Estimate of recharge to the freshwater lens of northern Guam

Ayers, Jerry

The groundwater hydrology of Guam has been studied by a number of investigators over several decades. One of the earliest studies was made by Peterman et al. in 1945 as part of a groundwater study sponsored by the military; this study estimated that the northern half of Guam received groundwater aquifer recharge at the rate of 0.39 mgpd per square kilometer. In 1964, Davis estimated a range of 0.73 to 0.89 mgpd reaching the freshwater lens of Northern Guam. In 1968, Davis and Huscel estimated the recharge rate at 0.97 mgpd per square kilometer in an administrative report prepared for the Air Force. Also, in 1968, Sheaham estimated that 50% of rainfall infiltrates the soil as recharge. In a report to the Public Utility Agency of Guam in 1970, Auston, Smith, and Associates, Inc. estimated a recharge rate of 1.04 mgpd per square kilometer. The present study was based on the chloride-ion concentration in rainwater. The chloride ion was used as a tracer which is concentrated evapotranspiration. The chloride-ion concentration of rainwater in Guam is 4.5 mg/liter, while the freshest part of the lens is about 11.8 mg/liter. Using the chloride-ion concentration as a tracer yields a recharge rate of 38% of rainfall, or about 83 cm/yr.