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Groundwater Availability in Guam

Project Chief: Stephen B. Gingerich
Project Period: January 2010 through December 2013
Cooperator: Headquarters, United States Marine Corps
Location: Island of Guam

Problem

Guam’s population is expected to increase substantially over the next decade during a proposed military forces relocation, which includes a major realignment of U.S. Marine Corps (USMC) personnel and their families to northern Guam. Groundwater production from the Northern Guam Lens Aquifer, currently about 40 million gallons per day, could significantly increase during this time. This prospective increase has prompted concerns over the sustainability of additional groundwater development. Recognizing these concerns, Headquarters, USMC has funded the U.S. Geological Survey (USGS) $1.2M to conduct a 3.5-year groundwater-availability study that will provide information and tools to more effectively manage Guam’s groundwater resources. The USGS has in turn engaged the University of Guam’s Water and Environmental Research Institute of the Western Pacific (WERI), with whom it has had a long-standing collaborative relationship, to provide local scientific expertise and coordinate with local cooperating agencies in developing a well database.

Objectives

The goals of this study are to (1) advance the understanding of regional groundwater dynamics in the Northern Guam Lens Aquifer, (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 will serve as a tool to assist water-resource managers in estimating the effects of selected groundwater-pumping and climate scenarios on the water supply. Although the main area of interest is the Northern Guam Lens Aquifer, the study will also provide a new water-budget estimate of recharge for southern Guam.

Relevance and Benefits

The numerical groundwater flow and transport model developed as part of this study will represent the best available tool that can be used to (1) effectively manage groundwater withdrawals from the Northern Guam Lens Aquifer and (2) develop long-range plans for future development of water resources.

Approach

A four-phased approach will be used to meet the study objectives: (1) compilation and review of existing hydrologic and geologic data; (2) collection of additional groundwater data; (3) water-budget calculations to estimate island-wide groundwater-recharge rates; and (4) development of the groundwater flow model for northern Guam. Groundwater data will be collected and analyzed to determine regional groundwater-flow patterns and estimate aquifer hydraulic properties, and these data will be used to construct the groundwater flow model. The model will be used to estimate the regional effects of selected pumping and recharge scenarios on groundwater availability, and the results will assist water-resource managers to plan, design, and manage water systems that will produce a sustainable and reliable freshwater supply.

Collaboration with WERI

Throughout this study, USGS and WERI hydrologists will collaborate to incorporate the latest knowledge of the unique characteristics of the Northern Guam Lens Aquifer. The USGS and WERI have had a close collaborative relationship since 1998, when the 24th Guam Legislature first provided permanent funding to support the jointly funded Comprehensive Monitoring Program for collection and analysis of hydrologic data in Guam. For this study, WERI scientists will coordinate with other Guam agencies to develop a comprehensive geographic information system and spreadsheet-based database on well locations, depths, and construction, along with hydrologic data, including water-level and salinity histories and pumping rates for each well. This database is essential for building the groundwater flow model, and it will also constitute a separate valuable tool for water-resource management and future hydrologic studies on Guam. Development of the new water budget and the groundwater flow model will also take into consideration recent WERI studies of groundwater-recharge mechanisms in the Northern Guam Lens Aquifer.

Stakeholder Technical Working Group

A stakeholder technical working group has been formed to foster local interagency communication and cooperation. This group is essential to the success of the study and to the long-term use of the well database and groundwater flow model. In addition to USGS and WERI scientists, the working group comprises local technical experts from the Department of Defense and Guam government agencies, including the Guam Environmental Protection Agency and the Guam Waterworks Authority. Besides facilitating the progress and eventual application of the study products, the working group will help to ensure that ongoing and future water-management decisions will be based on shared scientific information and common understanding of Guam’s hydrology. Working-group meetings will be held regularly throughout the study, and products of each phase will be shared through the working group and presented simultaneously to all stakeholders. Working group members will also be invited to observe or assist with fieldwork as practical, offer advice on the direction of the study, and suggest instructive and useful applications of the groundwater flow model. Study results, including the database and modeling tools, will be available to all stakeholders to help provide more reliable evaluations of the potential effects of groundwater production and preclude conflicts that can arise from insufficient information and differing interpretations of the data.

Progress

Data collection for this study began in March 2010 and continued through May 2011. Rain and groundwater samples were collected and analyzed for chloride concentration as part of the groundwater recharge study. Island-wide data on water inputs (for example rainfall, irrigation, water and sewer leaks) and outputs (for example plant water use, streamflow, runoff) were gathered for the water-budget analysis. A report of the water-budget analysis was published in 2012. WERI compiled data for over 300 wells and continues to further develop the well database as more data become available. Construction of the numerical groundwater-flow model began in 2011 and was completed in 2013.

Publications

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.

Rotzoll, K., Gingerich, S.B., Jenson, J.W., and El-Kadi, A.I., in press, Estimating hydraulic properties from tidal attenuation in the Northern Guam Lens Aquifer, Territory of Guam, USA: Hydrogeology Journal, DOI: 10.1007/s10040-012-0949-9, accessed Jan. 15. 2013.

Johnson, A.G., 2012, A water-budget model and estimates of groundwater recharge for Guam: U.S. Geological Survey Scientific Investigations Report 2012-5028, 53 p.

Gingerich, S.B., and Jenson, J.W., 2010, Groundwater availability study for Guam--Goals, approach, products, and schedule of activities: U.S. Geological Survey Fact Sheet 2010-3084, 4 p.

Presentations - Technical Working Group

Animations

Map of the island of Guam

Figure 1. The island of Guam, in the western Pacific Ocean, has a freshwater-lens system in the productive limestone aquifer (Northern Guam Lens Aquifer) underlying the island’s northern half, where most of the population resides and where population is expected to increase substantially as a result of military expansion. A groundwater-availability study will help guide sustainable management of this critical and increasingly used resource. The darker “window” in the middle of the aquifer is the extent of the volcanic basement rock above sea level, where groundwater pumping is precluded by the very low permeability of the rock. (Image from U.S. Department of Agriculture, Natural Resources Conservation Service, 20060714, Orthophoto Mosaic for Guam).

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