Environ. Sci. Technol. 2000, 34, 4152-4162
Field-Scale Demonstration of Enhanced MTBE Bioremediation through Aquifer Bioaugmentation and Oxygenation J O S E P H P . S A L A N I T R O , * ,† PAUL C. JOHNSON,‡ GERARD E. SPINNLER,† PAUL M. MANER,† HALINA L. WISNIEWSKI,† AND CRISTIN BRUCE‡ Equilon Enterprises LLC, Westhollow Technology Center, P.O. Box 1380, Houston, Texas 77251-1380, and Department of Civil and Environmental Engineering, Arizona State University, Tempe, Arizona 85287-5306
In situ bioaugmentation and biostimulation experiments were conducted at the USN Hydrocarbon National Environmental Test Site at Port Hueneme, CA (PH), where the dissolved MTBE groundwater plume is over 1500 m long. Laboratory microcosm experiments prepared with PH groundwater showed that MTBE was rapidly metabolized (t1/2 e 2 weeks) after inoculation with a high-activity MTBEdegrading bacterial consortium (MC-100). Microcosm studies also showed that natural ether degraders were present at PH; however, the rates were 3-5 times slower than with the bioaugmented treatment. The field pilot test was conducted to assess the efficacy of creating an MTBE biobarrier by inoculating with MC-100 and maintaining welloxygenated conditions. Three test plots located in the MTBEonly portion of the plume included control (no treatment), O2-only (intermittent O2 gas injection), and O2 + bioaugmented (MC-100) zones. Initial MTBE and dissolved oxygen (DO) concentrations in the plots prior to treatment varied from 2 to 9 mg/L and e1 mg/L, respectively. DO levels increased in the O2-only and O2 + MC-100 plots from 5 to >20 mg/L within a few weeks of O2 gas injection. MTBE levels decreased in the O2-only plot to 0.01-0.1 mg/L after a lag period of 186-261 days, indicating the apparent stimulation of naturally occurring ether degraders. In contrast, in the O2 + MC-100 plot, MTBE concentrations decreased after 30 days and throughout the 261-day experiment eventually to e0.001-0.01 mg/L. tert-Butyl alcohol (TBA) concentrations also declined in the bioaugmented plot to 0.2 µg/L and 3% had > 20 µg/L MTBE. Reports on the fate and transport of MTBE by workers at the Lawrence Livermore National Laboratory (6, 7), the U.S. Geological Survey (8), and the University of Texas (9) have also shown that in many fuel spill cases the migration of MTBE plumes exceeds those of BTEX. MTBE transport along an aquifer flow path depends on several factors including the spill release history, extent of source removal, and local hydrogeology (soil type, hydraulic conductivity, and water table fluctuations). On the basis of its physical-chemical properties, MTBE is more water-soluble (25-250×), has a lower Kow (1.2-2.5×), has a lower soil sorption coefficient (2-10×), and is more slowly biodegraded (5-10×) than the BTEX components of gasoline (8, 10). These observations also support the monitoring well findings that MTBE plumes can extend beyond those of BTEX. Indeed, there are reports of three MTBE plumes that have migrated 260 m [Borden CFB site in Canada (11)], 1900 m [East Patchogue, NY, site (12)], and 1500 m [Port Hueneme, CA, naval base site (13)]. Very few BTEX plumes, however, have traveled g100 m from the source (7) because of the well-known natural bioattenuation mechanisms in aquifers and the widespread abundance of BTEX degraders in soils (14). Three types of naturally occurring MTBE-degrading mixed or single bacterial cultures have been isolated from wastewater biosludges, biofilters, and soil. These microbial systems can metabolize MTBE either partially to TBA or completely to CO2. Cometabolic consortia or single cultures that require another substrate such as alkanes and isoalkanes to induce MTBE degradation have been reported (15-18). Mixed culture bacterial enrichments derived from refinery, chemical, or municipal plant biosludge have also been shown to completely degrade MTBE (19-22). Single culture bacteria isolated from biosolids enrichments that also utilize MTBE have been reported as belonging to species of Rhodococcus (23) and Rubrivivax (24). There are limitations, however, to the natural bioattenuation of MTBE in groundwater. The slow growth rate and low yield (mg of cells formed/mg of MTBE utilized) of aquifer microbes on MTBE (19, 23, 24) may not allow for sufficient bacterial enrichment of ether degraders either in or on the edges of a plume to cause significant and consistent contaminant reduction in some hydrogeologic settings. The low natural O2 transfer rates in groundwater also limit the growth of MTBE degraders. Thus, in situ treatment might be more quickly achieved through increased oxygenation alone (biostimulation) or through the inoculation (seeding) of 10.1021/es000925e CCC: $19.00
2000 American Chemical Society Published on Web 08/26/2000
sufficient concentrations of specialized MTBE-degrading microbes combined with increased oxygenation (bioaugmentation). In cases involving difficult-to-degrade groundwater contaminants such as MTBE, where there are uncertainties associated with biostimulation (e.g., natural degraders may not be present and the time to generate sufficient populations is uncertain and possibly long), bioaugmentation may be an attractive in situ treatment method. Indeed, the use of microbial inocula to stimulate degradation in aquifers has been demonstrated in field pilot tests for TCE using methanotrophic bacteria (25) and for CCl4 with a denitrifying Pseudomonas species (26). In these examples, high concentrations of cells were pumped down injection wells, and contaminant levels and microbes were monitored in downstream wells. In the bioaugmentation field experiment described in this study, we installed both oxygenated-seeded and oxygenatedonly biobarriers within an MTBE-only portion of a large gasoline source plume at the USN National Test Site in Port Hueneme, CA. In the bioaugmented barrier, several kilograms of high-activity MTBE-degrading culture were injected into the aquifer. In both treatments and within a control plot, MTBE and TBA concentrations were monitored with time upstream, within, and downstream of the treatment zones. The data obtained suggest that the extent and rate of decline of MTBE and TBA in the seeded plot are significantly faster than the control (no treatment) and oxygenated-only plots. In addition, spatial variability in treatment effectiveness is greater in the oxygenated-only plot than in the seeded plot.
Materials and Methods MTBE Plume at the Naval Test Site. During 1984-1985, several thousand gallons of leaded gasoline containing the additives MTBE and 1,2-dichloroethane were released from the fuel delivery lines connecting the UST and the dispenser pumps at the NEX gasoline station on the Port Hueneme, CA, Construction Battalion Center Naval Base. Groundwater and soil analyses data suggest that the areal extent of source zone soils (soils containing trapped immiscible-phase gasoline) is about 100 m wide × 300 m long. Analytical results from groundwater monitoring well samples from 1997 to 1999 indicate that the MTBE plume has already traveled more than 1300 m downgradient from the spill source. The width of the plume is about 150 m, and MTBE is the primary organic component in about 75% of the plume. The dissolved BTEX components of the spilled gasoline are apparently being attenuated within a relatively short distance (approximately 50 m) of the downgradient edge of the source zone soils. Dissolved oxygen concentrations are generally low (
