Experimental Investigation of Phosphorus Uptake onto Carbonate Minerals in Coastal Wastewater Injection Zones

The nearshore, oligotrophic waters of the Florida Keys National Marine Sanctuary (FKNMS) are sensitive to fluctuations in nutrient levels, especially to phosphate, a limiting nutrient in the Florida Bay. With over two million people visiting the archipelago each year, effective strategies are needed for managing anthropogenic nutrient inputs from runoff and wastewater and combating eutrophication of the nearshore waters. Shallow injection wells are the primary method for disposing wastewater in the FKNMS. These wells pump treated effluent 27 meters into the Key Largo Limestone (KLL) bedrock. This remediation method relies on the rapid adsorption of phosphate onto the carbonate mineral surfaces of the KLL before the effluent emerges into nearshore waters. However, laboratory studies have demonstrated there are potential drawbacks to this mode of remediation, including limits to phosphate adsorption in the bedrock or desorption under certain conditions. We combined field and laboratory experiments to better understand the permanent and temporary interactions between phosphate and carbonate karst.

We tested the uptake of wastewater-derived phosphate onto KLL prior to emergence into nearshore waters at the Area 3 Wastewater facility in Marathon, Florida. We tracked the migration and chemical evolution of the effluent plume following injection by analyzing concentrations of total and soluble reactive phosphate and sucralose, a conservative pharmaceutical tracer found in wastewater. We additionally tested the behavior and reaction kinetics of phosphate uptake via flow-through experiments on KLL cores using wastewater standards and phosphate-spiked seawater. These cores were investigated by synchrotron-based phase identification to determine the mode of phosphorus incorporation into carbonate minerals. We compare field and experimental data to determine the conditions under which wastewater injection into the KLL is an effective phosphorus remediation strategy.