentrations of chloride, chloroethenes, pH, alkalinity, redox sensitive species and major ions. The simulation results match this diverse set of observations at the injection well and at a downgradient well over the three year experiment. In addition, the observations and model simulations indicate that a significant pool of TCE initially sorbed to either the fracture surfaces or in the matrix was degraded during the field experiment. The calibrated reactive transport model is being used in alternative hypothetical scenarios to identify processes and parameters that control the removal of chloroethene mass from an aquifer undergoing enhanced bioremediation.