Karst aquifers with high primary-porosity matrix, such as the Floridan aquifer, have the potential for movement of water between conduits and matrix, with important implications for karst development and the maintenance of groundwater quality. The Santa Fe River Sink and River Rise conduit system, along with the surrounding unconfined Floridan aquifer in north-central Florida, provides a study area to test and quantify conceptual models of exchange between conduits and matrix. The Santa Fe River sinks underground and flows for ∼5 km before reemerging at a first-magnitude spring, the River Rise. During February and March 2003, we recorded discharge rates into the Santa Fe River Sink and out of the River Rise along with hydraulic heads at the River Sink, River Rise, and matrix monitoring wells. Comparison of conduit and monitoring-well hydraulic heads allowed us to track the changes in hydraulic gradient between conduits and wells as a discharge peak passed through the conduits, and the observed head differences between the wells and conduit show a linear relationship with gains and losses of water from the conduit system. The responses of heads at three of the monitoring wells to changes in head within the conduits suggest a transmissivity between 950 and 160,000 m 2 /d, and analysis suggests that the values depend on the scale of measurement. These results demonstrate the potential for transmissivity determinations in karst aquifers by passive monitoring and are consistent with previous observations that transmissivity of karst aquifers varies with the scale over which it is measured.

The regional Floridan aquifer system has been dewatered and otherwise altered extensively throughout much of Florida and coastal Georgia by groundwater pumpage (mining). An increasing threat to this karst aquifer system is structural mining of aquifer formations, primarily to produce fertilizers, titanium products, construction materials, and pet food supplements. These excavations often include mechanical dewatering to facilitate shallow and deep extraction of the aquifer formations. All include reduced aquifer levels, dewatering of the aquifer system, and altered hydroperiods at and surrounding the excavated pits, due to increased void space and evapotranspirative losses (nonmechanical dewatering). Only mechanical dewatering is considered by regulatory agencies during evaluations of applications for structural mining of the aquifer system. Despite refuting data, open pits resulting from these excavations increasingly are portrayed as subsurface “reservoirs” that create new or enhanced sources of water in areas where natural groundwater supplies have been depleted. Four permits and sites were evaluated for excavated and proposed pits in SE, NW, SW, and east-central Florida's natural areas used for groundwater supply. The combined surface area for pits under those four permits will result in ∼237,000 m 3 /d (∼62.7 million gallons per day [Mgd]) of induced discharge from the regional Floridan aquifer system due to nonmechanical dewatering. This volume is more than twice the reported pumpage from the combined three municipal supply wells at the Miami-Dade West Well Field. The ∼123 ha (∼308 ac) SW Florida mine, most recently excavated in an area designated as critical habitat for the federally listed Florida panther, will result in induced aquifer discharge of ∼1505 m 3 /d (0.4 Mgd) due to nonmechanical dewatering. This loss is equivalent to ∼5% of all water used by domestic supply wells in that county in 1990. That recently initiated excavation in SW Florida revealed environmental damage extending beyond the mine boundaries, to surrounding private property, and is the first documented case of such damage solely from aquifer formation mining and nonmechanical dewatering of the aquifer system. A federal court ruled on 22 March 2006 that the U.S. Army Corps of Engineers and U.S. Fish and Wildlife Service had failed to carry out their duty to protect the federal wetlands and protected species by issuing permits for mining in the SE case-study area.