Kansas Geological Survey, Open-file Report 88-39
Great Plains and Cedar Hills Aquifers--Page 8 of 25
For the purposes of this report the Great Plains aquifer system has been subdivided into two parts: an upper part, consisting of the Dakota Formation and a lower part, consisting of the Kiowa Formation and Cheyenne Sandstone. This subdivision has been made based on perceived differences between the Dakota Formation and the Kiowa Formation- Cheyenne Sandstone. In general, the Dakota Formation consists of thick lenticular channel sandstones enclosed in an alluvially deposited sequence of interbedded claystones, siltstones, and sandstones. These channel sandstones occur commonly in the upper part and near the base of the Dakota Formation. In contrast, the Kiowa Formation and Cheyenne Sandstone consist largely of interbedded shales, siltstones, and thin- bedded sandstones. Thicker lenticular sandstones do occur in these formations particularly near the base of the Cheyenne but they are infrequent and generally not widespread. We believe that these lithologic and architectural differences are reflected in the bulk porous media properties of the Dakota Formation and the underlying Lower Cretaceous formations. As a result, an upper and a lower hydrostratigraphic unit can be distinguished in the Great Plains aquifer.
Over most of the study area the Great Plains aquifer system is confined by overlying Upper Cretaceous rocks referred to by Helgeson et al. (in review) as the Great Plains Confining System. The Upper Cretaceous rocks that comprise the Great Plains Confining System are the Graneros Shale, the Greenhorn Limestone, the Carlile Shale, and the Niobrara Chalk. Not discussed in this report is the Codell Sandstone aquifer, which locally produces sufficient water to supply domestic and stock needs in the study area. The Codell Sandstone aquifer occurs at the top of the Carlile Shale.
Separating the Great Plains aquifer system from the underlying Cedar Hills aquifer are younger Permian and Jurassic rocks in Graham and Trego counties that appear to act as confining layers for the Cedar Hills aquifer. The formations that comprise the confining layer are the Flower-pot Shale, the Blaine Formation, the Whitehorse Formation, and the Morrison Formation.
Ground water is transmitted through pores within the rocks and through fractures in these aquifer systems in response and in the opposite direction of the hydraulic gradient according to Darcy's Law (Freeze and Cherry, 1979). The direction of water flow is from a point of greater to lesser hydraulic head. Temperature differences are assumed to influence the flow of ground water across the region only slightly.
The lateral direction of ground-water flow is determined by constructing a potentiometric surface map of the aquifer in the region of interest. The configuration of the potentiometric surface is determined by contouring a map of measured static water levels in wells that penetrate the aquifer. The direction of ground-water flow is perpendicular to the orientation of the contours in the down gradient direction if the anisotropy of the aquifer is negligible. In terms of regional systems, ground-water flow is from points of recharge to points of discharge (Freeze and Cherry, 1979).
The vertical component of the hydraulic gradient is determined by measuring the hydraulic heads at two vertically separated observation points within the ground-water system. As before, the component direction of flow is from a point of greater to lesser hydraulic head.
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