Numerical Simulation of the Anaerobic Transformation of Tetrachloroethene to cis-Dichloroethene in a Continuous Flow Aquifer Column

The anaerobic reductive dechlorination of tetrachloroethene (PCE) to cis-dichloroethene (c-DCE) in a laboratory column study was numerically simulated and compared with experimental observations. The column study was conducted with continuous flow and injection of PCE in synthetic groundwater. The column was packed with aquifer solids from the Hanford DOE site and bioaugmented with the Evanite (EV) dechlorinating enrichment culture. After the column was bioaugmented and fed lactate as an electron donor, c-DCE concentrations in the column effluent exceeded the influent PCE concentration. This high c-DCE concentration resulted from enhanced PCE desorption and transformation. A 1-D reactive transport model was developed that included the processes of dispersion, advection, rate-limited sorption and desorption, reductive dechlorination kinetics with competitive inhibition and microbial growth and decay. The model was validated by mass balances, comparisons with analytical solutions and batch kinetic models. Previously determined kinetic and inhibition constants for the EV culture of Yu and Semprini (2004) were input into the model simulations. Initial biomass concentration was assumed to be exponentially distributed along the column. The sorption parameters including the aquifer: water distribution coefficients (Kds) and first-order mass transfer coefficients for PCE, trichloroethene (TCE), and c-DCE were determined in batch laboratory studies. The system of model equations was solved numerically using COMSOL 3.3, which employs finite-element methods. The reactive transport model successfully simulated the initial results of continuous flow column experiment. The increase in c-DCE above the influent PCE concentration was simulated and TCE was shown not to accumulate in the column effluent. The simulations showed that microbial kinetic values generated in previous studies and the sorption parameters generated in batch tests, when used in a transport model, did a reasonable job estimating PCE, TCE, and c-DCE concentration histories in the column effluent. Currently a sensitivity analysis is being performed to better understand why some differences, such as the more rapid breakthrough of c-DCE, was observed in the experimental data, but not in the model simulations.