TRACER STUDIES AT HEADQUARTERS CAVE: VADOSE ZONE TRACING IN THE EDWARDS AQUIFER RECHARGE ZONE, TEXAS

Tracer tests were conducted in the vadose zone above Headquarters Cave in northern Bexar County, Texas during the spring of 2006. Extensive instrumentation of the cave for a multi-year recharge study allows calculation of flow in specific drip areas and precise estimation of travel times and tracer mass recovery. Results from these tests demonstrate important characteristics about karst vadose zone hydrology. Specifically, the cave collects recharge quickly and directly from the surrounding area, even though it is not primarily formed as a vadose cave. Vertical recharge rates through fractures directly correlate to antecedent precipitation, increasing from 3.1 to 4.4 m/hr with antecedent moisture. Closely spaced injection sites take discrete flow paths into the cave, some of which mix and some of which do not. The entire recharge zone of the Edwards Aquifer should be considered very vulnerable to contamination instead of just observable karst features. Non-sinkhole injection sites are clearly responsive to recharge events. In this study, the inter-sinkhole areas selected for dye injection produced more detections, and at a greater number of drip sites, than the sinkhole injection sites.