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Monitored Natural Attenuation For Inorganic Contaminants In Ground Water - Technical Issues
FORD, R. G. Monitored Natural Attenuation For Inorganic Contaminants In Ground Water - Technical Issues. Presented at EPA Technical Support Project Meeting and Training, Orlando, FL, November 16, 2010.
To discuss two case studies where subsurface characterization is being conducted to support the Remedial Investigation/Feasibility Study to consider MNA as a potential component of the groundwater remedy for uranium and arsenic contamination.
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 achieved 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 attentuation 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. Two case studies will be discussed where subsurface characterization is being conducted to support the Remedial Investigation/Feasibility Study to consider MNA as a potential component of the groundwater remedy for uranium and arsenic contamination.