Temporal Variability in the Quality of Produced Water from Wells Tapping the Ozark Aquifer of Southeast Kansas
State and local agencies have become concerned that the available water supply from the Ozark aquifer in the Tri-state region of southeast Kansas, southwest Missouri, and northeastern Oklahoma may become unusable or require additional water treatment because of deteriorating quality resulting from overdevelopment. Many southwest Missouri and southeast Kansas water supplies withdraw water from a 30-60-mi (48-96-km)-wide transition zone in the Ozark aquifer that separates calcium, magnesium-bicarbonate ground water with low dissolved solids to the east from sodium-chloride brines to the west. Water-quality deterioration within the transition zone could potentially come about as a result of eastward migration or upward movement from deeper horizons of saline water. This study assessed variability in the quality of water produced from wells within the transition zone in southeast Kansas across a variety of time scales. Water samples were collected monthly from nine wells located in the transition zone over a two-year period and frequently during two 50+ hr chemical-quality pumping tests of Pittsburg well 8. The samples were analyzed for conductance and pH, and all major and selected minor dissolved inorganic constituents. Mixing curves of chloride versus bicarbonate/chloride ratio and chloride versus sodium/chloride ratio demonstrate that the produced water from Ozark aquifer wells is a mixture of low dissolved solids, calcium, magnesium-bicarbonate ground waters and sodium-chloride brines. Produced water from wells tapping sources in the Ozark and the overlying Springfield Plateau aquifers is a blend of waters from these sources, although the data suggest that the contribution from the Springfield is small relative to the Ozark aquifer. Fluctuations in the quality of the produced water during pumping most likely result from complex mixing of waters of differing quality from different parts of the Ozark aquifer within the well bore. This is borne out by bicarbonate/chloride ratio versus chloride concentration mixing curves and the dissolved constituent ratios indicative of bicarbonate and sodium in excess of that required to balance calcium + magnesium and chloride, respectively. Comparison of the 1979-1980 data with the 2006-08 data from this project indicates that the chloride concentration in some of the sampled supplies has increased. Based on extrapolation of the maximum estimated rate of chloride concentration increase, the earliest that water from Pittsburg wells 8 and 10 would exceed the recommended drinking water limit for chloride is estimated to be by the years 2045 and 2060, respectively, assuming continuation of the present rates of pumping.
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