The Department of Defense (DoD) currently faces enormous costs for the cleanup of sites contaminated with chlorinated solvents. Some chlorinated solvent sites have successfully undergone enhanced biological reductive dechlorination through the addition of various electron donors (e.g., lactate, propionate, vegetable oils, hydrogen) to the aquifer. However, many sites show insignificant or incomplete dechlorination, especially those with high aquifer sulfate levels. Although the competition of sulfate with the chlorinated organics as an electron acceptor has been considered, the rapid conversion of sulfate to toxic free sulfide during bacterial reductive dechlorination has been overlooked. Accumulation of free sulfide could be especially important at DoD sites in marine or closed-basin environments, which display both high sulfate and low available iron due to high salinity, alkalinity, and pH. Reductive dechlorination inhibition by free sulfide has been observed in microcosms conducted for high sulfate field sites.

Free sulfide toxicity to microorganisms can be prevented if ferrous iron precipitates the free sulfide. Further, iron sulfide mineral precipitates have been shown to catalyze reductive dechlorination of chlorinated solvents (e.g., tetrachloroethene [PCE], trichloroethene [TCE]) at rates comparable to metallic iron, on a surface area normalized basis. Microcosms performed at high sulfate sites showed both removal of free sulfide toxicity to dehalogenating bacteria and catalytic reductive dechlorination when supplemented with ferrous chloride. This project was designed to evaluate the impacts of ferrous chloride addition to a site that had accumulated toxic free sulfide levels as a result of sulfate reducing conditions. A laboratory-scale demonstration was performed using samples from Dugway Proving Grounds, Utah, a high-sulfate site contaminated with TCE. The demonstration sought to determine changes in microbial populations following additions of electron donors, dehalogenating microorganisms, and/or ferrous chloride, and to measure catalytic activity rates in microcosms under controlled conditions as a proof of concept.

Samples from the site were incubated up to 5 months, with no evidence of TCE biodegradation (from a starting concentration of approximately 15 µM). The sulfate concentrations were approximately 11,000 mg/L, and free sulfide concentrations of approximately 500 mg/L were measured soon after addition of lactate or soybean oil. Addition of electron donor (lactate or soybean oil) and bioaugmentation (addition of a Dehalococcoides culture capable of reductive dehalogenation) was necessary for TCE biodegradation to occur. However, even in the donor-amended and bioaugmented microcosms, TCE degradation did not occur unless ferrous chloride also was added. The free sulfide levels were reduced to non-detect levels immediately after addition of the ferrous chloride. Some abiotic degradation of TCE occurred in microcosms amended with lactate and ferrous chloride, but none could be measured in the microcosms amended with soybean oil. Further, a substantial lag time (approximately 80 days) was required before degradation could be measured in the lactate-amended microcosms. The results suggested that sulfide toxicity had killed the native dechlorinators at the site and that the formation of iron sulfide precipitates was necessary for TCE biodegradation by dechlorinators added to the site samples. Added dechlorinating bacteria could survive and completely degrade TCE after both electron donor and ferrous chloride had been added. However, abiotic degradation by the iron sulfides produced after ferrous chloride addition was not a major contributor to the overall TCE removal measured.

This technology has the potential to treat both source areas and soluble plumes at any aquifer depth. Soluble ferrous chloride can be added to reduced groundwater in a manner that allows for significant lateral migration prior to forming precipitates with free sulfide. Free sulfide removal promotes biological reductive dechlorination, while the iron sulfide solids create a large, highly reactive surface area for abiotic dechlorination, which follows a pathway that limits vinyl chloride formation and results in accumulation of nontoxic acetylene and ethene. Abiotic dechlorination is also not significantly affected by the presence of residual high sulfate levels. Nothing harmful or reactive would be added to the subsurface, and well screen plugging could be minimized by maintaining sulfate-reducing conditions in the aquifer. Costs should be comparable to enhanced biological reductive dechlorination.

The results of this project suggest that enhanced bioremediation of chlorinated ethenes is possible at high-sulfate sites. However, substantial management will be required. Addition of electron donor and ferrous chloride will be necessary, with later bioaugmentation after the inhibitory conditions have been relieved and the proper redox conditions established. The abiotic degradation that can occur through reduction of TCE by the iron sulfides formed has proven to be a difficult process to control and was not a substantial contributor to the overall TCE removal measured. (Project Completed - 2005)