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

The analysis of groundwater systems on scales ranging from local to regional is greatly enhanced by the judicious use of geochemical data to delineate recharge areas and flow paths, identify zones of interaction between groundwater and surface water or between aquifers, identify natural and anthropogenic factors that can limit the suitability of water for various uses, and to estimate the residence time of groundwater. With regard to the implications for management of groundwater resources, such information can be of great significance to water and natural resource planners, hydrogeologists, regulators, legislators, and managers of city and regional water systems.

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.