Dakota Home Report Archive

1991 Symposium: The Dakota Aquifer in Kansas


Water Quality of the Dakota Aquifer

Donald Whittemore
Kansas Geological Survey, Lawrence, KS 66047

Water-quality data for the Dakota aquifer system have been assembled from many sources, including samples collected as a part of the Dakota Program research. A system for storing and accessing the information (KWATCHEM) was developed using INFO, the relational data-base-management system of the geographic information system ARC/INFO. Freshwaters in the Dakota aquifer occur in the eastern outcrop zone, part of the eastern subcrop area adjacent to the outcrop boundary, and in the outcrop and subcrop areas in southwestern Kansas. The ground waters become saline in the subcrop area where the Dakota aquifer is overlain by Upper Cretaceous strata. The salinity distribution is complex and the areal salinity change is great along the eastern subcrop adjacent to the outcrop/subcrop boundary. The salinity increases appreciably with depth in the eastern subcrop area and the outcrop zone near the subcrop line. Both areal and vertical changes in salinity are generally more subdued in southwestern Kansas.

The water quality in the aquifer reflects the composition of the aquifer sediments, the past and present flow rates of different sources of recharge, the order in which the recharge sources entered, and the degree of mixing of the different sources as affected by the relative vertical and horizontal permeabilities of the system. Stream-aquifer interactions and discharge zones of the aquifer are important controls on the rates and proportions of mixing of different water sources. Factors controlling the major constituent concentrations in Dakota aquifer waters include:

  1. Meteoric recharge in the outcrop zone dissolves carbonate minerals to produce calcium-bicarbonate type waters that are fresh.
  2. Pyrite in some Dakota strata is oxidized by meteoric recharge producing solutions that add sulfate and increase calcium by dissolving more calcite to give calcium-sulfate type waters.
  3. Recharge passing through and dissolving calcite and gypsum in overlying Upper Cretaceous rocks brings in calcium-bicarbonate type, calcium-sulfate type, or mixtures of the two type waters mainly near the top of the Dakota aquifer where the Upper Cretaceous strata is thin.
  4. Recharge from the overlying Ogallala aquifer brings in calcium-bicarbonate type, calcium-sulfate type, or mixtures of the two type waters.
  5. Recharge from underlying Permian rocks, primarily the Cedar Hills Sandstone, is the main souce of saltwater and produces sodium-chloride type water with high concentrations of calcium, magnesium, and sulfate. The saltwater source has been geochemically identified as primarily from the solution of halite (rock salt) in Permian rocks.
  6. Cation exchange and calcite equilibria adjust the cation and bicarbonate contents of Dakota aquifer waters. Resultant sodium-bicarbonate waters are common in the subcrop areas near the eastern outcrop band, as well as within the subcrop zone in west-central Kansas. Sodium-sulfate waters probably derived from the combined effects of cation exchange and mixing.

The quality of water in the Dakota aquifer for different uses is being assessed. The main problem is high dissolved-solids concentrations, and thus, chloride and/or sulfate contents, in many areas. Iron, manganese, and fluoride contents often exceed drinking-water standards. High-fluoride water is associated with sodium-bicarbonate water. Other constituents with concentrations over current drinking-water standards include nitrate (several percent of the well waters) and arsenic, selenium, and mercury (from one to a few percent of the wells). However, none of the selenium values exceeds the new, revised federal standard for selenium. No well waters exceeded the standards for other heavy metals and only one had a radioactivity content over the drinking limit. Radon concentrations have also been determined, but a suggested federal standard has not yet been set.

Computer programs are being developed to improve ease of access and processing of Dakota data in ARC/INFO. One program is a user-friendly interface for interactive mapping and query of hydrologic-related data programmed using the ARC macro language. The interface is named KHARCINS (Hydrologic ARC/INFO System) and comprises menus which can be pulled down for selection and screen display of geographically-referenced information, including water-chemistry data from KWATCHEM. A method of displaying water types based on color information theory was developed in another program.

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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/whittem.htm