WATER EXCHANGE BETWEEN CONDUITS AND MATRIX IN THE SANTA FE RIVER SINK-RISE SYSTEM OF THE FLORIDAN AQUIFER

The Floridan Aquifer is characterized by both high matrix permeability and extensive conduit development. At the boundary between the confined and unconfined Floridan Aquifer, sinking streams are a common feature. These streams allow the rapid movement of surface water into underground conduits, which can control water chemical compositions in the aquifer and may introduce surface contaminants to groundwater supplies. Consequently, constraining the parameters that control conduit-matrix exchange is critical for understanding dissolution and for designing groundwater management practices. This study investigates the connection between a sinking stream (the Santa Fe River) and its resurgence (the River Rise) in the unconfined portion of the Floridan Aquifer. The timing and magnitude of exchange between conduits and matrix are provided by discharge calculations and by mixing calculations based on measured specific conductivity. These estimates indicate that flow from matrix to conduits dominated most of the yearlong study. However, flow from conduits to matrix occurred following rainfall events as discharge rates peaked. Underground travel times during two events were calculated using correlation of temperature signals between the River Sink and River Rise and intermediate surface-water exposures. The velocity is on the order of km/day, suggesting large diameter conduits. As a first approximation, flow through the conduit system appears to be reasonably represented as flow through a closed-pipe, although flow may differ at higher discharge rates than those measured in this study. These results demonstrate that management of karst waters requires understanding flow in both the conduits and the matrix rocks as well as the exchange between conduits and matrix.