Subsurface Attenuation of Nutrients and Organic Wastewater Compounds beneath Septic Tank Drainfields in the Woodville Karst Plain, Florida

Effluent from about 18,000 septic tanks is a potential source of nitrogen loading in the Woodville Karst Plain (WKP), an area in northern Florida where numerous sinkholes and a thin veneer of sands and clays overlying the Upper Floridan aquifer (UFA) make ground water vulnerable to contamination. Water samples were collected from three septic-tank systems in the WKP during dry and wet periods to investigate the subsurface movement of chemical constituents [nutrients, organic wastewater compounds (OWCs), pharmaceutical compounds] and microbiological indicators (bacteria and viruses) to the UFA. Each sampled system included septic tank effluent (STE), shallow and deep lysimeters, a drainfield well, and a background well. In addition, water extracts of core material from the unsaturated zone from various depth intervals beneath each drainfield were analyzed for nitrate, chloride, ammonium, OWCs, and pharmaceuticals. Of the 64 OWCs analyzed, 24 were detected in STE samples in low micrograms per liter (μg/L) concentrations; whereas only six OWCs (flame retardants and fragrance compounds) were detected in water samples from lysimeters or drainfield wells. Concentrations (in μg/L) of caffeine (42-130), paraxanthine (7-30), acetominaphen (0.4-3.5), and cotinine (0.03-1.1) in the STE were higher than sporadic detections of these compounds in water samples from lysimeters and drainfield wells (<0.1 μg/L). Sulfamethoxazole was detected in two drainfield wells, but not in their corresponding STE samples. Wastewater indicator bacteria and human enteric viruses were detected in STE samples from each site and intermittently in the lysimeter and drainfield samples. Nitrogen loading rates to ground water were highly variable (3 to 25 kg/yr) at each site along with different nitrogen and chloride profiles with depth in the unsaturated zone. Movement of contaminants to ground water beneath each septic-tank system was related to differences in water use, soil characteristics, depth to ground water, and chemicals used at each site.