2010-04-20_REVISION - M1977300 (2)

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Whetstone Associates Technical Memorandum Water quality in the mine pool has shown a trend of decreasing uranium and molybdenum concentrations over time. Although the uranium concentrations are still significantly above water quality standards, the chemistry of the mine pool is expected to eventually return to an equilibrium chemistry that resembles pre-mining background, as oxygen is consumed and redox conditions become less oxidizing. A pump and treat system would increase oxidation of the mine pool and wall rock in the shaft, as water levels rise and fall during pump cycling. During the pilot feasibility study for pump and treat of the mine pool, the pump was set at 27 ft below the Steve Level shaft opening and the pumping rate was only 1.2 gpm. Significant drawdown would be induced during pumping, exposing wall rock to additional oxidation. The low flow rates achieved during the pilot test feasibility study and the trend of declining uranium concentrations indicate that uranium recovery from a pump and treat system would not be economical. Also, since hydrologic calculations indicate that the water level in the mine pool will not reach the Steve level, the pump and treat system is not considered to be necessary. Bench scale studies of in-situ treatment of the mine pool would be difficult to implement, since representative wall rock samples are not available. The underground workings include 208,600 cubic feet of void space, and are accessible only through a 64 square foot shaft opening. Mixing and distribution of reductants throughout the workings would not be feasible, since the extensive workings are accessible only through the Steve Level Shaft. Access to the Steve Level is limited and dangerous. Supporting structures within the underground mine (i.e., timbers and backs) are deteriorating rapidly, and the safety of personnel is a concern. Based on the trend in water chemistry and the hydrogeologic conceptual model, the most effective and practical method for treating water in the mine pool is to intercept and treat any seepage from the mine pool water that reaches the alluvium. Flow rates are low, and the source may be diffuse. However, the groundwater in the alluvium can be intercepted and treated using the mitigation strategies presented in the revised EPP. 3) Because the underground workings are containing the contaminants in the mine pool water, DRMS contends that the underground workings should be designated as an Environmental Protection Facility and therefore subject to the provisions of Rule 64.20(7)(e). Operator must describe any release response procedures, redundancies, and "backup" measures necessary, appropriate, and economically reasonable, to control, prevent and mitigate releases of the toxic forming materials from the containment facility (i.e., the underground workings) outside the permit area. The underground workings were included in Section 7 of the EPP submitted to DRMS in August 2009. Each section of the EPP corresponds to the sections in Rule 6.4.20 (EXHIBIT T - Designated Mining Operation Environmental Protection Plan). Section 7 (Facilities Evaluation) corresponds to Rule 6.4.20(7). Additional text has been added to the revised EPP referring to the monitoring program in place, planned additional monitoring, natural hydrogeologic controls on releases, and proposed mitigation of the mine pool via the alluvial groundwater pathway. 4109C.100416 3