Ground Water on Tropical Pacific Islands—

Understanding a Vital Resource

U.S. Department of the Interior

U.S. Geological Survey

Circular 1312
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Ground Water on Tropical Pacific Islands—

Understanding a Vital Resource

Circular 1312

U.S. Department of the Interior

U.S. Geological Survey

By Gordon Tribble
Cover—Perennial flow in Ko‘iawe Stream, Waipi‘o Valley on the Island of Hawai‘i. The sustained flow of 

water in the stream results in the discharge of dike-impounded ground water from high elevations at the 

back of the valley. Streams that flow perennially because of ground-water discharge are important sources 

of water for traditional and modern agricultural practices, provide habitat for a unique group of stream 

animals, and contribute to the aesthetic value of the islands. (USGS photograph by Gordon Tribble.)

U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

This report and any updates to it are available online at:

http://pubs.usgs.gov/circ/1312/

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod/

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov/

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright 

owners to reproduce any copyrighted material contained within this report.

Suggested citation:

Tribble, Gordon, 2008, Ground Water on Tropical Pacific

Islands—Understanding a Vital Resource: U.S. Geological Survey

Circular 1312, 35 p.

Cataloging-in-Publication data are on file with the Library of Congress (URL http://www.loc.gov/).

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, April 19, 2007

Text edited by James W. Hendley II

Layout and design by Jeanne S. DiLeo and Susan Mayfield
iii

CONTENTS

Foreword ........................................................................................................................................................v

Introduction ...................................................................................................................................................1

Importance of Ground Water .....................................................................................................................1

Precipitation and Ground-Water Recharge ............................................................................................2

Freshwater-Lens Systems ..........................................................................................................................4

Porosity, Permeability, and Hydraulic Conductivity ...............................................................................5

The Ghyben-Herzberg Principle and the Transition Zone .............................................................6

High-Level Ground Water ...........................................................................................................................8

Interactions Between Ground Water and Surface Water...................................................................12

Ground-Water Quality.................................................................................................................................13

Effects of Ground-Water Pumping...........................................................................................................14

The Need for Ground-Water Data ...........................................................................................................17

Ecological Importance of Ground Water ...............................................................................................17

Ground-Water Management....................................................................................................................20

Examples of Ground-Water Use on Tropical Pacific Islands..............................................................22

Laura Area of Majuro Atoll, Republic of the Marshall Islands—

A Freshwater-Lens System Provides Relief During Drought ..........................................22

‘Ïao Aquifer, Maui, Hawai‘i—

A Freshwater-Lens System Stressed by Rising Demand ................................................24

Saipan, Mariana Islands—

Challenges of Pumping From a Thin Freshwater Lens .....................................................26

Tutuila, American Samoa—

Contrasts in Aquifer Permeability Affect Well Performance ..........................................28

Acknowledgments .....................................................................................................................................30

Additional Information ...............................................................................................................................30

Glossary .......................................................................................................................................................32

iii
iv

Freshwater cascades from 

a spring on a cliff face into 

a pool next to the Pacific 

Ocean near Nähiku on the 

Hawaiian island of Maui. The 

water comes from rain that 

fell at higher elevations on 

Haleakalä Volcano, seeped 

into the ground, and has 

traveled slowly toward the 

coast. (USGS photograph by 

Gordon Tribble.)
v

Foreword

To a casual observer, tropical Pacific islands seem idyllic. Closer scrutiny reveals that their generally small size 

makes them particularly vulnerable to economic and environmental stresses imposed by rapidly growing populations, 

increasing economic development, and global climate change. On these islands, freshwater is one of the most precious 

resources. Ground water is the main source of drinking water on many islands, and for quite a few islands, it is the only 

reliable source of water throughout the year. Faced with a growing demand for this valuable resource, and the potential 

negative effects on its availability and quality from changes in global climate, increasingly sophisticated management 

approaches will be needed to ensure a dependable supply of freshwater for the residents of these islands.

