2010-12-17_REVISION - C1981019 (140)

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Hydrological system and available data DRAFT 7 2.3 Groundwater gradients and pressures In the vicinity of the mine site, water level elevations decrease from 7,800 to about 7,000 ft in the F sandstone and from about 7,600 to 6,900 ft in the F/G sequence. In the F sandstone, the direction of groundwater flow is north-northwest. In the F/G sequence, the direction of groundwater flow is north through Phase 1 and northeast across Phase 2. The direction of groundwater flow in the H and I sandstone is to the northeast. Horizontal groundwater gradients in the F sandstone and F/G sequence are approximately 0.3 and 0.14 respectively in the southern half of the site. Groundwater gradients in the central area flatten in the F sandstone and F/G sequence compared with gradients to the south. Gradients in the F sandstone and the F/G sequence are both about 0.04. Gradients in the H sandstone range from 0.02 to 0.08 to north. The limited water level data for the I sandstone show a gradient of 0.03 to the north. In the southern portion of the Collom area, groundwater gradients in the individual units reflect flow in perched zones and are not representative of the area-wide gradients. In the central portion of the pit footprint, these units intersect the area-wide groundwater table and are no longer perched. Thus, the central portion of the pit footprint represents a transition from localized perched groundwater flow condition to area-wide flow conditions. Vertical groundwater gradients increase with distance down-dip to the north, starting at approximately the location where the horizontal gradients start to flatten. Build-up of pressure appears to be due to the numerous mudstone and siltstone units between the coal seams and coarser sand interbeds. The degree of hydraulic connection between a coal seam and the sandstone, mudstone, or siltstone units above and below it is probably considerably variable. At the north edge of the proposed pit, there is 200 to 400 ft of pressure head in the F/G sequence and the H sandstone. Pressure head is expected to continue to increase down dip. Groundwater discharges out of the pressurized units laterally into the stream drainages or vertically as inter-unit leakage. 2.4 Water balance An Collom area water balance was performed as a cross-check for the numerical model. The volume of water flowing through the model area was estimated using a mass balance approach. Water balance information is provided in Table 2.2. The water balance was developed by comparing the inflow of water into each drainage basin to the outflow. The methodology for the calculation is described below. All the drainages extend beyond the numerical model boundary, either upstream (south of the modeled area) and/or downstream of the contact with the Trout Creek sandstone to a gauging station. The boundaries of each basin were defined in a Geographic Information System (GIS) using topographic data. The areas of each drainage basin above, within and below the model area were measured using a GIS (Table 2.2). The groundwater outflow from each drainage basin was estimated using stream flow data and estimates of groundwater flow out of each basin. 2572-R3 Colowyo Coal Company, L.P. Water Management Consultants