FRESHWATER RESOURCE EVALUATION ON AN OCEANIC ISLAND WITH RESISTIVITY

Resistivity is an excellent tool for studying the freshwater lens on an oceanic island. Over the last 10 years we have conducted a series of resistivity surveys to increase understanding on the freshwater resource of northern Andros Island, Bahamas. Using many resistivity soundings we have determined the continuity and thickness of the lens and the effects of pumping on it. We have found that the lens does not follow the Ghyben-Herzberg model. It is thinner (~15m) and more uniform in thickness rather than lenticular. The well field on Andros Island extracts large quantities of fresh water and we observed thinning of the freshwater zone in the area of the well field. Field measurements and numerical modeling show that careful resistivity measurements near the open trench wells can provide reliable depths to the freshwater-saltwater interface in the well field. A cross-island profile shows that a single freshwater zone extends completely across the island. We also conducted azimuthal surveys, which indicate the main fracture orientation in the carbonate bedrock. This orientation of higher hydraulic conductivity is important in understanding groundwater flow.