Effect of Moderate Heat on TCE Reductive Dechlorination Rates in Groundwater

Abstract Low‐temperature heating (in the mesophilic range of ~15 to 40 °C) of contaminated aquifers offers the prospect of increasing the rates for biotic and abiotic treatment of volatile organic compounds. Thermal In Situ Sustainable Remediation (TISR®) is one of the approaches available to implement low temperature heating. The effectiveness of implementing TISR depends in part on the magnitude of increase in bioremediation in response to heating. The objective of this study was to evaluate the effect of temperature on the kinetics of trichloroethene (TCE) dechlorination. Samples of groundwater and soil were collected from a site in southern California that has been undergoing treatment of a plume of TCE and other chlorinated volatile organic compounds (VOCs) using biostimulation and where a field test of TISR is being conducted. Microcosms were prepared in an anaerobic chamber with ~92 g of wet sediment and ~93 mL of groundwater with ~5.4 mg/L of TCE. Treatments included unamended and lactate‐amended microcosms, incubated at 18, 25, 30, 35, 40, and 50 °C. For the unamended microcosms, activity was limited to TCE reduction to cis ‐1,2‐dichloroethene (cDCE), indicating that at the time and location of sample collection, the aquifer was electron donor limited. The first‐order rate of TCE reduction to cDCE increased by approximately 6‐fold from 0.078 d −1 at 18 °C to 0.48 d −1 at 25 °C, and then declined to less than 0.11 d −1 at temperatures of 30 °C and above. For the lactate‐amended treatment, complete dechlorination of TCE to ethene occurred. First‐order dechlorination rate constants were determined based on a chloride mass balance. The rate constants increased approximately 2.5‐fold from 0.12 d −1 at 18 °C to 0.30 d −1 at 25 °C, and 3.8‐fold from 18 °C to 0.46 d −1 at 35 °C. No dechlorination occurred at 40 or 50 °C. Molecular analysis of the lactate‐amended microcosms confirmed the presence of microbes capable of TCE reduction to cDCE (e.g., Desulfitobacterium and Dehalobacter ) and complete reduction of TCE to ethene ( Dehalococcoides and Dehalogenimonas ), as well as the genes for key dehalogenases (PCE Reductase, tceA Reductase, and Vinyl Chloride Reductase). These results confirm the potential for significant improvements to the rate of complete biotic reductive dechlorination of TCE in response to moderate heating and biostimulation of an aquifer.