Kansas Geological Survey, Public Information Circular (PIC) 8
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In the eastern part of Suitability Area III, the Dakota is a shallow unconfined aquifer or is in contact with stream-aquifer systems (figs. 1, 4). In the western part of the suitability area, the Dakota is confined and the Upper Cretaceous aquitard is thin and relatively permeable. In the western (confined) part of this suitability area, the depth below the surface to the top of the Dakota aquifer is generally less than 150 feet (46 m). Where the Dakota is unconfined, recharge is estimated at a few tenths of an inch per year. Throughout this suitability area, ground-water quality in the upper part of the Dakota is locally variable, except where the salinity has been flushed from the aquifer by freshwater recharge. In the unconfined regions, ground water near the top of the Dakota is mostly fresh (fig. 3). Salinity generally increases with depth near the bottom of the aquifer in the confined Dakota, and in the unconfined aquifer it increases near some of the major streams that cross central Kansas. In this suitability area, the Dakota is used for drinking water, industry, and agriculture, including irrigation. Irrigation use is concentrated primarily in southwestern Washington, southeastern Republic, and northern Cloud counties. Water-level declines from development in Suitability Area III are believed to be less than 20 feet (6 m).
According to computer simulations, the primary impact of water-resources development in Suitability Area III is a reduction in discharge from the Dakota aquifer to streams, and the lateral movement of water from the confined to the unconfined Dakota aquifer from west to east. Further increases in development may only slightly increase water-level declines where the Dakota is a shallow aquifer.
Suitability Area IV includes that part of the confined Dakota aquifer located west of its regional discharge areas in north-central Kansas (figs. 1, 4). Depth below the surface to the top of Dakota aquifer is generally less than 400 feet (122 m). Freshwater recharge from overlying sources varies locally and may have flushed some of the salinity from the upper part of the aquifer. However, natural saltwater in the lower part of the Dakota from the Cedar Hills Sandstone is widespread in this suitability area. Consequently, ground-water salinity generally increases with depth. Based on salinity, the suitability area is divided into two parts: in Suitability Area IVa, the salinity near the top of the Dakota is less than 1,500 mg/L, whereas in Suitability Area IVb, the salinity ranges from 1,500 mg/L to 10,000 mg/L (fig. 3). In Suitability Area IVa, water quality is acceptable for most uses, but in Suitability Area IVb, the aquifer is marginally usable. Ground-water salinities in the lower part of the Dakota aquifer probably exceed 10,000 mg/L over the entire suitability area. Water-level declines from development are believed to be less than 10 feet (3 m). In Suitability Area IVa, the upper part of the Dakota is used primarily for drinking water and stock water. In Suitability Area IVb, water from the aquifer may require advanced treatment technologies to reduce its salinity.
Wells in the upper part of the Dakota aquifer near the eastern edge of Suitability Area IV may reduce the eastward flow of fresher ground waters into the regional discharge areas. This may allow deeper, more saline ground water to move into the upper part of the Dakota and may increase the salinity of ground-water discharge to streams.
In Suitability Area V, the confined Dakota aquifer receives negligible freshwater recharge from overlying sources (figs. 1, 4). Depth below the surface to the top of the aquifer ranges from 400 feet (122 m) to more than 1,500 feet (457 m) along the shared boundary with Suitability Area VI in northwest Kansas. The major source of freshwater is from the regional recharge area in southeastern Colorado. Ground-water salinites in the upper Dakota aquifer generally increase northeastward from less than 500 to 1,500 mg/L in Suitability Area Va, and from 1,500 mg/L to 10,000 mg/L in Suitability Area Vb (fig. 3). Ground-water salinities in the lower Dakota aquifer are greater than 2,000 mg/L and may locally exceed 10,000 mg/L in the far northern and eastern parts of the Suitability Area V. In Suitability Area Va, water quality is acceptable for most uses and the Dakota is used primarily for drinking and livestock, but in Suitability Area Vb, the water quality is marginal. In general, the upper Dakota is more saline with depth to the north and east. Water-level declines from development are believed to be less than 50 feet (15 m) in this suitability area due to its limited use.
Water-resources development in this part of the aquifer is not likely to induce additional freshwater recharge from the High Plains aquifer into the unconfined Dakota aquifer in Suitability Area I. Pumping rates greatly exceed recharge from overlying sources. As a result, pumping may locally deplete the aquifer if wells and wellfields are spaced too closely together.
