Reconnaissance of the geology and ground-water resources of the Pumpkin Creek area, Morrill and Banner Counties, Nebraska, with a section on the chemical quality of the water

摘要

The area described in this report includes the eastern, or lower, end of the valley of Pumpkin Creek and is in Morrill and Banner Counties, in western Nebraska. A reconnaissance of the geology and ground-water resources of the area was made to determine the possibilities of developing ground-water supplies for irrigation.Rocks that crop out in the Pumpkin Creek area are shown on a geologic map that is included in the report. They range in age from Oligocene to Recent and consist of the Brule, Arikaree, and Ogallala formations and of the alluvium. The Chadron formation, of Oligocene age, underlies the area but is not exposed at the surface. The Lance formation, of Late Cretaceous age, underlies the Chadron formation. Sandstones, which are thought to be in the Lance formation, contain ground water under artesian pressure and might yield sufficient water for irrigation on a small scale. Several thousand feet of Pierre shale, which is not known to yield appreciable quantities of water to wells, underlies the Lance formation. In some places sufficient water for irrigation can be obtained by wells from fractures in the Brule formation. Large supplies of water can be expected in places where the fractures are extensive or where they are overlain by saturated alluvial material. A few springs that are fed by recharge from the upland area issue at the contact of the Brule and Arikaree formations. The Ogallala formation yields water to domestic and stock wells in the upland areas south of Pumpkin Creek valley. Inasmuch as this formation is high topographically and is well drained, the depth to water is considerable generally 100 ft or more and the thickness of the saturated material probably is not great. It is doubtful, therefore, that sufficient water for irrigation could be obtained from this material; however, it may be possible in some places to obtain water for irrigation on a small scale.Alluvium, which underlies the floodplains of Pumpkin Creek and some of its major tributaries, is the principal aquifer in the area. The alluvium of Quaternary age consists of coarse sand and gravel and yields sufficient water for irrigation. In most places yields of 1,000 to 2,000 gpm probably could be obtained from properly constructed wells in this material.It is estimated that about 21,000 acre-ft of ground water annually leaves the area as stream flow. This represents approximately the amount of additional ground water available for development; however, if sufficient ground water is pumped to lower the water table appreciably, the additional water salvaged from loss by evaporation and transpiration also would be available for irrigation use. The development of any additional ground water from the unconfined ground-water reservoir will cause a reduction in the flow of Pumpkin Creek.Water from the alluvium and the Brule formation is moderately mineralized. Although most water is hard, some soft water is obtained from the Brule. The dissolved solids in water from the Brule formation and the alluvium ranged from 236 to 356 ppm. Hardness in these two water-bearing formations ranged from 6.0 to 20k ppm.Water obtained from deep wells in the Lance formation is more mineralized and softer than water obtained from the overlying formations. The dissolved solids, which consist largely of sodium bicarbonate and sodium chloride, range from 812 to 1,170 ppm. Although the water from the Lance formation is satisfactory for domestic needs, the high percentage of sodium restricts its use for irrigation.