Unconsolidated deposits of Pliocene and Pleistocene age comprise the principal aquifer in the Great Bend Prairie. Forty-five million acre-feet of ground water are estimated to be in storage in these deposits. Only about 60 percent of this amount may be usable owing to an insufficient saturated thickness and to high mineralization of water locally. Yields to irrigation wells are commonly between 500 and 1,000 gal/min, and yields of as much as 2,000 gal/min may be available in localities where the saturated thickness is greater than 100 feet.
In 1940, there were about 30 municipal and industrial wells and 50 irrigation wells in the area. In 1973, there were about 200 municipal and industrial wells and 950 irrigation wells; and in 1974, about 1,160 irrigation wells. Annual withdrawals of ground water by wells increased from about 5,000 acre-feet in 1940 to about 140,000 acre-feet in 1974. The steadily increasing withdrawal rate has not produced long-term declines of water levels that may be distinguished from the effects of climatic changes. Significant additional development for irrigation that could cause ground-water withdrawal to exceed long-term recharge and result in water-level declines is anticipated in some areas. The average annual recharge rate to ground water from precipitation is estimated to be about 2 inches.
The chemical quality of ground water from unconsolidated deposits in the western half of the area is generally suitable for most uses. The chemical quality of water in the eastern half may be unsuitable locally for most purposes as a result of upward leakage of highly mineral-ized water from underlying Permian rocks.
Locally, wells in rocks of Early Permian or Early Cretaceous age may yield from about 10 to 100 gal/min. However, the water from these wells generally contains more than 1,000 mg/L (milligrams per liter) of dissolved solids and more than 500 mg/L of chloride and sulfate.
