The objective of this project was to evaluate enhanced in-situ anaerobic bioremediation (EAB) as a tool to remediate groundwater sites impacted by fuel hydrocarbons, particularly benzene, toluene, ethylbenzene, and xylene (BTEX). The technology is based on introduction of electron acceptors such as sulfate and nitrate, coupled with removal of toxic or inhibitory compounds such as sulfide, in order to stimulate indigenous anaerobic ground water bacteria. Field tests were performed at the Naval Weapons Station Seal Beach, California from June 1997 through November 1998. To control conditions, extracted water was treated to remove oxygen, nitrate and sulfate, and compounds toxic to microorganisms, such as sulfide. Creation of three separate test zones allowed water to be amended with nitrate and sulfate, with sulfate alone, and without any electron acceptors. The amended water was then reinjected into the test zones. Multi-level sampling wells were used to monitor the in-situ concentrations of BTEX and electron acceptors.
Concentrations of all BTEX compounds were attenuated during the technology demonstration. Toluene concentrations were below the MCL of 10 μg/L from the beginning of the project indicating that it was preferentially biodegraded by existing aerobic and anaerobic conditions. Ethylbenzene was degraded by nitrate from 250 μg/L to less than 10 μg/L. O-xylene was degraded by sulfate from 400 μg/L to less than 10 μg/L. M- and P-xylene was degraded by nitrate from 500 μg/L to less than 20 μg/L. Benzene concentrations were reduced, though not below the MCL of 5 μg/L, mostly as a result of groundwater flushing. Degradation of benzene did however occur towards the end of the demonstration in the zone where both nitrate and sulfate were added, supporting the hypothesis that benzene can be biodegraded anaerobically but only after other preferentially-degraded hydrocarbons have been removed. Anaerobic methanogenic biodegradation did not play a significant role at this site.
EAB costs less than the typical pump-and-treat systems if site conditions favor anaerobic microorganism populations. Based on results of this demonstration, researchers have estimated the cost of using EAB to clean up a site where 3,000 m3 of ground water and aquifer solids have been contaminated by 250 gallons of fuel hydrocarbons. For EAB, the estimated clean-up time and cost per gallon of fuel hydrocarbons treated are 5 years and $4,300 per gallon of fuel, versus 15 years and $6,100 per gallon of fuel estimated for pump-and-treat. EAB could be the remedial alternative of choice where monitored natural attenuation (MNA) fails to stop the migration of BTEX compounds to downgradient receptors. Expectedly, EAB works much faster than MNA.
The results of this demonstration are promising although the design of an EAB system must include consideration of site-specific conditions such as contamination, hydrogeology, and geochemistry. If EAB involves the injection of nitrate at concentrations higher than its MCL of 45 mg/L, regulatory approval would be required. Regulators will require adequate hydraulic control due to the reinjection of hazardous substances. Monitoring for hydrogen sulfide gas and methane in below-ground vaults and basements is necessary whenever sulfate-reducing or methanogenic anaerobic conditions exist. (Project Completed - 1999)