PRELIMINARY RESULTS FROM A MULTI-TRACER EPIKARST RECHARGE EXPERIMENT: MCCARTY CAVE, TEXAS, USA

We used tracer and drip water analyses to investigate epikarst infiltration and recharge at McCarty Cave, in the Edwards Limestone on the Edwards Plateau of central Texas. Surface water and groundwater resources in the region are being impacted by rapid development, pumping, and cyclical drought conditions, and improved estimates of recharge rates at a variety of scales are required for more accurate groundwater modeling. We applied conservative and non-conservative tracers on the ground surface above the cave to estimate infiltration rates, storage capacity, nutrient uptake, and recharge at a site where soil depths average 30 cm.

An on-site precipitation sampler collects rainwater, and a weather station records environmental parameters at 10-minute intervals. Groundwater samples are collected at drip sites in the cave where drip rate and geochemical parameters are also logged at 10-minute intervals. Water samples for ion and stable isotope analyses are collected using an automatic sampler and analyses were performed at Texas State University. Tracer concentrations and discharge at the drip site were compared with the amount of tracers applied directly above (4 m) the in-cave sampling site, and data were analyzed to calculate tracer velocity and the percent of mass recovered.

Preliminary results show rapid and slow tracer responses. An initial pulse of tracers yielded a velocity of 0.01 m/s, indicative of fracture flow. The later breakthrough of a diffuse-flow tracer signal revealed varying tracer velocities and percent of mass recovered, dependent upon the tracer. Bromide, and chloride peaks result in an average velocity of 5.4014 x 10^-7 m/s. Sodium had an average velocity of 6.1993 x 10^-7 m/s, and nitrate had an average velocity of 9.0829 x 10^-7 m/s. At 264 days post-tracer application, total mass recovery was 0.28% for bromide and sodium, 0.79% for chloride, and 0.45% of nitrate.

These results suggest that a significant amount of bedrock storage exists in the shallow epikarst, which implies that there is much more diffuse matrix flow than we originally hypothesized. Current work is focused on measuring other hydraulic parameters at this site.