GEOCHEMICAL ANALYSIS OF THE DISTRIBUTION OF RECHARGE AREAS, THE RANGE OF APPARENT GROUNDWATER AGES, AND THE EVOLUTION OF HYDROCHEMICAL SIGNATURES ALONG FLOW PATHS IN THE EDWARDS AND TRINITY AQUIFERS OF CENTRAL TEXAS
An investigation, commissioned in 2016 by the Barton Springs Edwards Aquifer Conservation District (BSEACD), of geochemical indicators of interaction between the Edwards and Trinity aquifers of southern Travis, Hays, and Comal counties has done much to reinforce many important aspects of conceptual hydrostratigraphic and flow models developed over many years. The investigation has also yielded potentially new insight into the evolution of hydrochemical signatures along flow paths and to the use of major-ion chemistry, along with environmental isotopes (2H, 18O, 3H, 13C, 14C) and strontium isotope ratios (87Sr/86Sr) to refine conceptual models of recharge distribution, the range of groundwater ages within the Edwards and Trinity systems, and zones of natural and induced flow between formations. Of particular interest is a well-developed linear association between 3H and 14C that indicates regional mixing between young and old groundwaters, but also a large component of very young (50 to 30 years and less) groundwater distributed not only within the jurisdiction of the BSEACD but across many counties of the Edwards-Trinity plateau. This presentation will be an overview of work on this project but will emphasize implications of the 3H-14C association for understanding not only the distribution of recharge areas, but also the dynamics of the flow within the regional groundwater system.