A full-scale cleanup has been in progress using in situ bioremediation to treat MTBE, BTEX, and TBA in groundwater at the Naval Base Ventura County, Port Hueneme in Oxnard, California. Geology at the site consists of a shallow sand aquifer bounded on the bottom by a clay aquitard, through which groundwater flows at an average velocity of 0.3 m/d (1 ft/d). At a depth of 3-6 m (10-20 ft) below ground surface, the 1500 in. by 150 m (5000 by 500 ft) dissolved MTBE plume mixes with a smaller the BTEX plume that originates from sands contaminated with residual NAPL.
The in situ bioremediation system consists of a 150 m (500ft)-wide "biobarrier," which acts as a passive flow-through system and was installed just downgradient of the NAPL plume. Contaminated groundwater containing dissolved MTBE, TBA, and BTEX travels through the biobarrier and is injected with various combinations of oxygen, air, and conditioned microorganisms. Oxygen gas and bioaug-mented sections are located in the central core of the dissolved contaminant plume and air injections are used on the edge of the plume. Operation of the system began in the fall of 2000. Initial MTBE, BTEX, and TBA concentrations in the groundwater plume were >10,000 pg/L in the center of the plume.
In 2002, after 18 months, contaminant concentrations were reduced to <5 pg/L in monitoring wells downgradient of the biobarrier and extending across the length of the biobarrier. No significant differences in performance were observed for the differently operated sections of the barrier. Dissolved oxygen increased from a preinjection concentration of less than 1mg/L to 10-35 mg/L throughout the treatment zone, thereby increasing the potential for aerobic biodegradation to occur. In addition, the increased dissolved oxygen levels upgradient of the treatment zone due to dispersion of the injected gas appear to cause upgradient reductions in MTBE and benzene concentrations. Peripheral monitoring wells have not shown an increase in contaminant concentrations, indicating that groundwater is flowing through and not around the biobarrier.
The biobarrier system includes 252 gas injection wells, 174 monitoring wells, 25 satellite gas storage tanks, 154 solenoid valves, a 6.8m3/h (240 ft3/h)-capacity oxygen generator, automated timer circuits, and associated piping and electrical lines. The total installation cost of this equipment was USD435,000; initial year (2001) O&M costs were USD75,000 and are expected to continue for a service life of 40 years. A preliminary cost comparison with an existing pump-and-treat system at this site suggests savings of more than USD34 million over the project life. The state regulatory agency recently approved continued use of this biobarrier and installation of a second biobarrier (at the toe of the plume) as the final remedy for the MTBE plume.7273
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