CHARACTERIZATION OF EPIKARST STORAGE, INFILTRATION, AND RECHARGE USING APPLIED TRACERS AND DRIP WATER ANALYSIS: MCCARTY CAVE, SOUTH-CENTRAL TEXAS, USA

Analysis of drip waters and environmental data were used to investigate infiltration and recharge rates in the epikarst at McCarty Cave, on the eastern edge of the Edwards Plateau in central Texas, USA. Surface water and groundwater resources in the region are impacted by rapid development, pumping, and cyclical drought conditions. We applied both conservative and non-conservative tracer ions (Na, K, Cl, B, NH⁴⁺, NO^3-, δ²D) on the surface to estimate and model infiltration, storage capacity, nutrient uptake, and recharge in an area where soil depths average <30 cm over bedrock, and depth to an in-cave monitoring site is less than 4 m.

A weather station and precipitation sampler record environmental parameters at 10-minute intervals and collect rainwater. Drip water samples are collected with an autosampler at the in-cave site where drip rate and geochemical parameters are also logged at 10-minute intervals. Water samples were analyzed for ions and liquid water stable isotopes at Texas State University. Tracer concentrations and discharge data from the drip site were compared with the amount of tracers applied on the surface directly above the in-cave sampling site, and data were analyzed for the breakthrough times, evidence of nutrient uptake, amount of tracer recovered, and evapotranspiration.

Preliminary results reveal both rapid and slow tracer responses. An initial pulse of tracers yielded velocity of 0.01 meters/second, indicative of fracture flow, while a later breakthrough of a diffuse-flow tracer signal revealed varying tracer velocities ranging from an average velocity of 5.4014 x 10^-7 m/s to 9.0829 x 10^-7 m/s, dependent upon the tracer ion. Relative to conservative tracers, decreased concentrations of nitrate indicate that nutrient uptake and de-nitrification processes are occurring, and stable isotope analysis show that evaporation and mixing of shallow soil moisture and recent precipitation with waters stored in the deeper epikarst (>1-2 m) is occurring. Our findings from this and other studies suggest that a surprisingly large amount of storage exists in porous bedrock and soils in the shallow epikarst on the Edwards Plateau, which implies that there is much more diffuse flow and matrix storage than originally hypothesized. Current work is focused on characterizing the epikarst as a critical zone.