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Monitored Natural Attenuation For Radionuclides In Ground Water - Technical Issues
Citation:
FORD, R. G. Monitored Natural Attenuation For Radionuclides In Ground Water - Technical Issues. Presented at U.S. Nuclear Regulatory Commission, Information Briefing, Rockville, MD, March 11, 2009.
Impact/Purpose:
To provide an EPA perspective on site characterization data needed to support monitored natural attenuation for ground-water remedies.
Description:
Remediation of ground water contaminated with radionuclides may be achieved using attenuation-based technologies. These technologies may rely on engineered processes (e.g., bioremediation) or natural processes (e.g., monitored natural attentuation) within the subsurface. In general, remediation via monitored natural attenuation (MNA) may be achieved through a combination of radioactive decay and/or immobilization reactions, where the former mechanism may dominate for short-lived radionuclides. Selection of MNA for ground-water restoration relies on subsurface characterization to support a thorough engineering analysis to inform the understanding, monitoring, predicting, and documenting of the attenuation processes. Evaluating the performance characteristics of the MNA remedy for sites where contaminant immobilization is the dominant mechanism necessitates characterization of subsurface solids, since the technical evaluation must address both identification of the attenuation process(es) and the long-term stability of the immobilized contaminant to resist remobilization due to potential changes in ground-water chemistry. For sites where radioactive decay and/or immobilization are viable attenuation processes, site assessment will include determination of the flux of contaminant from source zones relative to the capacity of the aquifer to prevent plume expansion. The data collection and analysis process should support development of a detailed knowledge of the system hydrogeology, the mechanisms and rates of contaminant attenuation, the capacity of the aquifer to sustain attenuation of the mass of contaminant within the ground-water plume, and the long-term stability of immobilized contaminants. Determination of immobilization mechanism(s) can be realized through measurement of the ground-water chemical setting (including redox characteristics), the chemical speciation of the contaminant and key reactants in ground-water, and the solid components within the aquifer with which the contaminant reacts. Demonstration of adequate performance characteristics for an MNA remedy will thus depend on the adequacy of the design and installation of the monitoring network to delineate characteristics of the hydrologic system; the procedures for sampling, processing, and preserving aqueous and solid phase samples; and the types and accuracy of chemical measurements to characterize the speciation of contaminants and subsurface reactants in liquid and solid matrices. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.