Exchange of water between conduits and matrix in the Floridan aquifer

Flow through carbonate aquifers may be dominated by conduits where they are present, by intergranular or fracture porosity where conduits are missing, or may occur in conduits and matrix porosity where both are well developed. In the latter case, the exchange of water between conduits and matrix could have important implications for water management and hydrodynamic modeling. An extensive conduit system has been mapped by dye trace studies and cave diving exploration at the Santa Fe Sink/Rise system located in largely unaltered rocks of the Floridan aquifer of north-central Florida. In this area, the Santa Fe River flows underground at the River Sink and returns to the surface ∼5 km to the south at a first magnitude spring called the River Rise. Limited data show that discharge is greater by 27–96% at the River Rise than at the Sink and that the downstream increase in discharge is inversely related to discharge of the river. Natural SO42− concentrations indicate that ∼25% of the water discharging from the Rise originates from the Sink during low flow. Conversely, SO42− and other solute concentrations indicate that most of the water discharging from the Rise originates from the Sink during floods. Ar ∼40% decrease in Na+ and Cl− concentrations over a 5 1/2-month period at a down-gradient water supply well may reflect flow of dilute flood water from the conduits into and through the matrix at rates estimated to be between 9 and 65 m/day. Calcium concentrations remain constant through time at the well, although flood waters have ∼90% lower Ca2+ concentrations than ground water, perhaps reflecting dissolution of the matrix rocks. This apparent exchange of water between matrix and conduits is important for regional ground water quality and dissolution reactions.