Introduction

The U.S. Territory of Guam has a fragile source of freshwater from which ground water supplies about 80 percent of the drinking water. In northern Guam, water is obtained from wells that tap the upper part of a fresh ground-water lens in an aquifer composed mainly of limestone. In southern Guam, the main source of freshwater is from surface water that runs off the weathered volcanic rocks that are exposed over much of the area. In cooperation with the Water and Environmental Research Institute (WERI) at the University of Guam, rainfall, pumpage, water-level, chloride-concentration, and streamflow data for Guam through Summer 2007 are displayed here (click on the above tabs to see each section). Pumpage data are current through the end of October 2007 and chloride-concentration are current through September 2007. Rainfall and streamflow data are current through the end of August 2007. Water-level data are current through September 2007.

Summary of recent conditions

North: Above-average rainfall. During April - June 2007, rainfall was about 157 percent of average. As of June, rainfall near Dededo was above average 10 of the last 12 months. During July 2006 - June 2007, rainfall was about 146 percent of average. Due to equipment malfuntion rainfall totals are not available for July and August 2007.

South: Above-average rainfall. During May - July 2007, rainfall was about 161 percent of average. As of July, rainfall near Windward Hills was above average 10 of the last 12 months. During August 2006 - July 2007, rainfall was about 137 percent of average. Due to equipment malfuntion rainfall totals are not available for August 2007.

Pago River: Normal streamflow for August. August 2007 flow in Pago River was 132 percent of the median for August (1951-82, 1998-2007). Streamflow over the past 3 months (June - August 2007) was 80 percent of the median for those months and over the past year (September 2006 - August 2007) was 89 percent of the yearly median.

Ugum River: Below normal streamflow for August. August 2007 flow in Ugum River was 66 percent of the median for August (1977-2007). Streamflow over the past 3 months (June - August 2007) was 60 percent of the median for those months and over the past year (September 2006 - October 2007) was 92 percent of the yearly median.

Additional information about the hydrology of Guam can be found in "Hydrologic Resources of Guam" by Stephen B. Gingerich, 2003, U.S. Geological Survey Water-Resources Investigations Report 03-4126.

A description of a U. S. Geological Survey hydrologic data collection project during 2004-2005 in cooperation with the Guam EPA can be found here.

Rainfall

Data collected by the U. S. Geological Survey in cooperation with the Water and Environmental Research Institute (WERI) at the University of Guam. Data after September 30, 2006 are provisional and subject to revision.

Use the pull-down menus below to jump to real-time rainfall data (Fena dam rain gage), a discussion of recent rainfall conditions (Dededo and Windward Hills rain gages), or available historical data for all Guam rain gages in the USGS on-line database. Rain gages   Umatac Historic data Windward Hills Recent conditions Historic data Mt. Chachao Historic data Dededo Recent conditions Historic data Fena Gages Real-time data-Fena Dam Historic data-Fena Dam Historic data-Fena Pump Historic data-Fena Filter Plant Almagosa Historic data Mt. Jumullong Manglo Historic data Mt. Santa Rosa Historic data

The source of all freshwater on Guam is rainfall, which averages about 85 to 115 in/yr (rainfall distribution map). Although the island is relatively small and the mountains are low (1,300 ft or less), a significant orographic effect causes rainfall totals from recording stations to differ by as much as 30 in/yr. Guam has distinct wet and dry seasons. About 70 percent of the total annual rainfall is recorded during the wet season (July through December) with the rest falling during the dry season (January through June). The heaviest rainfall is usually associated with the passage of typhoons during the wet season but only about 12 percent of average annual rainfall can be attributed to typhoons passing within about 200 mi of Guam. Exceptionally dry years recur about once every 4 years in correlation with episodes of El Niño Southern Oscillation (ENSO) in the Pacific.

Additional information can be found in:

"Hydrologic Resources of Guam" by Stephen B. Gingerich, 2003, U.S. Geological Survey Water-Resources Investigations Report 03-4126

"Creation of a 50-Year Rainfall Database, Annual Rainfall Climatology, and Annual Rainfall Distribution Map for Guam" by Mark A. Lander and Charles P. Guard, 2003, Water and Environmental Research Institute University of Guam Technical Report No. 102

Summary of recent conditions

North: Above-average rainfall. During April - June 2007, rainfall was about 157 percent of average. As of June, rainfall near Dededo was above average 10 of the last 12 months. During July 2006 - June 2007, rainfall was about 146 percent of average. Due to equipment malfuntion rainfall totals are not available for July and August 2007.

