2010-12-17_REVISION - C1981019 (153)

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Executive summary iii South of the proposed mine, water levels are perched in the units down to the F/G coal sequence. Groundwater conditions in the southern part of the proposed Collom Mine are expected to be similar to those observed at the Colowyo Mine. The separate water tables merge and the heads starts to rise above the water bearing units towards the center of the mine. At the north edge of the proposed pit, the heads in the F/G sequence down to the H sandstone are 200 to 400 ft above their respective water bearing units. Groundwater gradients in the perched zones range from 0.3 to 0.12, but decrease to approximately 0.03 in the northern part of the proposed mine. Groundwater discharge appears to occur laterally into the valley fill within the stream drainages where the streams cut through the synclinal axis. The vertical extent of the groundwater system relevant to the Collom Mine appears to be defined by the KM, a 4 -ft thick clay layer with very low vertical permeability. The KM acts as an aquiclude, effectively separating the units containing the Collom deposit from the lower units of the Williams Fork Formation and Trout Creek Sandstone. Groundwater quality in the Williams Fork Formation is characterized by slightly alkaline pHs and TDS ranging from 500 to 1,000 mg /I. Groundwater quality is generally good, with only occasional and very slight exceedances of drinking water criteria for iron and manganese. Hydraulic testing of the valley fill wells show a wide range of hydraulic conductivities with C a geometric mean of 3.3 ft/day. In the upstream reaches, surface water infiltrates into the valley fill during the spring runoff period. Some of the valley fill groundwater may recharge the bedrock units while the balance flows into the lower valley fill. Groundwater stored in the valley fill is a source of stream baseflow after the spring runoff, as well as the source of springs. In the downstream reaches, the bedrock units appear to discharge to the valley fill, particularly in the vicinity of the north limb of the syncline. Discharge to the valley fill augments base flow to the stream channels. Dewatering of the mine pit as it advances below the water table and depressurization of the surrounding slopes were evaluated. Dewatering plans were developed for two different mine plans. The original mine plan (Option 1) involves a box cut on the north side of the site and mining from north to south. Option 2 starts in the perched units on the south side and progresses downdip to the south. Initial slope stability analyses for Option 1 indicated that excess pore water pressures in the pit slopes would result in unstable slope conditions. WMC conducted numerical 2D groundwater modeling to evaluate potential slope depressurization measures that could be implemented to improve slope stability. These measures include vertical wells, vertical drains, horizontal drains, and natural drainage to in -pit sumps. The model considered potential well spacing, dewatering rates, and the timeframe necessary to achieve depressurization. Results of the model showed that wells installed to depths of 100 to 150 ft below the planned bottom of the mine, at 1,000 -ft spacing intervals, could effectively depressurize the pit slopes. The preliminary plans also describe well distribution and construction, pumping schedules, pumping rates, and mine water management for both mine plan options. A preliminary plan was also prepared to address spoils drainage for Option 2. 2572 -R2 Colowyo Coal Company Water Management Consultants