ASSESSMENT OF PUMPING-INDUCED WATER QUALITY CHANGES IN THE OZARK PLATEAUS AQUIFER SYSTEM

Recent concerns have been raised that the available supply from the Ozark Plateaus aquifer system in southeast Kansas may become inadequate, rendered unusable, or require additional water treatment because of overdevelopment. The Ozark Plateaus system consists of an upper Springfield Plateau and a lower Ozark aquifer separated by a low-permeability, confining unit of variable thickness. Many southwest Missouri and southeast Kansas water supplies withdraw water from a 40-60-km wide transition zone in the Ozark aquifer that separates low dissolved solids calcium, magnesium-bicarbonate ground waters to the east from sodium-chloride brines to the west. A project was undertaken to assess the influence of pumping on the temporal variability in the quality of water produced from wells within the transition zone in southeast Kansas. Water samples were collected from nine wells located in the transition zone in during a 2-year period and during 2 chemical-quality pumping tests of a high-capacity well. The samples were analyzed for calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, bromide, boron, fluoride, and strontium. Temperature, specific conductance, and pH were measured in the field and laboratory. Chloride vs. bicarbonate/chloride and chloride vs. sodium/chloride ratio mixing curves demonstrate that the produced water from Ozark aquifer wells is a mixture of the low dissolved solids calcium, magnesium-bicarbonate ground waters and the sodium-chloride brines within the Ozark aquifer. Excess sodium and bicarbonate indicate minor leakage of water from the overlying Springfield Plateau aquifer to the Ozark aquifer. Produced water from multiaquifer wells is mixture of waters from both aquifers. The chloride vs. sodium/chloride ratio mixing curve and 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 aquifer within the wellbore. Comparison of the data from this project with 1979-80 data indicates that water quality has deteriorated in some of the sampled supplies.