The water table-______________________________________________ Configuration. ____________________________________________ Fluctuation. ______________________________________________ Recharge.____________________________________________________ Discharge. ___________________________________________________ Future development.__________________________________________ Chemical quality of ground water and of surface water, by John R. Tilstra_ _ Chemical characteristics of water._______________________________ Classification of water use._____________________________________ Domestic use_____________________________________________ Industrial use.____________________________________________ Irrigation_ _ _ ____________________________________________ Quality of ground water________________________________________ Quality of surface water________________________________________ Records of selected wells and springs_______________________________ References cited___________________________________________________ nr

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Ground water is being extensively developed for domestic, agricultural, and industrial use in a 2,320-square-mile area in Laramie County bounded approximately by Horse Creek on the north, Nebraska on the east, Colorado on the south, and pre-Tertiary outcrops on the west. Currently (1977) about 47,300 acres of land are irrigated with ground water. Groundwater levels are declining in some areas as much as 4 feet per year. The investigation was made to provide State water administrators with data on water-level changes resulting from present (1977) groundwater withdrawals and to provide a means of predicting the future effect of groundwater development. A digital model was developed of the hydrologic system in the post-Cretaceous rocks. The ability of the model to simulate the hydrologic system was determined by comparing the water-level changes measured at 37 observation wells located in areas of irrigation pumping with the water-level changes calculated by the model for 1971-77. Comparison of the measured and calculated changes showed agreement with a root-mean-square deviation of ±3.6 feet with 8 feet as the maximum deviation. It is concluded that the model adequately simulates present hydrologic conditions in the post-Cretaceous rocks and may be used to predict the effect of applied stress to the system.

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Prepared in cooperation with the Wyoming State Department of Economic Planning and Development. 16. Abstracts ^he Arikaree Formation of early Miocene age is an aquifer of large areal extent and is composed of very fine grained, poorly bedded, loosely to moderately cemented sandstone and interbedded silt, limestone, and many concretionary layers. Th(area studied is about 800 square miles in southern Niobrara and northern Goshen Counties. Long-term average annual recharge to the aquifer from streams and precipiti tion is estimated to total about 24,270 acre-feet. Pumpage from public-supply and irrigation wells is estimated to have totaled aboi 48,000 acre-feet from 1938 through 1972. This pumpage did not cause any noticeable decrease in natural discharge and it is assumed there has been no significant change : groundwater storage. Pumpage is estimated to total about 39,500 acre-feet for the 3-year period 1973 through 1975. A digital model was developed to simulate the groundwater system in the Arikaree Formation. The model can be used to indicate the general effect of applying hydraulic stresses to the system._____________ ____________________________________ 17. Key Words and Document Analysis. I7a. Descriptors Aquifer/ irrigation wells/ groundwater recharge/ transmissivity/ evapotranspiration/ specific yield/ discharge/ potentiometric surface/ seepage. 17b. Identifiers/Open-Ended Terms Digital model/ hydrologic system/ pumpage/ surface-and groundwater relationship 17c. COSATI Field/Group 18. Availability Statement No restriction on distribution.