ASSESSING SOURCES OF CONTAMINANTS IN A KARSTIC SPRINGS BASIN DOWNGRADIENT FROM THE LAND APPLICATION OF TREATED MUNICIPAL WASTEWATER

Approximately 6,800 cubic meters per day of treated municipal sewage effluent are applied at a 142-hectare sprayfield site in northern Florida.  Multiple chemical constituents (nutrients; stable isotopes of N, O, H, and C; 67 organic wastewater compounds, 24 pharmaceutical compounds) and microbiological indicators were used to assess the impact from the land application of wastewater on ground-water quality in a karstic springs basin.  Water samples were collected following high-flow conditions in May 2005 and during low-flow conditions in October 2005 from the treated sewage-effluent reservoir, and wells and springs downgradient from the sprayfield within the 960-km² Ichetucknee Springs basin. Enriched delta ¹⁸O and ²H values were found in water samples from the sprayfield effluent reservoir and a sprayfield monitoring well (MW-7); ground-water samples downgradient from the sprayfield had delta ¹⁸O and ²H concentrations that represented recharge of meteoric water. Boron and chloride concentrations were elevated in water from the effluent reservoir and MW-7, but concentrations in ground water decreased substantially with distance downgradient to background levels in the springs (about 12 km) and indicated at least a 10-fold dilution factor.  Nitrate-nitrogen isotope (delta ¹⁵N-NO₃) values greater than 10‰ were indicative of organic waste sources in water from all sites except Blue Hole Spring (delta ¹⁵N-NO₃= 4.6-4.9‰) indicating an inorganic source of nitrogen (fertilizers).  Elevated levels of total coliforms and enterococci were found in Blue Hole Spring during high-flow conditions.  Concentrations of the insect repellent, N,N-diethyl-metatoluamide (DEET) were detected in low micrograms-per-liter (µg/L) concentrations at all sampled sites (<0.5 to 5.4 µg/L); the highest concentration was measured in Devil's Eye Spring, the farthest downgradient site.  Detection of low concentrations (<0.5 µg/L) of other organic compounds in Devil's Eye Spring indicate possible leakage from a nearby septic tank drainfield.  Results indicate that the Floridan aquifer in the Ichetucknee Springs basin is highly vulnerable to contamination from several anthropogenic sources of nitrate contamination throughout the springs basin.