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Ground Water Issue Paper: Synthesis Report on State of Understanding of Chlorinated Solvent Transformation
Citation:
Pivetz, B., Ann Keeley, E. Weber, Jim Weaver, J. Wilson, AND Cissy Ma. Ground Water Issue Paper: Synthesis Report on State of Understanding of Chlorinated Solvent Transformation. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-13/237, 2014.
Impact/Purpose:
Chlorinated solvents are altered by biotic and abiotic processes. Biotic transformation can include reductive dechlorination, cometabolism, and limited oxidation. Abiotic transformation is less well understood but may play a role at some sites. Transformations may be limited such that endpoints fall short of complete degradation of the solvent to innocuous compounds. Determination of which endpoints are reached, the processes of transformation, and the needed site data are critical for assessing and modeling transport, and deciding on monitored natural attenuation (MNA) as a remedy. This Issue Paper summarizes the biotic and abiotic transformations of several important chlorinated solvents. It briefly describes the factors that affect the transformation mechanisms, as well as the measurements necessary to distinguish among the mechanisms. It serves as a guide for developing an advanced ground-water transport model, with governing equations for simulating these processes in models. The primary audience is the EPA remedial project managers (RPMs). The Issue Paper is intended to provide RPMs with a basic understanding of the fundamentals and terminology of chlorinated solvent transformation in the context of MNA.
Description:
The U.S. EPA (1999) provided clarification of its policy on the application of MNA as a remedy for contaminated sites, and defines this alternative as “the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods”. Natural attenuation (NA) processes that degrade or destroy contaminants are preferred over other processes (e.g., dilution and volatilization) that merely attenuate (i.e., diminish contaminant concentrations) contaminant mass (U.S. EPA, 1999). By definition, MNA does not include the use of any active remedial technologies; however, at most sites MNA is very likely to be just one component of the overall remedial strategy as it may be applied to only certain portions of the site, and/or after active technologies have been implemented. Thus, when investigating, modeling, or evaluating MNA it is imperative to take into consideration other remedial activities that have previously occurred or are currently taking place. Chlorinated solvents are altered by intrinsic biotic and abiotic processes. Transformations may be as such that endpoints fall short of complete degradation to innocuous compounds. The determination of which endpoints are reached, the processes of transformation, and the needed site data are critical for assessing and modeling transport, and deciding on Monitored Natural Attenuation (MNA) as a remedy. MNA is a component of 22% of all Record of Decision (ROD) in Superfund sites. Therefore, relevancy of MNA research to OSWER and others in terms of reducing uncertainty over field processes and better remedial decision-making are the expected impacts of this work. Many sites with chlorinated solvent contamination may never proceed to a contaminant fate and transport modeling stage, and therefore use the data to make statistical inferences. For those sites, a thorough recognition of transformation processes to form a strong foundation for the development of a conceptual site model and integrating site data to conceptualize fate and transport processes without the benefit of a computational model are essential. A quantitative conceptual model, based upon transformation knowledge and field observation provides the framework for understanding and remediating a site. The conceptual model also provides the basis for developing and applying numerical models. This document will briefly describe the process of applying models, given the uncertainty in processes and input parameters. It will continue by discussing alternative model formulations and their potential utility. Transformation endpoints are also summarized to facilitate in classification of observed plume behaviors and patterns.