In Suitability Area VI, the Dakota is a confined aquifer and contains saline or "mineralized" ground water, defined by state statute as water containing more than 5,000 mg/L chloride or more than 10,000 mg/L total dissolved solids. Mineralized ground water is considered undesirable for most uses except oil and gas operations (such as secondary recovery) and is not protected by state and local regulations. In the northwest part of Suitability Area VI, the Dakota receives negligible freshwater recharge from overlying sources, flow rates are very low, and the depth to the top of the Dakota exceeds 1,500 feet (457 m). In the central part, vertical and lateral flow rates through the aquifer are higher and the top of the aquifer is less than 500 feet (152 m) below the surface. However, the flow of freshwater through the aquifer has been insufficient to flush the salinity from the aquifer where it is hydraulically connected to the Cedar Hills Sandstone.
The Dakota is a major aquifer system in Kansas and may become an important source of water in areas where shallower aquifers are inadequate. However, unlike other major shallow aquifer systems in Kansas, conditions in the Dakota are highly variable. Consequently, delineation of water-supply-suitability areas within the aquifer on the basis of hydrology, water quality, and the impact of pumping is an important first step in proactive policy development to prolong the use of this water resource.
Additional information on the Dakota aquifer can be obtained by contacting the Geohydrology Section at the Kansas Geological Survey (785-864-3965; 785-864-5317, FAX). The Kansas Geological Survey also maintains a World Wide Web home page that features information on the Dakota aquifer: http://www.kgs.ku.edu/Dakota/vol1/.
Further information on the topics covered in this pamphlet can be found in the following publications.
Buchanan, R. C., and Buddemeier, R. W., compilers, 1993, Kansas Ground Water: Kansas Geological Survey, Educational Series 10.
Macfarlane, P. A., and Sawin, R. S., 1996, A User's Guide to Well-spacing Requirements for the Dakota Aquifer in Kansas: Kansas Geological Survey, Public Information Circular 1.
Macfarlane, P. A., 1997, The Dakota Aquifer System in Kansas: Kansas Geological Survey, Public Information Circular 7.
Aquifer: A part of a geologic formation, or one or more geologic formations, that is porous and permeable enough to transmit water at a rate sufficient to feed a spring or for economic extraction by a well. An aquifer transmits more water than an aquitard.
Aquitard: A part of a geologic formation (or one or more geologic formations) that is of much lower permeability than an aquifer and will not transmit water at a rate sufficient to feed a spring or for economic extraction by a well.
Confined aquifer: An aquifer that is bounded above and below by aquitard units; water levels in wells screened in the confined aquifer are higher than the top of the aquifer.
Discharge area: An area where ground water is lost naturally from an aquifer through springs, seeps, or hydraulic connection to other aquifers. The water leaving the aquifer is referred to as discharge.
Hydraulically connected: A condition in which ground water moves easily between aquifers that are in direct contact. An indication of this condition is that the water levels in both aquifers are approximately equal.
Pleistocene and Holocene deposits: Unconsolidated gravels, sands, silts, and clays in alluvial and wind-blown deposits that formed within the last 2 million years, mostly along the Arkansas River valley in southwestern and south-central Kansas.
Recharge area: A geographic area where water enters (recharges) an aquifer. Recharge areas usually coincide with topographically elevated regions where aquifer units crop out at the surface. In these areas infiltrated precipitation is the primary source of recharge. The recharge area may also coincide with the area of hydraulic connection where one aquifer receives flow from another adjacent aquifer.
Salinity: The sum of the dissolved materials in water expressed in milligrams/liter (mg/L). The upper limit for freshwater is 1,000 mg/L; natural seawater has a salinity of approximately 35,000 mg/L.
Unconfined aquifer: An aquifer that is not bounded above by an aquitard; water levels in wells screened in an unconfined aquifer coincide with the elevation of the water table.
Water-supply-suitability area: A region of the Dakota in which the hydrogeologic and water-quality characteristics of the aquifer and the impacts caused by pumping on the hydrologic system are relatively uniform.
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Kansas Geological Survey, Geology Extension
1930 Constant Ave., Lawrence, KS 66047-3726
Phone: (785) 864-3965, Fax: (785) 864-5317
Web version June1997