STRUCTURAL CONTROLS ON SALTWATER INTRUSION IN THE SURFICIAL AQUIFER ON ST. CATHERINES ISLAND, GEORGIA

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

Chloride and total dissolved solids data from the original 6-wells have revealed periodic saltwater intrusion events into the surficial aquifer on the marsh-side of the traverse as opposed to the ocean side. Chemistry data from the additional wells installed in 2016 show that salinity is highly variable at two wells, S4 and M6. Analysis of tidal data show that pulses of saltwater intrusion are associated with unusually large, spring-tide events. Due to the localized nature of the intrusion events in the vicinity of wells S4 and M6, it was hypothesized that saline water is moving into the shallow aquifer system along preferred structural or stratigraphic pathways.

To investigate the shallow subsurface near wells S4 and M6, ground-penetrating radar and electrical resistivity profiles were conducted. These data are consistent with the presence of fractures and faults near wells S4 and M6 along and a pronounced sag structure located near well M6. It is hypothesized that prior to modern pumping withdrawals from the regional carbonate aquifer system, artesian water from the Upper Floridan aquifer flowed upwards along regional joint and fault trends. Solution caverns naturally developed along these trends over time, some of which collapsed, creating sag structures in the overlying units and artesian springs at the surface. Data from this study indicates that large tidal events periodically cause saline water to move laterally, and perhaps vertically, into the surficial aquifer along fault and solution collapse features at wells S4 and M6. This study concludes that the primary mechanism of saltwater intrusion is not necessarily by diffuse lateral flow of modern seawater, but by the flow of more saline water along structural pathways.