USING SPECIFIC CONDUCTIVITY TO MONITOR RECHARGE AND SURFACE WATER/GROUNDWATER INTERACTIONS IN THE FLORIDAN AQUIFER SYSTEM

Specific conductivity records from monitoring wells in the karstic Floridan aquifer system in north-central Florida reveal significant heterogeneity in water chemistry at the water table and complex mixing of diffuse (autogenic) recharge, contributions from sinking streams (allogenic recharge), and older groundwater. Water levels and specific conductivities were monitored in surface water of the Santa Fe River and four pairs of nested wells screened at the water table and near the level of a conduit system connecting the Santa Fe River Sink and Rise. Specific conductivity data indicate variable responses to recharge events. Both the amplitude and time lag of the recharge signal vary temporally and spatially. Furthermore, the recharge signal can appear either as a specific conductivity drop, suggesting dilution, or a rise, suggesting flushing of the vadose zone. These variations likely reflect heterogeneities within the vadose zone and within the aquifer, differences in antecedent conditions, and interactions between flow driven by diffuse recharge and by allogenic recharge. Attempts to sample following large recharge events resulted in very complex specific conductivity responses, suggesting that pumping temporarily mixes these different water types. These results illustrate the insight that can be provided by specific conductivity monitoring but also illustrate the difficulties inherent in characterizing karst aquifers.