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-LU- The alluviwn of the North Furk of the Gunnison River consists of iluaternary Age deposits of mixed coarse sand, cobbles and boulders. These coarse sedirnents are composed primarily of igneous and metamorphic rock types, and have their source area in the headwaters and upper reaches of the North Furk. This coarse alluvium is capped by finer sands and silts. The North Fork alluvium in the area of the Hawk's Nest, Bear, Mt. Gunnison and Somerset Mines is fairly narrow in width and between 5U to 7U feet thick. Aoout a mile below the town of Somerset, Colorado, the width of alluvium increases while the thickness of alluvium decreases co aoout .is feet. Uescriotion of the Ground Water Reoime Four categories of potential aquifers occur in the Somerset Coal Field. These are: the alluvial and terrace deposits associated with the North Fork of the Gunnison River; the localized, shallow alluvium along creeks tributary to the North Fork; the discontinuous, lenticular and laminar sandstones of the Mesaverde Formation; and the Rollins sandstone. The most significant occurrence of ground water in the region is associated witn the alluvium of the North Fork of the Gunnison River (Figure 4). Significant thicknesses of alluvial sand and gravel between 3U to aD feet exist along the North Fork. Numerous wells are developed in the alluvium witn an average yield of 17.4 ypm. N pumping test conducted by dear Coal Loinpany on an alluvial well near the site of the proposed Bear No. 3 Mine yielded a value of 806.5 g/d/ft for transmissivity and U.2 for sturativity. Three water quality analyses were conducted for the Bear No. 3 P1ine on three wells completed in the North Fork alluvium. Two wells exceeded drinking water standards fur total dissolved solids, sodium and sulfate. The well located closest to the IVOrth Fork (the Bear No. 3 - office well) showed the lowest levels of these constituents. This is due to the dilution of alluvial ground water by North Fork River water at this well. Except fur areas in the Minnesota Creek Drainage, the alluvium along the lower reaches of tributaries to the North Fork is predo~~rinantly thin and confined to discontinuous narrow bands along the stream courses. The deeply incised channels of these trioutaries restrict the width of the alluvium, while the streaar gradient and the presence of resistant sandstone units in the stream channels limit the thickness and downstream extent of tiie alluvium.