2008-03-03_PERMIT FILE - C2006085 (16)

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Ms. Angela Bellantoni Apri125, 2007 Page 2 • Vermejo Formation dips to the west and northwest at the mine location and the depth to the Ocean Wave coal seam ranges from approximately 300 to 800 feet beneath the ground surface as illustrated in cross sections A - A' and B - B', presented in Appendices 1 and 2, respectively. We note that during the drilling of the on-site monitoring wells and drilling of a recently constructed water ,supply well (permit No. 222887-A), there was very little ground water encountered in the Vermejo formation including the Ocean Wave coal seam. The Vermejo and overlying units have low permeabilities based on information from the on-site core hole and monitoring well data as we as we records and previous studies in the area. As presented in the lithologic logs, impermeable mudstone is present both above and below the Ocean Wave coal seam. The mudstone above the coal ranges in thickness from 12 to 20 feet thick, and although sandstone is present in intervals above the mudstone, it has a very low hydraulic conductivity as indicated from borehole and pumping test data. The boreholes did not penetrate to sufficient depths to determine the thickness of mudstone below the coal, but mudstone was logged beneath the coal in the boreholes. The hydraulic conductivity of the material beneath the coal is probably very low. The impermeable mudstone acts as an aquitard and acts to restrict the vertical movement of ground water between the coal and the overlying and underlying sediments. Accordingly, the primary direction of ground water flow is horizontal. The ground water flow direction is generally from south to north towards the Arkansas River, as presented in Figure 6 of our March, 2006 hydrology report. • The low permeability of the formation material inhibits the flow of ground water in the vicinity of the mine. Accordingly, water level changes resulting from mine dewatering will be isolated to the areas very close to the mine and are not expected to propagate to nearby water supply wells. We note that in our March, 2006 hydrology report, we presented a range of possible inflow values to the mine based on a range of hydraulic conductivity values. Based on the work completed since that initial report, there is evidence that the hydraulic conductivity of the Vermejo formation in the vicinity of the mine, and the corresponding mine inflow, is in the mid to lower range of values presented in Table 4 of that report. Well Inventory and Water Level Drawdown Analyses Although there is not an aquifer present in the Vermejo Formation, low yielding water supply wells have been developed to provide local water supplies, primarily for residential and limited stock and commercial uses. In addition to wells completed in the Vermejo Formation, there are higher capacity wells completed in the alluvium of the Arkansas River and the alluvium of Chandler Creek. The wells completed in the alluvial material are in hydraulic connection with surface water systems including Chandler Creek and the Arkansas River. Conditions in these surface water systems will control water levels in the alluvial material. Water level changes in the Vermejo formation, if they occur, will not alter water levels in the alluvial aquifer systems.