Field Tests of Enhanced Intrinsic Remediation of an Mtbe Plume

We have been studying an MTBE plume at Vandenberg Air Force Base, CA. Microcosm studies with site sediments, conducted at the University of California at Davis and the University of Waterloo, suggest that native aerobic MTBE-degrading bacteria are present in the site sediments and can be stimulated to degrade MTBE solely by adding oxygen. In two separate field tests, dissolved oxygen has been released into the MTBE plume by diffusion through the walls of tubing pressurized with oxygen and in contact with the groundwater flowing through unpumped well screens or permeable walls. In both field tests, significant reductions in MTBE concentrations have been noted downgradient of the diffusive oxygen release systems in repeated sampling events, presumably from in situ biodegradation of the MTBE. Studies are underway to determine if potential breakdown intermediates such as TBA can be detected and if populations of MTBE-degrading bacteria have increased due to oxygenation. INTRODUCTION Laboratory studies have shown that MTBE can be degraded under aerobic conditions by direct metabolism (when the MTBE serves as the carbon and energy source for microbial growth). There is also evidence from field studies that MTBE plumes may be affected by biodegradation during transport through some aquifers (Borden et al., 1997; Schirmer and Barker, 1998), but the rates of degradation are low. When dissolved oxygen is limited or absent, as is often the case in groundwater impacted by petroleum hydrocarbon spills, natural attenuation of MTBE plumes may be insufficient for risk management. A remedy based on enhancement of in situ aerobic microbial treatment of MTBE must: 1) create steady aerobic conditions, and 2) generate enough microbial biomass to accomplish the treatment at a practically useful rate. Salanitro et al. (1999) injected a non-native MTBE-degrading bacterial culture at Pt. Hueneme, CA, and provided oxygen via a pulsed sparging system, yielding encouraging results. However, it is not yet clear how that method may compare to approaches that rely solely on native MTBE-degrading microorganisms. More generally, there has been little discussion or investigation of whether such in situ bioreactive walls in fact function as they are intended to, i.e. ideally allowing uniform flow of contaminated groundwater through a uniform treatment zone over the long term. At Site 60, Vandenberg Air Force Base, CA, we have found that the existing MTBE plume is weakly anaerobic and located within a relatively shallow and thin aquifer. Microcosm studies with site sediments (UC Davis and U Waterloo) suggested that native aerobic MTBE-degrading bacteria are present in the site sediments and can be stimulated to degrade MTBE solely by adding oxygen (Wilson et al., 1999). Thus, we initiated several field pilot tests of enhancement of aerobic intrinsic remediation near the apparent centerline of the existing MTBE plume and well downgradient of the source area where MTBE is the only VOC at significant concentrations. The main goals of our field tests were 1) to seek field evidence that release of dissolved oxygen into the natural flow of MTBEcontaminated groundwater stimulates and maintains a population of native MTBE-degrading microbes capable of significantly reducing the MTBE flux, and 2) to evaluate the field performance of two configurations of diffusive oxygen-releasing devices, cylindrical and rectilinear. The cylindrical method is described briefly below, while the rest of this paper focuses on the rectilinear “panel” test. Preprint Extended Abstract for Proceedings of ACS National Meeting in San Francisco, CA, March 26-3