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To be presented at 1999 Petru,eum Hydrocarbons Conference, Houston, Text,_ Introduction Methyl tert-butyl ether (MTBE), a fuel oxygenate required in reformulated gasolines in the US since 1990 as a part of the Clean Air Act Amendment, possesses all the characteristics of a persistent compound in the subsurface: high solubility, low volatility, low sediment sorption, and resistance to biodegradation. As such, it has rapidly become one of the most widespread groundwater contaminants currently on state regulatory lists. Its environmental persistence has created a challenge for those charged with managing the impact of these plumes. A couple studies have found that MTBE may degrade naturally (Borden et al., 1997; Schirmer and Barker, 1998), but rates are likely insufficient to make natural attenuation a viable plume management strategy. Aerobic mineralization of MTBE has been shown to occur in some mixed (Salanitro et al., 1994; Cowan and Park, 1996) and pure (Mo et,al., 1997; Hanson et al., 1998) cultures, as has degradation as a result of cometabolism (Hardison et al., 1997; Steffan et al., 1997, Hyman et al., 1998). It is conceivable that organisms capable of MTBE oxidation or cometabolism may be present at some sites, but at populations too low to affect the overall flux of MTBE. It may be possible to initiate or enhance such reactions in the subsurface by the addition of oxygen or oxygen and a co -substrate. Injection of oxygen and MTBE -degrading non-native bacterial cultures has also been demonstrated in the field (Salanitro et al., 1999). The MTBE plume at Site 60, Vandenberg Air Force Base, California (Figure 1) was selected as a site to, among other things, evaluate various techniques to initiate or enhance the aerobic mineralization of MTBE by a variety of methods. Einarson et al (these proceedings) describe the site history and plume characteristics in some detail. MTBE emanating from the source areas has formed a long plume, whereas other gasoline constituents appear to be naturally attenuated within 200 feet or less. The plume and surrounding groundwater appears to be devoid of significant concentrations of dissolved oxygen. Mackay et al (these proceedings) present details of ongoing and proposed field pilot -scale tests at VAFB of methods to stimulate the direct utilization of MTBE by either indigenous or non-native microbes, and cometabolism of MTBE by indigenous alkane -degrading organisms. MTBE Oxidation by Indigenous Microbes Materials and Methods — UCD tests For the microcosm experiments, representative samples of sediment were collected from two locations illustrated in Figure 1: between 5-15 feet below ground surface during auger installation of monitoring devices near UW9-ML ("contaminated" sediment from within the plume) and upgradient of the source area ("background" sediment). At that time, 6 L of water was collected from UW9-ML points 2-6 (representative of the depth interval of sediment sampling) and MW -1 (upgradient, near site of sediment collection) for use in these and other microcosm experiments. A subsample of the contaminated soil was sent to the University of California at Davis (UCD), where some of the tests were conducted. The remainder was returned to the University of Waterloo (UW) for these and other tests. The UCD microcosm experiment included three replicates of each treatment: sterile, uninoculated and PM1 bacterial isolate inoculated. In the UCD test, approximately 20 g (wet weight) of groundwater matrix material was placed in 250 -mL bottles, which were then sealed with teflon -lined mininert'u caps. MTBE was added as a saturated gas to achieve the concentration of 10 ppm based on soil volume. No nutrients were added. Sterile controls were established using I% Na Azide. This soil/water/headspace configuration provided sufficient water for reactions to occur, but allowed for optimal exposure of the soil microbes to oxygen. The bottles were incubated in a Lab -Line® Incubator -Shaker in the dark at room temperature between sampling intervals. At each sampling time, bottles were removed from the shaker and a 50 pL sample of the headspace was drawn via syringe and injected onto a Shimadzu GC -14A equipped with a photoionization detector and a 15-m 0.53 mm DB 1 column. The bottles were allowed to breathe for a few minutes in the hood to allow replenish headspace oxygen and allow re -spiking of 10 ppm Page 2