Ground water on tropical Pacific Islands: understanding a vital resource

Underground water or “ground water” is the primary source of water on many tropical Pacific islands. Contained in porous, regionally extensive geologic formations or “aquifers,” fresh ground water on these islands floats on and is surrounded by more dense saltwater from the ocean. The potential for an aquifer to provide a reliable source of good-quality water depends on the amount of recharge that occurs from rainfall, the physical properties of the aquifer, and how the water is pumped or removed from the ground. Because of their relatively small size and oceanic setting, ground-water resources on tropical Pacific islands are vulnerable to overpumping and saltwater intrusion, especially during droughts caused by climatic variations such as El Niño events. Also, this vulnerability is made worse because the effects of ground-water pumping are initially difficult to perceive. This book describes some of the factors that influence the availability of ground water on tropical Pacific islands, such as recharge, aquifer properties, patterns of flow, salinity distribution, and the effects of pumping. Some examples of U.S. Geological Survey (USGS) studies in Hawai‘i and other Pacific islands are used to illustrate these concepts and their relevance to the management of ground water resources.