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Kansas Geological Survey, Open-File Rept. 91-1
Annual Report, FY 90--Page 3 of 9

Regional Setting

The Dakota aquifer consists of interbedded sandstones and shales that were deposited in river valleys, in deltas, and in nearshore marine environments during the early part of the Cretaceous Period (108 to 94 million years ago, approximately) (Figure 3). The aquifer can be subdivided regionally into upper and lower aquifer units separated by a less permeable aquitard of shale that is present over much of area (Figure 4). The ability to transmit water laterally through the upper and lower aquifers depends on the hydraulic continuity of the framework which is directly related to the proportion of sandstone and shale. Vertical movement of water between aquifer units depends on the hydraulic connection of sandstone aquifers, including fractures that penetrate the shale. Well yields vary widely in the system and are controlled by the thickness and hydraulic properties of the sandstone units. The aquifer is unconfined or partially confined in central Kansas and widely separated areas of southwestern Kansas-southeastern Colorado and is confined where covered by younger less permeable strata elsewhere. The main pattern of ground-water flow in the Dakota aquifer is from recharge areas in southeastern Colorado and southwest Kansas to discharge areas in central and north-central Kansas river valleys (Figures 4 and 5). The Dakota aquifer is recharged by precipitation in the outcrop area, by the overlying water-table aquifer in southwest Kansas, and by the underlying Cedar Hills aquifer where both aquifers are hydraulically connected in the central part of the state.

Figure 3. Midcontinent geography during deposition of the middle portion of the Dakota aquifer approximately 97 million years ago.

Figure 4. Distribution of hydraulic head and ground-water flow direction in a cross-section of the shallow subsurface, including the Dakota aquifer, extending from southwestern into central Kansas.

Figure 5. Configuration of the pre-development potentiometric and water-table surfaces of the upper part of the Dakota aquifer in Kansas.

The Dakota aquifer is used widely for irrigation, public water supply, and industry in southwestern and central Kansas and southeastern Colorado. In Kansas approximately 96% of the current volume withdrawn occurs in southwest and south-central parts of the state. Water-level declines in observation wells have become appreciable in southwestern Kansas and adjacent areas as development of the High Plains and Dakota aquifers continues.

Freshwaters occur along the eastern outcrop area, part of the eastern subcrop adjacent to the outcrop zone, and in the subcrop area of the Dakota aquifer in southwestern Kansas. The rates of change in dissolved-solids contents with areal distance are greatest along the easternmost subcrop from Republic County to Barton County. Saltwater in the Dakota aquifer in central Kansas is geochemically identified as derived primarily from solution of halite (rock salt) in Permian rocks, and has flowed upward into the Dakota. The upward movement from the Permian is affected by pinching out of confining layers and the presence of fractures and other geologic structures.

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Kansas Geological Survey, Dakota Aquifer Program
Original report available from the Kansas Geological Survey.
Electronic version placed online Nov. 1998
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu