STRATIGRAPHY AND STRUCTURE OF A GEORGIA BARRIER ISLAND: HYDROLOGIC IMPACTS AND IMPLICATIONS

St. Catherines Island (SCI), Georgia consists of a Silver Bluff Pleistocene core flanked by Holocene ridge and swale, and salt marsh deposits. Holocene dunes cover the higher (4.3 – 7.9 m elevation) northern and eastern core, thinning to the west. A NE-SW trending lowland (2.5 – 5.0 m elevation) occupies the central and western portions of SCI and contains Holocene freshwater wetland deposits. Coring and radiocarbon dating in the lowlands revealed substantial variation in thickness (<1 m to > 3 m) of Holocene sediments and elevation (0.6 m to 4.3 m) of the Holocene/Pleistocene contact. Plant clasts from cores yielded conventional ages of > 37,410 BP for Pleistocene strata. A wood clast in a southern lowlands core gave the oldest Holocene conventional radiocarbon age of 8,410 +/- 30 BP. Some relief on the Pleistocene erosional surface may be due to fault displacement. Faults are indicated by NE-SW and NW-SE linear geomorphic features. The former artesian springs and ongoing saltwater intrusion in the Upper Floridan of SCI require faults. Coring revealed a water table aquifer (< 11 m depth) 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. Head, specific conductivity and chemical data from 24 monitoring wells indicated very high tides and storm surges drive saltwater along permeable pathways to specific wells in the SCI interior. Certain wells in the semi-confined aquifer also react to tidal events, but have a lower background salinity than the unconfined wells. Geophysical profiles (GPR, ER) indicated the permeable pathways are associated with faults and sag structures. The potentiometric surface of the Upper Floridan aquifer, no longer permits artesian springs and the present vertical gradient beneath SCI is downward. Consequently, the fault zone conduits that allow periodic incursion of saltwater into the shallow aquifers now have the potential to transfer saline water downward into Upper Floridan water. Periodic influx of saltwater into the shallow aquifers of SCI should increase in frequency and extent with rising sea level. Plans for new Floridan wells along the GA coast should include reviews of regional well data for signs of salt water intrusion and geophysical surveys to identify faults and sag structures.