The Dakota Aquifer

Sarah Pick

Spring 2008

GO 571 Geohydrology

Introduction

The Dakota Aquifer covers most of central North America. This aquifer covers two thirds of the counties in Kansas, and can be found in 59 of Kansas’s 105 counties (KGS, 1996), as shown in figure 2. The Dakota Aquifer is a part of the Great Plains Aquifer System, which “underlies most of Nebraska, about one-half of Kansas, the eastern one-third of Colorado, and small parts of New Mexico, Oklahoma, Texas, South Dakota, and Wyoming” (USGS, 1997).

Fig.1
Fig. 1 Area covered by the Dakota Aquifer. Picture from the Kansas Geological Survey, 1996.
Fig.2
Fig. 2 All the Kansas counties that overlie the Dakota Aquifer. Picture from the Kansas Geological Survey, 1996.

Hydrologic Setting

The water in this aquifer moves east and northeast, so the recharge for the eastern part of this aquifer comes from the recharge of southeastern Colorado. Most of this recharge is in the form of precipitation. Although there is a little recharge that comes from downward leakage from the Great Plains confining system.

Fig.3
Fig. 3 The mean annual natural recharge, in inches. The blue shows the location of the High Plains Aquifer. Picture from Michael T. Dealy, Kansas Geological Survey.

Most of the surface water in Kansas is in the eastern half of the state, which lies just out of reach of the Dakota Aquifer, see fig.4.

Fig.4
Fig. 4 This image illustrates where rivers and lakes are in Kansas. Most are located in eastern Kansas, while small portions lie in the Dakota Aquifer. Picture from Michael T. Dealy, Kansas Geological Survey.

Hydrologic and Geologic Medium

Geologically defined, the Dakota Aquifer is an unconfined aquifer made of sandstone. The Dakota Formation that makes us the aquifer is from the Cretaceous period, and is covered by “Graneros Shale, Greenhorn Limestone, and Carlile Shale” (Burkart, 1984), also by glacial till, loess, sand, and gravel from the Quaternary age. The size of sand grains in the sandstone part of the aquifer is very heterogeneous. It ranges from fine to coarse sand, gravel, and silt and clay. The sandstone part of the aquifer is very permeable because of the different grain sized; however, the shale and limestone are less permeable.

The Dakota Aquifer is separated into two parts, its upper and lower units. The upper unit is about 300 feet thick , and the lower part varies in thickness, but can get up to 200 feet thick. In western Kansas, these two units are separated by thick marine shale, called the Kiowa shale aquitard. The aquitard also varies in thickness and can get to over 300 feet thick.

Fig.5
Fig. 5 The upper and lower units of the Dakota Aquifer, separated by the Kiowa Shale aquitard. Picture from the Kansas Geological Survey, 1996. Fig.6
Fig. 6 Parts of Kansas covered by the Kiowa shale aquitard. Picture from the Kansas Geological Survey, 1996.

The hydraulic conductivity can range anywhere from 3.6 to 88 feet per day. Domestic wells in Colorado can pump anywhere from five to fifty gallons per minute, and some irrigation wells yield over 1,000 gallons per minute.

The chemical characteristics of the groundwater in this aquifer vary with each section of the aquifer. For example, the area that underlies north and northeastern Nebraska is rich in calcium-sulfate and calcium-sodium-sulfate. The water here is very old and, because it is under a thick confining layer, takes a long time to recharge. In the eastern part of Nebraska, the waters are rich in calcium-magnesium bicarbonate. Central and southeast Nebraska is highly saline. Where it is connected to the High Plains aquifer, the water has calcium bicarbonate, and where it is confined by the Niobrara-Graneros confining layer, it has high amounts of sodium bicarbonate.

Water Resources

The shallow parts of this aquifer are currently being used for irrigational purposes. Because these parts have been used so much, the water levels have decreased significantly. Now water is being taken from the deeper portions of the aquifer for irrigation. Other parts of the aquifer are used for public supply and industrial use. Although the number of wells used for public supply and industrial use are lower then that of irrigation wells. In a study done by the United States Geological Survey in 1989, 116 wells were used for irrigation, 13 for industrial supply, and 8 for public supply. Because most of the water in this aquifer goes to irrigation of crops and it takes a while for the aquifer to recharge, the water supply here is threatened. Fortunately, the Southwest Groundwater Management District #3 developed new policies and regulations for water use of this aquifer and the High Plains aquifer. The focus of these new regulations is on a “graduated well spacing based on the yearly withdrawal of groundwater from the well to be installed” (Kansas Geological Survey, 1996).

Ground Water Contamination

Currently, the Dakota aquifer is not contaminated. However, there is a slight risk that oil-brine disposed in the shallow zone in central Kansas could leak into the aquifer. Should there be a contamination of any sort, the Environmental Protection Agency is in charge of cleaning it up.

Resources

Bredehoeft, J. D., C. E. Neuzil, and P. C. D. Milly. United States Geological Survey Water – Supply Paper 2237. Regional Flow in the Dakota Aquifer: A Study of the Role of Confining Layers. Washington: GPO, 1983.

Burkhart, M. R. United States Geological Survey Water – Supply Paper 2215. Availability and Quality of Water from the Dakota Aquifer, Northwest Iowa. Washington: GPO, 1984.

Cech, Tom. Ground Water Atlas of Colorado. 2003. Colorado Geological Survey. 25 April 2008. http://geosurvey.state.co.us/wateratlas/chapter6_8page1.asp.

Dealy, Michael T. “Groundwater Basics and Kansas Aquifers”. Kansas Geological Survey. [Retrieved 25 April 2008].

Gosselin, David C., F. Edwin Harvey, and Charles Flowerday. The Complex Dakota Aquifer: Managing Groundwater in Nebraska. April 2003. Geotimes. 25 April 2008. http://www.agiweb.org/geotimes/apr03/feature_nebraska.html.

Macfarlane, P. Allen. Dakota Aquifer Program. October 1998. Kansas Geological Survey. 25 April 2008. http://www.kgs.ku.edu/Dakota/vol1/dakotaHome.html.

Miller, James A., and Cynthia L. Appel. Ground Water Atlas of the United States Kansas, Missouri, and Nebraska. 1997. U.S. Geological Survey. 9 April 2008. http://capp.water.usgs.gov/gwa/ch_d/index.html.

Watts, Kenneth R. Potential Hydrologic Effects of Ground – Water Withdrawals from the Dakota Aquifer, Southwestern Kansas. United States Geological Survey Water – Supply Paper 2304. Washington: GPO, 1989.