CHARACTERIZATION OF SURFICIAL AQUIFER PROPERTIES FOR INVESTIGATION OF NITRATE LOADING FROM SEPTIC SYSTEMS INTO CHARLOTTE HARBOR, FLORIDA

Charlotte County, Florida contains over 30,000 septic systems, many of which are in areas where the water table is less than 2 feet below surface. High water tables hinder effective treatment of septic effluent through downward percolation within a traditional drain field. Instead, septic effluent is directly conveyed to the water table which periodically intersects land surface during the rainy season, and has led to outbreaks of E.coli in and around Alligator Bay. Shallow ground water in coastal areas discharges along the fresh water – salt water interface and this raises concern for the transport of environmental contaminants into Charlotte Harbor. A dual tracer test is currently underway to characterize flow directions, velocity, and dispersivity within a sandy surficial aquifer. Ground water velocity of approximately 2 m/yr was estimated from site-specific hydraulic gradients permeameter and sieve size analyses of aquifer sediment. These data were used to design a well field comprised of 10 tracer monitoring wells, installed at distances from 1 to 15 feet down gradient. Conservative tracers, sodium bromide and Rhodamine WT, were simultaneously injected into the shallow aquifer in April, 2018 and monitoring wells are sampled biweekly by local volunteers. Preliminary results show bromide breakthroughs in all but the furthest downgradient well, with Rhodamine exhibiting much slower velocities caused by sorption to organic matter. Tracer concentrations appear sensitive to small-scale heterogeneities and precipitation trends which may lead to vertical smearing and redistribution of tracer mass. Longitudinal plume snapshots indicate that bulk bromide velocity is consistent with a priori velocity estimates with pronounced dispersion along the leading plume edge. Further work includes the characterization of ground water – surface water interaction with widespread canal systems which may serve as rapid transport pathways for septic-related nitrate to Charlotte Harbor.