South: Above-average rainfall. During May - July 2007, rainfall was about 161 percent of average. As of July, rainfall near Windward Hills was above average 10 of the last 12 months. During August 2006 - July 2007, rainfall was about 137 percent of average. Due to equipment malfuntion rainfall totals are not available for August 2007.

Recent rainfall record compared with historical data

Dededo rain gage description

The monthly maximum and minimum data show how recent conditions compare to the range of monthly rainfall amounts measured in the historical record. The entire period of record is used to estimate the maximum, minimum, and average monthly rainfall.

Historical rainfall record

Recent rainfall record compared with historical data

Windward Hills rain gage description

The monthly maximum and minimum data show how recent conditions compare to the range of monthly rainfall amounts measured in the historical record. The entire period of record is used to estimate the maximum, minimum, and average monthly rainfall.

Historical rainfall record

Well pumpage

Data provided by the Guam Waterworks Authority, Guam EPA, and the Naval Facilities Engineering Command, Marianas.

About 150 wells are used on Guam to withdraw ground water from the freshwater lens. The distribution and magnitude of pumping is based on available data provided by Guam Waterworks Authority, Guam Environmental Protection Agency (GEPA), and the U.S. Navy. The total average reported daily withdrawal during 2006 was about 41 Mgal/d with most of the wells individually withdrawing less than 0.5 Mgal/d.

Additional information can be found in:

"Hydrologic Resources of Guam" by Stephen B. Gingerich, 2003, U.S. Geological Survey Water-Resources Investigations Report 03-4126

Summary of recent conditions

Distribution of reported pumpage in wells

click here to view animation of historical pumpage (warning: animation file is about 17 Mb)

Table 1. Monthly reported pumpage by subbasin, in million gallons per day

(values greater than GEPA's estimated available yield for each subbasin are shown in red)

Estimated available yield numbers are from:

Camp Dresser & McKee Inc., 1982, Northern Guam lens study, Groundwater management program, Aquifer yield report prepared for the Government of Guam, Guam Environmental Protection Agency, variously paged.

Monthly reported pumpage for selected subbasins, total reported pumpage, 12-month moving average of total reported pumpage, and number of reported wells, 1980 to present.

Water levels

Data collected by the U. S. Geological Survey in cooperation with the Water and Environmental Research Institute (WERI) at the University of Guam. Data after September 30, 2006 are provisional and subject to revision. Selected miscellaneous water-level data provided by Guam EPA.

Use the pull-down menus below to select from up to three types of data for each well: 1. a display of recent water levels at four sites (A-20, ACEORP tunnel, M-11, and the Hagåtña Tide Gage), 2. a discussion of a 2004-05 monitoring study at four sites (EX-6, EX-7, EX-10, and GHURA-Dededo wells), or 3. available historical data for all Guam monitoring wells in the USGS on-line database. Wells A-16 Historic data A-20 Historic data Recent conditions ACEORP tunnel Historic data Recent conditions BPM-1 Historic data GHURA-Dededo 2004-05 monitoring study Historic data Hagåtña Springs Historic data Hagåtña 147 Historic data M-10A Historic data M-11 Historic data Recent conditions EX-1 Historic data EX-4 Historic data EX-6 2004-05 monitoring study Historic data EX-7 2004-05 monitoring study Historic data EX-8 Historic data EX-9 Historic data EX-10 2004-05 monitoring study Historic data Tide Gage Hagåtña Tide Gage  Historic data Recent conditions

Water levels in wells may rise and fall in response to short- and long-term changes in ocean level and to changes in recharge and pumping. In wells closest to the coast in the high-permeability limestone, water levels fluctuate daily as much as 0.5 ft in response to ocean tides. Wells in the high-permeability limestone interior of the island typically show much smaller daily fluctuations. Although data are sparse, wells in the low-permeability volcanic rocks probably have minor or no daily water-level fluctuations from ocean tides but show more response to local recharge.

