Isotopic tracers of sources of water for springs from the Edwards Aquifer, Central Texas, USA

The Edwards Aquifer (EA) in Central Texas provides water supply for over two million people and contains springs that are hydrologically and ecologically important to the region. The residence time of groundwater in the EA ranges from a few days to many thousands of years, since water in the aquifer is contained and transported within both matrix porosity and large conduits. In this study, stable isotopes of water from five springs are investigated for tracing the origin of water and hydrological processes in the EA system during 2017–2019. There is a quick response of the isotopic signals measured at these springs to changes in the isotopic compositions of precipitation. By utilizing an isotope mixing model, we have identified sources of water for these springs with a bi-modal distribution of groundwater supply in the EA: water supplied from deep groundwater with a longer residence time (an average of 67%) and supplemental epikarst interflow with a shorter residence time (an average of 33%). The evolution of hydrochemical water types from HCO3–Ca to HCO3·Cl–Ca·Mg along the EA flowpaths indicates that inputs from epikarst interflow are greater in springs within the artesian zone than the springs within the contributing zone.