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Summary and conclusions
Ground water is available throughout the lower Paleozoic stratigraphic section in varying amounts depending on the lithology of the aquifer and the development of secondary permeability in the carbonate rocks. The rocks that comprise the framework of these aquifers show evidence that they have undergone changes induced by solution and tectonic activity many times in the geologic past. Well yields are typically higher where the aquifer materials have been fractured and have interconnected solution channels. Relatively minor amounts of ground water are available from the undifferentiated Mississippian rocks as compared with Cambrian-Ordovician strata. Typical well yields to be expected from the Mississippian range from 10 to 100 gpm (1.3 to 13.4 ft3/min), whereas typical well yields from the Cambrian-Ordovician are in the range of 100-750 gpm (13.4-100 ft3/min).
Ground water flows through these aquifers in a westerly direction in the Tri-State area away from recharge areas in the Ozark region of southern Missouri where the Lower Paleozoic rocks crop out. Locally the movement of ground water in these aquifers is influenced by pumping centers. Ground-water withdrawal from pumping wells has produced large cones of depression in the Miami, Oklahoma-Baxter Springs, Kansas, and in the Pittsburg, Kansas, areas. In some areas static water-level declines in excess of 450 ft (135 m) have been recorded since pumping began. Static water-level declines are generally less than 50 ft (15 m) over most of the area.
Ground-water chemical quality in the lower Paleozoic aquifers varies widely across the region reflecting a change from fresh to saline conditions. The mixing of fresh and saline waters is regionally a simple process but may be more complex locally due to chemical interactions and the influence of geologic structure. Ground waters from the confined portion of the Mississippian aquifer were Na-Cl or Na-HCO3 type and contained sulfide; samples from the unconfined portion were a Ca-HCO3 type water. The ground waters in the freshwater portion of the Cambrian-Ordovician aquifer generally fell into a Ca-HCO3 classification. Ground water from the saline portion of the aquifer was sodium-chloride brines which may contain up to 100 mg/L sulfide. Chloride and total dissolved-solids levels increased in a westerly direction across a wide northeast-southwest-trending chemical-quality transition zone separating the fresh and saline portions of the Cambrian-Ordovician aquifer system. Waters in the Cambrian-Ordovician system westward of the Mississippian outcrop contained sulfide concentrations greater than 0.2 mg/L, as was noted for the overlying confined Mississippian system. The presence of sulfide in the aquifers indicated a reducing hydrogeochemical environment.
Some changes in ground-water chemical quality with time were documented during this study. A number of the multiaquifer wells showed significant changes in groundwater chemical quality during the sampling period. Na-HCO3 type waters from Mississippian aquifers seem to make significant and variable contributions to the total water chemistry of several wells open to both the Mississippian and Cambrian-Ordovician aquifer systems. These findings confirm the results of previous investigations in the area. Ground-water chemical-quality changes over time also were noted for two Cambrian-Ordovician wells in the study area.
The distribution of radium-226, uranium, and uranium-234/uranium-238 activity ratios in waters from the lower Paleozoic aquifers is consistent with the freshwater-transition zone and saline hydrogeochemical partitioning of the aquifer. The freshwater portion generally represents an oxidizing environment; whereas, the transition zone and saline portion, with sulfide-bearing waters, are reducing environments. Radium-226 activity is generally less than 2 pCi/L and uranium-activity ratios are anomalously high in the freshwater portion of the aquifer. In the eastern portion of the transition zone, the uranium-activity ratios remain anomalously high and the radium-226 activity ranges from 2 to 9 pCi/L. In the saline portion of the aquifer, uranium-activity ratios decrease and radium-226 activity ranges upward to 1,600 pCi/L in some of the oil-field brines. Uranium concentrations in the freshwater region are more than two orders of magnitude higher than they are in the saline portion of the Cambrian-Ordovician aquifer. Radium-226 activities range from 0.8 to 3.6 pCi/L for a series of samples collected from wells open only to the Mississippian. These findings suggest that the source of radium-226 in the Cambrian-Ordovician aquifers is not from water leakage from the Mississippian. Furthermore, an eastward movement of the reducing environment in recent geologic time is suggested from the distributions of the various radionuclide data. The distribution of radium-228 is difficult to interpret because of the small number of sample sites and their location.
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Kansas Geological Survey, Geohydrology
Placed on web Sept. 1, 2010; originally published 1987.
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