IMPLICATIONS OF ELECTRICAL-RESISTIVITY DATA REGARDING GROUNDWATER LENSES ON SAN-SALVADOR ISLAND, BAHAMAS

Fresh ground-water lenses in the Bahamas may be mapped using electrica l resistivity surveys utilizing the large resistivity contrast between fresh-water and salt-water saturated carbonate rock. Ideally, this co ntrast is a factor of 18 or more; however, porosity variations introdu ce even larger resistivity variations, The relevant resistivity-porosi ty relationship determined for surficial rock samples from San Salvado r Island is R-o = 1.04 R-w/Phi(2.19), where R-o and Rw are the resisti vities of rock and pore water, and Phi is the porosity, These criteria were used to determine the depth of the fresh-water/salt-water interf ace of fresh-water lenses in the Sandy Point area of San Salvador Isla nd, based on interactive inversion of vertical electrical sounding (VE S) data collected during the past 15 years, The resulting computed res istivity layer models indicate a major fresh-water lens of maximum thi ckness 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 c onfined fresh-water aquifers at depth beneath brackish near-surface gr ound water in parts of the interior, with fresh-water recharge from th e peripheral hills. A resistivity profiling survey of the interior ind icates possible conduits connecting several of the brackish blue holes .