Aerobic cometabolism is a promising technology for the in-situ remediation of chlorinated aliphatic hydrocarbons (CAH) at Department of Defense (DoD) sites. Low-cost methods are needed for generating the data required to design field-scale systems, a task that is complicated by the complexity of the cometabolic process and the different cometabolic substrates. The contaminants and their concentration are also important considerations, along with the transformation abilities of the indigenous microorganisms that are stimulated on a specific substrate. In this project, a single-well, push-pull test method for determining the parameters needed for full-scale remediation design was developed and tested in standard monitoring wells, providing a rapid, straightforward in-situ means of characterizing the cometabolic process of interest at a site.
The single-well, push-pull method consists of the controlled injection of a prepared test solution into an aquifer followed by the recovery of the test solution/groundwater mixture from the same location. Tests were performed in existing monitoring wells at McClellan Air Force Base (AFB), California, and at Fort Lewis, Washington. The test solution consisted of water containing non-reactive tracers such as bromide, the cometabolic substrate of interest, dissolved oxygen, and reactive solutes that were designed to permit estimations of the in-situ transformation rates of the CAHs of interest. A validated protocol for conducting push-pull tests was developed through this project and is now available.
In the McClellan AFB demonstration, propane was added as the cometabolic substrate, and ethylene and propylene were used as surrogate compounds. Biostimulation tests showed the initial rates of propane utilization were very low, and rates increased substantially following five sequential additions of dissolved propane and oxygen over a period of 75 days. Push-pull activity tests and natural drift activity tests provided similar results and showed that injected propane and oxygen were consumed and that injected ethylene and propylene were transformed to ethylene and propylene oxide. Transformation of cis-dichloroethene (DCE) and trichloroethene (TCE) proved more difficult to assess, since they were present in the injected groundwater at concentrations lower than were present in the aquifer. In a final test, the utilization of propane and the transformation of cis-DCE and ethylene were inhibited by acetylene, a known inhibitor of the propane monooxygenase enzyme.
The effectiveness of gas sparging to stimulate indigenous propane utilizers or methane utilizers was evaluated in the second McClellan AFB demonstration. Propane (or methane) utilization, oxygen consumption, and ethylene and propylene cometabolism were demonstrated in gas sparging activity tests, with ethylene oxide and propylene oxide observed as cometabolic by-products. The Fort Lewis tests demonstrated that indigenous toluene utilizers also could be stimulated. During the biostimulation tests, decreases in toluene concentration and the production of o-cresol as an intermediate oxidation product was observed. Indigenous microorganisms transformed the surrogate compound (i.e., isobutene) and both cis-DCE and trans-DCE. Similar rates of toluene utilization and cis-DCE and isobutene transformation were achieved using the push-pull activity tests and the natural-gradient tests. In a final test, the utilization of toluene and the transformation of isobutene, cis-DCE, and trans-DCE were all inhibited in the presence of 1-butyne, a known inhibitor of the toluene ortho-monooxygenase enzyme.
The single-well, push-pull test provides an in-situ means of generating the data needed to design in-situ cometabolic treatment systems. The method eliminates the need for obtaining core samples and conducting microcosm studies. Compared to microcosm tests, this in-situ test is more representative of the actual environmental conditions that will be encountered in the field. The method should permit more rapid deployment of this treatment process at DoD sites. (Project Completed - 2006)