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EVALUATION OF THE PROTOCOL FOR THE NATURAL ATTENUATION OF CHLORINATED SOLVENTS: CASE STUDY AT THE TWIN CITIES ARMY AMMUNITION PLANT
Citation:
Wilson*, J T., D H. Kampbell*, M. Ferrey, AND P. Estuesta. EVALUATION OF THE PROTOCOL FOR THE NATURAL ATTENUATION OF CHLORINATED SOLVENTS: CASE STUDY AT THE TWIN CITIES ARMY AMMUNITION PLANT. EPA/600/R-01/025 (NTIS PB2001-107960), 2001.
Impact/Purpose:
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Description:
At the request of staff in the EPA Regions, EPA?s Office of Research and Development carried out an independent evaluation of the Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water (EPA/600/R-98/128). The Protocol was developed around case studies on relatively small plumes in sand aquifers. Staff in the Regions wished to know if the Protocol could be usefully applied to large plumes, or to plumes in aquifers in fractured consolidated rock.
Disposal of waste solvents containing trichloroethylene and 1,1,1-trichloroethane at the Twin Cities Army Ammunition Plant (TCAAP) in Minnesota resulted in the formation of a ground water contaminant plume over four miles long. As part of the remedial response, the U.S. Army installed an extensive pump and treat containment system around the source areas intended to prevent additional loading of contaminants to the down gradient portion of the aquifer.
The site was selected for an independent evaluation of the Protocol for two reasons. First, the ground water plume was very well characterized and an extensive historical sampling database was available to complement the study. Second, although long term monitoring indicated that the concentrations of trichloroethylene and 1,1,1-trichloroethane attenuated with distance from the source, natural attenuation did not contain the ground water plume within the site boundary. Thus, the evaluation could determine if an analysis done according to the Protocol would predict a large ground water plume under existing site conditions.
The results show that the Protocol was successful in predicting the development of the ground water plume at TCAAP. The screening analysis in the Protocol predicts that the geochemical environment at TCAAP is not favorable to rapid reductive dechlorination. The modeling portion of the study indicated that the current ground water plume should be expected when the rate of reductive dechlorination is slow.
The study also shows that natural biodegradation complements the on-going efforts to extract contaminated ground water at the source, and should greatly reduce the time required to reduce the concentration of contaminants to U.S. EPA drinking water standards. If the rate of natural biodegradation exhibited in the last ten years continues for the next twenty years, the portion of the aquifer down-gradient from TCAAP will be reclaimed.