A study was conducted on Roi-Namur Island, Kwajalein Atoll to define the extent of the freshwater lenses and recharge zones and to asses potential contaminant migration from known sources of contamination. Rainfall, which is the sole natural source of freshwater, is strongly seasonal and occasional multi-year droughts are capable of disrupting the island's water supply. The supply of freshwater is produced by a joint system of rain catchments and shallow wells. From 1980-91, rain- catchment yield and ground-water withdrawal average 22,632 and 5,829 gallons per day, respectively. Maps were produced showing the areal extent of freshwater, the thickness of the freshwater lenses, the water-table configuration and directions of ground-water flow, and contamination sites and potential migration pathways of contaminants. Sectional views of freshwater lens thicknesses and seasonal freshwater lens thickness changes were also constructed. The freshwater lens attains a maximum thickness of 23 feet beneath the central area of Roi where recharge is high. The estimated amount of water in the lenses with chloride concentrations less than 250 milligrams per liter underlying Roi and Namur is 226 million and 4.2 million gallons, respectively. The presence of thick vegetation on Namur increases evapotranspiration losses significantly producing a smaller freshwater lens. Freshwater thicknesses shrank and expanded in a seasonal cycle as much as 3 feet near withdrawal wells. The water table forms broad mounds beneath Roi and Namur and freshwater heads reach a maximum of 1.4 feet. Most known sites of contamination lie near the periphery of the island where ground-water flow patterns will carry contaminants away from the withdrawal wells toward the shore.