Theme: 11. Advanced Diagnostic Tools

Topic Area: 11c. Using Omic Approaches and Advanced Molecular Tools to Optimize Site Remediation

Formatted Title:

Microcosms Reveal Complete Dechlorination Activity with Initial Dehalococcoides below Detection

Background/Objectives:

At a site in the midwestern United States that includes industrial, commercial, and residential properties, a plume of contaminated groundwater was identified that requires remediation. The plume is chiefly composed of trichloroethylene (TCE) along with lesser amounts of perchloroethylene (PCE), cis-1,2-dichloroethylene (DCE), and 1,1-dichloroethane (1,1-DCA). The depth to the water table at the site is generally from 4 to10 ft below ground surface (bgs), and the aquifer consists of unconsolidated sand and gravel materials. A microcosm study was performed with the following objectives: 1) to evaluate the potential for biostimulation and bioaugmentation to achieve the cleanup goals for TCE, as well as other VOCs that persist at the site; 2) evaluate toxicity factors that have precluded natural degradation of the site contaminants; and 3) to evaluate the potential for other measures that will most effectively address groundwater contamination at the site such as the use of zero valent iron (ZVI) to promote abiotic degradation, in combination with biological dechlorination.

Approach/Activities:

A sediment sample (~2 kg) was collected from 21-27 ft bgs. Average conditions for groundwater collected from several wells included a pH of 7.2, temperature of 19.0 ⁰C, an oxidation/reduction potential (ORP) ranging from 27 to 121 mV, nondetectable dissolved oxygen and nitrate, and sulfate at 14 mg/L. Microcosms were prepared in an anaerobic chamber in 160 mL glass serum bottles containing 70 mL groundwater and 50 g soil. Treatments were prepared in triplicate, as follow: unamended; lactate amended; lactate amended + bioaugmented; emulsified vegetable oil (EVO) amended; EVO amended + bioaugmented; nanoscale ZVI; nanoscale ZVI + EVO; autoclaved controls; and water controls. Resazurin (1 mg/L) was added as a redox indicator. TCE saturated water was added to bring the initial concentration to ~1 mg/L. A commercial bioaugmentation culture was used to raise the initial concentration of Dehalococcoides (Dhc) to ~10⁶ copies/mL. The ZVI dose was ~2.3% of the wet weight of the soil.

Results/Lessons Learned:

The unamended microcosms exhibited no sign of biotic dechlorination activity, consistent with an ORP >-110 mV (based on the resazurin color), and Dhc and Dehalogenimonas below 10⁴ copies/L. After adding electron donor and observing the redox level fall below -110 mV, the bioaugmentation culture was injected. Within 14 days, TCE was completely dechlorinated to ethene. The peak vinyl chloride (VC) concentration was ~10% of the initial TCE and declined below 2 µg/L. TCE decreased at a first order rate in the treatments with ZVI added, but after three months of incubation, the concentration persisted above the maximum contaminant level of 5 µg/L. Although acetylene was detected, ethene and ethane were the main products. Adding EVO along with ZVI did not improve the rate of dechlorination. After ~2 months of incubation, dechlorination activity started in the lactate only treatment, with complete dechlorination to ethene underway. This occurred in spite of the fact that the initial levels of Dhc and Dehalogenimonas were below detection. While the use of molecular diagnostic tools to assess the potential for bioremediation is now commonplace, these microcosm results serve as a reminder that a site may harbor the microbes necessary for complete dechlorination even if they are initially below standard methods of detection. Unlike the bioaugmented treatment, the rate of dechlorination in the lactate amended microcosms was slower and VC reached stoichiometric levels, indicating important advantages to using bioaugmentation. The treatment biostimulated with EVO alone did not exhibit dechlorination activity within the same timeframe.

Dominique Sorel

Affiliation

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(Group 1, Poster Board #31) Microcosms Reveal Complete Dechlorination Activity with Initial (ital)Dehalococcoides below Detection