2022-10-10_GENERAL DOCUMENTS - C1981008

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sediments. The youngest Cretaceous rocks in the area are Mancos shale, a dark gray marine shale, comprising the isolated hilltops south of the town of Nucla. The Mancos shale overlies the Dakota sandstone. Two hundred feet of Dakota sandstone outcrops extensively along the axis of the Nucla syncline. This formation consists of a yellowish-gray, fissile sandstone and conglomeratic sandstone interbedded with dark gray shale and coal. There are two coal seams within 100 feet of the base of the Dakota. The lower Cretaceous unit in the area, the Burro Canyon Formation, is separated from the Dakota by an unconformity. The Burro Canyon is comprised of white, gray and light brown sandstones and conglomerates interbedded with green and purplish shales, siltstones, mudstones and thin beds of limestone. This formation has a maximum thickness of 200 feet and outcrops on rims of canyons west and northwest of the mine. The Burro Canyon formation is difficult to differentiate from the Dakota in this area and is commonly considered hydrogeologically similar. The Jurassic Morrison Formation below is composed of 250 feet to 600 feet of varicolored siltstones and mudstones with beds of limestone and sandstone. The New Horizon Mine primarily produced coal from the Lower Dakota coal seam and utilizes the Upper Dakota coal seam when the seam is thick enough and of sufficient quality. The lower Dakota coal seam ranges in thickness from 4.2 feet to 6.9 feet and is 99 feet above the top of the Burro Canyon Formation. The coal dips 4.50 SW along a strike of N 450 W. C Surface Water The San Miguel River has its source in the San Juan Mountains. These mountains are primarily composed of Tertiary volcanic rocks. Irons et. al. (1965a) has shown these waters to be of a calcium bicarbonate-sulfate type during high flow periods. These waters contain less bicarbonate during low flows while the calcium and sulfate concentrations increase due to less dilution from snowmelt runoff. The San Miguel River traverses the interior portions of a basin that is chiefly underlain by Dakota Sandstone, the Morrison Formation and Mancos Shale. The Morrison Formation and Mancos Shale have the greatest potential for influencing the river water chemistry. The San Miguel River drains an area of 1,080 square miles. In addition, approximately 15,500 acres of irrigated land lies between Placerville and Naturita, Colorado. It is estimated that 2.8 tons of Total Dissolved Solids (TDS) per acre are contributed to the waters annually from this area. This results in increased levels of magnesium, potassium, sulfate and chloride. Water quality samples taken from the San Miguel River at Naturita, Colorado, have a weighted average of 316 milligrams per liter (mg/1) TDS. Specific conductance varies between 318 and 730 millimhos (mmhos). The mean sodium adsorption ratio (SAR)is about 0.7 at Naturita. Water curves indicate that 90 percent of the time, flow in this river exceeds 60 cubic feet per second (cfs), while flows exceed 1000 cfs about 10 percent of the time. The mean discharge of the river is 351 cfs. San Miguel River waters are suitable for domestic usage except at low flow periods when sulfate levels are high. The Colorado Cooperative Ditch Company diverts as much as 145 cfs of water from the San Miguel River approximately ten miles east of the town of Nucla. The main diversion ditch then flows westerly passing just north of the town of Nucla and then flows northwestward passing north of the New Horizon Mine. Diversion ditches distribute water to mainly agricultural users in the area. Since the completion of the irrigation system for this area in about 1910, these areas have been intensively reworked for irrigated agriculture. Additional uses include domestic, municipal and stock pond consumption. 4