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1. Description of the Hydrologic Environment <br /> a. Regional Geology <br /> For information regarding regional geology, see Section A, <br /> Description of the Environment, Geology in this Findings <br /> Document. <br /> b. Ground Water <br /> Six categories of potential aquifers occur in the Somerset <br /> Coal Field. These are; I) the alluvial and terrace <br /> deposits associated with the North Fork of the Gunnison <br /> River; 2) the localized, shallow alluvium along creeks <br /> tributary to the North Fork; 3) the discontinuous, <br /> lenticular and laminar sandstones of the Mesaverde <br /> Formation; 4) the Rollins Sandstone; 5) the coal seam; 6) <br /> shallow colluvial surficial deposits; and 7) fracture zones <br /> in bedrock. <br /> The most continuous sandstone aquifers of any significance <br /> in the region are the Rollins Sandstone and the massive <br /> sandstone separating the Upper and Lower Coal members of <br /> the Mesaverde Formation. However, the latter massive <br /> sandstone produces very little water, as evidenced by the <br /> dryness of the Bear Mine and the lack of springs associated <br /> with this unit. Also, the Rollins Sandstone has been found <br /> to have insufficient yield to sustain ground water supply <br /> wells (Orchard Valley Mine MRP) . This unit lies 270 feet <br /> below the F coal seam which has been mined. In the Jumbo <br /> Mountain area, the Rollins Sandstone appears to be <br /> hydraulically isolated. To the south and west, the Rollins <br /> Sandstone outcrops in the form of cliffs; and to the north <br /> and northeast, it discharges into the North Fork of the <br /> Gunnison River. In the Jumbo Mountain area, the average <br /> interburden thickness between the B-seam and the Rollins <br /> Sandstone is 150 feet. <br /> The most significant occurrence of ground water in the <br /> region is associated with the alluvium of the North Fork of <br /> the Gunnison River. Significant thicknesses of alluvial <br /> sand and gravel between 30 and 80 feet exist along the <br /> North Fork. Numerous wells are developed in the alluvium <br /> with an average yield of 17.4 gpm. A pumping test <br /> conducted by Bear Coal Company on an alluvial well near the <br /> site of the Bear No. 3 Mine yielded a value of 806.5 <br /> gallons/day/foot for transmissivity. <br /> Three water quality analyses were conducted for the Bear <br /> No. 3 Mine on three wells completed in the North Fork <br /> alluvium. Two wells exceeded drinking water standards for <br /> total dissolved solids, sodium and sulfate. The well <br /> located closest to the North Fork (the Bear No. 3 - office <br /> well) showed the lowest levels of these constituents. This <br /> is due to the dilution of alluvial ground water by North <br /> Fork River water at this well . <br /> Except for areas in the Minnesota Creek Drainage, the <br /> alluvium along the lower reaches of tributaries to the <br /> North Fork is predominantly thin and confined to <br /> discontinuous narrow bands along the stream courses. The <br /> deeply incised channels of these tributaries restrict the <br /> width of the alluvium, while the stream gradient and the <br /> presence of resistant sandstone in the stream channels <br /> limit the thickness and downstream extent of the alluvium. <br /> 19 <br />