THE HYDROLOGIC EVOLUTION OF ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island (SCI), Georgia is a 20 km by 2 to 4 km barrier island composed of a Pleistocene core flanked by Holocene ridge and swale terrain. Colonial era records describe freshwater springs and marshes along the central and western side of the Pleistocene core that were probably fed by the Upper Floridan aquifer. Pumping withdrawals that accompanied industrial development of the Georgia coastal plain reduced hydraulic head within the Upper Floridan. Springs and marshes were transformed to ephemeral ponds and wetlands, and drainage ditches further reduced these wetlands in the 1950’s. The hydrologic system of SCI has been investigated using geophysics, map and photo analysis, stratigraphy and palynology, and water chemistry. Vibracores extracted from remnants of the freshwater marshes contain an upper meter of organic-rich sediment with an open freshwater palynoflora that includes Sphagnum, Nuphar, Nymhaea and Myriophyllum. Deeper samples indicate mixing of warm- and cool-temperate plant communities known in Late Pleistocene strata. Ghost shrimp burrows in sand at 2 to 3 m depth typifies ancient near-shore marine deposits. Ground penetrating radar surveys identified sag structures compatible with 2 to 5 m subsidence of sandy surficial strata concomitant with filling of the sag basins. These sag structures are interpreted as a shallow manifestation of solution collapse structures that developed along joints or faults penetrating the Floridan aquifer. The alignment of ponds and wetlands on SCI topographic maps and photos (1957) coincides with the strike of joints cutting Coastal plain strata. Sampling of four Upper Floridan water supply wells on SCI reveals salt water intrusion; however, Piper diagram analysis eliminates downward intrusion from the surficial aquifer and lateral intrusion from the sea. The water chemistry is compatible with mixing between the Upper Floridan and more saline water of the Lower Floridan. The joint/fault systems and associated solution enhancement that provided conduits for Floridan water to reach the surface as artesian springs prior to industrialization, now appear to be conduits for salt water intrusion.