Island Groundwater: A Limited and Vulnerable Resource

A source of fresh potable groundwater on an isolated island is the most valuable of resources. Yet it is very limited and also the most vulnerable to contamination and destruction by mankind. The availability of fresh water on an island is directly related to the island size in conjunction with the geologic and climatic conditions. Islands can be generally classified as low islands or high islands. Most low islands are composed of relatively permeable sand or corals into which precipitation readily infiltrates; there are commonly no free flowing surface streams. In contrast, high islands are typically composed of volcanic rock with varying degrees of permeability, but tending to be relatively dense with only secondary fracture permeability; island mountains in oceanic environments also create their own high altitude climate, frequently with a wet side with running streams, and a much drier side from a wind generated "rain shadow" effect. The high islands usually offer more sustainable water development possibilities In oceanic islands, the fresh groundwater within the island is often in a unique balance with the sea water, known as the Ghyben-Herzberg principle. This is based on the principle that fresh water, being less dense than salt water, will tend to float in a lens configuration on the salt water that lies at some depth under islands. The most effective groundwater development is a "skimming" process, by shallow wide diameter wells or infiltration galleries. Since the groundwater lies at shallow depth in a low-lying island, it is highly susceptible to surface contamination from many sources. A well recharge protection zone free of contaminating disposals of any kind must be established and the well itself must be sealed and protected from direct outside contaminant sources.