Much scientific information has been collected by the U.S. Geological Survey (USGS) and other organizations 

about the ground-water resources of tropical Pacific islands. The aim of this Circular is to give members of the public, 

policymakers, and other stakeholders knowledge that will help ensure that this information can be used to make 

informed decisions about the management of these life-giving resources.

As the demand for freshwater grows, new monitoring and research efforts will be needed to (1) characterize the 

extent and sustainability of ground-water resources on different tropical Pacific islands, (2) better understand linkages 

between ground-water discharge and freshwater and nearshore ecosystems, and (3) prepare for the effects of climate 

change, which will likely include the loss of habitable land and reduced areas for the accumulation of ground water as a 

result of rising sea levels.

The welfare of people living on tropical Pacific islands is of great interest to the United States. These islands not 

only include the State of Hawai‘i but also the U.S. territories of American Samoa, Guam, and the Commonwealth of the 

Northern Mariana Islands, as well as the Freely Associated States of the Federated States of Micronesia, the Republic 

of the Marshall Islands, and the Republic of Palau. The Secretary of the U.S. Department of the Interior (DOI) has the 

administrative responsibility for coordinating Federal policy in the territories and for administering and overseeing 

Federal assistance provided to the Freely Associated States. The Office of Insular Affairs executes these responsibilities 

on behalf of the Secretary.

As the DOI’s science bureau, the USGS serves the Nation by providing reliable and impartial scientific 

information so that decisions can be made on an informed basis. We look forward to continued service to Pacific 

islanders by providing the science to help them manage their water resources to ensure a reliable supply of freshwater.

DIRK KEMPTHORNE

SECRETARY OF THE INTERIOR

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HawaiÔi

Indonesia

New Zealand

Yap

Chuuk Pohnpei

American

Samoa

Federated States

of Micronesia

Papua

New Guinea

New Caledonia

Fiji

Tonga

French

Polynesia

Australia

Kwajalein Atoll

Majuro Atoll

Tutuila

Guam

Saipan

Babelthuap

Kosrae

Republic of Palau

Commonwealth of the

Northern Mariana Islands

Republic of the 

Marshall Islands

Pacific Ocean

0 500 1,000 miles

0 500 1000 1,500 kilometers

20°N

0°

20°S

140° E 160° 180° 160° W

Map of the tropical Pacific Ocean, 

showing the locations of some 

island groups. On many tropical 

Pacific islands, ground water 

is the primary source of water. 

Because of their relatively small 

size and oceanic setting, these 

ground-water resources are 

vulnerable to overpumping and 

saltwater intrusion, especially 

during droughts caused by 

climatic variations.

22¡N

20¡N

160¡W 158¡W 156¡W

HAWAIÔI

Pacific Ocean

KauaÔi

NiÔihau

OÔahu

Maui

MolokaÔi

LanaÔi

KahoÔolawe

HawaiÔi

0

0 75 150 kilometers

45 90 miles
Ground Water on Tropical Pacific Islands—

Understanding a Vital Resource

By Gordon Tribble

Introduction

Underground water or “ground water” 

is the primary source of water on many 

tropical Pacific islands. Contained in porous, 

regionally extensive geologic formations 

or “aquifers,” fresh ground water on these 

islands floats on and is surrounded by 

more dense saltwater from the ocean. The 

potential for an aquifer to provide a reliable 

source of good-quality water depends on the 

amount of recharge that occurs from rainfall, 

the physical properties of the aquifer, and 

how the water is pumped or removed from 

the ground. Because of their relatively small 

size and oceanic setting, ground-water 

resources on tropical Pacific islands are 

vulnerable to overpumping and saltwater 

intrusion, especially during droughts caused 

by climatic variations such as El Niño 

events. Also, this vulnerability is made 

worse because the effects of ground-water 

pumping are initially difficult to perceive. 

This book describes some of the factors that 

influence the availability of ground water 

on tropical Pacific islands, such as recharge, 

aquifer properties, patterns of flow, salinity 

distribution, and the effects of pumping. 

Some examples of U.S. Geological Survey 

(USGS) studies in Hawai‘i and other Pacific 

islands are used to illustrate these concepts 

and their relevance to the management of 

ground-water resources.

