DEEP-SEATED STRUCTURAL CONTROLS AND SALTWATER INTRUSION ON ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island is a composite barrier island located along the Georgia coast consisting of a high-standing Pleistocene core surrounded by lower Holocene salt marsh and ridge and swale deposits. An approximately east-west transect of six monitoring wells, ranging in depth from 5-8 meters, were installed across the island and oriented parallel to the expected groundwater flow directions within the surficial aquifer. Hydraulic head and chemical data have been collected regularly from the surficial wells since 2011. Two additional transects were installed in 2016, creating a network of 18 wells.

Tidal and chemistry data from the original six wells revealed periodic saltwater intrusion events into the surficial aquifer well M6 on the marsh-side of the island. Data from the additional wells installed in 2016 show that tidal-driven saltwater intrusion is also occurring at well S4 in the interior of the island. Ground-penetrating radar and electrical resistivity profiles were conducted near the two wells and revealed the presence of faults and fractures along with a pronounced sag structure located near 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 developed along these trends over time, some of which collapsed, creating sag structures in the overlying units and artesian springs at the surface. Large tidal events periodically cause saline water to move laterally, and perhaps vertically, into the surficial aquifer along the faults and solution collapse feature found in this study.

Chemical data from an earlier study on the island showed that saltwater intrusion is also taking place within the Upper Floridan aquifer by the upward movement of more saline water from the Lower Floridan aquifer along near vertical faults and or solution collapse features. It is hypothesized that these pathways are related to the fault and collapse features found in the surficial aquifer near wells S4 and M6. It is further hypothesized that these vertical pathways are ultimately related to recurrent movement of Mesozoic basement faults located beneath the sedimentary sequence of the coastal plain.