The use of stable and radioactive isotopes in tracing nutrient sources and sinks in two urbanized coastal environments of Florida: The Florida Keys and Sarasota Bay

Two artificial tracer experiments were conducted in a groundwater system surrounding a high-volume wastewater disposal well in the Florida Keys. Groundwater transport rates are bimodal, both horizontally and vertically. Slow, dispersive-type flow rates were estimated to be below 0.3 m/d while the most rapid conduit type flow was characterized by flow rates as high as 20 m/d and even higher immediately adjacent to the point of injection. Radiolabelled phosphate experiments showed that 32PO₄ was rapidly adsorbed onto Key Largo limestone. The limestone matrix underlying the study site appears to act as a phosphate buffer once exposed to phosphate-rich water. Denitrification in the KCB subterranean wastewater plume was also examined. Estimated denitrification rates ranged from 2 to 258 uM m−3 hr−1 over the course of the study period. Isotopic enrichment of 15NO ₃, which is indicative of denitrification, was also observed at wells located in the wastewater plume. The isotope enrichment factor (ϵ _p/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 δ15N values for seagrass and macroalgae were observed in the southern portion of the Bay near Phillippi Creek. Enriched values were also observed 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 δ15N values were near zero (−4 to +4 per mil). Stormwater ammonium δ15N values were much heavier (+10 to +20 per mil) due to volatilization of ammonia from the stormwater. The isotopic composition of nitrate was usually slightly enriched compared to rainwater from the same storm event. Ammonium and nitrate δ 15N values from Phillippi Creek were similar to stormwater samples. 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.