Hydrogeochemical Analysis of the Dakota Aquifer in Northwest Kansas

The declining water table of the High Plains aquifer has emphasized the need for an alternate source of water supply. Although the Dakota aquifer in northwestern Kansas has been considered to be a potential source, very little is known about its hydrochemistry. The results from this study show that the aquifer can be divided into four zones. The northern part of the aquifer has a Na-Cl type water and the southern part a predominantly Na-HCO3 type. Between these two zones a mixing zone of Na-Cl-HCO3 type water occurs. In the eastern part of the aquifer Na-HCO type water is mainly found, with localized pockets of Na-Cl type water. The existence of these high-salinity pockets of water is attributed to leakage of fluids form underlying formations through fractures. Saturation models of CaCO3 suggest that calcium carbonate solubility is strongly influenced by varying pH, pCO2, and cation exchange phenomena. The occurrence of fluoride appears to be determined by the solubility of fluorapatite and the increased activity of water. Mathematical simulation suggests that semi-permeable membrane effect may have been a factor in the past in determining the chemical composition of water in the aquifer. In addition, membrane filtration phenomenon is thought to be important in influencing the chemical composition of Dakota water today, and could account for the chemical quality variations of Dakota waters