Nitrogen sources and sinks in a wastewater impacted saline aquifer beneath the Florida Keys, USA

Groundwater wells surrounding a high volume advance treatment wastewater (ATW) disposal well in the Florida Keys were monitored for nitrate, nitrite, and ammonium concentrations over a 14 month period. Nutrient concentrations in the shallow subsurface (9 m) show a bimodal distribution between the low salinity wastewater plume and the ambient brackish to saline groundwaters. High NO3− concentrations are found within the ATW plume while the highest NH4+ concentrations are found in shallow wells outside of the plume. Evidence suggests that the overlying mud layer unique to this study site contributes the bulk of the NH4+ observed in these wells. NO3− concentrations at 9 m wells varied by a factor of four in response to concurrent variations in ATW NO3− loads over the coarse of the study. Estimated NO3− uptake rates varied from 32 ± 29 to 98 ± 69 and did not directly correlate with ATW NO3− loading as we hypothesized. We estimate that 70 ± 34% of the NO3− from the treatment plant is removed from solution in the subsurface of the study site. Considerable decreases in NO3− concentration and enrichment of 15NO3− was observed in many wells, indicating significant denitrification or anaerobic ammonium oxidation is occurring in the subsurface. Dissolved inorganic nitrogen concentrations, distributions, and 15N compositions indicate that denitrification is likely the dominant mechanism for N removal in the ATW plume at Key Colony Beach, Florida.