In Situ Bioremediation of Chlorinated Solvents Source Areas with Enhanced Mass Transfer

ER-200218

Background

Cleanup of chlorinated solvent sources in groundwater is often considered technically (or economically) impracticable because of their density and hydrophobicity, often compounded by subsurface heterogeneity. As a result, many sites have resorted to pump and treat or other containment technologies. Operation and maintenance costs of such systems become very large over time, and due to the longevity of the subsurface sources, these costs have become a large proportion of Department of Defense environmental budgets.

Bioremediation has several advantages as a remedial technology, including (1) the potential to reduce capital costs compared to other technologies for chlorinated solvent source area cleanup, (2) no generation of secondary waste streams, (3) non-hazardous to workers and the environment, (4) contaminants are destroyed in situ, and (5) systems are low maintenance and minimize disturbance of the site. However, bioremediation has traditionally been viewed as very passive with respect to source area remediation. That is, conventional wisdom suggests that bioremediation is limited by the rate at which nonaqueous contaminants dissolve or diffuse to where bacteria can degrade them. Recent advances have shown, however, that mass transfer rates of chlorinated solvents from the nonaqueous phase to the aqueous phase (where they are bioavailable) can be substantially increased during bioremediation.

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Objectives of the Demonstration

The overall objective of this demonstration was to show that facilitating enhanced mass transfer allows bioremediation to be applied cost-effectively to chlorinated solvent source areas in groundwater. Two hydraulically isolated treatment cells were used to compare different whey powder injection strategies and quantify their performance. A phased approach ensured experimental control was sufficient to measure with confidence the effects of different whey injection concentrations on mass transfer. This demonstration represents the first time the phenomenon of enhanced mass transfer in chlorinated solvent source areas as a function of whey injection concentration has been thoroughly documented at the field scale. The results far exceeded expectations and demonstrate the potential impact enhanced mass transfer during bioremediation can have not only on source areas, but also on downgradient plumes.

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Demonstration Results

Two hydraulically isolated treatment cells—each consisting of a network of monitoring wells, an injection well, and an extraction well—were installed at the site. Two injection strategies were applied to each treatment cell. For Treatment Cell 1, the first injection strategy was high concentration (10%) whey powder injections, and the second strategy was low concentration (1%) whey powder injections. For Treatment Cell 2, the strategies were reversed: low concentration (1%) injections first and high concentration (10%) injections second.

The results allowed quantification of the potential for a high-concentration whey solution to enhance mass transfer and facilitated comparison to that for the lower concentration. The factor of increase in aqueous chloroethene concentrations from baseline to 10% whey injections ranged from 1.8 to 4.2, with only one sampling location showing an increase less than a factor of 2.4 and four locations were 3.0 or greater. These increases greatly exceeded those during the 1% whey injections, even though the extent of dechlorination was equivalent (i.e., dechlorination was complete to cis-DCE, with little vinyl choride or ethene production). Three statistical comparisons were performed that demonstrated contaminant molar concentrations were increased at the 95% confidence level as a function of electron donor concentration.

The demonstration was aided by the addition of a row of flux monitoring wells installed by the Army downgradient of the treatment cells that revealed the impact of high-concentration whey injections on contaminant flux. Three months after 10% whey injections began in Treatment Cell 1, concentrations downgradient increased by a factor of 3 to 8, while total chloroethene concentrations downgradient of Treatment Cell 2 changed only by a factor of 0.8 to 1.3. The injection concentrations in the two treatment cells were reversed in November 2005. When the downgradient wells were sampled again in January 2006, the highest concentrations were measured downgradient from Treatment Cell 2, and the lowest concentrations were measured downgradient from Treatment Cell 1. In fact, chloroethene concentrations at one well downgradient from Treatment Cell 2 were a factor of 16 higher than baseline. This change in concentrations downgradient from Treatment Cell 2 of a factor almost 3 to greater than 8 from December to January is nearly identical to the change observed downgradient from Treatment Cell 1 in November 2005 compared to the baseline.

A cost-effectiveness analysis was performed to compare the life cycle costs of four remediation technologies for treatment of NAPL Area 3 of the Fort Lewis East Gate Disposal Yard (EGDY). Three of the technologies—bioremediation, pump and treat, and electrical resistance heating (ERH)—have been applied within or near NAPL Area 3 at EGDY so costs developed were based on actual costs of implementing these technologies at the site. Monitored natural attenuation was the least expensive at $1.1 million, but would not meet cleanup goals in the 30-year life cycle. Bioremediation was estimated at $1.6 million, pump and treat was $2.9 million, and ERH was the most expensive at $3.1 million.

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Implementation Issues

This Bioavailability Enhancement Technology™ (B.E.T.™) (U.S. patent numbers 6,783,678; 7,045,339; 7,141,170; 7,449,114) is readily scaled to any size site, as evidenced by deployments at scales ranging from dry cleaner sites to large-scale plumes. The technology as implemented uses a licensed, commercially available electron donor; all other process equipment is non-proprietary and readily commercially available. When using powdered whey as an electron donor, specialized pumping and mixing equipment is helpful. B.E.T.™ was originally developed at the U.S. Department of Energy’s Idaho National Laboratory. In general, licensed electron donor products can be purchased through JRW Bioremediation, and in some cases, no royalty is required for using the technology at government sites.

The Interstate Technology & Regulatory Council (ITRC) has developed guidance for bioremediation in chlorinated solvent source areas through the Bioremediation of DNAPLs technical team ( http://www.itrcweb.org/teamresources_47.asp). This guidance incorporates many of the concepts validated in this demonstration.

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Points of Contact

Principal Investigator

Dr. Kent Sorenson

CDM Smith, Inc.

Phone: 303-383-2430

Fax: 303-293-8236

Program Manager

Environmental Restoration

SERDP and ESTCP

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