ASSESSING THE VULNERABILITY OF THE UPPER FLORIDAN AQUIFER TO CONTAMINATION USING SPRING WATER AGES AND MULTIPLE GEOCHEMICAL INDICATORS

The karstic Upper Floridan aquifer (UFA) is highly vulnerable to contamination where it is unconfined or partially confined. Sinkholes, sinking streams, and other karst features facilitate the movement of water and contaminants from the land surface to the UFA. In northern Florida and southern Georgia, numerous springs that discharge water from the UFA have served as early warning systems for the degradation of regional groundwater quality because springs integrate water temporally, spatially, and vertically from large parts of the UFA. Environmental tracer techniques (transient age tracers (CFCs, sulfur hexafluoride, tritium, and helium-3), nitrate isotopes, ion ratios, and other indicators) and lumped-parameter models have been used to assess the sources and chronology of nitrate contamination in over 60 spring basins. Spring waters represent complex mixtures of groundwater flow with a wide range of residence times in the UFA (days to centuries). During prolonged periods of relatively low rainfall, there is a higher contribution of older water with lower nitrate concentrations and longer residence times from deep parts of the UFA compared to normal rainfall periods, when nitrate concentrations are higher and springs contain a larger component of recent recharge. The older water component also contains low dissolved oxygen concentrations that negatively impacts biota in the spring run. Groundwater residence times were roughly proportional to spring magnitude, dissolved oxygen concentrations, and the calcite saturation index. Variability in elevated nitrate-N concentrations (> 0.1 mg/L) and N-isotope data in spring waters were related to the proximity and amount of nitrogen loading from various sources in large spring basins. However, spring response is typically delayed in time relative to inputs because the residence times of groundwater discharging from springs are on the order of decades. Temporal trends in nitrate concentrations in most spring basins were consistent with fertilizer usage in agricultural areas; although the land application of treated wastewater was a source of nitrate contamination in several spring basins.