AQUIFER SOURCES OF THE PEDERNALES RIVER, TEXAS

An important water resource for central Texas, the Pedernales River flows from the Edwards Plateau through agricultural communities and into the Colorado River upstream of the water intake for the state capital. Aquifer contributions to rivers in karst landscapes can be uniquely complex, and this study combines geospatial analysis with synchronous geochemistry sampling to evaluate source aquifers during baseflow conditions. The contributing aquifers originate from several geologic units with similar carbonate geochemistry and are the sole source of water for the majority of the watershed. Groundwater in this region has a distinct signature with respect to hydrogen and oxygen stable isotope ratios. The isotopic gradient from groundwater to river water follows an evaporitic trend and indicates a strong evaporation control on water chemistry within the watershed. Principal component analysis supports the importance of groundwater inputs on sample geochemistry. Due to somewhat homogeneous aquifer chemistry, interpreting the groundwater source is dependent on the geologic location of springs. The geochemical groundwater signature indicates surface locations that are near springs and allows enhanced spatial analysis of spring locations when exact spring access is limited due to restricted private property. By overlaying the spring locations with surface geology, and therefore identifying host geologic unit and source aquifer, groundwater in the Pedernales River was determined to originate from the Edwards-Trinity Plateau Aquifer, the Trinity Aquifer, the Marble Falls Aquifer, and the Ellenburger-San Saba Aquifer (encompassing Cretaceous, Ordovician, and Cambrian age rocks). This information will be useful for water conservation and planning because Pedernales River and tributary flows have been decreasing since the 1980’s, severe drought has affected the region, and population in central Texas is rapidly increasing. This project was supported by a grant to the Meadows Center for Water and the Environment from the Cynthia and George Mitchell Foundation.