EFFECTS OF ENVIRONMENTAL PARAMETERS AND PRECIPITATION DYNAMICS ON INFILTRATION AND RECHARGE INTO THE TRINITY AQUIFER OF CENTRAL TEXAS, USA

The karstic Trinity Aquifer is the dominant groundwater resource for communities and private well owners in the central Texas Hill Country. As the region’s population grows, increased pressure is placed on this aquifer system, and parts of the Trinity may experience water level declines of up to 100ft by 2050. Without better management of this resource, numerous spring-fed streams will cease to flow and large portions of the population may face severe water shortages. The goal of this study is to quantify how cumulative environmental effects, and their timing, influence recharge to this system. The ultimate goal is to use this information to improve predictive groundwater models and facilitate the development of sustainable management policies in the region.

At Cave Without A Name, near Boerne, TX, we used multi-year cave drip monitoring coupled with weather station and ecohydrological data to explore relationships between precipitation, environmental parameters, and recharge. A regression model was created to determine the importance of independent variables on the occurrence and amount of recharge into the Trinity. Additionally, we used specific conductance, δD, and δ¹⁸O data in two-component mixing models to investigate recharge composition over varying antecedent moisture conditions, rain amounts, and rain intensities.

Preliminary results show that, under ideal conditions, the system requires a 6mm minimum rain event to produce a recharge response. This is atypical, however, and events of this size only cause a response during winter months when ET is lowest and when the system is saturated by previous rainfall. Rainfall minimums for recharge are more commonly observed around 13mm. Preliminary analyses show that rain amount is the strongest predictor of response followed by soil moisture 25cm below the surface, which is an indicator of antecedent moisture conditions. Additionally, analysis of two-component mixing models shows substantial variation in event water contributions between in cave monitoring sites, as well as site specific variations between rain events. This is likely a result of extensive heterogeneity in the epikarst, variations in piston flow contributions under different antecedent moisture conditions, and changes in infiltration dynamics during rainfalls of different intensities.