Dakota Home Report Archive Report Start

Kansas Geological Survey, Open-File Rept. 96-1a
Proposed Management Areas--Page 5 of 16

Dakota Aquifer Water Quality

Figure 3 shows the distribution of total-dissolved solids concentrations in the upper Dakota aquifer. The present salinity pattern of Dakota waters is mainly dependent on the rate at which freshwater is able to enter from above and along the long flow paths in the aquifer in comparison with the rate of saltwater intrusion from the underlying Permian rocks. In some regions the saltwater is able to more rapidly intrude into the bottom of the Dakota, such as in parts of central to north-central Kansas where the Dakota directly overlies the Cedar Hills Sandstone (Figure 1). In northwest Kansas the thickness of the confining units is great and the flow of freshwater throughflow is low. The Dakota rocks contain saltwater in both of these regions. Surface recharge along the outcrop belt of the Dakota aquifer in southeast Colorado and central Kansas occurs at a much greater rate than underlying saltwater intrusion, resulting in essentially complete flushing of any previous saltwater. Fresh recharge flowing through the Dakota sandstones in southwest Kansas have also removed nearly all salinity. The freshwater flowing through sandstones in the confined aquifer between northwest and central Kansas has removed much of the saltwater but enough dissolved salt remains to make much of the water slightly to moderately saline. As a result, the age of ground water ranges from 10s to 100s of thousands of years nearer the outcrop/subcrop belt and may exceed a million years in far northwest Kansas (Macfarlane et al., 1995). In general, the greater that distance from the edge of the confining zone, the greater the salinity.

Figure 3. Distribution of total dissolved solids (TDS) concentrations in ground waters in the Dakota aquifer in western and central Kansas. Water less than 1000 mg/L TDS is defined as fresh. Water with 1000-2000 mg/L TDS is usable for many purposes but is less desireable than freshwater. A concentration of 10,000 mg/L TDS is defined in the state regulations of the Kansas Corporation Commission as the upper limit of usable water; above 10,000 mg/L a water is classified as unusable or mineralized.


Ground water in the areas of the upper Dakota aquifer with high TDS or salinity (greater than 5,000 mg/L) shown in Figure 3 are of sodium-chloride type. Waters in the area of the confined aquifer with 500-2,000 mg/L TDS are generally soft (low calcium and magnesium content), sodium-bicarbonate in chemical type, and usually have elevated fluoride concentrations. Ground water with 2,000 to 5,000 mg/L TDS in the confined area are typically transitional between sodium-bicarbonate and sodium-chloride type. Waters in the outcrop and subcrop areas with less than 500 mg/L TDS are usually of calcium bicarbonate and sometimes of calcium, magnesium-bicarbonate type. Concentrations of TDS between 500 and 2,000 mg/L in waters in the outcrop/subcrop areas are often due to high calcium and sulfate levels such that the waters can be calcium-sulfate in type. Elevated sulfate concentrations can also produce sulfate type waters in less saline portions of the confined aquifer.

Where saline waters exist in the Dakota aquifer or in underlying strata, the salinity in the Dakota aquifer generally increases with depth. The rate of change with depth is seldom uniform; TDS is often substantially greater below low permeability layers which impede the upward movement of salinity. This is especially true where the Kiowa Formation is mainly shale and separates saltwater in the Cheyenne Sandstone from fresh or much less saline water in the overlying Dakota Formation.

Previous page--Ground-Water Flow Patterns || Next page--Local Effects of Well Pumping
Start of this report || Table of Contents

Kansas Geological Survey, Dakota Aquifer Program
Original report available from the Kansas Geological Survey.
Electronic version placed online July 1996
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu