RECHARGE, RESIDENCE TIMES, AND REDOX: CONTROLS ON KARST GROUNDWATER COMPOSITIONS (Invited Presentation)

Karst aquifers recharge diffusely across the land surface and at point sites such as sinkholes; recharged water resides in the subsurface for hours to decades prior to discharge at springs. In addition, some springs reverse flow direction to become recharge points when stream elevations rise above the groundwater table and the intruded water resides in the subsurface for weeks to months. Variations in recharge modes and residence times should allow evaluations of how surface water, typically rich in dissolved oxygen (DO) and organic carbon (OC) relative to groundwater, may impact aquifer redox reactions. We evaluate links between recharge and redox reaction products at several locations in the Suwannee River watershed of north Florida. Following a flow reversal at a first magnitude spring, variations in conservative ion concentrations yield mixing proportions of surface water and groundwater. Deviations from conservative mixing of reactive solute concentrations show stoichiometric OC remineralization, calcite dissolution, and bicarbonate production and loss of dissolved Fe and Mn followed by a gain of Mn over six weeks following the reversal. Other spring systems that do not experience reversals discharge water with old CFC apparent ages ranging from 2 to 4 decades and lower DO and concentrations of the greenhouse gas nitrous oxide (N₂O) than springs that experience reversals. These concentrations suggest extensive denitrification during long subsurface residence times. At a sink-rise system of a Suwannee River tributary, recharged surface water flows through a series of conduits in 10 to 72 hours, with shorter subsurface residence times at elevated stages. Water discharging at the resurgence reflects mostly conservative mixing of recharged N₂O-rich surface water and N₂O-poor groundwater. However, slightly elevated N₂O concentrations occur in water that resides in the subsurface for 20 to 30 hours suggesting a potential time constraint on N₂O production from incomplete denitrification. These results suggest the mode of recharge and subsequent subsurface residence times of recharged water affect the extent of reduction of electron acceptors introduced with the recharge water, the redox potential of groundwater, and concentrations of redox sensitive solute of karst aquifers.