USING WELL HYDROGRAPHS FROM A KARST AQUIFER TO SEPARATE THE INFLUENCE OF NEARBY CONDUITS FROM A DIFFUSE RECHARGE COMPONENT

The relationship between matrix and conduits in karst aquifers is important because the amount of mixing between conduit and matrix water will have a direct effect on the ability of surface contaminants to reach groundwater storage. Diffuse recharge provides a source of fresh water, and also can carry surface contaminants to the aquifer. In O'Leno State Park and River Rise State Preserve, located in northern Florida, the Santa Fe River enters the Floridan aquifer at the River Sink, and re-emerges downstream at the River Rise. Between these locations, over 5 km of well-defined conduits have been mapped by cave divers. Water level monitoring of observation wells located in the aquifer near the conduits has shown that during high flow into River Sink, water flows from the conduit sytem into the aquifer. Previous research in this region has defined matrix transmissivities using an analytical method in which change in river stage is used to produce type curves of computed change in head in the aquifer for various transmssivities. These type curves are then matched to observed responses in the observation wells to constrain aquifer transmissivity. Results have produced transmissivities consistent with other large scale studies in the region. However, observed hydrographs commonly have recession limbs greater than predicted water levels. Preliminary calculations show a consistent difference between observed and predicted hydrographs at various well locations, which may reflect diffuse recharge not considered in the analytical solution. Because this region is an eogenetic karst area with high matrix porosity, it is expected that high matrix permeabilities will produce a significant diffuse recharge component. Recently installed shallow monitoring wells and specific conductivity loggers will allow us to use specific conductivity signatures along with well hydrographs to further distinguish diffuse recharge from conduit water.