Vegetable Oil Pilots to Enhance DNAPL Sequestration and Reductive Dechlorination

As part of the AFCEE Enhanced Bioremediation Initiative, two field pilots are being conducted at Solid Waste Management Unit (SWMU) 10 at Arnold Air Force Base, Tennessee. Tetrachloroethene (PCE), 1,1,1-trichloroethane (1TCA), acids, and other compounds used for degreasing operations at the site were disposed into a leach pit and contaminated the groundwater with dense, nonaqueous-phase liquids (DNAPL). About 8,000 L of pure soybean oil were injected into two wells screened within the DNAPL-impacted zone. Three multi-level sampling point wells were installed around the injection wells. The soybean oil was distributed throughout the injection area as evi- denced by its appearance at all levels of the three multi-level sampling point wells. After eleven months, the soybean oil has promoted the reductive dechlorination of the chlori- nated ethenes and chlorinated ethanes in all three of the multi-level sampling point wells at all three intervals based upon increases in daughter products including ethene and ethane. Sequestration of the contaminants has been observed based upon the high levels of PCE, 1TCA, and their daughter products in samples of the soybean oil recovered from the sequestration pilot. However, sequestration of contaminants into the vegetable oil appears to have only been effective in reducing the concentrations of chlorinated com- pounds in one well. An emulsion of soybean oil, surfactant, yeast extract, and lactate was prepared and injected into four injection wells in a barrier configuration to intercept the groundwater plume at the site. Although substrate was effectively distributed throughout much of the emulsion pilot area, there has been little stimulation of reductive dechlori- nation over the first eleven months of the pilot. The groundwater appears to becoming more anaerobic with the removal of sulfate and increases in dissolved iron. One possible explanation for the limited dechlorination may be due to the low pH levels of 4.4 to 4.9 inhibiting the dechlorinating bacteria.