GEOCHEMICAL AND PHYSICAL TRACING OF RAPID RESPONSE IN THE VADOSE ZONE OF THE EDWARDS KARST AQUIFER

An improved understanding of temporal variability of recharge in karst regions can enhance our ability to delineate vadose flow paths and better evaluate aquifer response to precipitation events. We use soil infiltration tracing in conjunction with temporal geochemical variations of vadose groundwater to investigate the movement of dissolved constituents through soils, and to determine relative groundwater residence time. The study area is located in an upland catchment within the Barton Springs Segment of the Edwards Aquifer of central Texas. Soils average 20 cm in thickness, are relatively clay rich and overlie cave-forming Lower Cretaceous limestones. Caves provide access to vadose groundwaters and thus eliminate disturbances typically associated with intrusive monitoring techniques such as lysimeters, monitoring wells, and trenches. Conservative optical brighteners and ionic tracers were injected by storm water at soil sites 5 meters above and up to 35 meters laterally upslope from the monitoring site. Tracer breakthrough occurred within 24 hours of rain-flushed injection, which highlights the rapid movement of water through the soil and vadose zone.

Mg and Sr concentrations provide relative indicators of groundwater residence time. We interpret waters with low Mg/Ca and Sr/Ca ratios (i.e., after initial tracer breakthrough) as having a short residence time, whereas waters with high Mg/Ca and Sr/Ca ratios have a longer residence time (i.e., representing baseflow conditions). A comparison of dripwater trace element concentrations with tracing results indicates that groundwater residence time exerts a primary control on local geochemistry, even on short timescales of days to weeks. These findings are consistent with drip-rate data, which shows that high Mg/Ca and high Sr/Ca dripwaters are correlated with relatively low driprates. The rapid transport of recharge water through soils and significant temporal geochemical variability in dripwater have important implications on water resource management, and the interpretation of trace element variations in speleothems as paleoclimate indicators.