INVESTIGATION OF THE SHALLOW HYDROGEOLOGIC SYSTEM ON ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island (SCI), Georgia is a 20 km by 2 to 4 km composite barrier island consisting of a Pleistocene core flanked by Holocene ridge and swale terrain and salt marsh on the northeast, east and southeast. The Pleistocene core is associated with the Silver Bluff paleoshoreline and is covered by Holocene eolian sands that thin westward and local Holocene to Pleistocene fresh water marsh deposits. In March 2016, an existing east to west traverse of six shallow (< 30 ft) research wells was expanded using a tractor mounted geoprobe to extract core and install 12 additional shallow wells forming an 18 well grid in the southern portion of the island. Subsequent water sampling revealed elevated concentrations of Cl, Na, and Mg, with corresponding higher specific conductivity, in some wells (S4, S5) located in the interior of the island, indicating salt water did not follow simple lateral pathways from the sea. Previous research on SCI indicated the deep Upper Floridan aquifer was experiencing salt water intrusion from the underlying Lower Floridan aquifer via structural pathways. The more brackish water discovered in the interior shallow wells also suggests structural pathways. The wells are 20-24 ft deep and all are in sand. Resistivity profiles conducted near wells S4 ands S5 reveal a low resistivity layer between 3 and 20 ft depth, correlative with the surficial sand aquifer. A higher resistivity zone underlies the surficial aquifer and may represent a clay aquitard. The high resistivity layer separates the surficial aquifer from another low resistivity layer interpreted as a deeper (>30 ft) aquifer. Aquifers and the aquitard exhibit significant lateral variation in resistivity and Ground Penetrating Radar (GPR) profiles reveal various degrees of signal attenuation in the sands of the shallow aquifer consistent with lateral and vertical variation in water saturated porosity. The presence of a sharp GPR reflector with extreme signal attenuation below is interpreted as the top of the clay aquitard. Resistivity profiles over a sag structure near the west end of the well traverse reveal a thickened, very low resistivity layer below the aquitard suggesting a structural pathway at this site. This research is supported by Georgia Sea Grant, the St. Catherines Island Foundation and Georgia Southern University.