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The topography of the region is characterized by steep canyons cu[ by the North Fork and its <br />tributaries, with several remnant alluvial terraces above the valley of the North Fork. Proceeding <br />downstream below Somerset, Colorado, the canyon widens. At Paonia, Colorado, the canyon has <br />given way to a broad alluvial plain with interspersed remnant alluvial terraces. The coal [o be <br />mined is located in the Somerset Coal Field. The strata exposed in the Somerset Coal Field <br />consist of the Mancos Shale and the coal-bearing Mesaverde Formation of Upper Cretaceous <br />Age, and of the Ohio Creek Conglomerate, the Wasatch Formation and the Quartz Monzonite <br />Porphyry of Early Tertiary Age (Figure 2). Coal is mined from the Mesaverde Formation, a <br />2,500 foot thick sequence of sedimentary strata overlain by the Ohio Creek Conglomerate and <br />underlain by the Mancos Shale. The strata in the Sanborn Creek Mine permit area dip three to <br />five degrees north-northeast. <br />The Mesaverde Formation contains two coal bearing members. The Somerset Mine mined coal <br />from the B-2 seam of the lower coal bearing (Bowie) member of the Mesaverde Formation. The <br />Sanbom Creek and Sanborn Creek East additions to the mine will extract coal from the B and C <br />seams of this member. The Lower Coal member ranges from 260 to 350 feet thick in the <br />Somerset Coal Field and bears three minable coal seams. This member consists of interbedded <br />and lenticular sandstones, siltstones and coals, and is overlain by a massive sandstone 25 to 225 <br />feet thick which lies directly on the C seam and mazks the bottom of the upper coal member. <br />Three categories of potential aquifers exist in the general azea: alluvial deposits associated with <br />the North Fork of the Gunnison River and its tributaries, the Rollins Sandstone, and lenticular <br />discontinuous sandstones of the Upper Mesaverde Formation. <br />The largest alluvial aquifers are associated with the North Fork of [he Gunnison River. Smaller, <br />more isolated alluvial aquifers are associated with several tributaries of the North Fork. <br />The Rollins Sandstone is the only known sandstone with sufficient porosity and lateral extent to <br />be considered a regional bedrock aquifer. The only wells in the region which are completed in <br />this aquifer aze located near the Hawk's Nest Mine along the North Fork. <br />Localized perched bedrock aquifers exist in the discontinuous, lenticulaz, fine-grained sandstones <br />of the Upper Mesaverde Formation. The amount of ground water in these sandstones is <br />controlled by faulting and fracturing of the strata (secondary porosity) and the topography of the <br />recharge azea. No known wells are completed in the sandstones of the Upper Mesaverde <br />Formation above the mine workings. <br />The valley in which the towns of Paonia and Somerset are situated is semi- azid, with annual <br />precipitation averaging about 15 inches per year. Mean annual precipitation increases with <br />elevation, reaching over 40 inches per year near the summit of Mt. Owen. The May-September <br />precipitation is five inches for the lowlands and 13 inches for the mountain peaks. This indicates <br />that snowfall patterns play an important part in determining the hydrologic conditions of the area. <br />Temperature extremes at Paonia have ranged from -28NF in January to 100NF during July and <br />August. The average annual temperature is approximately 49NF. Snowfall averages 58 inches <br />per year. <br />8 <br />