Apra Harbor

Talofofo Bay

Ylig Bay

Tumon Bay

Hagatna Bay

Agat Bay

Pago Bay

PHILIPPINE

SEA

PACIFIC

OCEA

144 N

o

40'

144

o

50'

13

o

30'

13

o

20'

Hydrologic Resources of Guam

Prepared in cooperation with the

WATER AND ENVIRONMENTAL RESEARCH INSTITUTE (WERI)

UNIVERSITY OF GUAM

U.S. Department of the Interior

Gingerich–

– Hydrologic Resources of Guam

–

–

WRIR 03-4126

By Stephen B. Gingerich

U.S. Geological Survey Water-Resources Investigations Report 03-4126

2003
A A'

SEA LEVEL

FEET

1000

1400

600

200

1000

600

200

EXPLANATION

LIMESTONE

VOLCANIC ROCKS

GENERAL DIRECTION OF FRESH GROUND-WATER FLOW

ZONE WHERE FRESHWATER IN LIMESTONE IS IN DIRECT CONTACT

WITH FRESHWATER IN UNDERLYING VOLCANICS (PARA-BASAL)

Figure 3. Diagram showing generalized ground-water occurrence, northern Guam.

Water table

Spring

Freshwater

Saltwater

Philippine Sea

NOT TO SCALE

Water table

Freshwater

Saltwater Brackish water

Pacific Ocean

Sea level

About

40h

h

Ta Ta Ta

Tbl Tbl QTm

QTm

Qa

QTm

Ta

QTm QTm

QTm Qa

Tbl

Tbl

Ta

Figure 5. Diagram showing the effects of pumping on the freshwater lens, northern Guam, (A) widely spaced wells pumping at uniform rate causing slight rise in transition zone, (B) closely spaced wells causing

brackish water from the transition zone to rise into the wells and reducing the area with para-basal water.

Water table

Freshwater

Saltwater

NOT TO SCALE NOT TO SCALE

?

Water table

Freshwater

Saltwater

Brackish water

Philippine Sea Water table

Freshwater

Saltwater

Philippine Sea Water table

Freshwater

Saltwater

Brackish water

A

West East

B

Pumped well

Pumped well

Pumped wells

Figure 4. Water levels in selected wells, Guam

EXPLANATION

SURFACE EXPOSURE OF VOLCANIC ROCKS (MODIFIED FROM

TRACEY AND OTHERS, 1964)

SUBSURFACE VOLCANIC ROCKS ABOVE SEA LEVEL IN NORTHERN GUAM

(modified from David Vann, 2000, University of Guam, written commun.)

WATER LEVEL IN WELL INDICATING FRESHWATER LENS IN:

Limestone (basal)

Limestone overlying volcanic rocks (para-basal)

Volcanic rocks

GENERALIZED DIRECTION OF GROUND-WATER FLOW

Water level information from Ward and others (1965);

Mink (1974); Camp Dresser & McKee Inc. (1982);

Guam Environmental Protection Agency (1986);

Torikai (1997); Guam Hydrologic Survey (unpublished

data); U.S. Geological Survey, Hawaii District

(unpublished data).

EXPLANATION

LINE OF EQUAL MEAN ANNUAL RAINFALL, IN INCHES PER YEAR

Southern boundary of

Vann study

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

900

900

900

900

900

900

900 900

1260

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

180

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

540

900

900

900

900

900

900

900 900

1260

B

EXPLANATION

BEACH DEPOSITS, REEF DEPOSITS (Merizo Limestone),

ALLUVIUM, AND ARTIFICIAL FILL (Holocene)

MARIANA LIMESTONE (Pliocene to Pleistocene)

BARRIGADA LIMESTONE (Miocene to Pliocene)

OLDER LIMESTONE (Miocene to Pliocene)--

includes Bonya Limestone, Alifan Limestone, and

Janum Formation

UMATAC FORMATION (Oligocene to Miocene)

ALUTOM FORMATION (Eocene to Oligocene)

FACPI FORMATION (Eocene)

CONTACT

FAULT-dashed where approximately located,

dotted where covered

LINE OF SECTION

RELATIVE DIRECTION OF MOVEMENT ALONG FAULT

TOPOGRAPHIC CONTOUR--In feet. Interval is 180. Datum is

Guam mean sea level.

