HYDROGEOCHEMICAL RESPONSES AT in-CAVE SITES AS INDICATORS OF EPIKARST PROCESSES: CAVE WITHOUT A NAME, CENTRAL TEXAS, USA

Since 2008, continuous and periodic hydrologic, biologic, geochemical, and isotope data have been collected at 6 sites in Cave Without A Name near Boerne, Texas. We used the data to investigate controls on recharge responses to precipitation events at each site, and to develop a conceptual model that describes how surface climatic conditions and drip-site responses contribute to the hydrogeochemical responses observed in the base-level cave stream.

Drip site hydrologic responses range from slow diffuse-flow drips to rapid storm-flow. All sites (including the stream) respond to precipitation events only after a precipitation threshold is passed. With the current data record, the threshold is at least 12mm of rain, and a maximum of 60mm. This range is dependent on short-term (recent precipitation) and long-term antecedent moisture conditions (baseflow), and is season independent. Stream and drip baseflow recession analyses indicate that the stream baseflow is supported by epikarst vadose flow, draining of perched aquifers, and bedrock matrix contributions. Geochemical and hydrologic responses are complex, reflecting a range of controls on the system that produce rapid (hours), delayed (months), and environmental (barometric pressure) signals at the drip sites.

Data from periodic geochemical and isotopic (2D/18O) analyses are being used to investigate changes in proportions of source water at each site, as well as to quantify dilution and homogenization of stored water with recent precipitation during discrete storm events.

Subsurface data are also being combined with continuous sap-flow data, periodic pre-dawn water potential data, and periodic stem-water stable isotope data from 24 trees of 4 species growing above the cave, to better characterize the effects of vegetation on the shallow vadose system and in-cave responses to precipitation events.

Results highlight the importance of long-term monitoring at a range of in-cave sites with different hydrologic properties in order to develop a reasonable conceptual model of flow and storage in the epikarst. Current and future work includes incorporation of surface-applied tracers in combination with periodic geophysical surveys to measure changes in water content, further refine the conceptual model, and to investigate heterogeneities in the epikarst.