Groundwater Monitoring, Modeling, and Development in Response to the Military Buildup in Guam

Gingerich, Stephen

An expected significant population increase on Guam raised concern about the sustainability of groundwater resources. In response, the U.S. Geological Survey, in collaboration with the University of Guam’s Water and Environmental Research Institute of the Western Pacific and with funding from the U.S. Marine Corps, conducted a 3.5-year study to advance understanding of regional groundwater dynamics in the Northern Guam Lens Aquifer, provided a new estimate of groundwater recharge, and developed a numerical groundwater-flow and transport model for northern Guam. Results of the study, including three USGS reports, a scientific journal article, a well database, and an updated aquifer basement map provide more reliable evaluations of the potential effects of increased groundwater withdrawal and help guide sustainable management of this critical resource.

Two withdrawal scenarios (predevelopment conditions and a 5-year drought) indicated that prior to pumping, the freshwater lens was 10 to 50 feet thicker in the Yigo-Tumon basin and more than 50 feet thicker in the Hagåtña basin. The 2010 withdrawal distribution during a 5-year drought would result in decreased water levels, a thinner freshwater lens, and increased salinity of water pumped from wells. Available water with an acceptable salinity would decrease from about 34 million gallons per day to 11.5 million gallons per day after 5 years but recover to pre-drought levels 5 years after the return of average recharge conditions.

Five additional scenarios to assess groundwater demand projections and proposed new well sites under average and drought conditions indicated decreased water levels, a thinner freshwater lens, increased water salinity, and unacceptable salinity at several current withdrawal sites. However, some scenarios indicated that more than 40 million gallons per day can be withdrawn and the salinity of this water will remain in the acceptable category, except during drought.