TRACING WASTEWATER PLUME MOVEMENT TOWARD COASTAL WATERS IN THE FLORIDA KEYS

Current wastewater practices in the Florida Keys include advanced wastewater treatment to remove solids and reduce nitrogen and phosphorus loading, followed by shallow gravity-driven injection into the highly permeable Key Largo Limestone between 18-27m. The premise of wastewater remediation by shallow injection is that dilution by ambient saline groundwater and continued microbial denitrification and phosphate removal by adsorption onto carbonate minerals effectively reduces potential surface loadings. However, pharmaceuticals and nutrients have been detected in the nearshore waters surrounding the populated Florida Keys. We performed two dye tracer experiments on Vaca Key (City of Marathon) to track the migration of wastewater effluent, sampling from clusters of monitoring wells at depths of 3, 6, 15, and 27 m within 500 meters of the injection well. Wells closest to the point of injection showed significant quantities of dye within hours of injection at the shallower depths (3 and 6m), presumably driven by the buoyancy of wastewater in the saline groundwater system. At more distant wells, dye breakthrough first occurred at 15m, suggesting an overlying confining layer. Indeed, drillers reported “hard rock” between 35-40 feet, likely one of the soilstone crusts developed during previous Pleistocene lowstands. We also observed low dye concentrations in wells to the south and west; concentrations remained below 2.5 ppb after both injections, suggesting predominant transport to the east and north. Lateral transport rates (tidally driven) ranged from 2-10 m/day. Analysis of the breakthrough curves (using Qtracer2) provided refined estimates of solute transport rates and dispersivities in the subsurface that we are using to refine our 3-dimensional transport and reaction modeling of the wastewater plume using FEFLOW™. Together, observed salinity, tracer and pharmaceutical distributions and numerical modeling demonstrate that wastewater is rapidly moving to the surface and migrating to the island margin. These results, together with nutrient measurements (in progress) will allow us to determine whether wastewater injection in the Florida Keys is further reducing nutrient loading on surface waters or instead is acting as the functional equivalent of direct surface discharge.