Hydrogeology
In the region of southeast Kansas, southwest Missouri, northeast Oklahoma and a small portion of northwest Arkansas lies the Ozark Plateaus aquifer system. 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 is subdivided into the Springfield Plateau aquifer, which forms the uppermost unit, and the Ozark aquifer, which forms the lower unit. The two systems are largely separated by the Ozark confining unit.
Springfield Aquifer
The Springfield Plateau aquifer is composed of Mississipian limestones and cherts with a thickness range of 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, while 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 where 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. As a result of this dewatering, the sulfide minerals oxidized and when the mines refilled, this allowed the sulfide minerals to dissolve into the water. Consequently, there are higher concentrations of contaminants 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, generally increasing from northwest to southeast. In southwest Missouri the strata that forms the Ozark aquifer is at the surface or at shallow depths with increasing 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 where 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, which 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
Above the Ozark aquifer is the Ozark confining unit that largely separates the 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, there are a small number of regions where these confining rocks are absent. Here the potential lies for mining-related contamination from the Springfield Plateau aquifer to enter the Ozark aquifer.
