2010-12-17_REVISION - C1981019 (153)

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68 Groundwater The groundwater quality in Straight Creek at both locations (MS -04 -01 and MS- 04 -02) is a magnesium — sulfate /bicarbonate with 10 percent or less sodium and chloride. The inorganic chemistry of the water was very similar for all samples, with a pH of 7.0 to 7.3. TDS ranged from of about 1,440 to 1,580 mg /l. Samples from the upstream sampling location (MS- 04 -01) have iron concentrations of approximately 10 mg /I and manganese concentrations of approximately 1.4 mg /I. These concentrations exceed their respective MCLs. During the spring runoff, the concentrations of both of these metals drops below their respective reporting limits. Downstream at MS- 04 -02, the concentrations are greatly reduced, with iron below the reporting limit and manganese just slightly above the secondary MCL of 0.05 mg /I. The primary controls on the valley fill groundwater chemistry appear to be the surface water chemistry and distance downstream. In the upstream reaches, surface water recharges the valley fill. The chemistry of the surface water varies over time and with location. Surface water that infiltrates into the valley fill reacts with the sediments to produce a distinctive chemistry. As the degree of mixing between surface water and groundwater increases with distance downstream, their chemistries become more similar. 5.3 Conceptual hydrogeologic model C The conceptual hydrogeological model of the site incorporates the available information on the hydrogeological units, groundwater recharge /discharge areas, hydraulic gradients, geochemistry, water level elevations, and hydraulic characteristics of each hydrogeologic unit. The conceptual model provides the basis for describing the overall groundwater system in the Collom area and for conducting groundwater modeling, mine dewatering and environmental evaluations described later in this report. 5.3.1 Hydrogeologic units in the Collom study area The Williams Fork includes numerous bedded sedimentary layers as described above and shown on Figures 2.1 and 5.2 through 5.5. For purposes of evaluating the site hydrogeology, the individual layers have been grouped into the following hydrogeological units from top to bottom: Upper sequence — This includes the sequence above the F sandstone. This unit contains perched groundwater zones and is locally discontinuous due to stream dissection. F sandstone — This includes the sandstone unit and minor coal seams above the Fab seam. It is saturated in the Phase 1 box cut area and will require dewatering. • FIG sequence — This unit extends from the Fab seam down to the Gc seam. The entire sequence is saturated in the Phase 1 box cut area. The sequence is confined, which leads to increases in the head above the sequence from south to north across the Collom Mine. The Gb seam is the lowermost coal seam targeted for mining at Collom. 2572 -R2 Colowyo Coal Company Water Management Consultants