STRUCTURAL CONTROLS ON THE HYDROLOGY OF TWO GEORGIA BARRIER ISLANDS

St. Catherines Island (20 km by 2 to 4 km) consists of a Pleistocene core associated with the Silver Bluff paleoshoreline, flanked by Holocene ridge and swale terrain. The Isle of Hope (6.2 km by 0.5 to 1.7 km) is surrounded by salt marsh, and is part of an older barrier island complex associated with the Princess Anne paleoshoreline. LiDAR images of the southern portion of the Isle of Hope (IoH), within the Wormsloe State Historic Site, reveal linear features shared with St. Catherines Island (SCI), including depressions with a N22-25^oE strike, and additional depressions and linear embayment margins with a N65-68^oW strike. LiDAR images of SCI reveal a N22-25^oE trending linear concentration of depressions, shown as former ponds and wetlands on older topographic maps. Ground penetrating radar (GPR) surveys across the NE trending lineaments on the IoH reveal offsets in reflecting horizons, suggesting low displacement faults. GPR signal attenuation across these lineaments suggests salt water incursion into the surficial groundwater system along faults and joints. Faults and joints also permit vertical communication between deep aquifers. Sag structures identified by GPR profiling on SCI are interpreted as products of collapse of deep solution cavities in the Floridan aquifer that were localized along faults and joints. Water chemistry data from SCI indicates vertical communication between the Upper Floridan aquifer and saline water of the Lower Floridan aquifer. Sag structures have also been identified in GPR profiles from the IoH. Sampling of Floridan wells on the IoH, and the installation of additional shallow well traverses and water sampling on both islands will be used to explore the role of joints and faults as conduits for vertical and lateral salt water intrusion.