1986-12-11_PERMIT FILE - C1981017

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-23- The hydraulic conductivity of the colluvial outwash deposits was measured by the permittee directly in the field. Field permeability tests were performed in both types of the colluvial outwash deposits, in silty clays and underlying clayey gravel . Results of field testing indicated that both types of the material �ave a low permeability in the range between 2.4 x 10- and 4.1 x 10' cm/sec. Due to the almost identical permeability of both layers, the silty clay stratum is not considered to be a confining bed, and the water bearing layer is under water table, or unconfined conditions. Results of field permeability testing are supported by thr 1g laboratory testing results which gave permeabilities ranging from 1 .3 x 1017 to 2.5 x 10'8 cm/sec. The total recharge potential of the water bearing layers in the outwash deposits was estimated to be 18.8 gallons per minute. This estimate was made using an average annual precipitation of 35 inches and a 5% rate of infiltration over the recharge area of 207.8 acres. The only ground water aquifers that have the potential to be impacted by mining (the aquifers discussed above) are those used by the applicant. Several mine production wells are completed into the alluvial aquifer of Coal Creek. See Table III-C-3 of the permit application for a description of all private water wells in the area. No wells are known to be completed in the coal-bearing strata of the Mesaverde Group in the permit and adjacent areas of the mines. The applicant proposes to mine both the Coal Basin coal seam ("B" seam) and the Dutch Creek coal seam ("V" seam) by underground mining methods (see Figure III-D-1 of the permit application for stratigraphic relationships) . The mine entries are located near the outcrop of the seams. No significant physical disturbance of adjacent strata has occurred as a result of the mine entries. In addition, the applicant proposes to construct a tunnel which will connect the 2 seams currently being mined. The tunnel entry will be located in the underlying Mancos shale formation and will continue up the stratigraphic section until it intersects with the Dutch Creek coal seam. It will cut through 3 sandstone units that are considered potential aquifers. They are, in stratigraphically ascending order, the Cozzette, Rollins, and Middle sandstones (see Figure III-0-1 of the permit application). The waste rock generated during the construction of this tunnel is being disposed of near Dutch Creek. The mines dewater the coal seams- and adjacent strata as part of normal operations. This dewatering process will cause water levels in some aquifers to be lowered. The decline in water levels should not extend more than a mile from the edge of the mine workings based on transmissivities of these strata as presented in the application. This effect should be temporary in nature. When mining is completed, the water levels in the aquifers should return to pre-mining elevations.