GROUNDWATER-SURFACE WATER INTERACTION FIELD TESTING: TIDAL FLUCTUATIONS AT LITTLE ST. GEORGE ISLAND, FLORIDA

Along marine coastlines, field studies have shown that tidal fluctuations drive groundwater fluctuations, influence exchange of groundwater with ocean water, and affect water chemistry variations. Little St. George Island, an undeveloped, seldom studied island located in the panhandle of Florida, was selected to evaluate natural physical and chemical relationships of shallow groundwater in response to tidal fluctuations. This project entailed the construction and installation of simple field devices, which monitored water-table elevations, water quality, and horizontal and vertical groundwater gradients. Monitoring tube networks were installed at two transects oriented perpendicular to the Gulf of Mexico and the Apalachicola Bay coastlines.

The bay side transect had 5 monitoring tubes installed over ~90 feet, with a total relief of ~7.5 feet. The ocean side transect had 9 monitoring tubes installed over ~340 feet, with a total relief of ~13.5 feet.

Water-level fluctuations were greatest near the shoreline, as influenced by tidal fluctuations. A mini-potentiometer provided detailed data on vertical groundwater gradients at selected sites. Each showed groundwater discharging into both the ocean and bay, and suggested that variations of this flux are out-of-phase with the tidal periodicity.

Temperature, specific conductance, salinity, dissolved oxygen (DO), and pH were measured. Each parameter, except pH, remained stable at specific ranges throughout the study. Salinity and specific conductance varied strongly with tidal fluctuations in tubes near the ocean, but not in the bay. Specifically, DO varied strongly in G1 in the gulf, while the others remained steady at specific levels. Groundwater beneath the bay floor consistently had high salinity and low DO, whereas in the ocean side varied, but remained fairly stable. The pH of both the bay and ocean water were basic (average pH 8.0), whereas groundwater was more acidic. Unusual chemistry trends were found in depressions on each side of the island, where pH was more acid. Salinity and temperature, as expected, decreased inland, but the depression's groundwater was noticeably warmer and more saline.