Mining of phosphate ore on Angaur Island by mechanized methods created large water-table 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 United States 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 ? F; the average annual rainfall is about 125 inches; and the average relative humidity is about 80 percent. 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 water table 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 ground water in province A is not of uniform quality, as large variations in salinity occur throughout the area. In contrast, the ground water in province B is of relatively uniform quality, and contains less than 1,000 ppm (parts per million) of chloride in most places. In province A removal of earthy phosphate which had acted as a seal allowed the infiltration of saline water through solution channels exposed at the bottom of several of the lakes. The sampling program showed no indication of large-scale movement of saline water out of the lakes into adjacent ground water. Provinces A and B apparently function as independent ground-water units, and the quality of the ground water in province B has not been noticeably affected by mining activities in province A. The lakes were backfilled with limestone rubble to reduce the possibility of underground movement of saline water out of the lakes. The gross permeability of the backfill material is less than that of the surrounding limestone in province A, and, as the rate of influx of saline water was curtailed, the recharge of fresh water from rainfall tended to reduce the salinity of the ground water in the fill. In parts of the backfilled area the salinity of the water at the water table decreased rapidly in less than 1 year, as a layer of fresh water was built up at the water table.