Effect of Recharge Events and Repeated Well Sampling on Monitoring Well Specific Conductivity in An Unconfined Karst Aquifer near a Sinking Stream

Interactions between groundwater, allogenic input, and diffuse recharge were studied where the upper Santa Fe River flows into a 36 meter sinkhole, through a conduit network in the Floridan Aquifer, and discharges at a first magnitude spring. Monitoring using CTD (specific conductivity, temperature, and depth) loggers included three surface water sites and four well pairs with shallow wells screened at the water table (~ 7.5 meters deep) and deeper wells screened at the level of the conduit network (~ 30 meters deep). Specific conductivity was monitored to determine whether low-conductivity surface water flows from the conduit to the deeper monitoring wells and to observe the influence of diffuse recharge on water composition at the shallow monitoring wells. Long-term monitoring was complemented by water sampling, consisting of background sampling through baseflow conditions and high-resolution sampling following storm events.

During most of the base-flow monitoring, specific conductivities in the shallow monitoring wells were higher than those in the deep wells, possibly reflecting evapotranspiration during times of little recharge. During minor rain events, dips in specific conductivity were seen in two of the shallow wells. These decreases are interpreted as recharge reaching the water table and diluting the matrix water. Response to a March 2008 storm event was variable in the monitoring wells. At one pair of monitoring wells, specific conductivity greatly increased two days after the storm. Other wells showed little response or slowly increasing specific conductivity. Perturbations due to water sampling also varied among the monitoring wells, with some wells showing little change during pumping and others showing step changes in specific conductivity that slowly decayed with time following sampling. These results suggest a complex flow system related to the heterogeneity of this karst aquifer, with fluids of different compositions mixed by natural perturbations or pumping.