ROAD

Qa

QTm

Qa

Qa

Qa

QTm

QTm

QTm

QTm

Tol

Tbl

Tbl

Tol

Tol

Tbl

Tu

Tu

Tf

A

B

A'

C'

A

C

B'

Ta

Tf

Ta

Ta

144 ̊40'

144 ̊50'

13 ̊30'

13 ̊20'

0

0

1

1

2 MILES

2 KILOMETERS

VERTICAL EXAGGERATION 1.25x

A'

Apra Harbor

Talofofo Bay

Ylig Bay

Tumon Bay

Hagåtña Bay

Agat Bay

Pago Bay

144

o

40'

144

o

50'

13

o

30'

13

o

20'

0

0

2 MILES

2 KILOMETERS

Cocos Island

Base from U.S. Geological Survey

Island of Guam, 1:62,500, 1953.

Elevation from Digital Elevation Model,

1:24,000

Base from U.S. Geological Survey

Island of Guam, 1:62,500, 1953

Base from U.S. Geological Survey

Digital Line Graph, Island of Guam, 2000

Figure 1. Rainfall distribution (1950–- 99), Guam (modified from Charles Guard, Water and Environmental Research Institute, University

of Guam, written commun., 2000).

100

105 110

95

90

85

105

90

95

100

85

90

9

1

5

05

100

110

115

115

110

105

100

100

95

90

95 105

120° 140° 160°E 180° 160°W

0°

20°S

20°N

40°

Philippines

Australia

Japan

Hawaii

Wake

Guam

PACIFIC OCEAN

0 2 Miles

0 2 Kilometers

INDONESIA

AMERICAN SAMOA

FEDERATED

STATES OF

MICRONESIA

PAPUA

NEW GUINEA

FIJI

Tutuila

REPUBLIC

OF PALAU

COMMONWEALTH

OF THE NORTHERN

MARIANA ISLANDS

REPUBLIC OF THE MARSHALL ISLANDS

PHILIPPINE

SEA

PACIFIC

OCEAN

PREPARED IN COOPERATION WITH THE

WATER AND ENVIRONMENTAL RESEARCH INSTITUTE (WERI),

UNIVERSITY OF GUAM

WATER-RESOURCES INVESTIGATIONS REPORT 03-4126

Climate, Geology, Hydrology–SHEET 1 OF 2

Gingerich, S.B., 2003, Hydrologic Resources of Guam

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

HYDROLOGY

Rainfall can infiltrate the soil, return to the atmosphere by evapotranspiration, or runoff to streams. The amount

of runoff is determined by rainfall, topography, soil type, and land use. Runoff is typically high where rainfall is

high and slopes are steep and where rain falls on less permeable land surfaces. On Guam, streams are present only in

the south where low-permeability volcanic rocks slow the infiltration of rainwater and allow ground-water discharge

to streams. In northern Guam, the exposed limestone at the surface allows virtually all rainfall to infiltrate.

Evapotranspiration is the loss of water to the atmosphere by the combination of transpiration by plants and direct

evaporation from land and water surfaces. Water stored in the soil is available to plants or can eventually flow

downward to recharge the ground-water system.

Ground Water

In Guam, the major fresh ground-water systems are below the lowest water table and are freshwater-lens systems.

Minor perched systems can exist above the lowest water table. Ground water flows from areas of high water level to

areas of low water level, which in general, is from the interior of the island to the coast.

