ESTIMATING GROUNDWATER INFILTRATION USING AUTOMATIC DRIP RATE LOGGING SYSTEM AND LIDAR AT NATURAL BRIDGE CAVERNS IN CENTRAL TEXAS

Natural Bridge Caverns is located in Comal County, Texas within Early Cretaceous limestone, marl, and shale units of the upper Glen Rose Limestone and the lower Kainer Formation. The caverns are in the recharge area of the Trinity Aquifer and the Contributing Zone of the Edwards Aquifer. The climate regime at the caverns is sub-humid to semi-arid, and recharge to the aquifers are primarily attributed to rainfall runoff. Average rainfall at this location based on records from 1856 to 2008 is 740 mm/yr. ranging from 250 to 1320 mm/yr. The south-central Texas region relies on karst groundwater, of the Edwards and Trinity aquifers as a water sourcefor over two million people in the greater San Antonio area.

This study was designed to evaluate water infiltration through the speleothems at the caverns, using automatic drip loggers and LiDAR. Twenty drip loggers were distributed at actively dripping sites, throughout the Castle of the White Giants and the Hall of the Mountain King chambers and connecting passageways. 3D scan images were collected using a terrestrial LiDAR instrument, and will be used to locate the stalactites feeding the drip loggers and categorize varying flow types based on speleothem morphology. Following eight months of data collection, we are calculating the discharge from all twenty loggers and creating a drip time-series for the observation period. The rainfall data provided by the on-site Texas Mesonet weather station (EA011) will also be used to create a time series for the study period. The drip logger data will then be extrapolated to the entirety of the chambers and passageway to predict the total infiltration within the study area. Results of this study will provide insight related to the frequency and duration of focused recharge into the aquifers, and identify antecedent conditions required to initiate recharge.