Electromagnetic profiling and groundwater sampling on Sugarloaf Key and Little Torch Key in the Lower Keys of Florida show that these small, low-lying islands support only brackish-water lenses. On both islands, the lenses have central cores with lower salinities surrounded by areas of increasing salinity. The uppermost few meters of these central zones are uniform and slightly brackish (∼3 ppt) year-round on Sugarloaf Key. On Little Torch Key, which is smaller, there is a similar central, uniform brackish zone during the wet season, with salinity slightly higher than on Sugarloaf Key. These lenses are similar in form to many freshwater lenses, except that the central zone is slightly brackish rather than fresh.
During the dry season Little Torch Key loses the central brackish zone, and salinities increase approximately linearly with depth. This dry-season pattern, in which the freshwater/saltwater mixing zone effectively extends throughout the lens, is similar to that documented on Enjebi Island on the Enewetak Atoll. Seasonal variations on Little Torch Key are less evident outside the central zone.
The central lower-salinity core on Sugarloaf Key coincides with elevations > 0.8 m. Using vegetation as a proxy for elevation, the central zone corresponds to the highest elevations on Little Torch Key as well. Tidal efficiencies on Sugarloaf Key indicate the tidal signal propagates through the high-permeability, buried Key Largo Limestone, and then upward through the overlying lower permeability Miami Limestone that contain the lenses. Mixing driven by tidal pumping is likely responsible for the absence of true freshwater lenses on these islands.