Groundwater contamination by chlorinated volatile organic compounds (CVOCs) and perchlorate is a significant environmental issue for the Department of Defense. Numerous laboratory and field studies have shown that CVOCs can be biodegraded by naturally occurring microorganisms under anaerobic conditions. The process can be enhanced by the addition of biodegradable organic substrates resulting in the production of less-chlorinated intermediate products. Several studies have also shown that microorganisms from a variety of aquifers can anaerobically biodegrade perchlorate when supplied with appropriate organic substrates and related amendments. This project investigated an innovative, low-cost approach for distributing and immobilizing a water miscible emulsion with a controlled droplet size as the biodegradable organic substrate in CVOC- and perchlorate-contaminated aquifers. The emulsion is prepared using food-grade edible oils and surfactants and distributed throughout the treatment zone using conventional wells or temporary direct-push points. A portion of the oil becomes trapped within the soil pores leaving a residual oil phase to support long-term anaerobic biodegradation of the target contaminants.
The objectives of this project were to (1) demonstrate and evaluate use of an edible-oil-in-water emulsion as the substrate for stimulating in situ biodegradation of perchlorate and CVOCs in groundwater and (2) develop a protocol for its implementation. The pilot tests evaluated the distribution of the emulsion in the aquifer, the impact of substrate injection on permeability and groundwater flow paths, and the changes in contaminant concentrations and biodegradation indicator parameters. Two pilot tests were performed: one at a site contaminated with perchlorate and CVOCs and one at a site contaminated with CVOCs.
The demonstrations involved the one-time injection of low solubility, slowly biodegradable, soybean oil-in-water emulsion to provide the primary source of organic carbon. A commercially available product, EOS®, was used in each demonstration. The EOS® was distributed throughout the treatment zone using either conventional wells or temporary direct-push points.
At an industrial site in Maryland, a 50-ft long by 10-ft wide by 10-ft deep emulsified oil permeable reactive barrier (PRB) was installed perpendicular to groundwater flow and monitored to determine the cost and performance for controlling the migration of dissolved contaminants in groundwater. High perchlorate concentrations were comingled with elevated levels of 1,1,1-trichloroethane (1,1,1-TCA) and low concentrations of trichloroethene (TCE) in the shallow groundwater. The PRB reduced perchlorate to below the regulatory target, but additional contact time was needed to achieve the same results for 1,1,1-TCA and TCE. There was no adverse change in pH and no evidence of flow bypassing around the PRB. The pilot study was extended to 42 months and showed that a single application of EOS® was effective in the PRB for almost 3 years without replenishment.
At a site at the Charleston Naval Weapons Station, EOS® was used to treat a TCE source area in a shallow, low permeability aquifer. A tightly spaced grid of injection wells was used to distribute EOS® in the 20-ft by 20-ft by 10-ft deep pilot test treatment cell. After 6 to 9 months, TCE degradation slowed, apparently as a result of a drop in groundwater pH to near 5. Laboratory studies evaluated potential buffering agents, and after 28 months, the treatment cell was re-injected with a buffered emulsified oil substrate formulation. After the aquifer was neutralized, TCE was rapidly reduced to cis-1,2-dichloroethene (cis-DCE) and vinyl chloride (VC) with some measurable ethene production. However, the absence of microorganisms with the VC-reductase enzyme appeared to limit further biodegradation. The results demonstrated the effectiveness of the technology as a source area treatment for TCE, but also pointed out the importance of thorough site characterization.
The pilot tests successfully demonstrated that this approach could provide good contact between the substrate and the contaminants resulting in effective rates of biodegradation. As designed, a portion of the emulsified oil was trapped within the soil pores leaving a residual oil phase to support long-term anaerobic biodegradation of target contaminants. The technology also offers the potential to substantially reduce both initial capital and long-term operation and maintenance (O&M) costs.
The unit cost to install the 50-ft long PRB was $226/yd3. The cost to create a 20 x 20-ft source area treatment cell ranged from $325/yd3 for direct injection to $428/yd3 for a recirculation design. The mass of contaminant treated in the PRB was much higher due to the rapid flow of contaminated groundwater through the barrier. Consequently, the cost per gram of contaminant treated was also less in the PRB.
In situ bioremediation using emulsified oil substrate is not the least expensive technology to install, but calculating the net present value for a given scenario demonstrated that long-term costs are expected to be lower due to the lower O&M requirements and the longevity of the substrate compared to other electron donor materials.
The major technical challenges and cost drivers identified in these demonstrations when applying emulsified oil substrate technology included:
- Contaminant types, concentrations, and vertical and lateral extent
- Impact of aquifer composition and permeability on oil retention and the effective distribution of the substrate throughout the target treatment zone
- Impact of substrate on aquifer pH, which can limit biodegradation and may require buffering
- Presence of native microorganisms to biodegrade the contaminants or the need to consider bioaugmentation
- Establishing a treatment zone that affords adequate contact time between the contaminant, substrate, and bacteria, especially in PRBs
- Impact of regulatory goals and monitoring requirements for the site as they affect the duration of the project.