ST. CATHERINES ISLAND, GEORGIA: SAG STRUCTURES, HYDROLOGY AND SEA LEVEL RISE

St. Catherines Island, Georgia is a 20 km by 2 to 4 km barrier island situated at the head of the Georgia Bight, between the mouths of the Savannah and Altamaha Rivers. The Pleistocene core of the island is flanked by Holocene ridge and swale terrains on the north, northeast and southeast. LIDAR topographic terrain models of the core show a higher (4.3 – 7.9 meters elevation) eastern portion and lower (2.4- 5.0 meters elevation) western portion. The models also reveal drainage patterns associated with former artesian springs and fresh water marshes that have given way to ephemeral ponds and wetlands. Ground Penetrating Radar profiling using a MALA system with 100 MHz and 250 MHz shielded antennae has identified sag structures compatible with a 2 to 5 meter subsidence of sandy surficial strata concomitant with filling of the sag basins. These sag structures are tentatively interpreted as the uppermost manifestation of solution collapse structures that may originate in the Floridan Aquifer. Solution features concentrate and grow along primary hydrologic conduits such as joints and faults; consequently, the sag structures and the linear concentration of former ponds at the surface may mark the conduits that once carried artesian waters to the surface. Joint systems and faults penetrating Coastal Plain strata are well documented (Bartholomew et al., 2007) and a fault in the Brunswick area is a known conduit for salt water intrusion. On St.Catherines Island, the potential communication (via joints and faults) between the shallow unconfined aquifer(s) and the deep confined Floridan Aquifer becomes a critical question as sea level rises. Will the shallow aquifer(s) feed salt water through the joints, faults and associated solution conduits into the Floridan Aquifer? This is a critical question with respect to the sustainability of the most important groundwater resource in the southeast Coastal Plain. Our research group is working to define the internal structure of the Island and model the hydrologic response to eroding shorelines and rising sea level to evaluate this potential threat.