FRESHWATER LENSES IN THE BAHAMAS: A VIEW FROM THE INSIDE

The classic configuration of the Dupuit-Ghyben-Herzberg freshwater lens in islands, which depicts the lens pinching out at the edges, are in conflict with what is observed in situ on Grand Bahama Island, Bahamas, during cave diving exploration of a portion of the island’s freshwater supply exposed within flooded cave passages. Marine-water, driven by tidal forces and currents, pushes into the interior of the island, and influences lens shape, lens margin morphology, and what is measured at the surface by hydrologists studying the subsurface water without actually going into the island except via wells. The lens does not pinch out to a thin line-like margin, and it is not smooth, but is rather more undulatory in nature. This irregular nature of the margin of the freshwater resource could result in over or under estimation of the freshwater resource. The Dupuit-Ghyben-Herzberg lens model does not take into account the impact of marine currents and tidal forces, or temperature differences between marine and freshwater, all of which modify lens shape. The lens is affected by tidal lift and drop, by expansion of warmer marine water beneath the cooler freshwater lens, and also by friction from differential movement between waters of differing densities. The effect of this pushing and lifting is a large variability in the thicknesses of the mixing zones that can be seen and measured. In one horizontal cave system the thickness of the mixing zone varied from 11.8m at 120m from the beach to less than 1m at 650m inland from the coast toward the north. The main cave entrance is 1085m from the coast where the mixing zone is 2.8m thick. Approximately 1700m from the coast north of the main entrance, the freshwater lens is contained in the rocks overlying the cave, and its maximum depth is marked by a thin dark line along the ceiling, and can no longer be chemically identified within the cave passage. Reasons for variation in lens configuration, thickness of the mixing zone, and the nature of the seaward edge may be: 1) varying permeability and porosity that either dampens or facilitates flow of marine water into the island, 2) distance from the coastline, or 3) some combination of both and/or other unidentified variables.