USE OF CHEMICAL TRACERS TO ASSESS NITRATE CONTAMINATION AND AGE OF GROUND WATER IN THE WOODVILLE KARST PLAIN

Nitrate contamination of ground water in the Woodville Karst Plain (WKP) has worsened during the past 30 years, based on an increase in nitrate-N concentrations (0.25 to 0.90 mg/L) in Wakulla Springs, a large regional discharge point for water from the Upper Floridan aquifer (UFA) in northern Florida.  Chemical and isotopic tracers were used with geochemical modeling techniques to determine: (1) the distribution, sources, and fate of nitrate in ground water and springs, and (2) residence times of ground water from springs and wells.  From 1997 to 2000, water samples from 46 wells and 4 springs were analyzed for isotopes (N, O, H, He, C), and major ions, nutrients, DOC, and dissolved gases.  Nitrate-N concentrations were highly variable both spatially and vertically in the UFA, which is oxic throughout the WKP, and ranged from <0.02 to 16 mg/L.  Generally, nitrate concentrations greater than 1.0 mg/L were associated with wells with open intervals less than 15 m below land surface.  Delta ¹⁵N-NO₃ values ranged from 1.7 to 13.8‰ (median 6.8‰) for ground-water samples and indicated that nitrate originated from land-surface applied synthetic fertilizer and wastewater effluent.  During low recharge conditions, geochemical models indicated that the chemical composition of spring water (Wakulla Springs and River Sink) was derived from mixtures of 6-30% shallow ground water and 70-94% deep ground water.  During and following a heavy-rainfall period in early September 2000, chemical changes (including decreased nitrate and increased DOC concentrations) in Wakulla Springs and River Sink were modeled with mixtures of 20 and 95% surface water, respectively.  Generally, young ³H/³He ages of 5-45 years (median 25 years) for well and spring waters indicated a dynamic flow system consistent with modeled mixing between surface water and ground water and between shallow and deep parts of the aquifer.  Significantly higher values of magnesium, phosphate, dolomite saturation index, and pH in springs and in water from deep parts of the aquifer (>45 m) relative to a shallow zone (<45m) were indicative of longer ground-water residence times, and likely result from deep regional flow paths where water was recharged through the overlying Hawthorn Formation upgradient from the WKP.