Groundwater Services, Inc.

Historical and Retrospective Survey of Monitored Natural Attenuation Line of Inquiry Supporting Monitored Natural Attenuation and Enhanced Passive Remediation of Chlorinated Solvents

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Overview

The Department of Energy (DOE) is sponsoring an initiative to facilitate efficient, effective and responsible use of Monitored Natural Attenuation (MNA) and Enhanced Passive Remediation (EPR) for chlorinated solvents. This Office of Environmental Management (EM) “Alternative Project,” focuses on providing scientific and policy support for MNA/EPR. A broadly representative working group of scientists supports the project along with partnerships with regulatory organizations such as the Interstate Technology Regulatory Council (ITRC) and the Environmental Protection Agency (EPA). The initial product of the technical working group was a summary report that articulated the conceptual approach and central scientific tenants of the project, and that identified a prioritized listing of technical targets for field research. This report documented the process in which: 1) scientific ground rules were developed, 2) lines of inquiry were identified and then critically evaluated, 3) promising applied reseach topics were highlighted in the various lines of inquiry, and 4) these were discussed and prioritized. The summary report will serve as a resource to guide management and decisionmaking throughout the period of the subject MNA/EPR Alternative Project. To support and more fully document the information presented in the summary report, we are publishing a series of supplemental documents that present the full texts from the technical analyses within the various lines of inquiry (see listing). “Historical and Retrospective Survey of Monitored Natural Attenuation” -- is one of those supplemental documents.

Summary Report:

  • Natural and Passive Remediation of Chlorinated Solvents: Critical Evaluation of Science and Technology Targets, WSRC-TR-2003-00238

Supplemental documents:

  • Baseline Natural Attenuation Processes: Lines of Inquiry Supporting Monitored Natural Attenuation of Chlorinated Solvents, WSRC-TR-2003-00329
  • Potential Enhancements to Natural Attenuation: Lines of Inquiry Supporting Enhanced Passive Remediation of Chlorinated Solvents, WSRC-TR-2003-00330
  • Multiple Lines of Evidence Supporting Natural Attenuation: Lines of Inquiry Supporting Monitored Natural Attenuation and Enhanced Passive Remediation of Chlorinated Solvents, WSRC-TR-2003-00331
  • Potential Enhancements to the Characterization and Monitoring of Natural Attenuation: Lines of Inquiry Supporting Monitored Natural Attenuation and Enhanced Passive Remediation of Chlorinated Solvents, WSRC-TR-2003-00332
  • Historical and Retrospective Survey of Monitored Natural Attenuation: A Line of Inquiry Supporting Monitored Natural Attenuation and Enhanced Passive
    Remediation of Chlorinated Solvents, WSRC-TR-2003-00333

Historically, the recognition, evaluation and reliance on natural processes for remediation and final polishing of contaminated sites has been problematical. Over the past fifteen years,however, significant progress has been made due to the efforts of regulatory and federal agencies such as the Environmental Protection Agency (EPA) and the Department of Defense (DOD), and others. This progress has taken the form of regulatory protocols and case studies from attempted implementation. To be successful, the DOE Alternative Project must link to, and build upon,this progress. A critical component of responsibly advancing the technical basis for the use of
MNA/EPR was documenting this historical development and assessing, using a retrospective survey, what has worked and where there are barriers to implementation.

Early variants took the form of “alternate concentration limits” (ACLs) or “mixing zones”. These concepts provided a useful tool for relatively low risk sites and were based on modeling – the goal was to calculate a target concentration in the plume that would not result in an exceedence of an applicable or appropriate standard at an agreed exposure location. The guidelines for ACL modeling did not emphasize the full range of natural attenuation and remediation mechanisms. As a result, scientists, regulators and policymakers began a process to develop improved guidelines that would encourage responsible and disciplined use of remediation and site management strategies that rely on natural processes. These efforts were squarely focused on natural attenuation processes and lead to the definition and understanding of the term “MNA” (EPA, 1998 and EPA, 1999). The efforts were customized, as appropriate, to major classes of common contaminants. This allowed the most applicable mechanisms to be identified and specific guidelines and protocols to be established for the most widespread problems. It is particularly instructive to examine the timing of protocol development efforts for various classes of organic contaminants.

Figure 1 provides a time line for the development and regulatory protocol development for monitored natural attenuation (MNA) for petroleum hydrocarbons and for chlorinated solvents. It is clear from the timeline that development of MNA for petroleum hydrocarbons, because the underlying processes are inherently more robust and simple, occurred earlier than the development of MNA for chlorinated volatile organic contaminants (CVOCs). Moreover, MNA of petroleum hydrocarbons is now widely accepted and used. Protocol development for CVOCs has proven to be more complex and the viability, robustness and utility of the CVOC protocols
has not been documented. How have they worked? How many sites have they been applied to? What percentage of those sites met the requirements? What parts of the protocol have been most useful in practice? What parts of the protocol have been least useful? Obtaining answers to these types of questions is an absolute requirement for any effort whose goal is to build on and contribute to this historical process and to the positive evolution of MNA/EPR. The answers to key questions are key input that will help determine the most promising research and regulatory path. Of particular interest are experiences in the period since 1998/1999 – after the EPA regulatory protocols and directives were released. A few historical evaluations have been performed – normally covering a case study or a few sites. To assist in developing a target list of critical science and technology needs and issues to support advancing CVOC protocols, a general historical survey was performed.

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