Freshwater-Lens System

On oceanic islands such as Guam, freshwater commonly forms a body of water called a freshwater lens that

floats on saltwater and is separated from the saltwater by a transition zone of brackish water (fig. 3). The freshwater,

brackish water, and saltwater are all part of the freshwater-lens system. Salinity in a freshwater-lens system is

gradational, from an upper freshwater core through the underlying transition zone to saltwater. A chloride

concentration of 250 mg/L is the maximum contaminant level (MCL) for drinking water recommended as a

secondary standard by the USEPA (U.S. Environmental Protection Agency, 1991). Secondary standards are not

mandatory requirements, but instead establish limits for constituents that may affect the aesthetic qualities of

drinking water (taste and odor, for example). In this report, freshwater is defined as water having a chloride

concentration less than or equal to 250 mg/L. Seawater has a chloride concentration around 19,200 mg/L (Thurman,

1990).

The Ghyben-Herzberg principle is commonly used to estimate the depth where brackish water in the transition

zone has a salinity about 50 percent that of seawater. The Ghyben-Herzberg principle describes a freshwatersaltwater relation for conditions in which the two fluids do not mix (no transition zone) and the freshwater flow is

horizontal. For these conditions, the freshwater-lens thickness below sea level is directly proportional to the height

of the top of freshwater above sea level. In principle, at a place where the water table stands 1 foot above sea level,

for example, 40 ft of freshwater will be below sea level, and the freshwater lens will thus be 41 ft thick. This relation

exists because seawater is about one-fortieth denser than freshwater. In a real freshwater-lens system, mixing creates

a transition zone and reduces the amount of freshwater above the transition zone. In some places, especially near the

coast, where freshwater is extensively mixed with saltwater, brackish water may exist immediately below the water

table.

Transition-zone thickness depends on the extent of mixing between freshwater and saltwater and is generally

several tens of feet thick in northern Guam. Mixing in the transition zone results from tidal and pumping fluctuations

superimposed on the gravity-driven flow of freshwater toward the shore. Under conditions of steady recharge and no

pumping, the lens would ultimately have a fixed size. In reality, rainfall is episodic and seasonal, and lens volume

fluctuates naturally with time. Ground water discharges continuously throughout the year, but the lens shrinks during

dry periods when recharge diminishes or ceases, and expands when recharge increases.

On Guam, freshwater-lens systems are found in limestone and volcanic rocks but the most important sources of

ground water are from the freshwater parts of these systems in the high-permeability limestone rocks of northern

Guam (fig. 4). In the most permeable limestone, the water table is no more than a few feet above sea level, and the

slope of the water table is nearly flat. The freshwater lens in these rocks is relatively thin and is commonly referred

to as “basal” water after similar occurrences in Hawaii (Meinzer, 1930). Wells and springs in this part of the system

generally have water-level altitudes less than 8 ft. In some areas, the freshwater lens is thick enough to extend into

the low-permeability volcanic rocks underlying the limestone. The freshwater in the limestone is commonly referred

to as “para-basal” water where it is in contact with the underlying poorly permeable volcanic basement (Mink,

1974). Ward and others (1965) recognized that the low-permeability volcanic rocks must be saturated below the

water table. Therefore, ground water in the limestone and the underlying volcanic rocks is all part of a freshwaterlens system but the depth of freshwater in the volcanics is unknown. Figure 4 shows wells in which the theoretical

thickness of the freshwater lens (40 times the water level) extends into the volcanic rocks, the extent of which was

delineated by Vann for a Master’s thesis project (David Vann, University of Guam, written commun., 2000). Water

levels and springs as much as several hundred feet in altitude are where the freshwater-lens system is vertically

extensive in low-permeability volcanic rocks (fig. 3). Ground water in these systems can also be considered basal

ground water using Meinzer’s (1930) original definition of basal ground water. In such systems, where lowpermeability rocks cause significant vertical hydraulic-head gradients, the Ghyben-Herzberg principle does not

accurately predict the depth where the brackish water in the transition zone has a salinity about 50 percent of

seawater. Although the depth to saltwater has not been measured in wells in these systems, where flow is downward,

the freshwater lens is likely thinner than predicted by the Ghyben-Herzberg principle.

