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Dolores River basin. The mine is located two to four miles northeast of the San Miguel River. Gently rolling terrain traverses the azea and the mine occupies a predominantly southwestern exposure. The mine is located in the Nucla-Naturita coal field. The coal is found in late Cretaceous sediments. The youngest Cretaceous rocks in the azea consist of Mancos shale, a dark gray marine shale, present only on isolated hilltops south of the town of Nucla. 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 dazk gray shale and coal. There aze two coal seams within 100 feet of the base of the Dakota. The lower Cretaceous unit in the azea, 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. This formation is difficult to differentiate from the Dakota in this azea and is commonly considered hydrogeologically similaz. The Jurassic Momson Formation is composed of 250 feet to 600 feet of varicolored siltstones and mudstones with beds of limestone and sandstone. The New Horizon Mine 1 produced coal from both the upper and lower Dakota coal seams. The upper Dakota coal seam ranges in thickness from 1.6 feet to 3.3 feet and is separated from the lower Dakota seam by 7 to 11 feet of interbedded sandstone and shale. The lower Dakota seam vazies in thickness from 5.9 feet to 7.9 feet. The base of the lower Dakota coal is 83 feet above the top of the Burro Canyon Formation. The coal dips 1° to 2° southwest and strikes N 25°W to N 45° W. The New Horizon Mine 2 primarily produces coal from the Lower Dakota coal seam and will utilize the Upper Dakota coal seam when the seam is thick enough and of sufficient quality. The lower Dakota coal seam at New Horizon Mine 2 ranges in thickness from 4.2 feet to 6.9 feet and is 99 feet above the top of the Burro Canyon Formation. The coa] dips 4.5° SW along a strike of N 45° W. Surface Water The San Miguel River has its source in the San Juan Mountains. These mountains aze primarily composed of Tertiary volcanic rocks. Iorns et. al. (1965a) has shown these waters to be of a calcium bicazbonate-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 Dako[a Sandstones, the Morrison Formation and Mancos Shales. The Morrison Formation and Mancos Shale have the greatest potential for influencing the river water chemistry. The San Miguel River drains an azea of 1,080 squaze 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 azea. 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 vanes 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 flows in this river exceeds 60 cubic feet per second (cfs), while flows exceed 1000 cfs about 10 percent of the