Importance of Ground Water

Ground water is a vital source of water 

on many tropical Pacific islands. For 

example, on the Island of O‘ahu (Hawai‘i), 

ground water provides more than 90 percent 

of the drinking water and about half of 

the water used for agricultural irrigation. 

Ground water provides 80 percent of the 

drinking water on the Island of Guam and 

nearly all of the public water supply on 

the Islands of Saipan (Northern Mariana 

Islands) and Tutuila (American Samoa). 

On many tropical Pacific islands, 

ground water is the preferred source of 

drinking water because its treatment to 

meet water-quality standards is generally 

easier and cheaper than that of surface 

water. Also, ground water is less affected 

by variations in rainfall than surface water 

and water collected by rain-catchment 

systems, making it more reliable during 

droughts. Overall, the use of surface water 

is limited because many islands lack the 

available land to construct reservoirs large 

enough to store enough water to meet 

their needs. Additionally, the diversion of 

surface water from streams can adversely 

affect the plants and animals living in 

them, the scenic beauty of the landscape, 

and the availability of surface water for 

other purposes, such as recreational use. 

Because of all of these factors, ground 

water has been and will continue to be 

desirable as a reliable, economical, and 

safe source of freshwater on many tropical 

Pacific islands.
2 Ground Water on Tropical Pacific Islands

As moisture-laden air moves over the high 

mountains found on some tropical Pacific islands it 

cools, forming rain clouds. The side of the mountains 

exposed to the prevailing wind (windward side) 

typically has more rainfall than the more protected 

side (leeward side), and this rain recharges nearby 

aquifers. This photograph shows a rainbow and 

clouds on Mount Wai‘ale‘ale on the Island of Kaua‘i, 

Hawai‘i. Wai‘ale‘ale typically receives more than 

430 inches of rainfall annually, making it one of the 

wettest places on Earth. Real-time rain data from 

this area and others in Hawai‘i can be found at 

http://hi.water.usgs.gov/. (Copyrighted photograph 

courtesy of Arjun Roychowdhury.)

Precipitation and 

Ground-Water Recharge

Precipitation, mainly rainfall, is the 

source of the freshwater that recharges 

aquifers. On islands with high mountains, 

rainfall is commonly heavier on the 

windward side (the side exposed to the 

prevailing wind) and over mountainous 

areas, and rainfall is lighter on the leeward 

side (the side protected from the prevailing 

wind by the mountains). In the Hawaiian 

Islands, the amount of annual rainfall can 

range from several hundred inches on 

windward mountains to less than 10 inches 

on leeward areas only a few miles away. 

At high elevations, moisture in clouds 

can directly condense on the ground 

and vegetation; known as fog drip, this 

precipitation is not recorded by conventional 

rain gages. The amount of fog drip is 

substantial in some areas, and measuring it 

and its contribution to recharging aquifers 

remains a significant challenge.

Most tropical Pacific islands have 

distinct wet and dry seasons, and rainfall 

can also vary considerably over time. One 

of the major causes of this variation is 

the El Niño phenomenon, which results 

from large-scale, anomalous warming of 

sea-surface temperatures in the tropical 

eastern Pacific Ocean. The fully developed 

interaction between the atmosphere and 

ocean is called the El Niño-Southern 

Oscillation (ENSO). Large variations 

in rainfall between different years are 

commonly associated with the ENSO 

Measuring the water “budget” is key to understanding 

the extent of ground-water resources. This picture 

shows a station in a remote part of West Maui that 

records information so that the total amount of 

precipitation, evaporation, and transpiration can be 

calculated. The specific instruments at the top of the 

pole measure (from left to right) rainfall, humidity, 

total solar energy, net solar energy, and wind speed. 

