Characterizing Freshwater Lenses Beneath a Series of Ridge Systems and Natural Communities on the Barrier Island of Cayo Costa, Florida

Shallow subsurface freshwater within the ridge systems of mixed-energy barrier islands are a unique source of groundwater for the natural communities in coastal environments. The higher, sandy ridges and low-lying topography promotes the accumulation of rainfall and localized recharge to store freshwater as lenses. Cayo Costa is an undeveloped, Holocene barrier island located in SW Florida with a wider prograding north end and multiple sets of ridge systems. The objective of this study was to identify and characterize the freshwater lenses beneath a series of ridge systems and corresponding natural communities on the island. Several sites were surveyed along a transect spanning the Beach Dune, Coastal Interdune Swale, Coastal Grassland, Coastal Strand, and Mesic Flatwoods natural communities. A survey was conducted along a transect on the north end of the island. The depth to the water table and salinity was measured using a temporary piezometer, water level meter, peristaltic pump, and handheld YSI probe. The depth to the freshwater/saltwater interface (FSI) was identified using a SibER 32K4 multi-electrode electrical resistivity imaging instrument and processed using Res2DInv inversion software. Preliminary results from the dry season (October - May) include no occurrence of freshwater in the Beach Dune community. A shallow freshwater lens (depth = 0.67 m, FSI = 4.46 m) also exists beneath the low-lying areas of the Coastal Interdune Swale community, which is wedged against a saline lagoon (37.17 ppt), acting as an impermeable boundary. Future work includes additional surveying during the wet and dry seasons and remaining natural communities. Electrical resistivity studies are useful in understanding how hydroclimatic shifts affect the occurrence and distribution of freshwater in unaltered, natural environments in addition to the ecohydrological changes within the ridge systems of mixed-energy barrier islands in SW Florida.