Reports

Report Number: 21
Year: 1981
 

Estimate of Recharge to the Freshwater Lens of Northern Guam

In Guam, as in other oceanic island environments, rainfall has a significant chloride ion concentration. As a result, there is a relatively simple and straightforward method of estimating groundwater recharge by considering the chloride ion as a tracer which is concentrated by evapotranspiration processes.

The chloride ion concentration of rainwater in Guam is about 4.5 mg/L. the freshest part of the northern lens is about 11.8 mg/L. Taking the 11.8 mg/L value as an indicator of the chloride ion concentration of recharge, the average annual recharge rate is estimated at 38 percent of the 218 cm/yr average annual rainfall, or about 83 cm/yr.

This estimate agrees very well with previous work which utilized other independent methods of determining recharge rates.

Author(s):
Jerry F. Ayers