EXAMINING THE SPATIAL DISTRIBUTION OF DENITRIFICATION IN THE UPPER FLORIDAN AQUIFER USING AQUIFER GEOCHEMISTRY AND ISOTOPES OF NITRATE

The hydrogeologically complex Upper Floridan Aquifer (UFA) includes well-developed karst conduits, highly permeable limestone, and varying degrees of aquifer confinement. This complexity leads to heterogeneous aquifer properties, geochemistry, and hydraulic connectivity with the land surface. Increased nitrogen loading in the UFA has contributed to observed ecological shifts in aquatic flora and fauna in many spring systems. Significant differences between the estimated nitrogen loads in springsheds and observed nitrogen export from spring vents have been identified, suggesting that denitrification is mitigating the effects of nitrogen loading. However, defining areas where significant denitrification is occurring or factors that influence its magnitude remain elusive. The objective of this study is to characterize areas of groundwater denitrification in a UFA springshed and examine factors that contribute to enhanced denitrification, using spatially distributed measurements of aquifer geochemistry, nitrate isotopes (δ¹⁵N and δ¹⁸O), and dissolved organic carbon. The subject of this study, Ichetucknee Springs, is typical of many UFA spring systems with increased NOx-N concentrations (0.05 to 0.85 mg/l since 1966), increased filamentous algae cover, and decreased plant diversity. The Ichetucknee Springshed is situated near the edge of a regional confining surface layer, resulting in a loss of aquifer confinement and increased land-surface connectivity. This springshed provides an ideal test bed for examining how the degree of UFA land-surface connectivity, heterogenous aquifer properties, and spatial variability in nitrogen loads to the aquifer influence the occurrence or magnitude of denitrification.