CONDUIT ENLARGEMENT IN EOGENETIC KARST AQUIFERS: AN EXAMPLE FROM THE SANTA FE RIVER SINK-RISE SYSTEM, NORTH-CENTRAL FLORIDA

We evaluated processes driving conduit enlargement in a 6-km long sink-rise conduit system in the eogenetic (high matrix porosity) Upper Floridan aquifer (UFA) of north-central Florida using chemical analyses and reaction models of surface water and groundwater. Three sources of water enter the sink-rise system, including allogenic water at a swallet, diffuse recharge through the vadose zone that equilibrates with limestone, and upwelling of water from deep in the aquifer. The deep water is more mineralized than the shallow water following reactions with calcite, dolomite, and calcium sulfate minerals. Although allogenic recharge provides significant amounts of water undersaturated with respect to calcite to the UFA, most of the recharge flows through the conduits from the sink to rise during baseflow conditions with little interaction with the surrounding aquifer rocks. This limited interaction is caused by water draining from the matrix porosity, thereby restricting the contact of undersaturated conduit water with the conduit walls. Reaction models indicate dissolution occurred along the conduit flow path only 35% of the 14 times the system was sampled between January 2003 and April 2007. In contrast, when hydraulic head within the conduit exceeds hydraulic head in the surrounding aquifer, allogenic water undersaturated with respect to calcite recharges the surrounding aquifer and dissolves matrix rock in the vicinity of the conduit. This process generates a friable halo around the conduit that is absent in telogenetic (low matrix porosity) karst aquifers, where low matrix permeability focuses flow along joints, fractures, and fissures. These results suggest that conduits may develop in eogenetic karst aquifers by mixing of multiple water sources that flow into and out of the high matrix porosity. This model contrasts with models describing conduit enlargement in telogenetic karst that assume an impenetrable boundary at the conduit wall. Our results provide insights on the difficulty in applying concepts of dissolution in telogenetic karst to eogenetic karst aquifers, and illustrate the influence of matrix permeability on groundwater flow and conduit enlargement in eogenetic settings.