A water-budget model and estimates of groundwater recharge for Guam

Johnson, Adam G.

doi:10.3133/sir20125028

Cited by 26 publications

(60 citation statements)

References 20 publications

(37 reference statements)

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“…The objectives of this 3.5-year study were to (1) advance the understanding of regional groundwater dynamics in the NGLA, (2) provide a new estimate of groundwater recharge for the entire island, and (3) develop a numerical groundwater flow and transport model for northern Guam that could serve as a tool to assist water-resource managers in estimating the effects of selected groundwater-pumping and climate scenarios on the water supply. Objective 2 was met with the prior publication of a USGS Scientific Investigations Report describing groundwater recharge estimates (Johnson, 2012). Objectives 1 and 3 are addressed in this report.…”

Section: The Effects Of Withdrawals and Drought On Groundwater Availamentioning

confidence: 99%

“…This estimate is slightly lower than estimate of 254 Mgal/d. For the drought estimation (Johnson, 2012), total recharge during drought conditions is 32 percent lower than mean baseline recharge. For the future land-cover water-budget simulation, which represents potential land-cover changes from the military relocation and population growth, estimated recharge for the entire island is nearly equal to the baseline recharge estimate that was based on 2004 land cover.…”

Section: Rechargementioning

confidence: 99%

“…If average groundwater recharge to the NGLA is about 238 Mgal/d (Johnson, 2012) and withdrawal is currently about 42 Mgal/d then roughly 196 Mgal/d of freshwater is discharging as coastal seeps and springs.…”

Section: Coastal Springsmentioning

confidence: 99%

“…The distribution of mean annual groundwater recharge for baseline conditions on Guam calculated by using the water-budget model (Johnson, 2012). Enhanced recharge (colored purple) is where recharge is greater than 78.7 inches, and is due to water input from water-main leakage, septic systems, or stormwater runoff.…”

Section: Figurementioning

confidence: 99%

“…This interval was chosen because it is below the top layer of nodes, avoiding assigning two sets of recharge (lateral and areal) at the same node in SUTRA. All recharge was assumed to have a chloride concentration of about 30 mg/L (expressed as a mass fraction of 0.00006069 kg/kg in SUTRA), the average concentration measured in groundwater samples considered most representative of recharge (Johnson, 2012).…”

Section: Simulation Of Groundwater Flow 23mentioning

confidence: 99%

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

Gingerich¹

2013

Scientific Investigations Report

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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...

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Section: The Effects Of Withdrawals and Drought On Groundwater Availamentioning

confidence: 99%

Section: Rechargementioning

confidence: 99%

Section: Coastal Springsmentioning

confidence: 99%

Section: Figurementioning

confidence: 99%

Section: Simulation Of Groundwater Flow 23mentioning

confidence: 99%

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