Abstract: Water Supplies in Limestone Terranes of Micronesia

Dan A. Davis

Micronesia, as considered here, includes the Mariana Islands (exclusive of Guam), the eastern and western Caroline Islands, and the Marshall Islands, all of which lie south of the Tropic of Cancer and north of the equator. They are scattered over a distance of 4,500 km eastward from 130°E long.

Water-bearing limestones that are significant in water supplies of the islands are present in the southern islands of the Marianas, in the southern high islands of Palau, and in most of the atolls of the Caroline and Marshall Islands. Rainfall, which supplies the freshwater recharge to the limestones, is about 2,000 mm annually in the Marianas and the northern Marshall Islands and increases southward to about 3,800 mm in the southern Marshall and Caroline Islands. Heavy rains from tropical storms supply much of the freshwater recharge. Droughts occasionally cause distress.

In the Marianas, the islands of Rota, Tinian, and Saipan, ranging in size from 85 to 124 sq km, are built of moderately to highly permeable coralline limestones that envelope older and much less permeable volcanic cores having irregular surfaces of high relief. In many parts of the islands limestones extend below sea level. Where they are near sea level they contain bodies of fresh to brackish water that are among the principal sources of water supply, especially on Saipan and Tinian. On Saipan, extensive development and pumping of these low-level water bodies have produced patchy patterns of seawater intrusion. Increases in salinity at individual wells are the result of pumping, and increases and decreases in pumping rates cause corresponding changes in salinity. Great differences in salinity and changes in salinity from one well to another appear to be independent of pumping rates and are attributed to undefined geologic features that control the movement of seawater and fresh water. Of smaller extent in the islands but of great importance in water supply are limestone terranes high above sea level in which water is held by the underlying volcanic rock.

The high limestone islands of Palau are small. Those that are inhabited, principally Koror, Peleliu, and Angaur, range in area from about 7 to 12 sq km. Throughout Peleliu and Angaur the base of the limestone is below sea level, and all the ground water is near sea level and subject to seawater intrusion. Only small use is made of the water. On Koror, water in the limestone is held at high levels by volcanic rock and discharges above sea level at ephemeral springs.

The islands of the atolls are low and small, and the quantities of water in them are small. The water occurs as thin, fresh to brackish layers in the limestone at sea level. Historically the water has been widely used by the island people. Wells dug to the water table are used for washing, bathing, and retting of fibers. Larger pits are dug into zones of freshest water for the growing of wetland taro. The same fresh water sustains breadfruit trees in dry seasons. On only a few islands has development of the water been elaborate.

AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii