Nauru, in the central Pacific Ocean, is a raised atoll capping a volcanic seamount arising from an ocean floor depth of 4300m. The land area is 22km, and the island rises to 70m above sea level. Drilling has proved dolomitised limestone of upper Miocene or younger age to a depth of 55m below sea level. Gravity and magnetic surveys indicate that the limestone probably overlies volcanic bedrock at a depth of about 500m.
Reverse-circulation drilling and geoelectrical probes indicate that there is a discontinuous freshwater layer averaging 5m thick beneath Nauru. This is underlain by a mixing zone of brackish water, 60-70m thick. The exceptional thickness of the mixing zone is ascribed to high permeability of the karstified limestone. The forthcoming cessation of phosphate mining will mean a shortfall in water supply which will probably have to be met by the desalination of brackish water. Groundwater beneath the mined-out area, and the settled coastal terrace, is highly vulnerable to pollution, and waste disposal management needs to be considered in relation to groundwater protection.