UNDERSTANDING GROUNDWATER FLOW PATTERNS IN THE FLORIDAN AQUIFER SYSTEM USING AGE AND GEOCHEMISTRY—PAST, PRESENT, AND FUTURE

During the past five decades, the Floridan aquifer system (FAS) has been used as a natural laboratory by numerous hydrogeologists and geochemists to test hypotheses regarding regional groundwater circulation patterns. The FAS currently supplies more than 4 billion gallons per day of fresh groundwater primarily for irrigation and drinking water throughout the southeastern U.S. This presentation focuses on the use of age-dating techniques and environmental isotopes to better understand flow dynamics in confined and unconfined parts of the FAS. Building on the early studies by Hanshaw, Back, and Rubin in the 1960s and 1970s involving radiocarbon measurements along groundwater flowpaths in confined parts of the FAS, in the late 1980s Plummer and Sprinkle combined radiocarbon measurements with stable isotope data and geochemical reaction modeling to quantify mixing of fresh groundwater with saline water in coastal areas, obtain more accurate radiocarbon ages, and determine residence times of deep groundwater. With the advent of new techniques to date young groundwater (CFCs, SF₆, tritium and helium-3) developed in the 1990s, several studies have quantified mixing of surface water and groundwater in karst areas where the FAS is unconfined or poorly confined and most vulnerable to contamination. Combining these techniques with nitrate isotopes and other chemical indicators, spring-water studies have delineated sources and chronology of nitrate contamination of groundwater and groundwater residence times in unconfined parts of the FAS. Future studies will rely on new transient tracer techniques currently under development, along with multiple inorganic and organic chemical tracers, to assess any changes to this high-quality drinking water source for more than 10 million people.