QUANTIFYING ALLUVIUM EFFECTS ON KARST AQUIFER RECHARGE: SHALLOW GROUNDWATER AND SURFACE WATER EXCHANGE IN THE UPPER NUECES RIVER, TEXAS

The Edwards and Trinity aquifers are crucial resources in south-central Texas, with the former serving as the primary water source for over 2 million people in the greater San Antonio area. The Nueces River basin is the largest contributor of recharge to the Edwards Aquifer, and recent studies suggest that areas in the Edwards Contributing Zone supply autogenic and allogenic recharge, with possible inter-aquifer flow. Alluvium mantled over the karstic Glen Rose limestone in the upper Nueces floodplain provides significant storage of river underflow and shallow groundwater, which become an important source of river discharge in low flow conditions and thus recharge to the Edwards and Trinity aquifers. To quantify the role of transient floodplain storage on recharge, we study Candelaria Creek, a spring-fed gaining stream and major tributary to the Nueces. Candelaria Creek flows around 30 cfs year-round (contributing up to 70% of Nueces River flow during drought conditions), while Nueces River discharge decreases by 50-80% upstream of the creek confluence. Fluorescent dye tracer testing indicates that subsurface velocities are greater than 300 m/day through alluvial gravel deposits or karst conduits between the Nueces and Candelaria. Analyses of radon and strontium concentrations and conductivity permit us to constrain the velocities and residence times of springflow, streamflow, and regional groundwater. We quantify alluvium thickness, and thus the potential volume of dynamic floodplain water storage, using geophysical surveys across the study site, including electrical resistivity, ground penetrating radar, and electromagnetic surveys. We upscale our intensive local observations to compute a regional water balance under drought and flood conditions using distributed climate, groundwater, and streamflow measurements at the regional scale. Ultimately, our analyses indicate that alluvium buffers discrete infiltration into karst features and promotes diffuse recharge.