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-z7- The alluvium of the North Fork of the Gunnison River consists of Quaternary Age deposits of mixed coarse sand, cobbles and boulders. These coarse sediments are composed primarily of igneous and metamorphic rock types, and have their source area in the headwaters and upper reaches of the North Fork. 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 50 to 70 feet thick. About a mile below the town of Somerset, Colorado, the width of alluvium increases while the thickness of alluvium decreases to about 35 feet. Description of the Ground Water Regime Several categories of potential aquifers occur in the Somerset Coal Field. These are: 1) the alluvial and terrace deposits associated with the North Fork of the Gunnison River; 2) the localized, shallow alluvium along creeks tributary to the North Fork; 3) the discontinuous, lenticular and laminar sandstones of the Mesa Verde Formation; 4) the Rollins sandstone; 5) the coal seam; and 6) shallow colluvial surficial deposits. The most significant occurrence of ground water in the region is associated with the alluvium of the North Fork of the Gunnison River (Figure 4). Significant thicknesses of alluvial sand and gravel between 30 to 80 feet exist along the North Fork. Numerous wells are developed in the alluvium with an average yield of 17.4 gpm. A pumping test conducted by Bear Coal Company on an alluvial well near the site of the Bear No. 3 Mine yielded a value of 806.5 g/d/ft for transmissivity and 0.2 for storativity. Three water quality analyses were conducted for the Bear No. 3 Mine on three wells completed in the North Fork alluvium. Two wells exceeded drinking water standards for total dissolved solids, sodium and sulfate. The well located closest to the North 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 for areas in the Minnesota Creek Drainage, the alluvium along the lower reaches of tributaries to the North Fork is predominantly thin and confined to discontinuous narrow bands along the stream courses. The deeply incised channels of these tributaries restrict the width of the alluvium, while the stream gradient and the presence of resistant sandstone in the stream channels limit the thickness and downstream extent of the alluvium. Two mines in the region draw water from the alluvium of tributaries to the North Fork, the Blue Ribbon Mine draws from the alluvium of Hubbard Creek and the Orchard Valley Mine draws water from the alluvium of Stevens Gulch. Pumping tests conducted by Colorado Westmoreland Incorporated at the Orchard Valley Mine indicate that the Stevens Gulch alluvium could support a pumping rate of 28 gpm. The Bear Mine and Terror Creek Loadout use some water drawn from wells completed in the alluvium along the North Fork of the Gunnison River. The Bear Mine pumps on the average of 2 acre feet a year for use in the mine. The water quality of this well would approximate the water quality of the monitoring wells that surround it.