Sources and Chronology of Nitrate Contamination of Spring Waters: Integrating Science and Policy Decisions

Human health and ecological concerns have arisen regarding spring waters in Florida as a steady increase in nitrate concentrations has been observed during the past 30 years. The extensive aesthetic, cultural, and recreational value of these springs, which also supply water for human consumption and support critical ecological habitats, could be threatened by the presence of nitrate. As part of the response to these concerns by the State of Florida, several research studies have used various chemical and isotopic tracers to determine sources of nitrate contamination and age of ground water discharging from springs. Since 1997, 60 water samples have been collected from 44 springs and analyzed for isotopic (¹⁵N, ³H/³He, ¹⁸O, ²H, 13C) and other chemical tracers (CFCs, major ions, dissolved gases, SF₆). Delta ¹⁵N values of nitrate ranged from 2.6 to 12.9 per mil (median = 5.8 per mil) and indicated that nitrate in most spring waters originated from synthetic fertilizers. CFCs, ³H/³He, and SF₆, used to estimate the residence time of ground water discharging from springs, indicated that spring-water ages ranged from 5 to 39 years. Concentrations of these multiple transient tracers are consistent with a two-component hydrologic model with mixtures of varying proportions of young water (less than 8 years) from the shallow part of the aquifer system and older water (20-50 years) from the deeper part of the flow system. Given residence times of 20-40 years for ground water discharging from most springs, it could take decades for nitrate concentrations to decrease to near background levels, even with immediate reductions in nitrogen inputs to the land surface. These research results are being used by the State of Florida to inform elected officials, water-resource mangers, and planners that decisions about land use today will affect the quality of ground water in springs for decades.