Hydrogeology of a raised coral atoll - Niue Island, South Pacific Ocean

Niue Island, in the south Pacific Ocean, is a raised coral atoll with an area of 259 km^2. The original atoll rim is preserved as a peripheral ridge about 60 m above sea level, and the original lagoon floor now forms an internal basin about 35 m above sea level. Drilling has proved limestone of Miocene age to a depth of more than 200 m. Gravity and magnetic surveys indicate that the limestone probably overlies volcanic bedrock at a depth of about 300 m below sea level. The classical Ghyben-Herzberg freshwater lens does not exist on Niue Island. Results of electrical resistivity depth probes indicate that in the centre of Niue the freshwater layer is 40-80 m thick and beneath the former atoll rim it is 50-170 m thick. It decreases to 0 within 500 m of the coast, where salt water mixing occurs along fissures in the limestone. The irregular configuration of the freshwater layer is ascribed to permeability differences in the limestone. Considering the recharge conditions on Niue, the safe yield of a freshwater layer 50 m thick would be about 4000 m^3 of groundwater per year per hectare. Aquifer tests on two specially constructed bores indicate specific capacity (yield-to-drawdown ratio) of about 12 I/s per metre of drawdown, and a safe long-term pumping rate of about 8 I/s.