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RELATIONSHIP BETWEEN DEHALOCOCCOIDES DNA IN GROUND WATER AND NATURAL ATTENUATION OF CHLORINATED SOLVENTS
Citation:
WILSON, J. T. AND X. LU. RELATIONSHIP BETWEEN DEHALOCOCCOIDES DNA IN GROUND WATER AND NATURAL ATTENUATION OF CHLORINATED SOLVENTS. Presented at 9th International Symposium, In-Situ and On-Site Bioremediation, Baltimore, MD, May 07 - 10, 2007.
Impact/Purpose:
To inform the public.
Description:
Biological reductive dechlorination is the primary mechanism for natural attenuation of chlorinated solvents in ground water. The only organisms that are known to completely dechlorinate cis-DCE past vinyl chloride to ethylene or ethane are members of the Dehalococcoides group. The density of bacteria in ground water that belong to the Dehalococcoides group can be determined through a biochemical assay based on the polymerase chain reaction (PCR), using DNA primers that are specific to the Dehalococcoides group. The bioassay is commercially available, and the results of the assay are being offered as a “line of evidence” in support of natural attenuation of chlorinated solvents in ground water. The critical element in achieving goals for natural attenuation is the sustained rate of natural biodegradation. It would be valuable if PCR technology could be used to document a rate of natural attenuation expected at field scale. We compared the distribution of Dehalococcoides cells in ground water from monitoring wells to the field scale rate of attenuation of cis-DCE in the plumes. Six plumes were selected for the survey. The BIOCHLOR decision support system was used to calculate the rates of reductive dechlorination of chlorinated ethylenes. The density of Dehalococcoides cells in the water samples was determined by SiREM (Guelph, Ontario) using Quantitative Real-time PCR for primer sets specific to DNA sequences in the 16S rRNA gene of the Dehalococcoides. A rate of biodegradation of 0.3 per year will be adequate for natural attenuation in most ground water plumes. However, at some sites, slower rates may still reach cleanup goals. We found two studies in the literature that could be used to compare the density of Dehalococcoides cells and the rate of removal of cis-DCE. One was a pilot scale demonstration of in-situ bioremediation using a bioaugmentation culture and the second was a laboratory culture study. Based on the literature studies, a density of Dehalococcoides cells near 1 x 107 per liter is necessary to produce a rate of cis-DCE dechlorination of 0.3 per year. When the rate of attenuation in the ground water plumes was normalized by the density of cells, the Dehalococcoides organisms in the aquifers at the field sites performed as well or better than the organisms reported in the literature. It is possible, but unlikely, that the native Dehalococcoides organisms at our survey sites were more efficient than the Dehalococcoides organisms growing under optimum conditions in the literature studies. It is possible, but unlikely, that other organisms were primarily responsible for natural attenuation of chlorinated ethylenes at our study sites. The most likely explanation is that the monitoring wells did not efficiently sample the Dehalococcoides organisms in the aquifer, and that the number of Dehalococcoides cells recovered in a liter of well water was a small fraction of the number of cells that were exposed to a liter of ground water in the aquifer. Whenever possible, aquifer solids should be sampled and assayed for Dehalococcoides DNA. If aquifer solids are extracted and analyzed, there is a better chance of measuring the true density of Dehalococcoides cells, and of measuring a density of cells that can be expected to achieve a useful rate on natural attenuation at field scale.