Fate of Effluent-Borne Contaminants beneath Septic Tank Drainfields Overlying a Karst Aquifer

Groundwater quality effects from septic ranks were investigated in the Woodville Karst Plain, an area that contains numerous sinkholes and a thin veneer of sands and clays overlying the Upper Floridan aquifer (UFA) Concerns have emerged about elevated nitrate concentrations in the UFA, which is the source of water supply in this area of northern Florida At three sites during dry and wet periods in 2007-2008, water samples were collected from the septic tank, shallow and deep lysimeters, and drainfield and background wells in the UFA and analyzed for in chemical indicators including nutrients. nitrate isotopes, organic wastewater compounds (OWCs), pharmaceutical compounds, and microbiological indicators (bacteria and viruses) Median NO3-N concentration in groundwater beneath the septic tank drainfields was 20 mg L-1 (8 0-26 mg L-1) After adjusting for dilution, about 25 to 40% N loss (from dentrification, ammonium sorption, and ammonia volatilization) occurs as septic tank effluent moves through the unsaturated zone to the water table Nitrogen loading rates to groundwater were highly variable at each site (3 9-12 kg N yr(-1)), as were N and chloride depth profiles in the unsaturated zone Most OWCs and pharmaceutical compounds were highly attenuated beneath the drainfields: however, five OWCs (caffeine, 1,7-dimethylxanthine, phenol, galaxolide, and tris(dichloroisotopropyl)phosphare) and two pharmaceutical compounds (acetaminophen and sulfamethoxazole) were detected in groundwater samples Indicator bacteria and human enteric viruses were detected in septic tank effluent samples but only intermittently in soil water and groundwater Contaminant movement to groundwater beneath each septic tank system also was related to water use and differences in lithology at each site.