329-337 Gw M-j 04

Site-specific data are needed to perform feasibility assessments and remedial design for in situ bioremediation of chlorinated aliphatic hydrocarbon (CAH) contamination in ground water. Site-specific data are needed because treatment effectiveness is determined by the metabolic capabilities of native microorganisms, the combinations and concentrations of contaminants, and a variety of other factors, such as trace nutrient availability. For example, the use of aerobic cometabolism to oxidize CAH compounds like trichlorethene (TCE) to harmless byproducts requires the selection of a site-specific cometabolic growth substrate (e.g., propane, methane, or toluene). Laboratory microcosm tests performed with sediment samples collected by coring are often used for this purpose (Hopkins et al. 1993; McCarty et al. 1998). In previous field applications, microcosm tests were followed by pilotscale studies using well-to-well recirculation tests (Hopkins et al. 1993). Although the approach has been successfully applied at a few sites (McCarty et al. 1998), it has several disadvantages that limit its routine use for feasibility assessment and remedial design. For example, core samples are often difficult to obtain, and may be too small to provide representative information on subsurface conditions. Well-to-well recirculation tests provide more representative information, but are expensive and logistically complicated. This study adapts the single-well push-pull test for use in conducting rapid, low-cost feasibility assessments for in situ enhanced aerobic cometabolism of CAHs. Push-pull tests have been used to obtain quantitative site-specific Abstract Three types of single-well push-pull tests were developed for use in assessing the feasibility of in situ aerobic cometabolism of chlorinated aliphatic hydrocarbons (CAHs). These included transport tests, biostimulation tests, and activity tests. Transport tests are conducted to evaluate the mobility of solutes used in subsequent tests. These included bromide or chloride (conservative tracers), propane (growth substrate), ethylene, propylene (CAH surrogates), dissolved oxygen (electron acceptor), and nitrate (a minor nutrient). Tests were conducted at an experimental wellfield of Oregon State University. At this site, extraction phase breakthrough curves for all solutes were similar, indicating apparent conservative transport of the dissolved gases and nitrate prior to biostimulation. Biostimulation tests were conducted to stimulate propane-utilizing activity of indigenous microorganisms and consisted of sequential injections of site ground water containing dissolved propane and oxygen. Biostimulation was detected by the increase in rates of propane and oxygen utilization after each injection. Activity tests were conducted to quantify rates of substrate utilization and to confirm that CAH-transforming activity had likely been stimulated. In particular, the transformation of injected CAH surrogates ethylene and propylene to the cometabolic byproducts ethylene oxide and propylene oxide provided evidence that activity of the monooxygenase enzyme system, responsible for aerobic cometabolic transformations of CAHs, had likely also been stimulated. Estimated zero-order transformation rates decreased in the order propane > ethylene > propylene. The series of push-pull tests developed and field tested in this study should prove useful for conducting rapid, low-cost feasibility assessments for in situ aerobic cometabolism of CAHs.