The effects of withdrawals and drought on groundwater availability in the Northern Guam Lens Aquifer, Guam

Abstract

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit http://store.usgs.gov Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.Suggested citation: Gingerich, S.B., 2013, The effects of withdrawals and drought on groundwater availability in the Northern Guam Lens Aquifer, Guam: U.S. Geological Survey Scientific Investigations Report 2013-5216, 76 p., http://dx.doi.org/10.3133/ sir20135216. ISSN 2328-0328 (online) iii Executive SummaryMost of the public water supply on Guam is pumped from a freshwater lens in the Northern Guam Lens Aquifer, a limestone aquifer. Because of population growth and a potential military realignment involving the U.S. Marine Corps (USMC), groundwater withdrawals from wells are expected to increase. Currently (2013), the salinity of water pumped from some wells in the area is higher than acceptable for drinking water. The expected increase in demand for water has led to concern over the long-term sustainability of withdrawals from existing and proposed wells. In 2010, to aid in management of groundwater resources and to plan for sustainable growth on the island, the United States Marine Corps, Headquarters entered into a cooperative agreement with the U.S. Geological Survey (USGS) to study groundwater availability in northern Guam. The objective of this 3.5-year study was to estimate the effects of several hypothetical withdrawal scenarios on water levels, the transition zone between freshwater and saltwater, and salinity of pumped wells.On Guam, freshwater-lens systems are found in limestone and volcanic rocks, but the most important groundwater sources are from the freshwater parts of these systems in the high-hydraulic-conductivity limestone rocks of northern Guam. The most intensely developed groundwater system in this area comprises a lens-shaped freshwater body, an intermediate brackish-water transition zone, and underlying saltwater. For management purposes, the Guam Environmental Protection Agency (GEPA) delineated the Northern Guam Lens Aquifer (NGLA) into six hydrologically connected aquifer basins: Agafo Gumas, Andersen, Finegayan, Hagåtña, Mangilao, and Yigo-Tumon. Hydrologic DataKnowledge of the NGLA hydrology comes from compilation and analysis of data on water levels, salinity profiles in deep monitoring wells, production well pumping rates and salinities, and geologic information gleaned from decades of well drilling and hydrologic monitoring. The NGLA...

Water resources on Guam—Potential impacts of and adaptive response to climate change

“…The island's northern province is a broad limestone plateau bordered by steep cliffs and discontinuous coastal lowlands. Most of the plateau lacks stream channels, but it has many closed depressions and is a groundwaterdominated hydrographic setting underlain by the NGLA (Ward and others, 1965;Gingerich, 2003, Gingerich, 2013.…”

“…Guam's most important groundwater sources are from the freshwater lens of the NGLA, comprised of highly permeable limestone rocks (Ward and others, 1965;Jocson and others, 2002;Gingerich 2013;Rotzoll and others, 2013). Here, the water table is no more than six feet (1.8 meters) above sea level, and the hydraulic gradient is nearly flat.…”

“…Discharge from the NGLA occurs as withdrawals from wells, coastal springs, diffuse seepage to the ocean, and minor discharge to the Hagåtña Swamp. Average groundwater withdrawals in 2010 totaled about 42 Mgal/d, which consists of about 36 Mgal/d withdrawn by the GWA, about 4 Mgal/d by the DoD, and about 1.8 Mgal/d by private wells; groundwater discharge to the ocean bordering the NGLA is estimated to be about 196 Mgal/d (Gingerich, 2013).…”

“…On account of historical and projected population growth, freshwater demand on Guam has increased in the past and will likely increase in the future. During the early 1970s to 2010, groundwater withdrawals from the limestone of the Northern Guam Lens Aquifer (NGLA), the main freshwater source on the island, nearly tripled to almost 45 million gallons per day (Mgal/d) (Gingerich, 2013). Because of a proposed military relocation to Guam and expected population growth, freshwater demand on Guam is projected to increase further.…”

Fena Valley Reservoir watershed and water-balance model updates and expansion of watershed modeling to southern Guam

“…Although 80 percent of the potable water in Guam comes from groundwater (Jocson and others, 2002), surface-water resources are important because of the potential increased demand for freshwater as a result of population growth and proposed military expansion (Gingerich, 2013). For over two decades the U.S. Geological Survey (USGS), in cooperation with the U.S. Navy, has provided periodic projections of water availability for the Fena Valley Reservoir (FVR) in south-central Guam ( fig.…”