USING CHEMICAL AND ISOTOPIC TRACERS TO DETERMINE GROUND-WATER TRANSIT TIMES AND SOURCES OF CONTAMINATION IN KARST SYSTEMS

Concerns about nitrate contamination of the karstic Upper Floridan aquifer (UFA) in northern Florida have prompted the use of isotopic (³H/³He, ⁴He, ¹⁵N) and various other chemical tracers in several cooperative studies to determine transit times of ground water to wells and springs, mixing of waters from different zones, and nitrogen sources.  One area of intensive study was the Woodville Karst Plain where nitrate-N concentrations have steadily increased (0.25 to 0.90 mg/L) during the past 30 years in Wakulla Springs, a large regional discharge point (9.6 m³/s).  Based on water samples from 46 wells and 4 springs, collected from 1997 to 2000, nitrate-N concentrations were highly variable both spatially and vertically in the oxic UFA.  Delta ¹⁵N-NO₃ values (1.7 to 13.8‰) indicated that local sources of nitrate include both inorganic fertilizer and human/animal wastes.  Higher nitrate-N concentrations (>1.0 mg/L) were associated with shallow wells (open intervals less than 15 m below land surface).  Elevated nitrate concentrations in deeper wells were consistent with mixtures of water from shallow and deep zones as indicated from geochemical mixing models and the distribution of mean transit times (5-90 years) estimated using various lumped-parameter flow models. Mean transit times were positively correlated with the calcite saturation index and inversely correlated with dissolved oxygen concentrations indicating mixtures of water from shallow and deep parts of the flow system.  These trends also were consistent with significantly higher values of pH, magnesium, dolomite saturation index, terrigenic ⁴He, and phosphate in springs and deep water (>45 m), which have ground-water transit times of 50-90 years.  The combined use of age-dating techniques with a multi-tracer approach has provided invaluable information about ground-water flow patterns, sources, and time scales of contamination in this complex karstic aquifer system.