Identification of nitrogen sources and transformations within karst springs using isotope tracers of nitrogen

Isotope analyses of nitrate and algae were used to gain better understanding of sources of nitrate to Florida's karst springs and processes affecting nitrate in the Floridan aquifer at multiple scales. In wet years, delta N-15 and delta O-18 of nitrate ranged from +3 to +9aEuro degrees in headwater springs in north Florida, indicating nitrification of soil ammonium as the dominant source. With below normal rainfall, the delta N-15 and delta O-18 of nitrate were higher in almost all springs (reaching +20.2 and +15.3aEuro degrees, respectively) and were negatively correlated with dissolved oxygen. In springs with values of delta N-15-NO3 and delta O-18-NO3 greater than +10aEuro degrees, nitrate concentrations declined 40-50% in dry years and variations in the delta N-15 and delta O-18 of nitrate were consistent with the effects of denitrification. Modeling of the aquifer as a closed system yielded in situ fractionation caused by denitrification of 9 and 18aEuro degrees for Delta O-18 and Delta N-15, respectively. We observed no strong evidence for local sources of nitrate along spring runs; concentrations declined downstream (0.42-3.3 mu mol-NO3 L-1 per km) and the isotopic dynamics of algae and nitrate indicated a closed system. Correlation between the delta N-15 composition of nitrate and algae was observed at regional and spring-run scales, but the relationship was complicated by varying isotopic fractionation factors associated with nitrate uptake (Delta ranged from 2 to 13aEuro degrees). Our study demonstrates that nitrate inputs to Florida's springs are derived predominantly from non-point sources and that denitrification is detectable in aquifer waters with relatively long residence time (i.e., matrix flow).