ANALYZING DISTRIBUTION OF SULFATE IN GROUNDWATER USING CHEMICAL CROSS-PLOTS AND GIS, THE HILL COUNTRY, TEXAS

We use published and new geochemical analyses from wells in the Hill Country near Austin, Texas, to create cross-plots of ions, analyze spatial trends in groundwater chemistry, and test the hypothesis that zones of high sulfate (up to 3,000 mg/L) are the result of dissolution of localized anhydrite layers within the Glen Rose Formation. The Hill Country surrounding Austin relies heavily on groundwater to supply potable water to citizens. Current pumping rates have already begun to affect local spring flows. Groundwater Management Area-9 is charged with optimizing groundwater usage for both residential and environmental purposes. Because of a linear (east-west) zone of groundwater with an unusually high concentration of sulfate across Blanco and Hays Counties, the ability to drill new potable wells has been compromised. Geochemical data was separated by county and by the geologic unit from which sampling took place. Waters are predominately Ca-SO4 and Ca-HCO3, indicating dissolution of gypsum and limestone respectively. Analysis of spatial trends using GIS shows that most high-sulfate samples occur on interfluves between streams, especially in Blanco County. Groundwater in the region generally flows to the east-southeast. High-sulfate samples in Hays County (east of Blanco County) are found both on interfluves and near springs, consistent with upgradient dissolution of gypsum. Preliminary analyses of well logs in the study area confirm the presence of localized anhydrite layers interbedded in the Glen Rose Formation.