The artesian basin of the Dakotas has been studied for many years. The widely held concept has been that recharge enters the equivalents of the Dakota Sandstone, where they are exposed on the flanks of the Black Hills, and moves through this formation to the area of maximum development of the aquifer in eastern North Dakota and South Dakota. Some anomalies, difficult to explain by this concept, have remained unresolved.
According to the theory here proposed, recharge enters the Lower Mississippian Pahasapa Limestone and the underlying Englewood Formation where they are exposed on the flanks of the Black Hills. The Pahasapa is in part the equivalent of the Lower and Upper Mississippian Madison Group. The limestones are very cavernous where exposed on the flanks of the Black Hills, and seemingly in the subsurface as well. Streams flowing east from the Precambrian core of the Hills lose virtually all their flow in crossing the cavernous limestones. Widespread karst topography was developed on the Madison during the erosion period which preceded deposition of the Pennsylvanian and Permian Minnelusa Formation. Water moves freely through the cavernous limestone with little head loss for more than 100 miles east of the Black Hills.
Pre-Dakota erosion beveled all older rock units and removed many of the intervening beds younger than the Madison. Oil tests indicate that less than 200 feet of strata separate the Madison and Dakota in northern South Dakota east of the Missouri River; this is in marked contrast to the 1800 feet of intervening beds near Rapid City.
It is believed that in a zone east of the Missouri River in northern South Dakota and adjacent North Dakota, water which has moved through the limestone more than 150 miles from the Black Hills moves upward into the basal part of the Dakota Sandstone. Much of the water developed in the area of major water use from the Dakota Sandstone has moved a relatively short distance through this formation.
The chemical character and artesian pressure of water in the Dakota are influenced significantly by recharge from the Madison Group. West of the zone of recharge the water is highly mineralized and largely of a sodium chloride type. Along the zone of recharge and toward the southeast corner of South Dakota, an area of natural discharge, the water generally is of the calcium sulfate type. North of the Precambrian Sioux Quartzite and east of the calcium sulfate water is an area which has sodium sulfate water. It is inferred that this change results from natural base-exchange softening of the calcium sulfate water as it moves eastward through the Dakota Sandstone.