Modeling in Situ Bioremediation of TCE at Savannah River:  Effects of Product Toxicity and Microbial Interactions on TCE Degradation

An in situ bioremediation field demonstration was performed at the U.S. DOE's Savannah River site in 1992−1993 to remediate subsurface TCE contamination. This demonstration involved stimulating indigenous methanotrophic bacteria with injection of methane, air, and air-phase nutrients below the water table and vacuum extraction in the vadose zone. We model this field demonstration using TRAMPP, a numerical code that differs from those used in previous studies in that it includes both vadose and groundwater zones, unsteady air and water flow, limited nutrients, and airborne nutrients, in addition to toxicity, cometabolic kinetics, kinetic sorption, and predator grazing. We conclude that the total amount of TCE extracted or biodegraded in the Savannah River demonstration was about 25% higher than would have occurred by air sparging and vacuum extraction alone and that lower residual levels of TCE were achieved. Sensitivity analysis indicates that toxicity of the methanotrophs to TCE epoxides is an important factor affecting ef ficiency of remediation. Predation of methanotrophs by protozoa provide the best explanation for the oscillations in methanotroph population observed at the site, although competition, which we did not consider in our model, may also have played a key role. Neglecting the effects of microbial predation can lead to overestimates of TCE degradation on the order of 25%. Because rapid biodegradation only occurs where methanotrophs, TCE and “food” are all present, models that couple subsurface flow and transport with microbial processes are an important tool for assessing the effectiveness of bioremediation in field applications in which the environment is heterogeneous.