Two artificial tracer experiments were conducted in a groundwater system surrounding a high-volume (500-1200 m3/d) wastewater disposal well in the Florida Keys. Salinity and nutrient data indicate that the wastewater plume extends beneath the entire well field, as far as 175 m from the injection well. The dual tracer experiment using radiolabelled phosphate showed that 32PO4 was rapidly adsorbed onto the Key Largo Limestone. The extremely porous limestone matrix underlying the study site appears to act as a phosphate buffer once exposed to phosphate-rich water. The mechanisms controlling this buffering capacity are poorly understood at this time. Estimated denitrification rates were determined from two different methods and ranged from 2 to 258 uM m-3 hr-1. Isotopic enrichment of 15NO3 was also observed at wells located in the wastewater plume. The isotope enrichment factor (εp/s) estimations for in situ denitrification at KCB ranged from -14.4 to –34.75.
In Sarasota Bay, stable nitrogen isotopes were employed to examine the two largest sources of nitrogen to the Bay: wet atmospheric deposition and stormwater runoff. Seagrass and attached benthic macroalgae were used as proxies for water column DI15N. The heaviest d15N values for seagrass and macroalgae were observed in the southern portion of the Bay near Phillippi Creek, which runs through the middle of Sarasota Bay’s watershed and near Whitaker Bayou, which receives effluent from a wastewater treatment plant. Rainwater ammonium was found to be very light isotopically (0 to -10 per mil) while rainwater nitrate d15N valued were near zero (-4 to +4 per mil). Stormwater ammonium concentrations were usually much lower than the corresponding rainwater concentrations and the d15N values were much heavier (+10 to +20 per mil) due to volatilization of ammonia from the stormwater. Stormwater nitrate concentrations were typically elevated compared to the corresponding rainwater sample due to the entrainment of nitrogen oxides from surfaces. The isotopic composition of nitrate was usually slightly enriched compared to rainwater from the same storm event. Ammonium and nitrate d15N values from Phillippi Creek were similar to stormwater samples with d15NH4 values of approximately +15 per mil and d15NO3 values close to 0 per mil. Mass balance calculations support previous estimates of the percentage of nitrogen loadings to Sarasota Bay from stormwater, wet deposition, and wastewater although better estimates of the wastewater and groundwater DI15N compositions still need to be obtained.