Freshwater-lens systems are recharged by direct infiltration of rainfall, and by inflow from perched ground-water

systems. Discharge from the freshwater-lens system in northern Guam is by diffuse seepage near the coast and to

subaerial and submarine coastal springs. In southern Guam, much of the fresh ground water discharges directly to

stream valleys above sea level where the ground surface intersects the water table. In highly permeable rocks,

freshwater flow is mainly horizontal and toward the coast, but in low-permeability rocks, vertical flow can be

significant where there are substantial vertical head gradients.

Ground-water withdrawal from wells, saltwater upconing, and regional lens depletion.--Some fraction of

recharge can be withdrawn continuously by wells for drinking water, in effect capturing a fraction of the natural

discharge. The most efficient means of pumping water from a thin lens is to locate widely spaced shallow wells

where the lens is thickest and to maintain low, uniform pumping rates at each well (fig. 5a). This configuration of

wells spreads withdrawal over a wide area and skims freshwater from the lens. Uniform pumping rates will cause

less mixing in the transition zone when compared to variable pumping rates. Saltwater intrusion can contaminate

wells if the lens is too thin, if wells are too deep, or if too much water is withdrawn from a given area (fig. 5b).

The degree of saltwater intrusion depends on several factors, which include the hydraulic properties of the rocks,

recharge rate, pumping rate, and well location and depth. Even if wells are designed and placed to minimize local

upconing, the lens will gradually shrink to a size that is in balance with the withdrawal. This regional shrinkage

raises the transition zone closer to the wells, potentially can change the size and distribution of the “para-basal”

zone. Shrinkage of the freshwater lens due to dry weather can also contribute to high salinity in wells.

Perched Ground-Water Systems

Perched water is found in areas where low-permeability rocks impede the downward movement of ground water

sufficiently to allow a perched water body to develop within otherwise unsaturated rocks. Minor perched systems are

found in some of the higher-altitude limestone overlying the volcanic rocks of southern Guam. The height of the

water table above sea level and the extent of the perched system are respectively dependent on the altitude and areal

extent of the low-permeability rocks and the rate and duration of recharge. Discharge from a perched system can be

to springs, streams, downward to a lower water table, or to the atmosphere by evaporation.

INTRODUCTION

The U.S. Territory of Guam, which lies in the western Pacific Ocean near latitude 13°28’N and longitude

144°45’E (fig. 1), is the largest (211 mi2) and southernmost of the islands in the Mariana chain. Ground water

supplies about 80 percent of the drinking water for the island’s 150,000 residents and nearly one million visitors per

year. 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. About 180 wells, nearly all in the north, withdraw about 35 Mgal/d of water

with chloride concentrations ranging from 6 to 585 mg/L. 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. About 9.9 Mgal/d of

freshwater is obtained using surface reservoirs. The island’s freshwater resources are adequate to meet current

(2003) needs, but future demands will eventually be higher. To better understand the hydrology of the island, the

U.S. Geological Survey (USGS) entered into a cooperative study with the Water and Environmental Research

Institute of the Western Pacific (WERI) at the University of Guam. The objective of the study was to provide a better

understanding of the water resources of the island through analysis of data collected by the USGS on Guam.

This report provides a description of the general hydrologic principles of the island’s ground-water systems, as

well as of the rainfall and geology of Guam. Hydrologic data described in the report include water levels, chloride

concentrations, and pumpage from ground-water wells and streamflow data from southern Guam.

RAINFALL

The source of all freshwater on Guam is rainfall, which averages about 85 to 115 in/yr (Charles Guard,

University of Guam, written commun., 2000). 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 (fig. 1). 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 180 nautical mi of Guam (Lander,

1994). Exceptionally dry years recur about once every 4 years in correlation with episodes of El Niño/Southern

Oscillation (ENSO) in the Pacific (Lander, 1994).

