INFLUENCE OF GEOLOGIC STRUCTURE ON SALTWATER INTRUSION IN THE SURFICIAL AQUIFER ON ST. CATHERINES ISLAND, GEORGIA

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

General chemistry data obtained from the original 6 wells reveal pronounced saltwater intrusion events in the surficial aquifer on the marsh side of the traverse, but not on the ocean side. Additional chemistry data from the new transects installed in 2016 show significant variations in salinity at three wells: N5, M6, and S4, all which lie on the marsh side of the island. Analysis of tidal data show that pulses of saltwater intrusion are driven by large spring-tide and storm-surge events. Because the intrusion occurs in the vicinity of wells N5, M6, and S4, and therefore somewhat localized, it is hypothesized that saline water is moving into the surficial aquifer along preferred structural or stratigraphic pathways.

To help delineate the saltwater intrusion pathways, ground-penetrating radar and electrical resistivity profiles were conducted near wells M6 and S4. Data from these profiles indicate the presence of a pronounced sag structure located near well M6, whereas faulting was found near both M6 and S4. It is hypothesized that prior to modern pumping withdrawals from the Floridan aquifer, artesian water from this carbonate system flowed upwards along regional faults and joints. Over time, solution caverns developed along the faults and joints, some of which collapsed and created sag structures and artesian springs at the surface. Data indicate that large tide events periodically drive saline water laterally, and perhaps vertically, into the surficial aquifer along faults and solution collapse features. Furthermore, it is concluded that the primary mechanism of saltwater intrusion is not diffuse lateral flow of modern seawater, but rather by the more rapid and localized flow of saline water along structural pathways.