HydrologyHydrology

Hydrology

The water resources of the Cocos (Keeling) Islands consist essentially of groundwater and rainwater. Where conditions are favourable, fresh groundwater on coral islands occurs in the form of shallow freshwater lenses beneath the ground surface. Such lenses are found in some of the larger islands within the Cocos (Keeling) Group. The groundwater from these lenses is the major source of freshwater for potable and other uses, on both Home and West Islands (Falkland 1994).

Due to the generally porous nature of the soils and underlying geology, there is no significant surface run-off. Run-off only occurs in localised areas where the ground is compacted and only for very short periods after heavy rain.

The upper surface of a freshwater lens is the water table and the lower surface is a boundary between freshwater and saline water. The lower boundary is not a sharp interface but is in the form of a transition zone. Within the transition zone, over the space of a few metres, the water salinity increases from freshwater to seawater (Falkland 1994).

The size and salinity distribution of freshwater lenses, particularly the depth of freshwater and transition zones, are dependent on many factors including: rainfall; the amount and nature of surface vegetation; the nature and distribution of soils (these factors influence the evapo-transpiration); and the island’s size, particularly the width from sea to lagoon (Falkland 1994).

Preliminary investigations on North Keeling Island (Falkland 1988, 1992) indicate the presence of a very thin freshwater lens on at least part of the island. It is not known whether this lens is permanent.

Gibson-Hill (1948) states that the North Keeling Island wells were sunk to a depth of 3.6-4.5 metres, before the Second World War, when between 40 and 60 Cocos-Malay people lived there for up to 3 months at the end of each year, collecting coconuts and timber. The water in the wells then was reported to be brackish, though not heavily salted, and was used for washing and cooking rather than drinking.

The broader western side of the island is an average 300-400 metres wide. This roughly equates to the minimum 400 metre width of larger islands on the southern atoll, which is necessary to sustain a freshwater lens on tidally-influenced saltwater (Jacobson 1976).