UNDERSTANDING CONTAMINANT SOURCES, GROUND-WATER RESIDENCE TIME, AND FLOW PATTERNS IN A KARSTIC SPRINGSHED

Human health and ecological concerns have arisen due to a steady increase in nitrate-N concentrations (0.29 to 4.5 mg/L) during the past 40 years in Fannin Springs, a regional discharge point (mean flow 2.8 m³/s) for water from the karstic Upper Floridan aquifer (UFA).  Multiple chemical tracers (³H/³He, CFCs, SF₆, ¹⁵N and ¹⁸O of nitrate; dissolved gases; 66 pesticides and degradates; and 64 organic compounds typically found in domestic and industrial wastewater) were analyzed in samples from three ground-water flow transects and in spring water to identify contaminant sources and estimate ground-water age.  A preliminary ground-water contributing area for Fannin Springs (590 km²) was delineated from a potentiometric surface map (0.3 m contours) of the UFA based on high-resolution water level data. Nitrate-N concentrations were highly variable in the oxic UFA and ranged from <0.02 to 4.7 mg/L.  Delta ¹⁵N-NO₃ values (3.4 to 9.9‰) indicated that local sources of nitrate included both inorganic fertilizer and human/animal wastes.  Higher nitrate concentrations in spring water relative to water from upgradient wells indicate that fertilizers or septic tank effluent inputs likely enter parts of the aquifer that contain conduits close to the spring. Other evidence for agricultural sources included the presence of atrazine at slightly above or below analytical reporting limits (0.007 mg/L) in water from Fannin Springs and three upgradient wells.  An atrazine degradate, deethylatrazine, was detected at concentrations less than 0.006 mg/L in the spring water and in one well water sample.  Wastewater sources of contaminants to ground water included the quantifiable presence of very low concentrations of disinfectants, fragrances, flame retardants, and insect repellants.  Estimated recharge ages were younger in ground water samples taken closer to the spring (<8 years compared to 10-50 years for water from other wells) indicating that these wells likely intersect conduits containing more recent recharge.