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Coal in the Perron Sandstone Member of the Mancos Shale of Cretaceous age has traditionally been mined by underground techniques in the Emery Coal Field in the southern end of Castle Valley in east-central Utah. However, approximately 99 million tons are recoverable by surface mining. Ground water in the Perron is the sole source of supply for the town of Emery, but the aquifer is essentially untapped outside the Emery area. The Perron Sandstone Member crops out along the eastern edge of Castle Valley and generally dips 2° to 10° to the northwest. Sandstones in the Perron are enclosed between relatively impermeable shale in the Tununk and Blue Gate Members of the Mancos Shale. Along the outcrop, the Perron ranges in thickness from about 80 feet in the northern part of Castle Valley to 850 feet in the southern part. The Perron also generally thickens in the subsurface downdip from the outcrop. Records from wells and test holes indicate that the full thickness of the Perron is saturated with water in most areas downdip from the outcrop area. Tests in the Emery area indicate that transmissivity of the Perron sandstone aquifer ranges from about 200 to 700 feet squared per day where the Perron is fully saturated. Aquifer transmissivity is greatest near the Paradise Valley-Joes Valley fault system where permeability has been increased by fracturing. Storage coefficient ranges from about 10~6 to 10~3 where the Perron sandstone aquifer is confined and probably averages 5 x 10~2 where it is unconfined. The largest source of recharge to the Perron sandstone aquifer in the Emery area is subsurface inflow from the Wasatch Plateau to the west (about 2.4 cubic feet per second during 1979), most of which moves laterally through the more permeable zone along the Paradise Valley-Joes Valley fault system. Little water is recharged to the aquifer by the 8 inches of normal annual precipitation on the outcrop area. Natural discharge from the aquifer is mainly leakage to alluvium along streams in the outcrop area and leakage to the enclosing shales in the Tununk and Blue Gate Members. Discharge from wells that tap the Perron in Castle Valley averaged about 0.3 cubic foot per second during 1979. Discharge from the underground Emery Mine averaged about 0.7 cubic foot per second during 1979 and was the largest manmade discharge from the aquifer. The largest quantities of water are available from the Perron sandstone aquifer within about 2 miles of the Paradise Valley-Joes Valley fault system in the Emery area. Most wells in this area naturally flow at the land surface at rates less than 100 gallons per minute, but yields could be increased by pumping. Wells that fully penetrate the aquifer in this area could be expected to produce 100 to 500 gallons per minute if pumped. In the northern two-thirds of Castle Valley the Perron would probably not yield more than 10 gallons per minute to individual wells. The concentration of dissolved solids in water from the Perron sandstone aquifer in the Emery area increases eastward from the Paradise Valley-Joes Valley fault system toward the outcrop area of the Perron, in the general direction of ground-water movement. Dissolved-solids concentrations also increase upward in the aquifer in areas downdip from the outcrop. In the Emery area, dissolved-solids concentrations in water from the Perron ranged from less than 500 to more than 8,000 mg/L (milligrams per liter) during 1979. Deterioration in water quality in the Emery area usually is due to increased concentrations of dissolved sodium and sulfate. In the northern two-thirds of Castle Valley, dissolved-solids concentrations usually exceed 3,000 mg/L, and several test holes and gas wells have yielded water from the Perron with chloride concentrations greater than 10,000 mg/L and dissolved-solids concentrations greater than 20,000 mg/L. Quitchupah Creek, near the underground Emery Mine, and Christiansen Wash, downstream from a proposed surface-coal mine in the Emery area, were gaged during the 1979 wateryear, and stream discharges averaged 6.7 and 2.8 cubic feet per second. There were large seasonal variations in water quality in both streams during the water year. Observed dissolved-solids concentrations at the gaging station on Quitchupah Creek ranged from 695 to 3,960 mg/L, and observed suspended-sediment concentrations ranged from 111 to 27,000 mg/L. At the station on Christiansen Wash, observed dissolvedsolids concentrations ranged from 582 to 4,470 mg/L and suspended-sediment concentrations ranged from 3 to 4,870 mg/L. A three-dimensional digital-computer model was used to simulate ground-water flow in the Perron sandstone aquifer in the Emery area. The model also was used to predict the effects of dewatering of a proposed surface mine on aquifer potentiometric surfaces and the base flow of streams. The computer model was calibrated with water-level data collected during 1979. Mainly because

Hydrology of the Ferron Sandstone aquifer and effects of proposed surface-coal mining in Castle Valley, Utah, with a section on stratigraphy and a section on leaching of overburden