2018-08-22_REVISION - M1977493 (7)

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Eagle Park Reservoir Chemical Loading and Mixing Analysis cut-off wall, collection pond, and pump station, is designed to contain and collect seepage from the downstream toe of Robinson Dam and convey this water back to Robinson Lake. Based on bedrock topography and estimated aquifer thickness, the general flowpath of shallow, alluvial groundwater to EPR is from the southeast. Bedrock mapping information in the vicinity of the Robinson Lake Dam and field investigations conducted in 2013 and 2014, indicate that most groundwater follows two alluvial drainage channels to the east and west of a bedrock ridge. Kumar and Associates (1993) indicated that alluvial valleys in the Eagle Valley watershed are comprised of silts, sands and gravels. The westernmost, alluvial flowpath, identified as Section A -A' (Figure 1), follows a small alluvial drainage channel northwest towards EPR, from Robinson Lake Dam to below monitoring well EVMW-1S. The second alluvial flowpath, identified as Section B -B' (Figure 1), represents the eastern side of the valley below Robinson Lake. The upper reach of this eastern area leads towards surface water monitoring location EVS-1, located at the base of the alluvium at the southern edge of a former alluvial material borrow area to the east of EPR. POC Monitoring Well EV -MW -004 is located in the area where these two flowpaths enter EPR. Also considered is groundwater that may occur in the deeper, fractured bedrock system beneath the alluvial valleys just described. For purposes of the analysis presented in this document, the width of the flowpath extends approximately from point A to point B' (Figure 1). Groundwater Inflows Tables 1 and 2, below, present hydrogeologic parameter inputs for the modeled groundwater flow. All inputs are considered to be conservative. Flow gradients were established by evaluating surface topography, and the saturated width and length for each flowpath conservatively assume that the alluvium is contiguous across the entire width and length. Despite the observed bedrock outcrops indicating that alluvium is very thin, a conservative value of 20.3 feet was used for the saturated thickness for the entire length and width of the aquifer below the Robinson Seep Cut- off wall and monitoring well EVMW-1 (Section A -A') . This value is based on the water column measured in well EVMW-1S in August 2014. The saturated thickness of alluvium logged in that well was approximately 8 feet, and the well extends an additional 12.5 feet into the weathered and fractured bedrock beneath the alluvium. Consequently, the saturated thickness value accounts for flow in the alluvium as well as in the uppermost part of the weathered bedrock. A hydraulic conductivity (K) value of 1.15 feet per day (ft/day) was calculated from a slug test performed on the EVMW-1S well in August of 2014 (Tetra Tech, 2014). Recovery tests performed on DP1-BR and DP2-BR, two drive points installed to respective depths of 6 feet and 3 feet in the alluvium downstream from EVMW-1S, indicated hydraulic conductivity values of 0.19 and 0.08 ft/day for the alluvium, so the 1.15 ft/day values used for the flow calculations is very conservative (i.e., predicts higher flows). Much of the alluvial fill was removed from the alluvial valley represented by Section B -B' below EVS-1 for dam construction, and bedrock is exposed nearly continuously across the valley floor in parts of the middle and lower segments of the valley. Consequently, it is unlikely that alluvial groundwater flow is continuous from the upper part of the valley near EVS-1 to EPR across the Climax Molybdenum Company Page - 2 -