The objective of this project was to install and operate a full-scale biobarrier across a mixed methyl-tert-butyl-ether (MTBE)/benzene, toluene, ethylbenzene, and xylene (BTEX) dissolved plume and assess the effect of aeration with air or oxygen (biostimulation) as well as aeration with oxygen plus seeding with MTBE-degrading organisms (bioaugmentation) on reducing MTBE, BTEX and tertiary butyl alcohol (TBA) groundwater concentrations.

Technology Description

A 500 foot (ft) wide biologically reactive groundwater natural-gradient flow-through barrier (biobarrier) was installed downgradient of a gasoline-spill source zone at the Naval Base Ventura County, Port Hueneme, CA. The site had sandy soil, depth to groundwater of 10 to 25 ft below ground surface (bgs), BTEX/MTBE/TBA levels of 100-10,000 micrograms per liter (μg/L), and groundwater velocities ranging from 0.1 to 1.0 ft per day. A gas injection system was installed to supply either air or oxygen along the entire length of the biobarrier, and the higher MTBE concentration region was seeded with an MTBE-degrading microbial culture (MC-100 or SC-100). Various design permutations were operated along the biobarrier.

Demonstration Results

Dissolved oxygen (DO) was originally below 1 milligram per liter (mg/L) but increased to above 4 mg/L and in the oxygen gas injection zones was generally less than 20 mg/L within 5 ft of the gas injection row. This DO level, necessary for aerobic biodegradation, was maintained during the 15 month testing period. Groundwater MTBE concentrations decreased after 1 to 3 months of bioaugmentation and 4 to 6 months after gas injection. After 7 months, MTBE concentrations leaving the biobarrier were less than the detection limit. Benzene concentrations decreased more rapidly with time than MTBE. The results showed that air injection and oxygen injection stimulated naturally-occurring MTBE degraders and alone was sufficient to decrease MTBE and benzene concentrations to below target levels along certain sections of the biobarrier. The combination of oxygen gas and a one-time microbial seeding was equally effective. Projected costs to install a similar biobarrier range from $800 to $1,500 per linear foot for aquifers at depths less than 30 ft bgs. For a 500 ft wide biobarrier system, installation costs are estimated at $553,096, including $169,342 for the air/oxygen delivery system and $383,753 for the culture injection. Total operations and maintenance (O&M) costs for one year were estimated at $75,523 as compared to pump and treat with annual O&M costs of $250,000. Installation costs are sensitive to the depth to groundwater and the soil type, while operational costs should be similar for most systems.

Implementation Issues

The periodic gas delivery system was designed to create trapped gas pockets that continuously supplied oxygen to groundwater. It was speculated that this trapped gas could supply oxygen for several days in case of equipment failure. In all zones of the biobarrier, DO levels were increased to 4 mg/L and higher and degradation to non-detect levels occurred. While air injection was sufficient to treat the lower MTBE concentrations, the project team felt that oxygen gas injection offered additional advantages at higher MTBE concentrations; namely that the higher maximum DO concentrations resulted in a more uniform and high treatment effectiveness than might be achieved by air injection. (Project Completed - 2003)