Complete Reduction of TCE to Ethene in Acidic Aquifer Using Vegetable Oil Emulsion and Dehalococcoides sp. Inoculum

摘要

The Brandywine Defense Reutilization and Marketing Office (DRMO) site at Andrews Air Force Base (AFB) in Maryland is contaminated with chlorinated solvents, mainly trichloroethene (TCE) and its biodegradation daughter product cis-1,2-dichloroethene (DCE). TCE concentrations run in the tens of milligrams per liter (mg/L). The groundwater has very low alkalinity and the pH typically ranges from 4.5 to 5.5, but drops below 4 in the central portion of the plume. Chloride concentrations suggest that considerable biodegradation of chlorinated solvents has occurred in the past, but no further progress has been observed in recent years, probably due to low alkalinity, low pH, and lack of carbon substrate/electron donor. As part of its enhanced remediation program, Andrews AFB contracted with URS to conduct a microcosm study using site saturated soil and groundwater. URS tested the addition of pH buffer without electron donor compounds, buffer + lactate, and buffer + vegetable oil emulsion at two dosages. All treatments were inoculated with a microbial culture enriched in Dehalococcoides sp. The microcosm study included blanks, container controls, and sterile controls. The results showed that: 1. The addition of pH buffer and inoculum without organic electron donor compound did not have any measurable effect over the 98-day span of the study, indicating that addition of electron donor compounds will be necessary. 2. Lactate addition caused a rapid transformation of TCE to cis-l,2-DCE, but then the dechlorination slowed down and, by the end of the study, high vinyl chloride concentrations were still present. 3. Within less than two months (I.e., between 28 and 63 days), vegetable oil emulsion at both concentrations tested led to a nearly complete (99.96% and 99.97%, respectively) mole-per-mole dehalogenation of TCE to ethene. We concluded that injecting vegetable oil emulsion at a concentration corresponding to the soil's presumed adsorption capacity (soil-to-emulsion mass ratio of 1:0.0015), accompanied by a chloroethene-degrading inoculum, can completely remediate the site subsurface to below maximum contaminant levels (MCLs) in less than 12 months with a sufficient safety factor. If the vegetable oil emulsion is injected as a barrier, it should be wide enough to provide a groundwater flow through time of 28 to 63 days. The findings of the microcosm study are being implemented at the site.