SALTWATER INTRUSION ALONG PERMEABLE PATHWAYS IN THE SURFICIAL AQUIFER ON ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island is a barrier island located along the Georgia coast and consists of a Pleistocene core surrounded by Holocene salt marsh and ridge and swale deposits. Since 2011, hydraulic head and chemical data have been collected from the surficial aquifer along an east-west transect of six monitoring wells, which range in depth from 5-8 meters. In 2016, two additional transects were installed in the shallow aquifer, creating a network of 18 wells.

Data from the original 6-well traverse reveal periodic saltwater intrusion events whereby unusually high tides produce salinity spikes in the surficial aquifer. Of considerable interest is how the intrusion events are much more pronounced on the marsh-side of the traverse compared to the ocean side, indicating the presence of permeable pathways within the shallow aquifer system. Preliminary data from the additional wells installed in 2016 show anomalously high chloride concentrations at select locations within the island, thereby supporting the hypothesis that saltwater is intruding along preferred pathways.

Analysis of old topographic maps of the island show the linear alignment of former freshwater ponds and marshes that coincide with regional joint trends mapped by other researchers. Historical records indicate that the former ponds and marshes were fed by artesian springs whose source was the deep carbonate aquifer system. In addition, ground-penetrating radar profiles reveal sag structures in the surficial aquifer on the island. It is believed that prior to major pumping withdrawals from the deep aquifer, artesian water flowed upward along joint and fault traces, creating artesian springs at the surface. Over time, solution caverns developed in the carbonate aquifer, some of which collapsed and caused sag structures in the overlying layers.

It is hypothesized that the observed saltwater intrusion events on St. Catherines Island occur when unusually high tides allow saltwater to preferentially flow into the surficial aquifer along joint and fault trends. Future work will include electrical resistivity profiling and installation of additional wells to help delineate the movement of saltwater within the aquifer. Finally, it can be expected that the saltwater intrusion events will occur more frequently in response to accelerated sea level rise.