A solar panel on the pole provides electricity for 

the instruments and data recorders. The tripod on 

the right holds a collector that accumulates cloud 

moisture, also known as “fog drip.” By collecting 

this information in different settings, hydrologists 

can determine the relation between precipitation, 

vegetation, and ground-water recharge. (USGS 

photograph by Toby Vana.)
cycle. El Niño events generally correspond 

to decreased rainfall and droughts on many 

islands. During droughts, people must rely 

more heavily on ground water because 

surface-water sources and water collected 

in rain-catchment systems are depleted 

more rapidly.

On reaching the ground, precipitation 

can evaporate to the atmosphere, run 

off to the ocean, or drain into the 

soil. Water in soil is subject to further 

evaporation and transpiration (the 

evaporation of water from the leaves and 

stems of plants). Water not removed by 

these processes becomes ground-water 

recharge. Estimating the amount of 

recharge an aquifer receives is complex 

and commonly limited by lack of data. 

The amount of recharge is typically 

calculated by estimating the amount of 

total precipitation and then subtracting the 

estimated amounts of evapotranspiration 

(evaporation and transpiration) and 

direct runoff. Although such calculations 

are limited by knowledge of the actual 

amounts of total precipitation, direct 

runoff, and evapotranspiration, the use 

of a computer geographic information 

system (GIS) allows for more accurate 

estimates of these processes by 

including their geographic variation in 

the calculation of recharge. Estimates 

During years when the El Niño atmospheric phenomenon is active, 

many tropical Pacific islands experience decreased rainfall and droughts. 

During droughts, people must rely more heavily on ground water because 

surface-water sources and water collected in rain-catchment systems are 

depleted more rapidly. This diagram shows annual rainfall (in inches) on 

Majuro Atoll, Republic of the Marshall Islands, during the past 50 years, 

compared to the El Niño-Southern Oscillation (ENSO) index. Negative 

ENSO values indicate that the phenomenon is active. Periods when El 

Niño has been active are shaded green, and in general the island has 

received less rainfall during these years. The photograph shows many 

Marshall islanders waiting at a water distribution site in 1998 after local 

water resources on their island were depleted during a severe drought 

caused by an El Niño. The water used in the relief effort was pumped 

from a larger and carefully managed aquifer on a nearby island. (FEMA 

photograph by Angel Santiago.)

El Niño Events Can Cause Water Shortages

0

2

4

50

100

0

150

200

1960 1970 1980

Year

1990 2000

Majuro rainfall, in inches

Southern Oscillation Index

-4

-2

Precipitation and Ground-Water Recharge 3
4 Ground Water on Tropical Pacific Islands

of recharge on tropical Pacific islands 

typically range from 10 to 50 percent of 

total precipitation. Accurate estimates 

of recharge are extremely important 

in determining the availability and 

sustainability of ground-water resources.

Freshwater-Lens Systems

As freshwater recharges an island 

aquifer, saltwater is displaced, and the 

freshwater forms a “lens” that floats 

on underlying, denser saltwater. Under 

natural conditions, water in the lens flows 

toward the ocean, such that the amount 

of recharge is balanced by discharge 

to nearshore springs or as widespread 

seepage into the ocean. Frictional 

resistance to water flow within the 

aquifer elevates the “water table” relative 

to sea level. In general, the water table 

is the surface where water pressure is 

equal to atmospheric pressure, but more 

simply, the water table is the top of the 

underground body of water. The elevation 

or level of the water table generally 

increases with distance from the coast. 

Near the coast, water levels may undergo 

daily changes driven by ocean tides. 

Water levels in a freshwater-lens system 

are also influenced by seasonal and 

annual variations in sea level.

This diagram shows the major components of the ground water or “hydrologic” cycle 

on most tropical Pacific islands. Arrows show the movement of the rainfall and other 

precipitation that recharges aquifers and the generalized direction of ground-water 

flow toward the ocean. Water vapor is also shown returning to the atmosphere, where 

it again becomes precipitation.

The Hydrologic Cycle

Pacific Ocean

stream

infiltration

discharge

fog

rain

surface runoff

ground-water flow

water table

evaporation

transpiration
Ground water on 

tropical Pacific islands 

is pumped from a wide 

variety of wells. At top 

left, an inexpensive 

hand pump is being 

used to draw small 

volumes of water 

from a simple well in 

a shallow aquifer on 

an atoll island (Majuro 

Atoll, Marshall Islands) 

(USGS photograph 

by Stephen Anthony). 