GEOLOGY

Volcanic rock forms the foundation of the island and is exposed over about 35 percent of the island’s surface,

predominantly in southern Guam (fig. 2). Neogene limestone overlies the volcanic rock and is exposed over about 60

percent of the island, mainly in northern Guam. The high porosity of the limestone gives it high permeability,

whereas the texture and poor sorting of the volcanic basement material usually gives it much lower permeability. All

major limestone units have undergone pervasive meteoric diagenesis, thus most porosity and permeability is

secondary. Faults transect the island throughout, complicating the structure and permeability of the rock units. Age

determinations of Guam geologic units are based mainly on fossil remains and correlations with other islands in the

Pacific.

Seven major geologic units are shown in figure 2. The Facpi and Alutom Formations contain the oldest exposed

rocks, are of Eocene and Oligocene age, and underlie all other exposed rock units (Tracey and others, 1964; Reagan

and Meijer, 1984). These units crop out in the highlands from central to southern Guam and in the highest elevations

of northern Guam. These exposures cover about 20 percent of the surface of the island. The maximum thickness of

each unit is at least 1,000 ft (Reagan and Meijer, 1984). The formations contain a series of pillow basalts and waterlaid pyroclastic rocks of volcanic origin ranging from tuffaceous shale to coarse boulder conglomerate and breccia.

The permeability of the formations is considered low (Ward and others, 1965).

The Umatac Formation of Oligocene – late Miocene age (Tracey and others, 1964) is separated from the

underlying Alutom Formation by a structural unconformity. The Umatac Formation crops out over about 15 percent

of the land surface, principally in the south-central highlands and plateaus. The unit thickens to the south away from

the Alutom Formation and is at least 1,050 ft thick along the southwest side of the island (Tracey and others, 1964).

Here, the Umatac Formation is composed (from oldest to youngest) of about 250 ft of reef and forereef limestone

(Maemong Limestone Member), about 750 ft of tuff breccia and volcanic conglomerate (Bolanos Pyroclastic

Member, Schroeder Flow Member), and about 50 ft of basalt flows (Dandan Flow Member) (Meijer and others,

1983; Reagan and Meijer, 1984). The permeability of the formation is considered low (Ward and others, 1965).

Three limestone units, which locally overlie the Alutom and Umatac Formations, are grouped together as Older

Limestone for this report. These three units, from Miocene to Pliocene age, are the Bonya and Alifan Limestones,

and the Janum Formation (fig. 2). These three units are facies distinctions of limestones of the same age. Together

these units cover only about 5 percent of the island’s surface and individually the units are each only about 70 to 200

ft thick. The Bonya Limestone (periplatform and embayment facies of mostly jointed and fractured detrital

limestone) and the Janum Formation (deep forereef facies of mostly tuffaceous limestone) are considered permeable

but contain little water because of the limited extent of the deposits (Ward and others, 1965). The Alifan Limestone

(shallow reef margin facies of poorly to well-consolidated detrital limestone and conglomeritic clay marl [Talisay

Member]) is moderately to highly permeable and feeds some perennial contact springs in southern Guam.

The Barrigada Limestone, of late Miocene to Pliocene age, covers about 9 percent of the island’s surface but

forms the bulk of the aquifer underlying northern Guam. This formation consists of fine-grained, pure foraminiferaldetrital limestone with a thickness of greater than 540 ft (Tracey and others, 1964). The formation supplies most of

the wells in northern Guam and the permeability is high (Ward and others, 1965; Contractor and Srivastava, 1990;

Jocson and others, 2002). In places, the Barrigada Limestone grades upward and outward into the overlying Mariana

Limestone.

The Mariana Limestone is of Pliocene to Pleistocene age and is the most extensive unit at the surface. The

Mariana Limestone comprises about 45 percent of the surface area, covering most of the northern half and parts of

southeast Guam. The maximum thickness of the formation is at least 500 ft and may thin to zero inland. The

Mariana Limestone is composed of reef and lagoonal limestone containing a wide range of lithologies (Tracey and

others, 1964). The limestone is clay-rich near the older volcanic uplands. The permeability of the deposits without

clay is considered very high due to the presence of many solution fissures and channels (Ward and others, 1965;

Contractor and Srivastava, 1990; Jocson and others, 2002). The permeability of the clay-rich deposits is considered

moderate to high.

