USGS Publications Warehouse

Abstract

Coal in the Ferron 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 Ferron is the sole source of supply for the town of Emery, but the aquifer is essentially untapped outside the Emery area. The Ferron Sandstone Member crops out along the eastern edge of Castle Valley and generally dips 2 ? to 10 ? to the northwest. Sandstones in the Ferron are enclosed between relatively impermeable shale in the Tununk and Blue Gate Members of the Mancos Shale. Along the outcrop, the Ferron ranges in thickness from about 80 feet in the northern part of Castle Valley to 850 feet in the southern part. The Ferron also generally thickens in the subsurface downdip from the outcrop. Records from wells and test holes indicate that the full thickness of the Ferron is saturated with water in most areas downdip from the outcrop area. Tests in the Emery area indicate that transmissivity of the Ferron sandstone aquifer ranges from about 200 to 700 feet squared per day where the Ferron 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 Ferron sandstone aquifer is confined and probably averages 5 x 10 -2 where it is unconfined. The largest source of recharge to the Ferron 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 Ferron 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 Ferron 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 Ferron would probably not yield more than 10 gallons per minute to individual wells. The concentration of dissolved solids in water from the Ferron sandstone aquifer in the Emery area increases eastward from the Paradise Valley-Joes Valley fault system toward the outcrop area of the Ferron, 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 Ferron 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 rag/L, and several test holes and gas wells have yielded water from the Ferron 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 rag/L, and observed suspended-sediment concentrations ranged from 111 to 27,000 mg/L. At the station on Christiansen Wash, observed dissolved-solids 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 Ferron 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 was not possible to verify the model with historic data for aquifer response to manmade discharges, predictions made with the model are considered to be semiquantitative. Discharge from the proposed surface mine is predicted to average about 0.3 cubic foot per second during the 15 years of mine operation. Dewatering of the mine would affect the potentiometric surfaces of all sections of the Ferron sandstone aquifer, but the greatest effects would be in the upper section. Drawdowns in the potentiometric surface of the upper section of the aquifer greater than 5 feet are predicted to extend about 2 miles from the surface mine after 15 years of operation. Mine dewatering would also induce downward leakage of water into the Ferron from shale in the Blue Gate Member, and this could cause a deterioration in water quality in the upper section of the aquifer in some areas. West of the surface mine, however, the quality of water in the upper section of the aquifer might improve as the amount of saline water leaking downward from the Blue Gate Member would be small in comparison to the amount of water that would move laterally through the aquifer from the west. Modeling results indicate that, except for Christiansen Wash, the dewatering of the proposed surface mine would not affect the base flow of streams. If water from the mine were discharged into Christiansen Wash, the base flow would increase accordingly. The dissolved-solids concentration of water in Christiansen Wash also would be increased, at least during some periods, if mine water were discharged into the stream. Laboratory experiments indicate that if only precipitation were allowed to infiltrate mine spoil, water in the spoil would be of better quality than most ground water in the mine area and about the same quality as water in Christiansen Wash. However, the management of the spoil to reduce surface-water infiltration and spoil placement so that pyritic material is mixed with calcareous material would minimize the deterioration of water quality in Christiansen Wash. Sediment loads of streams downstream from the mine would not increase significantly if reclaimed slopes were graded to the least possible angle, if revegetation were prompt so as to stabilize the stockpiled topsoil and backfilled overburden, if runoff were channeled from the disturbed mine area through sediment ponds, and if Christiansen Wash were permanently diverted around the mine area. The long-term sediment yield from the disturbed area could actually decrease if vegetative cover were improved from premining conditions and if sediment ponds were properly maintained.