Sources of nitrate in water from springs and the Upper Floridan aquifer, Suwannee River basin, Florida

In the Suwannee River basin of northern Florida, nitrate-N concentrations are 1.5 to 20 mg l(-1) in waters of the karstic Upper Floridan aquifer and in springs that discharge into the middle reach of the Suwannee River. During 1996-1997, fertilizers and animal wastes from farming operations in Suwannee County contributed,approximately 49% and 45% of the total N input, respectively. Values of delta(15)N-NO3 in spring waters range from 3.9 parts per thousand to 5.8 parts per thousand, indicating that nitrate most likely originates from a mixture of inorganic (fertilizers) and organic (animal waste) sources. In Lafayette County, animal wastes from farming operations and fertilizers contributed approximately 53% and 39% of the total N input, respectively, but groundwater near dairy and poultry farms has delta(15)N-NO3 values of 11.0-12.1 parts per thousand, indicative of an organic source of nitrate. Spring waters that discharge to the Suwannee River from Lafayette County have delta(15)N-NO3 values of 5.40-8.39 parts per thousand, which are indicative of both organic and inorganic sources. Based on analyses of CFCs, the mean residence time of shallow groundwater and spring water ranges between 8-12 years and 12 25 years, respectively.