INFLUENCE OF SOIL GEOCHEMISTRY ON GROUNDWATER EVOLUTION IN THE EDWARDS AQUIFER OF CENTRAL TEXAS

Systematic differences in the geochemistry of vadose groundwaters in the Edwards aquifer of central Texas are traceable to local soil variability. The Edwards aquifer is developed in karstified Cretaceous limestone and is well-suited to examine groundwater evolution processes using vadose groundwaters (i.e., cave dripwaters), phreatic groundwaters, and soil leachates. Variations in Sr isotopes and Mg/Ca and Sr/Ca ratios offer insight into the influence of soils on groundwater geochemistry, sources of dissolved constituents in groundwater, water-rock interaction pathways, and groundwater residence time. Samples from multiple caves across the region, up to 300 km apart, provide the potential to distinguish between local (e.g., within a single cave) versus regional effects.

The Sr isotope composition of both vadose groundwaters (mean=0.7086) and phreatic groundwaters (mean=0.7079) generally falls between values for host carbonates (mean=0.7076) and exchangeable Sr in overlying soils and cave sediments (mean=0.7089). Variations in Sr isotope values likely reflect changes in the relative Sr flux from soils versus carbonate host rocks, which varies in response to residence time and the corresponding extent of water-rock interaction. Mg/Ca and Sr/Ca ratios of vadose dripwaters (e.g., Mg/Ca=0.01 to 0.22) are generally lower than values for phreatic groundwaters (Mg/Ca=0.06 to 0.46), consistent with the longer residence time of the phreatic groundwaters.

Strontium isotope values for cave dripwaters correlate inversely with both Mg/Ca and Sr/Ca ratios. Mass-balance modeling suggests that variations in fluid compositions are regionally controlled by groundwater residence times and water-rock interaction with overlying soils and host aquifer carbonate rocks. Local geochemical differences in dripwaters between individual caves are similar to differences in leachates of soils overlying the caves. These differences in the soils determine the starting point of a fluid evolution model in which waters evolve along a compositional pathway from soil water to vadose dripwaters to phreatic groundwaters. Although soils affect local variability in Sr isotopes and Mg/Ca and Sr/Ca ratios, the controlling processes on dripwater geochemistry and groundwater evolution are regionally extensive.