Groundwater flow and phosphate dynamics surrounding a high discharge wastewater disposal well in the Florida Keys

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Two artificial tracer experiments (June 1998 and September 1999) were conducted in a groundwater system surrounding a high-volume (500–1200 m 3/d) wastewater disposal well in the Florida Keys to determine the fate of wastewater borne phosphate in the subsurface. SF 6 served as the conservative groundwater tracer in both experiments. Groundwater transport rates are bimodal in nature, both horizontally and vertically. Diffusive/dispersive-type flow attributed to the limestone's primary porosity was estimated to have rates below 0.3 m/d. The most rapid conduit flow due to the limestone's secondary porosity was as high as 20 m/d in the well field and even higher, up to 123.3 m/d, immediately adjacent to the point of injection. Typical transport rates were approximately five times higher in the 1999 experiment than those observed in 1998 commensurate with a wastewater discharge increase from 0.63×10 6 to 2.32×10 6 l/d. The second experiment used radiolabelled phosphate as the reactive tracer and showed that 32PO 4 was rapidly adsorbed onto Key Largo limestone. Recirculation experiments using core material from the site and phosphate-rich water showed a rapid initial uptake of phosphate followed by a slower adsorption until an equilibrium concentration of approximately 26 μM was reached. Addition of phosphate-free water to the same core material showed a release of PO 4 into solution until the same equilibrium concentration was reached. The limestone matrix underlying the study site appears to act as a phosphate buffer once exposed to phosphate-rich water. The mechanisms controlling this buffering capacity are poorly understood at this time.