However, to draw 

large volumes of water 

from wells in deeper 

aquifers, such as those 

found on the Island 

of Hawai‘i, complex 

high-voltage pumps 

may be needed (see 

USGS photograph at 

left by Gordon Tribble). 

Regardless of how 

water is pumped from 

an aquifer, certain 

principles apply that 

determine the quality 

and quantity of water 

that can be withdrawn 

on a sustainable basis.

Most wells on tropical Pacific islands 

pump water from aquifers that have a 

freshwater-lens system. To provide a 

reliable source of freshwater, these aquifers 

need to be carefully managed so that 

overpumping does not draw up the salty 

or “brackish” water that is beneath the 

freshwater lens. Within an aquifer, mixing 

of seaward-flowing water with underlying 

saltwater forms a brackish zone of 

transitional salinity. The thickness of this 

“transition zone” depends on the physical 

properties of the aquifer, the amount of 

ground-water flow, and the extent to which 

water mixes within the aquifer.

Porosity, Permeability, and

Hydraulic Conductivity

The porosity and permeability of an 

aquifer affect the shape of the freshwater 

lens and how water flows through the 

aquifer. “Porosity” is the percentage of 

a geologic formation (sediment or rock) 

that consists of small pores or spaces that 

can contain water, and “permeability” is 

a measure of the ease with which water 

can flow through a geologic formation. 

Effective porosity reflects the volume of 

an aquifer that consists of interconnected 

spaces through which water can flow. All 

Porosity, Permeability, and Hydraulic Conductivity 5
The Ghyben-Herzberg Principle and the Transition Zone

The principle commonly used to 

estimate the thickness of freshwater 

in a freshwater-lens system is called 

the Ghyben-Herzberg principle. This 

principle was independently formalized 

by Captain Willem Badon Ghijben (Dutch 

Army) in 1889 and A. Herzberg (German) 

in 1901, but it was originally described 

in 1818 by Joseph Du Commun, an 

instructor at the United States Military 

Academy at West Point. 

Because seawater is about 2.5 percent 

denser than freshwater, the fresh ground 

water in a freshwater-lens system “floats” 

on the underlying saltwater. However, the 

weight of the freshwater depresses the 

surface or top of the saltwater downward 

40 feet for every foot the water table is 

above sea level. Therefore, the estimated 

thickness of a freshwater lens is about 40 

times the elevation of the water table above 

sea level. For example, a freshwater lens 

that is elevated 3 feet above sea level will 

extend to a depth of 120 (3×40) feet below 

sea level (left image on page 7). In reality, 

a broad “transition zone” of brackish water 

commonly exists between freshwater 

and saltwater. In this situation, the 

Ghyben-Herzberg principle estimates 

the depth in the transition zone where 

the brackish water has a salinity about 50 

percent of that of seawater (known as the 

“midpoint,” right image on page 7). 

Although a useful approximation, the 

Ghyben-Herzberg principle assumes no 

mixing between freshwater and saltwater 

and does not calculate the actual thickness 

of the drinkable or “potable” part of the 

lens, which is typically much less than the 

total thickness of the lens to the midpoint of 

the transition zone. The Ghyben-Herzberg 

principle also assumes that freshwater flow 

is predominantly horizontal. Thus, the 

principle does not apply in settings with 

complex geology or where vertical groundwater flow is significant.

The thickness of the transition zone 

is determined by several factors, such as 

the physical properties of the aquifer, flow 

rates, and the distribution of wells pumping 

ground water from the aquifer. Predicting 

the thickness and dynamic movement of 

the transition zone in response to pumping 

and changes in recharge is a considerable 

challenge. In general, a high-permeability 

aquifer will have a thicker transition zone 

than a low-permeability aquifer. In addition, 

ocean tides and ground-water pumping can 

cause vertical mixing of water, also resulting 

in a thicker transition zone.

