ESTIMATING CONDUIT DIAMETER IN AN EOGENETIC KARST AQUIFER FROM MODIFIED WATER TEMPERATURE SIGNALS

Although rapid flow occurs through high permeability zones of karst aquifers, the transfer of physical and chemical characteristics of water may not travel at the same rate as the water itself. The various travel times can be evaluated in river sink-rise systems, such as the Santa Fe River in north-central Florida, which sinks underground at the sinkhole River Sink and then flows through a series of water-filled conduits that have been mapped by cave diving exploration to be ~15-20 m in diameter (conduit width is slightly larger than the height). The conduits are connected to a series of karst windows where water reemerges briefly and then sinks into the subsurface again before ultimately discharging at River Rise, a first magnitude spring ~8 km downstream of River Sink. We have an on-going monitoring program (since May 2018) in which water level, electrical conductivity, and temperature are measured at two-minute intervals at River Sink, five intermediate karst windows, and River Rise. In the records, we identified distinctive maxima and minima in the electrical conductivity and temperature data around the same time and at similar discharge values to calculate the time required for these features to move from upstream to downstream monitoring stations. In general, temperature signals travel more slowly than electrical conductivity signals. In addition, the difference in signal arrival time increases at stations further downstream, providing field support that thermal retardation increases with flow path length. Using the thermal retardation data, we also estimated the effective conduit diameter as ~0.1-10 m, lower than known diameters from existing cave surveys. The thermal retardation analytical solution (Luhmann et al., 2015) employed does not account for the movement of water into and out of the porous and permeable eogenetic karst walls that surround the conduit. Such exchange would likely enhance the retardation of the temperature signals without significantly modifying the less reactive solute transport reflected in the electrical conductivity, potentially providing an opportunity to quantify the exchange.

Luhmann, A.J., M.D. Covington, J.M. Myre, M. Perne, S.W. Jones, E.C. Alexander, Jr., and M.O. Saar. 2015. Thermal damping and retardation in karst conduits. Hydrology and Earth System Sciences 19, 137-157.