Ground Water – Surface Water Exchange – Abstract

Ground Water – Surface Water Exchange

Abstract

Ground Water – Surface Water Exchange
A View from Inside the Unconfined Karstified Floridan Aquifer with Implications for Ground Water Protection

Abstract

Surface water that directly intrudes into the unconfined parts of the Floridan aquifer represents a significant component of ground water discharge in regions where the aquifer is dissected by surface streams. A two year investigation of the Devil’s Ear cave system, an extensive saturated conduit network in the Floridan aquifer which underlies a 1.5-km reach of the Santa Fe river in north-central Florida, revealed that there is an appreciable and rapid exchange of water between the river and the underlying Floridan aquifer. Natural tracers Radon-222 (222Rn) and d18O were used to quantify these exchanges. Cave diving was employed to collect 50 water samples which were analyzed for tracer content and to observe water clarity conditions within the saturated karst conduits as far as 1.2 km from the cave entrance.

222Rn concentrations measured in the cave system revealed three distinct zones where river water is rapidly intruded into the Floridan aquifer. A two-component mixing model was used to quantify the intruded river water which was found to account for as much as 62 percent of the discharge at Devil’s Ear spring. Observations of diminished water clarity in the cave system following large precipitation events in the highland provinces of the Santa Fe river basin indicate that river water intrusion to the aquifer can occur in as little as one day. The results of this investigation imply that, in regions such as the western Santa Fe river basin, intruded river water provides a significant vehicle for contamination of the unconfined Floridan aquifer.