Controls on the spatial and temporal variability of vadose dripwater geochemistry: Edwards aquifer, central Texas

Abstract

A 4-yr study of spatial and temporal variability in the geochemistry of vadose groundwaters from caves within the Edwards aquifer region of central Texas offers new insights into controls on vadose groundwater evolution, the relationship between vadose and phreatic groundwaters, and the fundamental influence of soil composition on groundwater geochemistry. Variations in Sr isotopes and trace elements (Mg/Ca and Sr/Ca ratios) of dripwaters and soils from different caves, as well as phreatic groundwaters, provide the potential to distinguish between local variability and regional processes controlling fluid geochemistry, and a framework for understanding the links between climatic and hydrologic processes. The Sr isotope compositions of vadose cave dripwaters (mean 87Sr/ 86Sr = 0.7087) and phreatic groundwaters (mean 87Sr/ 86Sr = 0.7079) generally fall between values for host carbonates (mean 87Sr/ 86Sr = 0.7076) and exchangeable Sr in overlying soils (mean 87Sr/ 86Sr = 0.7088). Dripwaters have lower Mg/Ca and Sr/Ca ratios, and higher 87Sr/ 86Sr values than phreatic groundwaters. Dripwater 87Sr/ 86Sr values also inversely correlate with both Mg/Ca and Sr/Ca ratios. Mass-balance modeling combined with these geochemical relationships suggest that variations in fluid compositions are predominantly controlled by groundwater residence times, and water-rock interaction with overlying soils and host aquifer carbonate rocks. Consistent differences in dripwater geochemistry (i.e., 87Sr/ 86Sr, Mg/Ca, and Sr/Ca) between individual caves are similar to compositional differences in soils above the caves. While these differences appear to exert significant control on local fluid evolution, geochemical and isotopic variations suggest that the controlling processes are regionally extensive. Temporal variations in 87Sr/ 86Sr values and Mg/Ca ratios of dripwaters from some sites over the 4-yr interval correspond with changes in both aquifer and climatic parameters. These results have important implications for the interpretation of trace element and isotopic variations in speleothems as paleoclimate records, as well as the understanding of controls on water chemistry for both present-day and ancient carbonate aquifers.