Additional information can be found in:

"Hydrologic Resources of Guam" by Stephen B. Gingerich, 2003, U.S. Geological Survey Water-Resources Investigations Report 03-4126.

"Responses of Well Water Levels on Northern Guam to Short-Term and Long-Term Variations of Rainfall and Tide" by Mark A. Lander, John W. Jenson, and Colette Beausoliel, 2001, Water and Environmental Research Technical Report No. 94.

Summary of recent conditions

Water levels at the ACEORP Tunnel and well M-11 have been higher than the long-term median water levels for these wells since September 2006. Water levels in well A-20 decreased below the long-term median water levels for this well in December 2006 and remained lower until September 2007.

These ground-water stations are operated by the USGS Honolulu field office.

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Recent water-level record compared with historical data

Hagåtña tide gage description

ACEORP tunnel description

Well M-11 description

Well A-20 description

The daily maximum and minimum data show how recent conditions compare to the range of daily water levels measured in the historical record. The entire period of record is used to estimate daily maximum, minimum, and median water levels.

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Historical water-level record

Water levels for well ACEORP tunnel, well M-11 and Hagåtña tide gage, 1982 to present.

Noted significant weather events were determined from the Naval Pacific Meteorology and Oceanography Center, Joint Typhoon Warning Center (NPMOC/JTWC) Annual Tropical Cyclone Reports (ST, supertyphoon; TS, tropical storm; TY, typhoon)

Most recent daily average water-level above sea level:

ACEORP tunnel: 2.98 ft, September 20, 2007;

Well M-11: 3.97 ft, September 19, 2007;

Tide gage: 0.46 ft, September 19, 2007

Well A-20: 41.94 ft, September 19, 2007

Chlorides in pumped water

Data provided by the Guam Waterworks Authority and Guam EPA.

Distribution of chloride concentrations in Guam wells

click here to view animation of historical chloride concentrations in pumped wells (warning: animation file is about 7 Mb)

On an island, the chloride concentration of water typically is an indicator of the amount of seawater that has mixed with the fresh rainwater infiltrated to the freshwater-lens system. For the wells with recent data, chloride concentrations ranged from 6 to over 900 mg/L. The samples with the highest concentrations are generally clustered near the center of the island. Factors that may contribute to the high concentrations include: (1) excessive rates of ground-water withdrawal from a single well, (2) a cluster of wells that are too closely spaced, (3) wells that are drilled too deeply into the freshwater lens, or (4) higher permeability geologic formations that allow increased mixing of freshwater with the underlying saltwater.

Additional information can be found in:

"Chloride History and Trends of Water Production Wells in the Northern Guam Lens Aquifer" by Mauryn Q. McDonald and John W. Jenson, 2003, Environmental Research Institute of the Western Pacific University of Guam Technical Report No. 98.

Nitrates in pumped water

Data provided by the Guam Waterworks Authority and Guam EPA.

Distribution of nitrate concentrations in Guam wells

click here to view animation of historical nitrate concentrations in pumped wells (warning: animation file is about 0.5 Mb)

Nitrate-nitrogen (NO3-N) is a naturally occurring form of inorganic nitrogen that is found in fertilizers and organic matter but is also produced from the breakdown of organic matter and fecal material. At concentrations over 10 mg/l, NO3-N can cause methemoglobinemia or "bluebaby syndrome", a blood disorder that most commonly affects infants. Methemoglobinemia decreases the oxygen supply to cells and tissues, resulting in convulsions and death in extreme cases. Because of this, the United States Environmental Protection Agency (USEPA) set the NO3-N Safe Drinking Water (SDW) maximum contaminant level (MCL) at 10 mg/l. The SDW standards also require that a parameter’s monitoring frequency be increased if 50% of its MCL is reached. Guam Environmental Protection Agency’s (GEPA) 1992 Revised Guam Water Quality Standards indicates that Guam’s ambient groundwater NO3-N concentration can be as high as 5 mg/l.

Additional information can be found in:

"Nitrate-Nitrogen Concentrations in the Northern Guam Lens and Potential Nitrogen Sources" by Mauryn Q. McDonald, 2002, Environmental Research Institute of the Western Pacific University of Guam Technical Report No. 95.

"Hydrologic Resources of Guam" by Stephen B. Gingerich, 2003, U.S. Geological Survey Water-Resources Investigations Report 03-4126.