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

Two artificial tracer experiments (June 1998 and September 1999) were conducted in a groundwater system surrounding a high-volume (500-1200 m3/d) wastewater disposal well in the Florida Keys to determine the fate of wastewater borne phosphate in the subsurface. SF6 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 limestoneʹs primary porosity was estimated to have rates below 0.3 m/d. The most rapid conduit flow due to the limestoneʹs 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*106 to 2.32*106 l/d. The second experiment used radiolabelled phosphate as the reactive tracer and showed that 32PO4 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 (mu)M was reached. Addition of phosphate-free water to the same core material showed a release of PO4 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.