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Redox conditions in heated and unheated microcosm experiments were studied to evaluate the effect of thermal remediation treatment on biogeochemical processes in subsurface environments. The results were compared to field-scale observations from thermal treatments of contaminated sites. Trichloroethene-contaminated aquifer material and groundwater from Ft. Lewis, WA were incubated for 200 days at ambient temperature (i.e., 10 degrees C) or heated to 100 degrees C for 10 days and cooled slowly over a period of 150 days to mimic a thermal treatment. Increases of up to 14 mM dissolved organic carbon were observed in the aqueous phase after heating. Redox conditions did generally not change during heating in the laboratory experiment, and only minor changes occurred as an effect of heat treatment in the field. The conditions were slightly manganese/iron-reducing in two sediments and possibly sulfate-reducing in the third sediment based on production of up to 0.20 mM dissolved iron and 0.15 mM dissolved manganese and consumption of 0.08 mM sulfate. The calculated energy gain of less than -20 kJ/mol H2 for iron and sulfate reduction as well as methane production indicated that these processes were thermodynamically favorable. Sulfate reduction and methane production occurred in the unheated microcosms upon lactate amendment. Little or no reduction of the redox level was identified in heated lactate-amended microcosms, possibly because of limited microbial activity. Because the redox conditions, pH, and alkalinity remained within normal aquifer levels upon heating, bioaugmentation may be feasible for stimulating anaerobic dechlorination in heated samples or in future field applications.

Anaerobic microbial dechlorination of trichloroethene (TCE) by a mixed, Dehalococcoides containing culture was investigated at different temperatures (4-60 degrees C) using propionate and lactate as a slow- and fast-releasing hydrogen (H(2)) source, respectively. Distinct temperature-dependent dynamics of substrate fermentation and H(2) levels could explain observed patterns of dechlorination. While varying the temperature caused changes in rate, the overall pattern of dechlorination was characteristic of the supplied electron donor. Feeding cultures with a rapidly fermentable substrate such as lactate generally resulted in high H(2) concentrations and fast and complete dechlorination accompanied by rapid methanogenesis. In contrast, low H(2) release rates resulting from fermentation of propionate were associated with 2 to 3-fold longer time frames necessary for complete dechlorination at intermediate temperatures (15-30 degrees C). A lag-phase prior to dechlorination of cis-dichloroethene (cDCE), together with a characteristic build-up of H(2) and methane, was consistently observed at slow H(2) supply. At temperatures of 10 degrees C and lower, the system remained in this lag phase and no dechlorination past cDCE was observed within the experimental time frame. However, when lactate was the substrate, complete dechlorination of TCE occurred within 74 days at 10 degrees C, accompanied by methane production. The choice of fermentable substrate decisively influenced the rate and degree of dechlorination at an electron donor/TCE ratio as high as 666:1. Temperature-dependent H(2) levels resulting from fermentation of different substrates could be satisfactorily explained through thermodynamic calculations of the Gibbs free energy yield assuming a constant metabolic energy threshold of -20 kJ/(mol reaction).

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