Minor reef beach deposits, reef limestone, and alluvium are of Holocene age. These deposits cover about 7

percent of the surface of Guam, and may be as much as 200 ft thick at the mouths of some rivers. The beach deposits

are composed of poorly consolidated sediments, mostly calcareous sand and gravel thrown onto beaches by waves

but some deposits of volcanic sand can be found where streams drain volcanic uplands (Tracey and others, 1964).

The Merizo Limestone is a reef limestone up to 12 ft thick. Deposits of alluvial clay fill stream valleys and cover the

inner parts of coastal lowlands. The overall permeability of alluvial deposits may be moderate but hydrologic

information is limited (see Ayers and Clayshulte, 1983).

?

?

?

?

?

?

?

?

?
1,000

0.1

1

10

100

MONTHLY MEAN DISCHARGE, IN CUBIC FEET PER SECOND

STREAM-GAGING STATION 85800

Ylig River near Yona

TOTAL FLOW

BASE FLOW

1

10

100

STREAM-GAGING STATION 84850

Maulap River near Agat

TOTAL FLOW

BASE FLOW

Apra Harbor

Talofofo Bay

Ylig Bay

Hagåtña Bay

Agat Bay

Pago Bay

Fena Valley

Reservoir

Pago

River

Ylig River Nam o River

Hagåtña

R

Talofofo River

Imong R

Maulap R

PHILIPPINE

SEA

PA

CIFIC

OCEAN

Hagåtña

Umatac

Talofofo

Agat

Inarajan Merizo

Almagosa R

144

o

40'

13

o

20'

0

0

2 MILES

2 KILOMETERS

ACTIVE STREAM-GAGING STATION (2003)

DISCONTINUED STREAM-GAGING STATION

ABBREVIATED STATION NUMBER--Complete

number is 16835000

DRAINAGE BASIN

EXPLANATION

Base from U.S. Geological Survey

Island of Guam, 1:62,500, 1953

85000

84900 84600

85800

85450

84000

85500

83500

83500

80960

80940

84800

86500

84500

80830

86200

82100

81600

84810

84850

84700

WELL AND AVERAGE WITHDRAWAL RATE

EXPLANATION

less than 0.2 Mgal/d

between 0.2 and 0.4 Mgal/d

between 0.4 and 0.6 Mgal/d

age and chloride concentration relative to seawater at various

wells, Guam. (Andersen AFB data from Western Regional

DATE

er Service)

