Tools to Monitor In situ Bioremediation of Fuel Oxygenates

The potential for in situ biodegradation of tert-butyl alcohol (TBA) by creation of aerobic conditions in the subsurface with recirculating well pairs was investigated in two field studies conducted at Vandenberg Air Force Base (VAFB). In the first experiment, a single recirculating well pair with bromide tracer and oxygen amendment successfully delivered oxygen to the subsurface for 42 days. TBA concentrations were reduced from approximately 500 μg/L to below the detection limit within the treatment zone and the treated water was detected in a monitoring transect several meters downgradient. In the second experiment, a site-calibrated model was used to design a double recirculating well pair with oxygen amendment, which successfully delivered oxygen to the subsurface for 291 days and decreased TBA concentrations to below the detection limit. Methylibium petroleiphilum strain PM1, a known TBA-degrading bacterium, was detectable at the study site but addition of oxygen had little impact on the already low baseline population densities, suggesting that there was not enough carbon within the groundwater plume to support significant new growth in the PM1 population. Given favorable hydrogeologic and geochemical conditions, the use of recirculating well pairs to introduce dissolved oxygen into the subsurface is a viable method to stimulate in situ biodegradation of TBA or other aerobically-degradable aquifer contaminants.;Aquifer microbial communities can be investigated using Bio-traps, passive samplers containing Bio-Sep® beads ("bio-beads") that are deployed in wells to be colonized by bacteria delivered via groundwater flow through the well. When bio-beads are "baited" with organic contaminants enriched in 13C, stable isotope probing allows assessment of the composition and activity of the microbial community. This study used an ex situ system fed by groundwater continuously extracted from an adjacent monitoring well within an experimentally created aerobic zone treating a TBA plume. The goal was to evaluate aspects of bio-trap performance that cannot be studied quantitatively in situ. The measured groundwater flow through a bio-trap housing suggests that such traps might typically “sample” about 0.9 liters per month. The desorption of TBA or MTBE bait from bio-traps during a typical deployment duration of 6 weeks was approximately 45% and 90%, respectively, of the total initial bait load, with initially high rate of mass loss that decreased markedly after a few days. The concentration of TBA in groundwater flowing by the TBA-baited bio-beads was estimated to be as high as 3,400 mg/L during the first few days, which would be expected to inhibit growth of TBA-degrading microbes. Initial inhibition was also implied for the MTBE-baited bio-trap, but at lower concentrations and for a shorter time. After a few days, concentrations in groundwater flowing through the bio-traps dropped below inhibitory concentrations, but remained 4-5 orders of magnitude higher than TBA or MTBE concentrations within the aquifer at this site. Desorption from the bio-beads during ex situ deployment occurred at first as predicted by prior sorption analyses of bio-beads, but with apparent hysteresis thereafter, possibly due to mass transfer limitations caused by colonizing microbes. These results suggest that TBA- or MTBE- baited bio-traps could be baited at lower initial total mass loading with no detriment to trapping ability. The bio-traps were able to collect detectable amounts of microbial DNA, and thus allow some insight into the sparse microbial community present in the aquifer during remediation of the low concentration plume.