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1991 Symposium: The Dakota Aquifer in Kansas


Regional Hydrogeology of the Dakota Aquifer in Southeastern Colorado and Kansas

P. Allen Macfarlane
Kansas Geological Survey, Lawrence, KS 66047

In this presentation the discussion of ground-water flow in the Dakota aquifer in southeastern Colorado and southwestern and central Kansas is presented in the larger context of the hydrogeology of the upper portion of the regional ground-water flow system. The lower boundary of this upper portion is a stream-line that defines a smaller intermediate-scale flow system within the more extensive regional-scale system. The depth to this boundary ranges from approximately 2,000 ft. below land surface in southeastern Colorado and southwestern Kansas to approximately 200 ft. in central Kansas. The focus of this presentation is on two vertical cross sections that begin in southeastern Colorado and extend through southwestern into central Kansas.

The "shallow" hydrostratigraphy consists of a sequence of aquifer and thick aquitard units. The surficial aquifer is the High Plains and Alluvial Valley aquifers where present. This aquifer is separated from the underlying Dakota and Morrison-Dockum aquifers by the Upper Cretaceous confining unit in much of western Kansas. In the southeastern Colorado-southwestern Kansas area and in the alluvial valleys of central Kansas, these aquifers are hydraulically connected. Underlying the Dakota and Morrison-Dockum aquifers over most of the region is the Permo-Pennsylvanian aquitard. In most of western and central Kansas thick-bedded evaporites and a Permian Sandstone aquifer are present in the Permo-Pennsylvanian aquitard. In central Kansas, the Dakota aquifer is hydraulically connected to the Permian Sandstone aquifer.

The hydrostratigraphy and geologic structure of the subsurface and the surface topography strongly influence the flow of ground water in the intermediate-scale ground-water flow system. Ground water moves through a hydraulically continuous framework from points of higher elevation to lower elevation due to the force of gravity. The water table is a reasonable reflection of the eastward-sloping surface topography from recharge areas in southeastern Colorado-southwestern Kansas to discharge areas in central Kansas and the Arkansas River valley in southeastern Colorado. However, the potentiometric surfaces of the deeper confined aquifers are significantly lower than the eastward-sloping water table in much of western Kansas. This is because the deeper aquifer units are relatively isolated from significant overlying sources of recharge by thick aquitards. An exception to this is the Dakota aquifer in the outcrop/subcrop areas of southeastern Colorado and southwestern and central Kansas. A simple analysis of the ground-water flow system using the conceptual model of Belitz and Bredehoeft (1989) shows that the deeper aquifers that are recharged through overlying confining units transmit significantly less water per unit area than the surficial aquifer.

In the Dakota and Permian Sandstone aquifers eastward increases in the hydraulic conductivity and transmissivity properties indicate better hydraulic connection between the aquifers and discharge areas in central Kansas than recharge areas in southeastern Colorado-southwestern Kansas. The greater topographic relief in central Kansas and in the Arkansas River valley in southeastern Colorado has promoted the development of local flow systems involving the Dakota aquifer. The local flow systems are superimposed on and dominate the intermediate-scale flow system where streams have cut down through the Upper Cretaceous aquitard. In central Kansas they help focus the discharge of deeper more saline ground waters into stream valleys. These local-scale systems are situated in areas where the Dakota aquifer is most transmissive. As a result, ground water is discharged at higher rates in central Kansas than can be recharged from adjacent confined areas. Thus, the stream valleys behave as drains for the shallow regional flow system, including the Permian Sandstone aquifer.

At present the Dakota aquifer is used for irrigation, public water supply, and industry in southeastern Colorado and southwestern and central Kansas. Reported well yields range up to 2,000 GPM in the outcrop and subcrop regions. Very little long-term historical water-level records are available to assess the effects of development in this area. What limited data exist suggest that after a century of use water-level declines are on the order of 50 ft. or less in the southeastern Colorado-southwestern Kansas area. In these areas flowing artesian well conditions no longer exist where development was the heaviest. Elsewhere in the area undergoing development the data suggest that water-level declines are on the order of 10 ft. or less.

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Kansas Geological Survey, Dakota Aquifer Program
Original symposium held October 1991
Electronic version placed online April 1996
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu
URL=http://www.kgs.ku.edu/Dakota/vol3/symp/macfar.htm