Implications of Electrical Resistivity Data Regarding Ground-Water Lenses on San Salvador Island, Bahamas

Fresh ground-water lenses in the Bahamas may be mapped using electrical resistivity surveys utilizing the large resistivity contrast between fresh-water and salt-water saturated carbonate rock. Ideally, this contrast is a factor of 18 or more; however, porosity variations introduce even larger resistivity variations. The relevant resistivity-porosity relationship determined for surficial rock samples from San Salvador Island is R_o = 1.04 R_w/Φ²¹⁹, where R_o and R_w are the resistivities of rock and pore water, and Φ is the porosity.

These criteria were used to determine the depth of the fresh-water/salt-water interface of fresh-water lenses in the Sandy Point area of San Salvador Island, based on interactive inversion of vertical electrical sounding (VES) data collected during the past 15 years. The resulting computed resistivity layer models indicate a major fresh-water lens of maximum thickness of approximately 30 m under the peripheral hills of Sandy Point with little or no fresh water in the low-lying interior of the region. Pseudo-depth sections and high-resistivity layers at depth suggest confined fresh-water aquifers at depth beneath brackish near-surface ground water in parts of the interior, with fresh-water recharge from the peripheral hills. A resistivity profiling survey of the interior indicates possible conduits connecting several of the brackish blue holes.