Paper No. 2
Presentation Time: 8:20 AM
STRUCTURAL, STRATIGRAPHIC AND ANTHROPOGENIC CONTROLS ON THE HYDROLOGY AND WETLAND EVOLUTION OF A GEORGIA BARRIER ISLAND
St. Catherines Island (SCI) is a 20 km by 2 to 4 km barrier island composed of a 2.4 m-7.9 m in elevation Pleistocene core flanked by Holocene ridge and swale terrain and salt marsh on the northeast, east and southeast. Pleistocene and Holocene strata are exposed by 3 m/yr beach retreat and erosion by meandering tidal creeks. Vibracores and Ground Penetrating Radar (GPR) profiles provide subsurface data and hydrologic data are obtained from shallow and deep wells in the Pleistocene core. Sands of the island core thin to the west and southwest where the water table becomes more shallow. Fresh water marsh deposits occupy depressions in the central and western areas of the core. Marsh muds below the sands are important aquitards in both the Pleistocene core and Holocene sand ridges, ponding water in swales, controlling lateral fresh water outflow and tidal flux of salt water. Holocene hydrologic environments have been altered by shoreline retreat, meandering tidal creeks and construction of causeways, dikes and ditches for agriculture and wildlife habitat. Springs and associated fresh water marshes (verified by palynoflora) described in pre-industrial historical accounts of the island core suggest artesian flow from the Upper Floridan aquifer (UFA). Alignment of natural ponds with Coastal Plain joint trends, and recognition of joints in surface exposures suggest joints and faults were conduits for the springs. Water chemistry data reveal that the UFA below SCI has been subject to salt water intrusion by mixing with saline water from the Lower Floridan aquifer, a process requiring vertical conduits. Sag structures observed in GPR profiles are interpreted as resulting from collapse of solution features localized by these conduits. Upward flow and mixing of fresh and saline waters suggests classification of related solution features as hypogene. Sharp lateral changes in the water table observed in GPR profiles and salinity flux in shallow wells may also reflect fault/joint influence. Groundwater flow along joints is indicated by joint-parallel humate concentrations exposed along a Pleistocene scarp, and brackish water grass in the salt marsh at the scarp base. Vertical and lateral variations in structure and stratigraphy and rapid change in hydrochemistry and head indicate a complex and dynamic hydrologic environment.