Progress report on the geology and ground-water hydrology of the lower Platte River Valley, Nebraska, with a section on the chemical quality of the ground water

Abstract

The occurrence of abundant ground-water supplies in the lower Platte River Valley has made possible the present agricultural and industrial economy of the area. Likewise, the future development of the area is dependent on the wise use of this important resource. The current investigation, on which this report is based, is a necessary step in the planning for the greatest ultimate utilization of the water resources in the lower Platte River Valley. The area covered by this study is the floor of the lower Platte River Valley between North Platte and Fremont and embraces about 2,500 square miles. The entire valley floor is underlain by unconsolidated Pleistocene sediments which consist of clay, silt, sand and gravel and range in thickness from less than 20 feet to nearly 200 feet. Westward from Cozad these sediments were deposited in a valley entrenched in bedrock, but east of Cozad they are continuous with similar deposits which underlie the adjacent uplands. Bedrock formations of Tertiary age are in contact with the basal Pleistocene sediments from the west end of the area to about Central City. From Central City eastward, formations of Cretaceous age immediately underlie the Pleistocene deposits. The Pleistocene sediments and underlying pervious formations are water saturated below depths which range from less than 1 foot to about 90 feet below the surface. In general, the configuration of the water table is similar to the topography of the land surface, but the relief on the water table is considerably less by comparison. Movement of ground water is either toward the river or parallel to it. Based on present information, movement of ground water out of the valley is not indicated but additional water-level control is needed south of the valley between Grand Island and Columbus to determine the possibility of ground-water loss in this stretch of the valley. Periodic observations of water-table fluctuations have constituted an important phase of ground-water studies in the lower Platte River Valley. Examination of water-level data collected by the Geological Survey in cooperation with the Conservation and Survey Division of the University of Nebraska indicates that water levels throughout most of the valley between Gothenburg and Grand Island declined during the 9-year period, December 1930 to December 1939. The maximum net declines observed during this period were a little greater than 4 feet. During the period December 1939 to December 1946, water levels recovered throughout much of the same area; in local areas on the south side of the Platte River between North Platte and Overton water levels rose in excess of 10 feet as a result of seepage losses tram canals and irrigated lands. However, north of Wood River in western Hall County water levels continued to decline to the extent that in at least 3 observation wells water levels in December 1946 were more than 5 feet lower than in December 1930. Water levels were measured monthly in 1946 and bimonthly in 1947 in observation wells located throughout the area covered by this report. During this period water levels fluctuated through a range of about 3 feet, the lowest levels being reached in September 1946 and the highest levels being reached in July 1947. Average fluctuations between successive measurements were a little less than 0.4 foot. The amount of ground-water discharge in the summer months of 1947 was approximately double that of the previous summer. However, replenishment to groundwater storage during the fall of 1946 and the spring of 1947 was sufficiently great that the amount of water in storage in the valley as a whole at the end of the 2-year period was essentially equal to the amount in storage at the beginning of the period. At the end of 1947 the Dawson and Buffalo County areas showed gains to storage whereas the remainder of the valley suffered vary slight losses during the same 2 years. The quantity of ground water that a water-bearing material will yield is dependent upon the hydrologic properties of the material. Two hydrologic properties of greatest importance are permeability and specific yield. During the course of the present investigation 5 pumping tests were conducted to determine the coefficients of permeability and storage of the Pleistocene deposits. The values of the former range from 955 to 4,925 and the latter tram 0.007. to 0.236 attar 24 hours of pumping. The mineral character of the ground water has been determined from analyses of samples, of which 15 represented municipal supplies and 3 were irrigation waters. Ground waters analyzed for this report contained variable amounts of dissolved solids ranging from 240 to 1,060 parts per million, with hardness values above limits considered desirable tor public supplies. The increased concentration of dissolved solids for ground waters in the western part of the valley as a result of return flow conditions, is noted. Ground waters in the valley are discussed as to suitability for municipal and irrigation uses, and analyses of samples do not reveal any serious condition that would adversely affect the use of these waters for the purposes intended. As irrigation increases, periodic chemical analyses of ground waters in this area will be required to evaluate the effects of the re-use of drainage waters and to provide information for salinity control. Ground-water supplies are drawn upon extensively for irrigation purposes, for municipal supplies, and for rural, domestic and stock supplies. Nearly 4,000 irrigation wells are known to exist in the area, the greatest concentrations of these being in Dawson, Buffalo, and Hall Counties.