River water intrusion to the unconfined Floridan aquifer

Rapid infiltration of river water into unconfined parts of the Floridan aqu ifer represents a significant component of subsequent ground-water discharg e in regions where the aquifer is dissected by surface streams. A two-year investigation of the Devil's Ear cave system, an extensive saturated condui t network in the Floridan aquifer which underlies a 1.5-km reach of the San ta 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 (Rn-222) and delta(18)O were used to qua ntify these exchanges. Cave diving was employed to collect 50 water samples which were analyzed for tracer content and to observe water clarity condit ions within the saturated karst conduits as far as 1.2 km from the cave ent rance. Rn-222 concentrations measured in the cave system revealed three distinct z ones where river water is rapidly intruded into the Floridan aquifer, A two -component mixing model was used to quantify the intruded river water that 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 fol lowing large precipitation events in the highland provinces of the Santa Fe River basin indicate that river water intrusion to the aquifer can occur i n as little as one or two days, The results of this investigation imply tha t, in regions such as the western Santa Fe River basin, there can be no cle ar distinction between ground and surface waters and intruded river water p rovides a significant vehicle for contamination of the unconfined Floridan aquifer.