High concentrations of sulfate in groundwater, up to ten times the amount recommended by the EPA, in Blanco and Hays Counties, Texas, are of concern as groundwater pumping and population increase. The goals of this study are to characterize the chemistry of groundwaters in Blanco and Hays Counties within the context of Texas Groundwater Management Area-9 and to determine chemically and hydrogeologically the explanation for the spatial distribution of sulfate between the Pedernales River, the Blanco River, and Onion Creek. Insights gained by examining sulfate distribution in Blanco and Hays Counties can be applied to the other counties on the Edwards-Trinity Plateau with similarly high concentrations of sulfate in groundwater. Hydrochemical data from the Cretaceous Edwards and Trinity Groups and water level measurements were used to analyze groundwater chemistry and flow. PHREEQC was used to examine whether phase changes in aquifer minerals could explain the observed geochemical patterns. COMSOL was used to develop a simplified groundwater flow model for a cross-sectional area between the Pedernales River and Onion Creek in Hays County. Water levels indicate that groundwater generally flows southeast in the study area and most streams are gaining. The groundwater flow model indicates a zone of slow-moving groundwater beneath the topographic high between the Pedernales River, the Blanco River, and Onion Creek. Chemical analyses of well data show the presence of four groundwater chemical endmembers in Groundwater Management Area-9; a Ca-Mg-HCO3 fresh endmember, a Ca-Mg-SO4 endmember, a Ca-Mg-SO4-Na-Cl endmember, and a Na-Cl endmember. High sulfate waters generally come from the Upper and Middle Trinity aquifers while fresher waters are from the Edwards aquifer. Physical and chemical analyses indicate that the zone of high sulfate in Blanco and Hays Counties may be the result of gypsum dissolution and dedolomitization in the Upper and Middle Trinity aquifers combined with low rates of groundwater flow beneath the topographic high. Groundwater flow analyses are consistent with those for the Groundwater Availability Models published for the region. Chemical analyses, specifically SO4 distributions and Ca/Mg ratios, are consistent with those found by Nance(2010) on the Edwards Plateau, farther west of the study area. / text

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