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Subsurface Characterization To Support Evaluation Of Radionuclide Transport And Attenuation
Citation:
FORD, R. G. Subsurface Characterization To Support Evaluation Of Radionuclide Transport And Attenuation. Presented at U.S. Nuclear Regulatory Commission (USNRC) 20th Annual Regulatory Information Conference (RIC), Rockville, MD, March 11 - 13, 2009.
Impact/Purpose:
To provide a technical perspective for evaluation of monitored natural attenuation as a potential ground-water cleanup remedy.
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 attenuation) within the subsurface. In general, remediation via monitored natural attenuation (MNA) may be acheived through a combination of radioactive decay and immobilization reactions, where the former mechanism may dominate for short-lived radionuclides. Both types of remedial technologies rely 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 attenuation-based remedies that rely on contaminant immobilization 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. 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. Demonstration of adequate performance characteristics for an attenuation remedy that relies on contaminant immobilization 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 of attenuation-based technologies.