Anomalous fresh water lens morphology on a strip barrier island

Abstract

Electromagnetic surveys of a young, wholly sedimentary, strip barrier island (St. George Island) on Florida's Gulf Coast reveal a Dupuit-Ghyben-Herzberg fresh water lens with the thickest part located close to the ocean side of the island at both undeveloped and developed sites. This result is at odds with those obtained on other barrier beaches, where the lens is either approximately symmetric or skewed toward the lagoon side due to higher sea level and greater upper beach storage on the ocean side. The inferred morphology of the fresh water lens on St. George Island can be attributed to variations in hydrogeologic (e.g., hydraulic conductivity, presence of semipermeable confining layer) and/or boundary conditions (e.g., recharge, higher sea level on the lagoon side), but the effects of variations in these factors on the shape of the lens are not separable. We therefore use independent constraints on hydraulic conductivity (from tidal pumping analyses), hydrofacies distribution, and recharge patterns to assess the plausibility that lateral variations in various physical parameters produce the observed lens shape. Dune ridge topography, which may constitute a significant portion of the width of a narrow island, perturbs recharge and shallow ground water flow patterns and may contribute to the development of a thick fresh water lens on an island's ocean side. However, topographic variations alone cannot account for the anomalous form of the island's fresh water lens.