CONDUIT ENLARGEMENT IN EOGENETIC KARST AQUIFERS BY RIVER REVERSALS INTO SPRINGS

Most conceptual models of conduit enlargement assume dissolution in conduits occurs when water that is undersaturated with respect to carbonate minerals flows from recharge points to springs. This process cannot dissolve conduits that are connected to springs fed by distributed recharge because long residence times in the matrix porosity allows recharge to equilibrate with aquifer rocks prior to discharge into the conduits. Becasuse discrete inputs of undersaturated water are lacking for many springs discharging from the Upper Floridan Aquifer another explanation for their development is needed. Springs that normally discharge into rivers in the Suwannee River Basin can reverse flow if allogenic runoff increases the river stage faster than groundwater heads. To determine if the reversal of river water into karst springs can dissolve conduits in eogenetic karst aquifers, we installed conductivity, temperature, depth (CTD) loggers in underwater conduits up to 1.1 km from the entrance of Peacock and Madison Blue Springs to collect high resolution records of specific conductivity (SpC) and temperature. These records were collected twice at Peacock Springs and once at Madison Blue Springs since April 2009. During reversals at both locations, the temp and SpC of conduit water dropped rapidly at the onset of the reversal from stable background values to approach the temperature and SpC of the river water. Water samples at Peacock Springs indicate the calcite saturation index of conduit water dropped to -4.96. Dissolved oxygen (DO) increased to saturation during the reversal and then dropped rapidly to near-zero when water began flowing out of the matrix. These changes in water chemistry indicate that reversals can drive dissolution from the discharge end of conduits as well as alter the redox potential of the aquifer water. Changes in redox potential should be influence microbial communities in the subsurface, oxidation of organic carbon, and overall ecosystems.