10 percent

50 percent

90 percent

10 percent 50 percent

90 percent

10 percent

50 percent

90 percent

2000

1990

PREPARED IN COOPERATION WITH THE

WATER AND ENVIRONMENTAL RESEARCH INSTITUTE (WERI),

UNIVERSITY OF GUAM

WATER-RESOURCES INVESTIGATIONS REPORT 03-4126

Water Levels, Salinity, Ground-Water Withdrawals –SHEET 2 OF 2

Gingerich, S.B., 2003, Hydrologic Resources of Guam

and chloride concentration, indicators of salinity, in deep wells

er transition zone provide information about the thickness and

f water from various depths in the deep wells were collected

ecific conductance and chloride concentration. Beginning in

conductivity/temperature/pressure sensor lowered into the deep

hloride concentration or salinity of water in the well was equal to

or salinity of seawater. Each sample date allows the approximate

or these plots) and the approximate thickness of the transition

imated and the time series of samples shows the seasonal

n zone. The freshwater thickness is largest at well EX-1, ranging

ne thickness also is the largest, ranging from about 100 to 250 ft

e not understood but may be related to the relatively lower

e freshwater lens farther to the north is significantly thinner,

seasonal variability is less pronounced. The transition-zone

. The data from all three wells show how the freshwater lens gets

rge is reduced and thicker after increased recharge during the

tion Data

ground water from the freshwater lens (fig. 10). The distribution

a provided by Guam Waterworks Authority (GWA), Guam

Navy (USN), and the U.S. Air Force (USAF). The pumping

ell between January 1996 and December 1998, the last period

daily withdrawal during that period was about 35 Mgal/d with

n 0.4 Mgal/d. The two wells that withdrew the most water are

is an indicator of the amount of seawater that has mixed with

system. For the 180 wells with recent data, chloride

34 wells and springs for which historical chloride-concentration

ve exceeded 250 mg/L in at least one sample (fig. 11). Another 5

aching the 250-mg/L guideline. The samples with the highest

r of the island. Factors that may contribute to the high

d-water withdrawal from a single well, (2) a cluster of wells that

deeply into the freshwater lens, or (4) higher permeability

freshwater with the underlying saltwater (McDonald, 2001).

The Fena Valley Reservoir, constructed in 1951, captures surface water from three main streams with a combined

drainage area covering 5.88 mi2 and is the major source of domestic water in southern Guam (Nakama, 1992).

Combined flow into the reservoir averaged about 21.3 ft3/s since 1972, about 15.3 ft3/s (9.9 Mgal/d) of which was

diverted from the reservoir and a nearby spring for domestic and military use during 1999-2000 (Naval Facilities

Fena Water Treatment Plant, written commun., 2000).

Surface Water

The USGS has at various times operated 22 continuously recording gages in southern Guam to record streamflow

(fig. 12 and table 1). Hydrographs of two rivers, the Ylig and Maulap Rivers, show total flow and base flow

estimated from a computerized hydrograph-separation technique (Wahl and Wahl, 1995) for the period January

1980–September 2000 (fig. 12). The gaged streams of southern Guam have a ratio of base flow to total flow ranging

from 0.24 to 0.57 indicating that the low-permeability aquifer causes a significant part of streamflow to be from

ground-water discharge (table 1). In other words, ground-water discharge constitutes 23 to 57 percent of total

streamflow in the gaged basin. Similar ratios (0.38 to 0.56) are reported from eastern Kauai, Hawaii, where the lowpermeability lava flows that form the aquifer have allowed water levels to build up hundreds of feet above sea level

(Izuka and Gingerich, 1998).

Table 1. Data for selected gaged stream basins, Guam

[ft

3

/s, cubic feet per second, drainage basins shown in figure 12; complete gaging-station number is preceded by 16 and ends in 0; nd, not

determined; na, not applicable; values of n and f in the base-flow index program (Wahl and Wahl, 1995) for all streams were 4 and 0.9,

respectively; rates of flow expressed in ft

3

/s can be converted to Mgal/d by multiplying the rate by 0.64632; currently (2003) active

continuous-record stations shown in bold.]

USGS

gagingstation

number

Gaging-station

location

Period of record available

for base-flow analysis

Average

total flow

(ft

3

/s)

Average

base flow

(ft

3

/s)

Ratio of base

flow to total

flow1

(in percent)

80765 Aplacho River at

Apra Heights 10/1999 to 09/2001 nd nd nd

80830 Finile Creek at Agat 04/1960 to 12/1982 1.41 0.80 57

80940 Cetti River near

Umatac 03/1960 to 09/1967 4.44 1.21 27

80950 La Sa Fua River near

Agat na nd nd nd

80960 La Sa Fua River near

Umatac

05/1953 to 06/1960,

10/1976 to 04/1984,

06/2000 to 09/2001

4.56 1.31 31

81600 Umatac River at

Umatac 10/1952 to 12/1976 8.66 3.07 35

82100 Geus River near

Merizo 05/1953 to 09/1975 3.01 0.72 24

83500 Inarajan River near

Inarajan 10/1952 to 01/1983 17.44 6.61 38

84000 Tinaga River near

Inarajan 11/1952 to 02/1986 5.62 2.03 36

80765

80950