THE INFLUENCE OF DIFFUSE RECHARGE AND INTRUDING RIVER WATER ON OXYGEN AND NITRATE CONCENTRATIONS AT TWO KARST SPRINGS

Decreased dissolved oxygen (DO) and increased nitrate (NO₃^-) concentrations have been identified as potential drivers of ecosystem degradation in Florida’s springs, but controls and processes altering DO and NO₃^- concentrations in the aquifer are poorly understood. Our research focuses on two karst springs in north-central Florida (Madison Blue and Peacock Springs) located adjacent to major rivers (Suwannee and Withlacoochee). Both springs receive diffuse recharge in their uplands and point recharge into the spring vents from flooding rivers following storms. Temperature, DO, and conductivity sensors installed at various distance into the water-filled conduit systems that source the springs provided concentration measurements on half hour intervals through multiple storm events. These data and limited grab sampling of the water at the spring vent allowed us to evaluate how the interplay between diffuse and point source recharge affects surface and groundwater DO and NO₃^- concentrations. Diffuse recharge raises the water table, increases DO and NO₃^- concentrations in the groundwater and increases spring discharge. Rising river levels and the resulting intrusion of river water into the conduits provide a source of DO as well as reactive organic carbon to the aquifer. The oxidation of the organic carbon consumes DO and NO₃^-, thereby altering the redox state in the aquifer. The flow rate of the intruding river water and the rate of organic matter breakdown have a greater control on the magnitude of the alteration of the redox state of the aquifer than diffuse recharge in the upland. In contrast, the duration of the altered redox state depends on the extent of diffuse recharge, which forces the intruded river water from the aquifer. Distribution of rainfall controls the type of recharge and thus is critical for redox sensitive components of both groundwater and surface water in fluvial systems characterized by spring discharge.