Effects of Environmental Parameters and Precipitation Dynamics on Infiltration and Recharge into the Trinity Aquifer of Central Texas
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Good predictive models of infiltration and recharge through the vadose zone in karst systems are required for improving groundwater models and sustainable management policies in karst regions. The goal of this study was to estimate annual recharge rates and quantify how cumulative environmental effects, and their timing, influence epikarst infiltration and recharge in the karstic Trinity Aquifer of the central Texas Hill Country. The Trinity Aquifer is the most important groundwater resource for communities and private well owners in the region, and as the region’s population grows, increased demands for groundwater are causing spring-fed streams to cease flowing, wells to run dry, and large portions of the population are facing water shortages, especially during drought periods. Stable isotope and hydrologic data from Cave Without A Name, near Boerne, TX, allowed me to quantify relationships between recharge and antecedent moisture conditions, environmental parameters, and rainfall characteristics. A mixed effects logistic regression model incorporating precipitation sum from rainfall events, antecedent soil moisture, and the sum of potential evapotranspiration for eight weeks prior to rainfall events is successful approximately 85% of the time at predicting whether or not a precipitation event will result in a hydrologic response in the cave. Data were randomly divided into a test set to build the final model, and then validated on the remaining data: both performed equally well. Additionally, a multiple linear regression model was found to be moderately successful (Adj. R2=0.67, P<0.01) at predicting the magnitude of hydrologic responses in an in-cave stream which drains a large groundwater basin. Combined, these models enable me to predict if a recharge response will occur, and if it does, what its magnitude will be. Chloride mass balance calculations were also made using nearly two and a half years of precipitation and in-cave groundwater samples. These calculations produce an average recharge rate of around 8%, which is consistent with previously published literature for the region. As a check, measured discharge from an in-cave stream over the same time period was combined with recharge estimates to calculate a watershed area of ~27 km2, which is realistic given the known extent of the associated cave system and the local geology.