Laserfiche WebLink
Eagle Park Reservoir Chemical Loading and Mixing Analysis <br /> cut-off wall, collection pond, and pump station, is designed to contain and collect seepage from <br /> the downstream toe of Robinson Dam and convey this water back to Robinson Lake. <br /> Based on bedrock topography and estimated aquifer thickness,the general flowpath of shallow, <br /> alluvial groundwater to EPR is from the southeast. Bedrock mapping information in the vicinity <br /> of the Robinson Lake Dam and field investigations conducted in 2013 and 2014, indicate that <br /> most groundwater follows two alluvial drainage channels to the east and west of a bedrock ridge. <br /> Kumar and Associates (1993) indicated that alluvial valleys in the Eagle Valley watershed are <br /> comprised of silts, sands and gravels. <br /> The westernmost, alluvial flowpath, identified as Section A-A' (Figure 1), follows a small alluvial <br /> drainage channel northwest towards EPR, from Robinson Lake Dam to below monitoring well <br /> EVMW-1S. The second alluvial flowpath, identified as Section B-B' (Figure 1), represents the <br /> eastern side of the valley below Robinson Lake. The upper reach of this eastern area leads <br /> towards surface water monitoring location EVS-1, located at the base of the alluvium at the <br /> southern edge of a former alluvial material borrow area to the east of EPR. POC Monitoring Well <br /> EV-MW-004 is located in the area where these two flowpaths enter EPR. <br /> Also considered is groundwater that may occur in the deeper,fractured bedrock system beneath <br /> the alluvial valleys just described. For purposes of the analysis presented in this document, the <br /> width of the flowpath extends approximately from point A to point B' (Figure 1). <br /> Groundwater Inflows <br /> Tables 1 and 2, below, present hydrogeologic parameter inputs for the modeled groundwater <br /> flow. All inputs are considered to be conservative. Flow gradients were established by evaluating <br /> surface topography,and the saturated width and length for each flowpath conservatively assume <br /> that the alluvium is contiguous across the entire width and length. Despite the observed bedrock <br /> outcrops indicating that alluvium is very thin, a conservative value of 20.3 feet was used for the <br /> saturated thickness for the entire length and width of the aquifer below the Robinson Seep Cut- <br /> off wall and monitoring well EVMW-1 (Section A-A') . This value is based on the water column <br /> measured in well EVMW-1S in August 2014. The saturated thickness of alluvium logged in that <br /> well was approximately 8 feet, and the well extends an additional 12.5 feet into the weathered <br /> and fractured bedrock beneath the alluvium. Consequently, the saturated thickness value <br /> accounts for flow in the alluvium as well as in the uppermost part of the weathered bedrock. A <br /> hydraulic conductivity (K) value of 1.15 feet per day (ft/day) was calculated from a slug test <br /> performed on the EVMW-1S well in August of 2014(Tetra Tech,2014). Recovery tests performed <br /> on DP1-BR and D132-BR,two drive points installed to respective depths of 6 feet and 3 feet in the <br /> alluvium downstream from EVMW-1S, indicated hydraulic conductivity values of 0.19 and 0.08 <br /> ft/day for the alluvium, so the 1.15 ft/day values used for the flow calculations is very <br /> conservative (i.e., predicts higher flows). <br /> Much of the alluvial fill was removed from the alluvial valley represented by Section B-B' below <br /> EVS-1 for dam construction, and bedrock is exposed nearly continuously across the valley floor <br /> in parts of the middle and lower segments of the valley. Consequently, it is unlikely that alluvial <br /> groundwater flow is continuous from the upper part of the valley near EVS-1 to EPR across the <br /> Climax Molybdenum Company Page-2- <br />