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Site Characterization for MNA of Radionuclides in Ground Water
Citation:
Ford, R. G., R. T. Wilkin, S. Acree, AND R. Ross. Site Characterization for MNA of Radionuclides in Ground Water. Presented at Federal Remediation Technologies Roundtable Meeting, Arlington, VA, November 15, 2007.
Impact/Purpose:
Presentation/Abstract
Description:
Monitored natural attenuation is often evaluated as a component of the remedy for ground water contaminated with radionuclides. 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. For many radionuclide contaminants, sorption onto immobile aquifer solids is the primary attenuation process, resulting in the persistence of the contaminant within the aquifer. This characteristic of radionuclide contaminants necessitates a different approach to site characterization, 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. 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 attenuation 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. Successful evaluation of MNA as a component of the ground-water remedy will thus depend on the adequacy of the design and installation of the monitoring network; 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. Several potential pitfalls in the site characterization effort will be highlighted, as well as technical areas in need of further development for routine implementation as part of MNA assessments.