The Ozark Plateaus aquifer system is in southeast Kansas, southwest Missouri, northeast Oklahoma and northwest Arkansas. In southeast Kansas and western Missouri, it consists of karstic and fractured carbonite rock units of Upper Cambrian, Lower Ordovician and Mississippian age. The aquifer system has two different aquifers. The Springfield Plateau aquifer forms the uppermost unit. The Ozark aquifer forms the lower unit. The two systems are largely separated by the Ozark confining unit.
Springfield Aquifer
The Springfield Plateau aquifer is composed of Mississippian limestones and cherts. The thickness ranges 200 to 400 feet thick in the tri-state region of southeast Kansas, southwest Missouri, and northwest Oklahoma. In Missouri, extreme southeast Kansas and parts of Oklahoma these rocks are exposed at the surface and are capped by Pennsylvanian shales farther to the west. The strata that form the Springfield Plateau aquifer are at the surface in southeast Cherokee County and the top of the Ozark aquifer is within 300 feet of the surface. At Pittsburg, Kansas, the top of the Springfield Plateau aquifer is within 200 feet of the surface. The depth to the top of the Ozark aquifer is around 450 feet.
Most of the recharge occurs to the Springfield Plateau aquifer in the form of precipitation where the rocks crop out at the surface. The water then enters the aquifer and moves underground to the west. Later, it discharges into the Spring and Neosho Rivers. In addition, the aquifer is also recharged by surface water entering lead and zinc mining-related shafts and pits. In the late 19th century, the Springfield Plateau aquifer was pumped to dewater the mines. Because of this dewatering, the sulfide minerals oxidized. When the mines refilled, sulfide minerals dissolved into the water. Consequently, concentrations of contaminants are higher in local areas of the Springfield Plateau aquifer.
Ozark Aquifer
The Ozark aquifer is composed of a thick sequence of water-bearing dolomites, limestones and sandstones. Throughout the tri-state region, the thickness of the Ozark aquifer varies from 800 to 1,500 feet. In general, it increases from northwest to southeast. In southwest Missouri the strata that forms the Ozark aquifer is at the surface or at shallow depths. It increases depth in the direction of southeast Kansas. The topographically higher region of southern Missouri where the aquifer's rocks crop out near Springfield, Missouri, serves as the recharge area for the Ozark aquifer.
The outcrop area serves as a route for rainwater to enter the aquifer. It moves by gravity in a westerly direction into the deeper part of the aquifer in southeast Kansas and northeast Oklahoma. There it encounters saltwater moving east from deeper rocks in western Kansas and Oklahoma. These deeper rocks are referred to as the Arbuckle group. It is an important source of hydrocarbons further west. Where these two water masses meet, a 20-30-mile-wide fresh-to-saline transition zone stretches northeast to southwest across the region. Rocks of the Precambrian age confine the Ozark aquifer from below.
Ozark Confining Unit
The Ozark confining unit separates the Ozark aquifer from the overlying Springfield Plateau aquifer. It is composed of shale and dense limestones and dolomites that are Devonian and Mississippian in age. In most regions, the confining unit forms an effective permeability barrier. However, confining rocks are absent in some areas. Here the potential lies for mining-related contamination from the Springfield Plateau aquifer to enter the Ozark aquifer.