Effects of Phosphate Mining on the Ground Water of Angaur, Palau Islands Trust Territory of the Pacific Islands

Mining & Minerals - Evaluations / Lithium, Potash, Halite, & P2O5 Mining Projects

Date posted: 12/25/16

Ted Arnow (USGS) reports that mining of phosphate ore on Angaur Island by mechanized methods created large watertable lakes, which became filled with brackish or saline water. A hydrologic investigation was started in 1949 to determine whether the saline water in the lakes would spread to surrounding areas and cause damage to agricultural lands and the water supply. Angaur, which is in the Palau Islands in the southwestern part of Micronesia, is administered as part of the Trust Territory of the Pacific Islands, under a trusteeship granted to the U.S. by the United Nations. The island has an area of 3.2 square miles and has a maximum altitude of about 150 feet. The climate is tropical oceanic. The average annual temperature is 82 degrees F, the average annual rainfall is about 125 inches, and the average relative humidity is about 80%. The northwestern third of Angaur (province A) consists topographically of a series of concentric ridges and depressions which are underlain largely by well-cemented coralline limestone of Pliocene, Pleistocene, and Recent ages. The remaining two-thirds of Angaur (province B) is a low plain underlain in the northern and central parts by a low platform of coralline rubble of Pleistocene and Recent age and in the southern part by sandy and rubbly beach deposits of Recent age. Province A contained extensive phosphate deposits of which more than 3 million tons were mined in 1909-55. Weekly water-level measurements at 35 wells, test holes, and lakes indicate that the watertable averages about 2 feet above mean sea level in the beach deposits, about half a foot above mean sea level in the rubble deposits, and about 1.35 feet above mean sea level in the coralline limestone. Water samples obtained weekly at the observation sites indicate that the groundwater in province A is not of uniform quality, as large variations in salinity occur throughout the area.