APPLICATIONS OF MG/CA AND SR/CA RATIOS TO GROUNDWATER EVOLUTION IN THE EDWARDS AQUIFER OF CENTRAL TEXAS

An improved understanding of the influence of soil composition on water chemistry can enhance our ability to trace flow paths in vadose environments. We applied major and trace element analysis in this regard to the Edwards aquifer in central Texas. This karst aquifer is developed in Lower Cretaceous limestone, and caves in this aquifer provide access to vadose dripwaters. The central Texas region is characterized by thin and stony mollisols, which range in thickness from 3 to 50 cm at the study sites. Soil leachates and waters were analyzed by ICP mass spectrometry for Mg/Ca and Sr/Ca ratios at 11 total dripsites in three caves. Soil samples were collected from directly above dripsite locations.

Soils from above the three study caves have distinct ranges of Mg/Ca and Sr/Ca ratios. Dripwater Mg/Ca ratios at each cave range from values equivalent to those for the cave’s soil leachates to higher values. The largest increase in Mg/Ca occurs at Natural Bridge Caverns. Here, soil leachates yield Mg/Ca ratios of 0.025 to 0.059 and Sr/Ca ratios of 1.0 x 10^-4to 2.2 x 10^-4. Corresponding dripwaters have Mg/Ca ratios of 0.015 to 0.36 and Sr/Ca ratios of 7.72 x 10^-4 to 57.2 x 10^-4.

The results indicate that the regional differences in soil chemistry influence regional differences in vadose water composition, and that waters all subsequently evolve via water-rock interactions with host limestone, albeit to different extents in the different caves. Water compositions at Natural Bridge Caverns have evolved to the greatest extent.