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Monitored Natural Attenuation of Inorganic Contaminants in Ground Water (147 pp, 5.56 MB) (EPA/600/R-10/093) September 2010

Volume 3: Assessment for Radionuclides Including Tritium, Radon, Strontium, Technetium, Uranium, Iodine, Radium, Thorium, Cesium, and Plutonium-Americium

The term “monitored natural attenuation,” as used in this document and in the Office of Solid Waste and Emergency Response (OSWER) Directive 9200.4-17P, refers to “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.” When properly employed, monitored natural attenuation (MNA) may provide an effective knowledge-based remedy where a thorough engineering analysis informs the understanding, monitoring, predicting, and documenting of the natural processes. In order to properly employ this remedy, the Environmental Protection Agency needs a strong scientific basis supported by appropriate research and site-specific monitoring implemented in accordance with the Agency's Quality
System. The purpose of this series of documents, collectively titled “Monitored Natural Attenuation of Inorganic Contaminants in Ground Water,” is to provide a technical resource for remedial site managers to define and assess the potential for use of site-specific natural processes to play a role in the design of an overall remedial approach to achieve cleanup objectives.

The current document represents the third volume of a set of three volumes that address the technical basis and requirements for assessing the potential applicability of MNA as part of a ground-water remedy for plumes with nonradionuclide and/or radionuclide inorganic contaminants. Volume 3, titled “Assessment for Radionuclides Including Tritium, Radon, Strontium, Technetium, Uranium, Iodine, Radium, Thorium, Cesium, and Plutonium-Americium,”
consists of individual chapters that describe

  1. 1) the natural processes that may result in the attenuation of the listed
    contaminants and
  2. data requirements to be met during site characterization.

Emphasis is placed on characterization of immobilization and/or radioactive decay processes that may control contaminant attenuation, as well as technical approaches to assess performance characteristics of the MNA remedy. A tiered analysis approach is presented to assist in organizing site characterization tasks in a manner designed to reduce uncertainty in remedy selection while distributing costs to address four primary issues:

  1. Demonstration of dissolved plume stability via radioactive decay and/or active contaminant removal from ground water;
  2. Determination of the rate and mechanism of attenuation by immobilization;
  3. Determination of the long-term capacity for attenuation and stability of immobilized contaminants; and
  4. Design of performance monitoring program, including defining triggers for assessing MNA failure, and establishing a contingency plan.

Where feasible, Agency-approved analytical protocols currently implemented for waste site characterization are identified, along with modifications that may be warranted to help insure the quality of site-specific data. In situations where Agency methods or protocols are unavailable, recommendations are made based on review of the existing technical literature. It is anticipated that future updates to these recommendations may be warranted with
increased experience in the successful application of MNA as part of a ground-water remedy and the development of new analytical protocols.

This document is limited to evaluations performed in porous-media settings. Detailed discussion of performance monitoring system design in fractured rock, karst, and other such highly heterogeneous settings is beyond the scope of this document. Ground water and contaminants often move preferentially through discrete pathways (e.g., solution channels, fractures, and joints) in these settings. Existing techniques may be incapable of fully delineating the pathways along which contaminated ground water migrates. This greatly increases the uncertainty and costs of assessments of contaminant migration and fate and is another area of continuing research. As noted in OSWER Directive 9200.4-17P, “MNA will not generally be appropriate where site complexities preclude adequate monitoring.”

The directive provides additional discussion regarding the types of sites where the use of MNA may be appropriate. This document focuses on monitoring the saturated zone, but site characterization and monitoring for MNA or any other remedy typically would include monitoring of all significant pathways by which contaminants may move from source areas and contaminant plumes to impact receptors (e.g., surface water and indoor air). Nothing in this document changes Agency policy regarding remedial selection criteria, remedial expectations, or the selection and implementation of MNA. This document does not supercede any guidance. It is intended for use as a technical reference in conjunction with other documents, including OSWER Directive 9200.4-17P, “Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites."


Robert G. Ford
Land Remediation and Pollution Control Division

Richard T. Wilkin
Ground Water and Ecosystems Restoration Division

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