CHARACTERIZING THE SURFICIAL AQUIFER ACROSS THE PLEISTOCENE CORE OF ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island is a 20 km by 2 to 4 km barrier island located between the Savannah and Altamaha Rivers on the Georgia coast. The Pleistocene core of the island is flanked by Holocene ridge and swale terrain on the north, northeast, and southeast. LIDAR topographic 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 former ponds, marshes and drainage patterns along topographic lows that trend parallel to the long axis of the core. It is believed that artesian springs fed the fresh water ponds and marshes that have since given way to ephemeral wetlands due to post-industrialization loss of artesian pressure. Ground-penetrating radar profiling has identified sag structures in the Pleistocene core that are compatible with 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 features that may originate at depth in the Floridan Aquifer, the principal artesian aquifer for the region.

Recent work has involved installing an east-west transect of four, 4.5 to 7.5 meter deep monitoring wells across the Pleistocene core. Head data indicate that shallow groundwater moves uniformly away from the topographic high and towards the adjoining salt marshes during wet periods. As the water table lowers during dry periods, a small groundwater divide develops, creating a reversal in groundwater flow within the topographic low adjacent to the core. Water chemistry data show that within the topographic low, the surficial aquifer contains Na-Cl type waters that are acidic (pH 4.5 to 5.0) and under reducing conditions. On the topographic high portion of the core, the shallow aquifer contains Na-Cl-SO₄ type water that is less acidic (pH 5.5) and under oxidizing conditions. From these data it is hypothesized that any chemical signature from the previous discharge of alkaline, Ca-HCO₃ type water associated with the springs and sag structures has been erased by the loss of positive artesian pressure within the Floridan Aquifer over the past 40 years.