SOURCES OF NITRATES AND AGE OF YOUNG FRACTION OF GROUND WATER FROM SILVER SPRINGS, FLORIDA

          Nitrate concentrations in Silver Springs, Florida, have increased from about 0.5 milligrams per liter (mg/L) in the 1970s to about 1.0 mg/L in 2003. During 2001-2002, nitrate concentrations in water from 56 wells in the spring basin ranged from less than 0.02 to 12 mg/L, with a median of 1.2 mg/L. Sources of nitrogen in the basin include atmospheric deposition, fertilizers, and human and animal wastes. Determining the relative influence of the various nitrate sources is problematic. The ratio of ¹⁵N/¹⁴N in ground-water samples ranged from -0.5 to 11.5 per mil, with a median of 4.9, indicating a mixture of both organic and inorganic sources. Four water samples from the Main Spring of Silver Springs analyzed for ¹⁵N/¹⁴N showed two distinct values, one (3.8 per mil) indicating inorganic N and the other (8.7 per mil), indicating mixed sources with a strong influence of organic N. The values did not seem to be related to spring discharge. Water samples from 35 wells in the basin and 3 springs of the Silver Springs group were analyzed for a suite of organic compounds commonly found in wastewater. The most commonly detected compound was N,N-diethyl-meta-toluamide (DEET), which was detected in 27 wells and all 3 springs. Samples collected from the three springs in January 2002 also were analyzed for CFCs, SF₆, and ³H/³He_trit. CFC concentrations in the spring-water samples higher than present-day atmospheric levels were indicative of wastewater influence and precluded the use of CFCs for age dating. SF₆ and ³H/³He_trit data yielded apparent ground-water ages (based on a piston-flow model which may not adequately represent the complex flow in the karstic aquifer system) ranging from about 8-27 years for the young fraction of ground water.