Consideration of Geochemical Issues in Groundwater Restoration at Uranium In-Situ Leach Mining Facilities (NUREG/CR-6870)
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Manuscript Completed: December 2006
Date Published: January 2007
James A. Davis and Gary P. Curtis
Water Resources Division
U.S. Geological Survey
345 Middlefield Road, Mail Stop 465
Menlo Park, California 94025
A.L. Schwartzman, NRC Project Manager
Division of Fuel, Engineering, and Radiological Research
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code Y6462
Some mining processes use fluids to dissolve (or leach) a mineral from an ore deposit in the ground. Although these "in-situ" leach mining techniques are considered more environmentally benign then traditional mining and milling practices they still tend to contaminate the groundwater. For this reason, the U.S. Nuclear Regulatory Commission (NRC) requires licensees to ensure that sufficient funds are maintained by the licensee for restoration of the site to initial conditions following cessation of in-situ leach mining operations. Because groundwater restoration represents a substantial portion of these costs, a good estimate of the necessary volume of treatment water is important for approximating the overall cost of decommissioning. This report discusses the in-situ leach mining process, common restoration methods, historical information on in-situ leach mine restoration, and analytical techniques that may be used for estimating the future costs for restoring these sites.
Groundwater restoration costs are a significant portion of the overall restoration costs at an in-situ leach mining facility. One method for estimating the groundwater restoration portion of the costs is to select a conservative dollar amount based on experiences with previous decommissioning activities at nonconventional uranium production facilities. A table of estimated costs from previously decommissioned sites is included in the report.
A second approach discussed in this report is the use of analogous sites which have already undergone decommissioning. A detailed discussion of the geologic and hydrologic similarities and differences associated with uranium mining sites throughout the United States are also included in this report. A table of redox, dissolution, sorption, and aqueous complexation reactions that may occur during the mining process is also included.
Of the three approaches discussed in this report, the third approach, developing and applying a conceptual model that considers the groundwater flow, solute transport, and geochemical reactions associated with a particular site, provides a quantitative and dynamic method for estimating the number of pore volumes and therefore costs associated with groundwater restoration as a function of both historical conditions and potential variations (i.e. under different assumptions of future site conditions). Once the conceptual model has been developed and populated with data collected from the site to gain a physical and chemical understanding of the system, this information can be input into a computer code such as PHREEQC Interactive (Parkhurst and Appelo, 1999) to do the necessary calculations for, in this case, estimating the number of pore volumes that must be removed to return the system to initial conditions.
In order to accurately model the groundwater system, this report also evaluates the main geochemical processes that need to be considered. Typically, the mined ore region is conservatively modeled as a well-mixed linear reservoir with homogeneous properties. However, these assumptions are not always accurate. For example, field observations have shown that lixiviant solutes are not always withdrawn at consistently declining concentrations and tailing can be observed in the extraction of chemically reactive solutes following the removal of the initial pore volume. Therefore adjustments to the conservative model are needed to more accurately model the groundwater restoration process. A series of ten reactive transport simulations using groundwater restoration data from the Ruth ISL pilot scale study were used to evaluate variations in the iv geochemical processes that may be associated with a specific mining site. These calculations demonstrate that a computer code such as PHREEQC can be used to make predictive calculations of how different geochemical conditions may impact evolving water quality during groundwater restoration. It is important to remember, however, that both the PHREEQC code and the conceptual model used in this report were examples only; other geochemical modeling codes and conceptual models could be used.
The information and analytical techniques discussed in the report may be used by licensees, state regulators, and NRC staff who oversee uranium leach mining facilities and assess the costs associated with their restoration. Chapter 6 in the report provides a description of the general approach for modeling the restoration process.