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within 100 feet of the base of the Dakota. The lower Cretaceous unit in the area, the Burro <br />Canyon Formation, is separated from the Dakota by an unconformity. The Burro Canyon is <br />comprised of white, gray and light brown sandstones and conglomerates interbedded with green <br />and purplish shales, siltstones, mudstones and thin beds of limestone. This formation has a <br />maximum thickness of 200 feet and outcrops on rims of canyons west and northwest of the mine. <br />This formation is difficult to differentiate from the Dakota in this area and is commonly <br />considered hydrogeologically similar. The Jurassic Morrison Formation below is composed of <br />250 feet to 600 feet of varicolored siltstones and mudstones with beds of limestone and <br />sandstone. <br />The New Horizon Mine 1 produced coal from both the upper and lower Dakota coal seams. The <br />upper Dakota coal seam ranged in thickness from 1.6 feet to 3.3 feet and was separated from the <br />lower Dakota seam by 7 to 11 feet of interbedded sandstone and shale. The lower Dakota seam <br />n to 7.9 n r feet. iThe b iase of r . the > lower Dakota i coal is ZSnn J r i <br />varies in 111i ckness from 5.9 feet above <br />the top of the Burro Canyon Formation. The coal dips 1 ° to 2° southwest and strikes N 25°W to <br />N 45° W. <br />The New Horizon Mine 2 primarily produces coal from the Lower Dakota coal seam and will <br />utilize the Upper Dakota coal seam when the seam is thick enough and of sufficient quality. The <br />lower Dakota coal seam at New Horizon Mine 2 ranges in thickness from 4.2 feet to 6.9 feet and <br />is 99 feet above the top of the Burro Canyon Formation. The coal dips 4.5° SW along a strike of <br />N 45° W. <br />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. loins et. al. (1965a) has shown these waters to be of a <br />calcium bicarbonate-sulfate type during high flow periods. These waters contain less <br />bicarbonate during low flows while the calcium and sulfate concentrations increase due to less <br />dilution from snowmelt runoff. The San Miguel River traverses the interior portions of a basin <br />that is chiefly underlain by Dakota Sandstones, the Morrison Formation and Mancos Shales. The <br />Morrison Formation and Mancos Shale have the greatest potential for influencing the river water <br />chemistry. The San Miguel River drains an area of 1,080 square miles. In addition, <br />approximately 15,500 acres of irrigated land lies between Placerville and Naturita, Colorado. It <br />is estimated that 2.8 tons of Total Dissolved Solids (TDS) per acre are contributed to the waters <br />annually from this area. This results in increased levels of magnesium, potassium, sulfate and <br />chloride. Water quality samples taken from the San Miguel River at Naturita, Colorado, have a <br />weighted average of 316 milligrams per liter (mg/1) TDS. Specific conductance varies between <br />318 and 730 millimhos (mmhos). The mean sodium adsorption ratio (SAR) is about 0.7 at <br />Naturita. Water curves indicate that, 90 percent of the time flows in this river exceeds 60 cubic <br />feet per second (cfs), while flows exceed 1000 cfs about 10 percent of the time. The mean <br />discharge of the river is 351 cfs. San Miguel River waters are suitable for domestic usage except <br />at low flow periods when sulfate levels 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 />9