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To be presented at 1999 Petroleum Hydrocarbons Conference, Houston, Texas temperature. 24 hours were permitted to pass before the first samples were collected to allow the systems time to equilibrate. Subsequent samples were drawn at intervals determined on the basis of results from the previously collected sample. Samples were collected using a glass -barrel syringe inserted through the mininert valve. A 5 -mL aliquot of water withdrawn and introduced into a 10 -mL vial, sealed with a cap and septa, and placed in a HP 5263 automated headspace sampler. A 1 -mL sample of the headspace in the vial is automatically collected and injected onto a HP 5890 GC equipped with a flame ionization detector and a 60 m 1313-5 capillary column. Approximate MTBE detection limit for this method is roughly 10 ppb. After sampling the microcosm systems the mininert valve was left open for a short time to allow ambient air to replace the volume of water removed. This helped to replenish the oxygen supply and prevent a negative pressure from forming in the system that would upset partitioning equilibria. These tests were ongoing at the time this paper was written and as such the results are not shown here, but will be presented at conference. Early data show no significant MTBE degradation to date, which was expected in this system where microbe -oxygen contact is less favorable than that achieved in the UCD tests. MTBE Oxidation by Non -Native Microbial Isolate Materials and Methods A subsample of the contaminated sediment was used to construct microcosms inoculated with a gram-negative proteobacteria strain isolated from a compost -filled biofilter treating volatile emissions from a water treatment plant. This strain, called PM1, has shown the ability to aerobically degrade MTBE in a range of subsurface media. These microcosms were constructed in the same fashion as the oxygen -only experiments conducted at UCD described above, except that approximately 107 cells of the isolate was added to the soil material before adding the MTBE. Results Figure 2 shows the average headspace MTBE concentration in the inoculated and sterile control systems over the 320 -hour experiment. MTBE was completely degraded within 45 hours, and more rapidly removed after 5 successive re -spikes. The 10 ug MTBE per mL inputs were degraded in as little as 20 hours by the acclimated population. These data suggest that the bacterial isolate can thrive and degrade up to 60 ug MTBE per mL in VAFB soils without requiring the addition of nutrients. Cometabolism by Indigenous Alkane Degraders Materials and Methods Sediment and groundwater collected from both contaminated and background locations were used to construct a series of batch microcosms at UW to determine whether native VAFB microbes can cometabolize MTBE when grown on simple, branched, and/or cyclic alkanes. For each sediment/groundwater combination, four series (3 active, 1 sterile control) of four sets of duplicate 60 mL hypovials were assembled in a sterile flow cabinet, consisting of roughly 20 g of site sediment, 30 mL of groundwater, and approximately 20 mL of headspace. The groundwater collected at the site was homogenized, purged of volatile organics, and partitioned into four 1- L bottles. The bottles were spiked with MTBE and either hexane (simple alkane), isopentane (branched alkane), cyclohexane (cyclic alkane), or all three compound (combined sterile controls). Materials for the sterile controls were triple autoclaved and the organic -spiked groundwater was amended with sodium azide. The bottles were mixed for 4 hours with magnetic stirrers and then added to the appropriate batch vials. The vials were sealed with cap and septa and incubated in the dark at room temperature. At each sampling interval 2 bottles from each series were sacrificed for analysis. 16 mL of water was withdrawn from each vial with a glass barrel syringe, introduced to a 22 -mL vial, sealed with cap and septa and analyzed using the same method and equipment as previously described. Results The results of the hexane -MTBE batch microcosm test are presented in Figure 3. In the contaminated sediments, hexane is largely depleted by day 37, after an apparent acclimation period of 10-12 days. Once hexane degradation began, MTBE concentrations also began to decrease and continued to drop to values near the detection limit by day 77 when the last set of vials were sacrificed. In the background sediments, hexane is rapidly degraded by day 12, and there is some evidence of MTBE degradation in one of the two duplicates over Page 5