Vocabulary:

• catchment—a structure or natural area in which fresh water is collected
• drought—long period of low rainfall and very dry conditions
• pollution—anything harmful or poisonous that is put into an environment

On low-lying coral atolls, fresh water is a precious resource that must be carefully managed. These islands receive less rain than high volcanic islands, where mountains force winds upward, causing moisture in the air to form rain clouds. For most atoll communities, the main source of fresh water is rainfall that is stored in catchments.

When catchments run dry because of drought or overuse, islanders must use ground water. Ground water fills the spaces between soil particles and cracks in rocks below the surface. When rain falls, it seeps through these openings until it reaches the saturated zone—where the spaces and cracks are already filled with water. The areas that water seeps through to reach the saturated zone are called recharge zones. On atolls, much of the ground water is salty like the surrounding sea. Fresh water from rain floats on top of the salty water because salt water is denser than fresh water. The weight of the fresh water pushes down on the salt water beneath it, forming a freshwater lens.

Freshwater lenses are not very deep and do not contain a very large volume of water. When islanders need fresh water, they drain it from the lens through wells. This empties the lens a little at a time. The lens gets refilled when it rains.

There are several threats to an atoll’s freshwater supply. Digging wells and drawing ground water from them must be done with care. If the wells are dug too deep, or if too much water is drawn out at a location where the lens is not thick enough, salt water may mix with the fresh water, making the water unsuitable for drinking or irrigation. Seepage from septic tanks and cesspools, waste dumps, underground gas tanks, farms, and cemeteries can also pollute the freshwater lens.

In addition to these human-made threats are threats caused by climate change. Scientists predict that atolls will get less rainfall overall. This means that freshwater lenses and catchments will not be recharged as frequently. When rain does come, scientists predict it will be heavier than normal and cause flooding. This may cause pollutants to contaminate recharge zones.

Atoll communities are taking action to secure their freshwater resources—for example, by eliminating leaks along water supply lines and expanding household rainwater catchment systems. Some communities are exploring technologies such as desalination, a process that converts salt water to fresh water. Others are working with scientists to better predict the impact of climate change on the atolls.

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