Timescales for nitrate contamination of spring waters, northern Florida, USA

Residence times of groundwater, discharging from springs in the middle Suwa nnee River Basin, were estimated using chlorofluorocarbons (CFCs), tritium ((3) H), and tritium/helium-3 (H-3/He-3) age-dating methods to assess the c hronology of nitrate contamination of spring waters in northern Florida. Du ring base-flow conditions for the Suwannee River in 1997-1999, 17 water sam ples were collected from 12 first, second, and third magnitude springs disc harging groundwater from the Upper Floridan aquifer. Extending age-dating t echniques, using transient tracers to spring waters in complex karst system s, required an assessment of several models [piston-flow (PFM), exponential mixing (EMM), and binary-mixing (BMM)] to account for different distributi ons of groundwater age. Multi-tracer analyses of four springs yielded gener ally concordant PFM ages of around 20 +/- 2 years from CFC- 12, CFC- 113, H -3, and He-3. with evidence of partial CFC- 11 degradation. The EMM gave a reasonable fit to CFC- 113, CFC- 12. and H-3 data, but did not reproduce th e observed He-3 concentrations or H-3/He-3 ratios, nor did a combination PF M-EMM. The BMM could reproduce most of the multi-tracer data set only if bo th endmembers had H-3 concentrations not much different front modern values . CFC analyses of 14 additional springs yielded apparent PFM ages from abou t 10 to 20 years from CFC- 113, with evidence of partial CFC- 11 degradatio n and variable CFC-12 contamination. While it is not conclusive, with respe ct to the age distribution within each spring, the data indicate that the a verage residence times were in the order of 10-20 years and were roughly pr oportional to spring magnitude. Applying similar models to recharge and dis charge of nitrate based on historical nitrogen loading data yielded contras ting trends for Suwanee County and Lafayette County. In Suwance County, spr ing nitrate trends and nitrogen isotope data were consistent with a peak in fertilizer input in the 1970s and a relatively high overall ratio of artif icial fertilizer/manure whereas in Lafayette County, spring nitrate trends and nitrogen isotope data were consistent with a more monotonic increase in fertilizer input and relatively low overall ratio of artificial fertilizer /manure. The combined results of this study indicate that the nitrate conce ntrations of springs in the Suwannee River basin have responded to increase d nitrogen loads from various sources in the watersheds over the last few d ecades, however, the responses have been subdued and delayed because the av erage residence time of groundwater discharging from springs are in the ord er of decades. (C) 2001 Published by Elsevier Science B.V.