Abstract
The 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. The 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 precipitation is estimated to total about 24,270 acre-feet. Pumpage from public-supply and irrigation wells is estimated to have totaled about 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 in ground-water 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 ground-water system in the Arikaree Formation. The model can be used to indicate the general effect of applying hydraulic stresses to the aquifer. INTRODUCTION The area described in this report covers about 800 mi2 in southern Niobrara and northern Goshen Counties, Wyoming (fig. 1). The area includes that part of the Arikaree Formation of early Miocene age between Rawhide fault and the Nebraska State line where potential exists for the development of ground water for domestic, industrial, and agricultural uses. Well yields of more than 800 gal/min are common. The number of irrigation wells in the study area has increased significantly since 1966 (fig. 2), and it is anticipated that the withdrawal of ground water will increase substantially in the next several years. Surface-water supply of the upper Niobrara River basin available to Wyoming and to Nebraska is allocated by the Upper Niobrara River Compact, 1962. The term "Upper Niobrara River Basin" is defined as the area west of Range 55 of the 6th principal meridian in Wyoming and Nebraska that is drained by the Niobrara River. Article VI A of the Compact states, "Nebraska and Wyoming recognize that future use of ground water for irrigation in the Niobrara River Basin may be a factor in the depletion of the surface flows of the Niobrara River and, since the data now available are inadequate to make a determination in regard to this matter, any apportionment of the ground water of the Niobrara River Basin should be delayed until such time as adequate data on ground water of the basin are available." State water administrators need additional information about characteristics of the aquifer, rate of ground-water withdrawal, and effect of withdrawal, to help them regulate ground-water development in the area. A study was needed that would describe the ground-water system in the Arikaree Formation in sufficient detail to provide information on the effects of ground-water use. Purpose and Scope The purposes of this investigation are (1) to define the groundwater system in the Arikaree Formation, (2) to determine the hydrologic cause-and-effect relationships resulting from the current ground-water development, and (3) to provide a means of indicating the hydrologic effect of future ground-water development. Data from previous investigations, along with data compiled during this study, were used to develop a digital model of the hydrologic system in the Arikaree Formation. The model is used to indicate the effect of ground-water withdrawal at the 1975 rate in future years. 110° |09° 108° 107° 106° 105° 104° t~ 'r I" -t ' N---i i ' \ j I V ' V W YOJM I NG INIOBRARAT COUNTY GOSJHEN I |COUNTY ] , ]~i_i 0 50 100 MILES i i i i i i i i i i i I I II M I I M I 0 50 100 KILOMETERS
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