Aerobics to the Rescue - Using Compatible Technologies to Improve Groundwater Remediation


Increased opportunities for contaminated property redevelopment have propelled property owners to evaluate options for accelerating site remediation. In some cases a containment technology may have been implemented to mitigate the off-property migration of contamination as the most practical option at the time. Focusing on the property's redevelopment potential, there is new interest in technologies that supplement or improve the effectiveness of existing remediation systems.
Compatible aerobic technologies


Figure 1: Sampling well locations.

Supplemental technology must be compatible with the original design and current site conditions. For example, aerobic bioremediation is naturally compatible with air sparging, soil vapor extraction or, under the right conditions, chemical oxidation by peroxide compounds or potassium permanganate. While the availability of oxygen may limit the applicability of bioremediation under natural conditions, in combination with an oxygen-producing technology, the effectiveness of both technologies may be improved. Selecting an oxidizing remediation technology for primary treatment increases the number of options for additional treatment, including stimulated and enhanced bioremediation. In contrast, technologies that drive groundwater conditions to become anaerobic, may limit the range of alternatives.

Chemical oxidation of the aquifer by peroxide or potassium permanganate have been shown to reduce the natural microbial population and diversity[1]. After application of a chemical oxidizer, the microbial population shift favors aerobic organisms. This observed shift in conditions presents an opportunity for the application of bioaugmentation with aerobic organisms selected for their ability to degrade the contaminants of concern.
Cometabolism of chlorinated solvents

Bioremediation of chlorinated solvents, especially trichloroethylene (TCE) and tetrachloroethylene (PCE), has been successful under anaerobic conditions. However, there has been limited success in the anaerobic degradation of the breakdown products dichloroethylene (DCE) and vinyl chloride. Aerobic cometabolism is an approach in which enhanced bioremediation degrades PCE, TCE and related breakdown products to inorganic constituents.

The bioaugmentation in the included case study used a commercially available blend of microbial strains that degrade chlorinated solvents by cometabolism. The microbes were isolated from contaminated soils where the naturally occurring microbes had adapted and developed the ability to cometabolize the contaminants.

The microbes in the proprietary blend are Pseudomonas sp., which are known for their ability to metabolize a wide range of organic compounds[2]. The specific microbes were selected for their ability to degrade chlorinated solvents in an aerobic environment and in the presence of dextrose. Aerobic cometabolism of chlorinated solvents has been demonstrated using microbes stimulated by methane, phenol, toluene and other regulated organic compounds. CL-Out bioremediation, however, is supported by the addition of dextrose, which may be added without the concerns raised by the addition of regulated or potentially toxic chemicals.

The cometabolic process is a fortuitous effect of the microbial metabolism of a primary substrate, in this case dextrose. The energy transfer that takes place as the microbes break down organic compounds involves enzymatic activity. With cometabolism, one pseudomonad strain reduces PCE to cis 1,2-DCE and HCl with a dehalogenating enzyme. The breakdown of DCE continues via monooxygenase. Monooxygenase is an enzyme that adds a molecule of oxygen to a hydrocarbon to form an epoxide. The epoxide is an unstable compound that converts to an alcohol or ketone, which is readily degraded to CO2 and water.
Case Study


Table 1: Well data.

Soil and groundwater contamination at a former dry cleaners site in Houston were initially addressed through the implementation of soil vapor extraction (SVE)and soil excavation. While these technologies reduced the volume of contamination in the source area, a significant level of groundwater contamination persisted down-gradient of the source area. Bioaugmentation was used to address this residual contamination.

The site featured shallow soils consisting of fairly recent floodplain deposits. The uppermost aquifer below the site was associated with the Chicot Aquifer, which had been known to generally consist of discontinuous layers of sand, silt and clay. The site was relatively flat, with surface elevations varying by no more than one foot across the property.

The first saturated unit was encountered at a depth of about 35 feet below grade, extending to a depth of approximately 50 feet. This geological unit was classified as silt to sandy silt. The groundwater flow had been consistently measured at a gradient of approximately 0.002 feet per foot toward the east-northeast. Groundwater yield from the uppermost saturated unit was expected to be greater than 150 gallons per day.

After two years of SVE system operation, the building was vacated, allowing for access to the contaminated soils under the building. That soil was excavated to the water table.

In anticipation of future treatments, the excavation was backfilled in a manner suitable for use as an infiltration gallery. Potassium permanganate was introduced through this infiltration gallery into the groundwater to provide treatment in the source area. As a result of the combined treatments in the source area, the concentrations of PCE was reduced from 1.8 to 0.026 mg/l (MW-2, Table 1 and Figure 1). Down-gradient of the source area, however, the PCE concentration in groundwater after permanganate treatment remained as high as 3.4 mg/l (MW-16).

Following the potassium permanganate injections, the dissolved oxygen (DO) level in the groundwater down-gradient of the source treatment was 1.1 mg/l. Aerobic bioaugmentation was selected in part based on its compatibility with the aerobic conditions and compatibility with the successful source area treatments.

Project engineers used 14 injection wells to provide uniform distribution over an area of approximately half an acre. An area in the middle of the injection field where existing businesses were located was inaccessible. The injection points were placed on 25-foot grid squares over approximately 10,000 square feet on two sides of the existing businesses. Engineers injected 110 gallons of hydrated microbes containing 1x109 to 1x1011 colony-forming units per milliliter (cfu/ml) into the groundwater three times at nine-week intervals between June and November 2003.

Using an oxygen release compound, the DO levels increased from 1.1 mg/l prior to the bioaugmentation to as high as 3.9 mg/l following the first inoculation (MW-17, Table 1). Ten months after the last inoculation, the average DO level was 0.8.

The average PCE concentrations in the treated groundwater were reduced from 1.6 mg/l-3.4mg/l to below detection limits (<0.005mg/l) after the three applications (see Table 1). While there was an apparent increase in the cis-1,2 DCE concentration and vinyl chloride concentrations, when the DO levels decreased to less than 1.0 mg/l, there was no vinyl chloride accumulation. The relationship between the presence of daughter products and the dissolved oxygen levels confirmed the dependence of this combined approach on the availability of dissolved oxygen to complete the co-metabolic breakdown of PCE. PE

1) Waddell, J. P. and G.C. Mayer (2003). Effects of Fenton's Reagent and Potassium Permanganate Application on Indigenous Subsurface Microbiota: A Literature Review. Proceedings of the 2003 Georgia Water Resources Conference, held April 23-24, 20003, at the University of Georgia. Kathryn J. Hatcher, editor, Institute of Ecology, The University of Georgia, Athens, Georgia.

2) Cookson, J.T., Jr. (1995). Bioremediation engineering design and application. New York: McGraw-Hill Inc.

3) U.S. Environmental Protection Agency. (2000, July). Engineered approaches to in situ bioremediation of chlorinated solvents: Fundamentals and field applications (EPA 542-R-00-008). Washington, DC: Office of Solid Waste and Emergency Response, Technology Innovation Office.

For more information, contact John D. Brusenhan, P.E., at InControl Technologies Inc, or Michael T. Saul at CL Solutions,


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