Potential hydrologic effects of ground-water withdrawals from the Dakota Aquifer, southwestern Kansas

Abstract

A study was conducted to evaluate the effects of potential development of the Dakota aquifer on the layered-aquifer system above Permian rocks in a 5,000-square-mile area of southwestern Kansas. This aquifer system, which consists of five layers, includes the Cheyenne aquifer, the Kiowa confining unit, the Dakota aquifer, the Niobrara-Graneros confining unit, and the High Plains aquifer. Water supplies from the sandstone aquifers thus far have been developed mainly in parts of Hodgeman and Ford Counties. Management restrictions placed on further development of the High Plains aquifer could lead to additional development of the sandstone aquifers in the study area. The upper sandstone aquifer, the Dakota aquifer, consists of sandstone and shale of the Lower Cretaceous Dakota Sandstone and is as much as 400 feet thick. Transmissivity of the Dakota aquifer, determined from analyses of pumping tests, ranges from 100-7,100 feet squared per day. The Dakota aquifer is confined where it is overlain by the shales and limestones of the Upper Cretaceous Niobrara-Graneros confining unit, but locally it is unconfined. The lower sandstone aquifer, the Cheyenne aquifer, consists of the sandstone and shales of the Lower Cretaceous Cheyenne Sandstone in the eastern half of the study area plus undifferentiated Middle and Upper Jurassic rocks (sandstone, siltstone, shale, and limestone) in the western half of the study area. Maximum thickness of the Cheyenne aquifer is more than 300 feet, and maximum transmissivity is estimated at 3,000 feet squared per day. Estimated water use in the study area was about 8,800,000 acre-feet from the High Plains aquifer and about 160,000 acre-feet from the Dakota aquifer during 1975-82. The Cheyenne aquifer is not developed in the study area, and no water use from it is reported. The chemical characteristics of water in the sandstone aquifers are highly variable in the study area. Water in the Dakota aquifer is a calcium bicarbonate type water, similar to water in the High Plains aquifer, in the subcrop area. However, in areas distant from the subcrop, water in the Dakota aquifer is a sodium bicarbonate type water with dissolved-solids concentrations in excess of 500 milligrams per liter. In some parts of the study area, water from the Dakota presents high to very high salinity and sodium hazards to crops and soil when it is used for irrigation. The Cheyenne aquifer locally contains mineralized water, as indicated by the response of resistivity curves on geophysical logs. Hydrographs of wells completed in the Dakota aquifer indicate that the Dakota and High Plains aquifers are hydraulically connected in and near subcrop areas. Locally, the Dakota aquifer has converted from confined to unconfined conditions as a result of declining water levels due to pumpage from the Dakota aquifer and as the result of depletion of the High Plains aquifer in subcrop areas. Gradual declines in the potentiometric surface of the Dakota aquifer have occurred since the onset of pumpage in the 1960's; however, water levels in some wells have risen during the late 1970's. A digital computer model of three-dimensional groundwater flow was developed to simulate hydrologic conditions of a five-layer hydrologic system for 1975-82 conditions. The major components of the simulated 1975-82 water budget were well discharge from the High Plains aquifer and loss of ground water from storage in the High Plains aquifer. Although downward leakage from the High Plains aquifer in the study area represented only 18,000 acre-feet of the 1,365,000 acre-feet discharged from the High Plains aquifer during 1982, it was a major source of inflow to the Dakota aquifer. Changes in storage in the Dakota aquifer in the study area during 1982 were about 5,000 acre-feet. A baseline projection was made using 1982 simulated hydraulic heads from the calibrated model and 1982 rates of pumpage from both the High Plains and the Dakota aquifer