EVIDENCE FOR STRATIGRAPHIC PARTITIONING OF THE SURFICIAL AQUIFER ON ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island is a 20 km long by 2 to 4 km wide barrier island located along the Georgia coast. The island consists of a high-standing Pleistocene core surrounded by lower Holocene salt marsh and ridge and swale deposits. LIDAR elevation data show that the Pleistocene core is subdivided topographically into two parts along the north-south trending axis of the island, with the eastern side about 2-3 meters higher than the western. An east-west transect of six monitoring wells, ranging in depth from 5-8 meters, were installed across the core and oriented parallel to the expected groundwater flow directions within the surficial aquifer. Hydraulic head and chemical data have been collected regularly from the wells since 2011.

Chemical data reveal that the surficial aquifer contains a Na-Cl type water whose chemistry appears to correspond to the island topography and, to a lesser extent, with seasonal changes in the water table. The surficial monitoring wells located near the crest of the Pleistocene core are under more oxidizing conditions and have an average pH of 4.9 and total dissolved solids (TDS) of 50 mg/L. In contrast, samples from monitoring wells in topographically low areas of the core are under more reducing conditions and have an average pH of 4.7 and TDS of 95 mg/L. Plotting the general chemistry data on a trilinear diagram shows that in the elevated areas of the core, the surficial groundwater is a more mixed-type, and then transitions to a strong Na-Cl type water in topographically low areas. In addition, ground-penetrating radar profiling reveals a poorly consolidated sand layer capping the topographically high areas of the Pleistocene core.

It is hypothesized that the observed differences in water chemistry within the surficial aquifer on St. Catherines are related to lithologic changes. Based on LIDAR and ground-penetrating radar data, it is suspected that the lithology of the higher portion of the core is capped by Holocene eolian sands and that the topographically lower areas are dominated by marine deposits. This interpretation is supported by the fact that on the trilinear diagram, groundwater samples from the topographic lower areas plot very close to modern seawater. Future work will involve obtaining lithologic cores along with radon and isotopic data to help verify this interpretation.