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siltstones, mudstones and thin beds of limestone. This formation has a maximum thickness of 200 <br />feet and outcrops on rims of canyons west and northwest of the mine. The Burro Canyon formation <br />is difficult to differentiate from the Dakota in this area and is commonly considered <br />hydrogeologically similar. The Jurassic Morrison Formation below is composed of 250 feet to <br />600 feet of varicolored siltstones and mudstones with beds of limestone and sandstone. <br />The NHM primarily produces coal from the Lower Dakota coal seam and utilizes the Upper <br />Dakota coal seam when the seam is thick enough and of sufficient quality. The lower Dakota coal <br />seam ranges in thickness from 4.2 feet to 6.9 feet and is 99 feet above the top of the Burro Canyon <br />Formation. The coal dips 4.5° SW along a strike of N 45'W. <br />C. Surface Water <br />The San Miguel River has its source in the San Juan Mountains. These mountains are primarily <br />composed of Tertiary volcanic rocks. Irons et. al. (1965a) has shown these waters to be of a <br />calcium bicarbonate -sulfate type during high flow periods. These waters contain less bicarbonate <br />during low flows while the calcium and sulfate concentrations increase due to less dilution from <br />snowmelt runoff. The San Miguel River traverses the interior portions of a basin that is chiefly <br />underlain by Dakota Sandstone, the Morrison Formation and Mancos Shale. The Morrison <br />Formation and Mancos Shale have the greatest potential for influencing the river water chemistry. <br />The San Miguel River drains an area of 1,080 square miles. In addition, approximately 15,500 <br />acres of irrigated land lies between Placerville and Naturita, Colorado. It is estimated that 2.8 tons <br />of Total Dissolved Solids (TDS) per acre are contributed to the waters annually from this area. <br />This results in increased levels of magnesium, potassium, sulfate and chloride. Water quality <br />samples taken from the San Miguel River at Naturita, Colorado, have a weighted average of 316 <br />milligrams per liter (mg/1) TDS. Specific conductance varies between 318 and 730 millimhos <br />(mmhos). The mean sodium adsorption ratio (SAR) is about 0.7 at Naturita. Water curves indicate <br />that 90 percent of the time, flow in this river exceeds 60 cubic feet per second (cfs), while flows <br />exceed 1000 cfs about 10 percent of the time. The mean discharge of the river is 351 cfs. San <br />Miguel River waters are suitable for domestic usage except at low flow periods when sulfate levels <br />are high. <br />The Colorado Cooperative Ditch Company diverts as much as 145 cfs of water from the San <br />Miguel River approximately ten miles east of the town of Nucla. The main diversion ditch then <br />flows westerly passing just north of the town of Nucla and then flows northwestward passing north <br />of the NHM. Diversion ditches distribute water to mainly agricultural users in the area. Since the <br />completion of the irrigation system for this area in about 1910, these areas have been intensively <br />reworked for irrigated agriculture. Additional uses include domestic, municipal and stock pond <br />consumption. <br />The NHM is located just northwest of the town of Nucla in the mildly sloping upland area between <br />Tuttle Draw and Calamity Draw, in the northwest corner of First Park. The surface water system <br />is characterized by a "disjointed" tributary drainage system almost completely controlled by return <br />flow ditches of the local irrigation network. <br />The West Lateral irrigation ditch is part of the Colorado Cooperative Company's main ditch system <br />10 <br />