SINGLE-WELL-PUSH-PULL TESTS FOR ASSESSING THE FEASIBILITY FOR IN-SITU AEROBIC COMETABOLIC TREATMENT OF CHLORINATED ALIPHATIC HYDROCARBONS

Single-well-push-pull tests were performed to assess the feasibility of in situ aerobic cometabolism of chlorinated aliphatic hydrocarbons (CAHs), such as trichloroethylene (TCE) and cis-1,2-dichloroethylene (c-DCE), using propane and toluene as growth substrates. Propane tests were performed in the saturate zone at McClellan Air Force Base, CA, while toluene tests were performed at Fort Lewis, WA. The effectiveness of dissolved substrate addition to stimulate the indigenous propane-utilizers and toluene-utilizers was evaluated in standard monitoring wells. Transport characteristics of dissolved solutes were evaluated using bromide as a conservative tracer. Propane and toluene-utilization as growth substrates was evaluated by observing repeated uptake under both natural gradient flow conditions, and during push-pull activity tests. For the push-pull activity tests the injected solution was amended with the substrates of interest, and after injection was permitted to reside in the formation for 19 to 24 hours and then extracted. Decreases in propane and toluene concentrations, normalized to bromide as a conservative tracer, indicated utilization of these growth substrates. When toluene was utilized, o-cresol was observed as an intermediate oxidation product. Ethylene, propylene, nontoxic surrogates to probe for CAH transformation activity, was added in the propane tests, while isobutene was added in the toluene tests. The stimulated propane-utilizers cometabolized ethylene and propylene to produce ethylene oxide and propylene oxide as cometabolic by-products. The stimulated toluene-utilizers produced isobutene oxide, which provides evidence that microorganisms with an ortho-monooxygenase were stimulated. Propane results confirmed that microorganisms with a propane-monooxygenase enzyme were stimulated. In order to further demonstrate the involvement of monooxygenase enzymes, acetylene blocking tests were also performed. Propane utilization and ethylene and propylene oxidation were essentially completely inhibited by the presence of acetylene. Similar inhibition tests are planned for the Fort Lewis tests. The results at both sites demonstrated that push-pull tests can be used to evaluate the potential of in situ cometabolic treatment.