VIRGINIA L MCGUIRE https://orcid.org/0000-0002-3962-4158
Date of this Version
U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 97-4266
From 1991 to 1992, the U.S. Geological Survey participated with the U.S. Department of Agriculture and the University of Nebraska-Lincoln in research activities at the Nebraska Management Systems Evaluation Area (MSEA) site near Shelton, Nebraska. The purpose of the U.S. Geological Survey study was to define the hydrogeologic system in the vicinity of the Nebraska MSEA site to aid the interpretation of ground-water sampling results by other researchers. The primary aquifer in the study area is the High Plains aquifer, which consists of an unconfined part, the shallow aquifer; a silt and clay unit that acts as a confining layer; and a confined part, the Ogallala aquifer. The shallow aquifer is the focus of this study. To determine aquifer properties for the shallow aquifer, two constant-discharge aquifer tests and about 450 straddle-packer slug tests were conducted. The test results indicate horizontal hydraulic conductivity values for the shallow aquifer ranged from 340 to 390 feet per day, and horizontal hydraulic conductivity values for 1.6-foot vertical intervals in the aquifer ranged from 92 to more than 620 feet per day.
A ground-water flow model of the shallow aquifer in the study area was developed using historical data and the hydrologic data collected for this study. The model quantified the water budget for the shallow aquifer in the study area. Aquifer properties were estimated for each cell, based on analysis of drillers' logs frorrrlrrigation wells and test holes and the results of constant-discharge aquifer tests and straddle-packer slug tests. Recharge and pumpage were estimated using a soil-moisture budget model.
Water budgets calculated by the model indicated that in 1931, the shallow aquifer discharged 25 percent of its outflow to the main channel of the Platte River and, in 1991, the main channel of the Platte River supplied more than 12 percent of the total inflow to the aquifer. In 1991, the largest sources of water to the system were recharge from precipitation (42 percent) and ground-water inflow from the western boundary of the model area (31 percent) and the losses were due to pumpage (81 percent) and ground-water outflow through the eastern boundary of the model (19 percent).
The study indicates that water collected from wells screened near the bottom or middle of the shallow aquifer at the MSEA site likely entered the system far upgradient of the site and that a vertical gradient is not measurable in the shallow aquifer except possibly in the immediate vicinity of a pumping irrigation well. Therefore, concentrations of agricultural chemicals in ground water collected from wells screened only at the top of the shallow aquifer probably are influenced by the effects of farming practices at the site.