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-33- The alluvium of the North Fork of the Gunnison River consists of Quaternary Aged 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 headwater 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 drops to about 35 feet. Description of the Ground water Regime Three 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 and lenticular sandstones and the Rollins Sandstone of the Mesa Verde Formation. The Trost significant occurrence of ground water in the region is associated with the alluvium of the North Fork of the Gunnison River. Sicmificant thicknesses of alluvial sand and gravel in excess of 30 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 proposed Bear No. 3 mine yielded values of 806.5 g/d/ft. for transmissivity and 0.2 for storativity. Three water quality analyses were conducted by Bear on the three wells completed in the North Fork alluvium. Two wells exceeded drinking water standards for total dissolved solids, sodium and sulfate, but the well located closest to the North Fork had acceptable levels of these constituents. This is due to dilution by North Fork river water. 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. 4tao 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 from the alluvium of Stevens Gulch. Pumping tests conducted by Colorado Westmoreland at the Orchard Valley mine indicate that the Stevens Gulch alluvium could support a pumping rate of 28 gpm. No pumping test was conducted by Western Associated Coal at the Blue Ribbon mine on the alluvium of Hubbard Creek. Terror Creek may have alluvial aquifers that could be developed as water supplies. However, no detailed analysis of this alluvium has been conducted. The Terror Creek alluvium may be used as a source of water at the Morrell Coco Camp to the north of the Orchard Valley mine. Water quality analysis for the Stevens Gulch well water indicates good quality water, with none of th_e parameters exceeding recommended drinking water standards. The only treatment necessary is chlorination, as required by the Colorado Department of Health for public drinking water supplies, The water quality for the Hubbard Creek well water is fair to poor and contains total dissolved solids and sodium levels which exceed secondary water quality standards for drinking water.