GEOLOGIC FRAMEWORK AND ANTHROPOGENIC IMPACTS ON THE HYDROLOGY AND ECOLOGY OF ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island (SCI) is a 20 km by 2 to 4 km composite barrier island consisting of a Pleistocene core flanked on the northeast, east and southeast by Holocene salt marsh and ridge and swale terrain. The Pleistocene core is associated with the Silver Bluff paleoshoreline and is covered by Holocene eolian sands that thin westward. Holocene to Pleistocene fresh water marsh deposits and ephemeral wetlands are concentrated in a series of depressions aligned along a N22-25^oE strike. This strike coincides with the N24^oE fault strike described by Maslia and Prowell (1988) from geophysical data at Brunswick, Georgia. Coring linked to the development of a 24 well monitoring system reveals a water table aquifer (<11 m depth) hosted in Holocene and Pleistocene sands and a semi-confined aquifer (>12.2 m depth) in Pleistocene sands, separated by an aquitard of Pleistocene clay and clayey sand. Deep well logs suggest the top of the Upper Floridan is ~ 107 to 122 m below the surface. Colonial period accounts of SCI describe crystal springs feeding extensive fresh water wetlands in the island interior. Analyses of palynoflora obtained from cores confirms the existence of these former wetlands which must have been fed by artesian flow from the Upper Floridan aquifer. Reduction of the Upper Floridan potentiometric surface by coastal industrialization eliminated artesian flow on SCI by 1970. Perennial wetlands were replaced by ephemeral wetlands, a process hastened by construction of drainage ditches in the 1950’s, and shrubs and trees now cover most of the former open wetlands. Monitoring and sampling of 4 Upper Floridan wells and the shallow well network indicates salt water intrusion in both systems. Sag structures identified by GPR profiling are the surface manifestation of solution collapse produced by a hypogene Upper Floridan system. Intense dissolution was localized along faults that breeched confining layers and permitted artesian flow prior to 1970. Those same faults permit upconing intrusion from the brackish Lower Floridan sytem and localize lateral salt water intrusion in the shallow aquifers.