Geochemical Clues to Groundwater Sources of the Pedernales River, Texas
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Interactions between aquifers and rivers are recognized as important components of the hydrologic system. Central Texas rivers and aquifers are especially well connected due to karstic carbonate geology where gaining and losing streams, springs, and caves are common. The Pedernales River is an important source of water for local communities, the city of Austin, and downstream water users of the Colorado River, to which it drains. The Pedernales River Basin is surrounded by rapidly developing areas with increasing water demands, but the majority of the watershed is developed only for agriculture. Identifying critical areas for water quality and quantity protection while the land is still relatively undeveloped creates an opportunity for proactive water resource protection. The primary objectives of this study were: 1) to compare water quality and geochemistry of water from the Pedernales River, its tributaries, springs across the basin, wells screened in specific aquifers, and historic data; 2) to identify groundwater sources of the river; and 3) to assess whether or not changes in water quality have occurred in the Pedernales River between 1962 and 2015. By conducting this study during baseflow conditions, the water sources are assumed to originate exclusively from groundwater, as opposed to runoff or soil interflow. Geospatial information was evaluated for springs, surface geology, and river gains and losses. Stable isotope ratios, principal component analysis, and spatial analysis highlight the importance of groundwater contributions and human impacts to the river and indicate that evaporation is controlling the geochemical evolution of surface waters. Water quality throughout the watershed was generally good. Hydrogen and oxygen stable isotope ratios for spring and well samples defined a regional groundwater signature, and tributary and main river samples plotted along an evaporative trend with the groundwater signature as the source. Principal component analysis identified two groups of variables which strongly represent variation in sample geochemistry: humanly impacted variables and groundwater signature variables. Spatial analysis also revealed patterns of human impacts and groundwater inputs. The groundwater source locations were combined with surface geology to identify host geologic unit, and therefore source aquifers, of groundwater in the river basin. Groundwater in the Pedernales River is derived from the Edwards-Trinity Plateau Aquifer, the Trinity Aquifer, the Marble Falls Aquifer, and the Ellenburger-San Saba Aquifer.