Preliminary digital model of the Arikaree aquifer in the Sweetwater River basin, central Wyoming

Abstract

Potentially large supplies of ground water are available in the Sweetwater River basin from the Arikaree aquifer, which consists of the upper part of the White River Formation (conglomerate and sandstone) of Oligocene age, the Arikaree Formation (sandstone) of early Miocene age, and the Ogallala Formation (thin beds of limestone, sandstone, claystone, and tuff) of late Miocene age. The Arikaree aquifer underlies an area of about 1,500 square miles that is approximately bisected by the Sweetwater River. The aquifer is divided into the western, northeastern, and southeastern areas in this report. Gain and loss studies during November 1975 indicate that the river gains about 17 cubic feet per second between the gaging station near Sweetwater Station and the gaging station near Alcova. The gain in streamflow in this reach is assumed to be the total discharge from the Arikaree aquifer in the western area. The recharge from precipitation to the aquifer is assumed to be about 2 percent of the estimated mean annual precipitation of 8.5 inches. Along the southern boundary, the Arikaree aquifer is assumed to receive additional recharge from greater precipitation near the mountains and from creeks flowing off the mountains into the study area. A preliminary digital model was developed for the Arikaree aquifer using a small amount of poorly distributed data, an estimated distribution of recharge, and a conceptual model of the Arikaree aquifer flow system. Calibration of the model was based on the reproduction of the potentiometric surface and the base flow of the Sweetwater River in November 1975. Calculated steady-state hydraulic heads were within 50 feet of the observed heads in about 98 percent of the nodes. The calculated leakage from the Arikaree aquifer to the Sweetwater River in the western area was within about 12 percent of the leakage determined by gain and loss studies. Observed data are not available to estimate leakage to the river from the northeastern and southeastern areas. In order to develop a comprehensive digital model that would respond to hydraulic stress in nearly the same manner as the actual aquifer flow system, measured responses of the aquifer to stress are needed. Also needed are additional data for aquifer characteristics, recharge to the aquifer, and stream-aquifer relationships.