ADVANCING THE SCIENCE OF NATURAL AND ENHANCED ATTENUATION FOR CHLORINATED SOLVENTS

Abstract

This report summarizes the results of a three-year program that addressed key scientific and technical aspects related to natural and enhanced attenuation of chlorinated organics. The results from this coordinated three-year program support a variety of technical and regulatory advancements. Scientists, regulators, engineers, end-users and stakeholders participated in the program, which was supported by the U.S. Department of Energy (DOE) and the Interstate Technology and Regulatory Council (ITRC). The overarching objective of the effort was to examine environmental remedies that are based on natural processes--remedies such as Monitored Natural Attenuation (MNA) or Enhanced Attenuation (EA). A key result of the recent effort was the general affirmation of the approaches and guidance in the original U.S. Environmental Protection Agency (EPA) chlorinated solvent MNA protocols and directives from 1998 and 1999, respectively. The research program did identify several specific opportunities for advances based on: (1) mass balance as the central framework for attenuation based remedies, (2) scientific advancements and achievements during the past ten years, (3) regulatory and policy development and real-world experience using MNA, and (4) exploration of various ideas for integrating attenuation remedies into a systematic set of ''combined remedies'' for contaminated sites. These opportunities are summarized herein and are addressed in more detail in referenced project documents and journal articles, as well as in the technical and regulatory documents being developed within the ITRC. Natural attenuation processes occur in all soil and groundwater systems and act, to varying degrees, on all contaminants. Thus, a decision to rely on natural attenuation processes as part of a site-remediation strategy does not depend on the occurrence of natural attenuation, but on its effectiveness in meeting site-specific remediation goals. Meeting these goals typically requires low risk, plume stability, and documentation of accepted and sustainable attenuation processes. Plume stability and sustainability depend on the balance between contaminant loading into the plume and contaminant attenuation within the plume. This ''mass balance'' is a simple and powerful idea that developed into the central framework for all aspects of the DOE MNA/EA program. The centrality of mass balance has been advocated by Chapelle and others (e.g., 1995) for several years, and the concepts proved to be critical to conceptualizing natural attenuation remedies, designing enhancements, developing characterization and monitoring strategies, and developing regulatory decision frameworks that encourage broader use of MNA/EA with clarified technical responsibility.