GROUNDWATER FLOWPATHS AND AQUIFER INTERACTIONS IN THE UPPER, MIDDLE, AND LOWER FLORIDAN AQUIFERS, SOUTHERN FLORIDA

Dissolved noble gases, TDS, radiocarbon, and stable isotopes of water were sampled in the Upper, Middle, and Lower Floridan aquifers in southern Florida to determine groundwater ages, flow patterns, and intra-aquifer interactions. Near the recharge area in Osceola County, Upper Floridan He concentrations were only slightly above the He solubility equilibrium value. He concentrations increased with age along the groundwater flowpaths towards the eastern and western coasts, providing a relative chronology. Noble gas recharge temperatures of groundwater near the recharge area were similar to the present annual mean air temperature, and lower noble gas recharge temperatures downgradient reflected cooler recharge during the last glacial period, demonstrating groundwater residence times between 15,000 and 25,000 years. The glacial and interglacial noble gas recharge temperature difference was approximately 5 degrees C. Few samples had recharge temperatures intermediate between the glacial and Holocene end members, indicating that there is little leakage from the surficial aquifers to the Upper Floridan downgradient from the recharge area. TDS in Upper Floridan groundwaters were 200 to 858 ppm in the recharge area and increased towards the coasts, where seawater intrusion, in some cases, increased the seawater content of the groundwater to 15 to 75%. Increasing He concentrations and noble gas recharge temperatures from the recharge area to the coasts in the Middle Floridan aquifer resembled the Upper Floridan data. However, Middle Floridan TDS concentrations ranged from 2,000 to 15,000 ppm, likely reflecting mixing between the Middle Floridan and the saline Lower Floridan. The Lower Floridan has low He concentrations, high radiocarbon concentrations, and noble gas recharge temperatures identical to modern seawater collected from the Straits of Florida, consistent with previously published seawater circulation models in the Lower Floridan.