Fate of wastewater nitrogen upon injection into a coastal saline groundwater system, Florida Keys, <scp>USA</scp>

Abstract Injection of treated wastewater into the shallow subsurface on islands and along coastlines is a common practice in areas like the Florida Keys, yet little is known about the fate of contaminants after injection. The strong chemical contrast between typically well‐aerated, nitrate‐rich and low‐density wastewater and anoxic, higher‐density, low‐nutrient, saline groundwater creates a dynamic environment demonstrably leading to buoyancy‐driven flow and microbial transformation. Whether injected wastewater discharges to surface waters and leads to their impairment depends on the efficiency of mixing and biogeochemical reactions in the subsurface. This study demonstrates that nitrate concentrations are significantly reduced through these processes at a study site on Vaca Key (Marathon), FL, USA. However, wastewater tracers and wastewater‐derived nutrient nitrogen are found at wells adjacent to Florida Bay. This study, together with previous work on the fate of wastewater phosphate, can inform decision‐making about the need for alternative wastewater disposal methods in the Florida Keys and elsewhere, where considerations of whether shallow injection is the functional equivalent of direct discharge of treated wastewater to coastal waters is a pressing issue.