Willem Badon Ghijben (circa 1900). 

(Photograph provided by Royal Netherlands 

Academy of Arts and Sciences.)

6 Ground Water on Tropical Pacific Islands
Idealized freshwaterlens system, showing a 

sharp boundary between 

freshwater and saltwater

Freshwater

Brackish water

Saltwater

Water table

Ground-water flow

Midpoint

Explanation

ocean

Sea level

(0 feet)

Bottom of

freshwater lens

(-120 feet)

freshwater

Top of

freshwater lens

(+3 feet) water table

saltwater

Typical freshwater-lens 

system, showing a “transition 

zone” of brackish water

Porosity, Permeability, and Hydraulic Conductivity 7

ocean

saltwater

midpoint

water table

brackish water

freshwater
8 Ground Water on Tropical Pacific Islands

Hawaiian Islands

LhuÔe Basin, Island of KauaÔi

Hawaiian Islands

Marshall Islands

Marshall Islands

Island of HawaiÔi

Mariana Islands

¥ Volcanic-dike complexes

¥ Basin-filling volcanic rock

¥ Volcanic-ash formations

¥ Atoll-island Holocene-age sedimentary deposits

 (sand- to boulder-sized coral-reef debris)

¥ Atoll-island Pleistocene-age marine limestone

¥ Layered volcanic rock (mostly basalt)

¥ Uplifted marine limestone

0.01 Ð 0.1

0.3

1Ð500

20Ð200

2,000Ð3,000

1,000Ð10,000

100Ð19,000

Aquifer material Location Hydraulic conductivity 

in feet per day (ft/d)

The permeability of an aquifer is typically described in terms of its hydraulic 

conductivity, which accounts for both the permeability of a geologic 

formation and the fluid properties of the water flowing through it. This table 

shows examples of hydraulic-conductivity values for some common aquifer 

materials found on tropical Pacific islands.

other factors being equal, a zone of high 

effective porosity results in higher rates 

of ground-water flow than does a zone of 

low effective porosity. The permeability of 

an aquifer is typically described in terms 

of its “hydraulic conductivity” (K), which 

accounts for both the permeability of a 

geologic formation and the fluid properties 

of the water flowing through it. Typical K

values range from less than 0.1 ft/d (feet 

per day) for low-permeability volcanicdike rocks to more than 10,000 ft/d for 

some types of highly permeable limestone. 

All other factors being equal, highpermeability aquifers have higher rates of 

ground-water flow and lower water-table 

levels, and low-permeability aquifers 

have lower rates of ground-water flow and 

higher water-table levels. 

High-Level Ground Water

In addition to freshwater-lens systems, 

ground water may occur at elevations 

high above sea level on some tropical 

Pacific islands. High-level ground-water 

bodies can form where low-permeability 

geologic features or structures block or 

slow the movement of ground water, 

either vertically downward or horizontally 

toward the shoreline. High-level groundwater bodies are not typically subject to 

contamination by saltwater and in some 

areas are important sources of freshwater. 

Some high-level ground-water systems 

and the geologic features that can lead to 

their formation are described below:

Perched systems—Ground water may 

occur at high elevations where lowpermeability rocks block or slow the 

downward movement of water, so that a 

perched water body develops. The lowpermeability material may be a thickHydraulic Conductivity on Tropical Pacific Islands
Volcanic “dikes” are vertical sheets of volcanic rock 

that form when magma (molten rock) is “intruded” 

into cracks in the Earth’s crust. These dikes can 

slow the flow of ground water and cause watertable levels to rise behind them. This trapped or 

“impounded” water can be an important source 

of freshwater on some tropical Pacific islands. 

This photograph shows people collecting water 

dripping from the roof of a tunnel used to reach 

and transport water impounded behind dikes on 

the windward side of the Island of O‘ahu, Hawai‘i. 

(Photograph courtesy of the Honolulu Board of 

Water Supply.)

bedded lava flow, a weathered-soil or 

volcanic-ash layer, or a sedimentary 

deposit. Perched ground-water bodies that 

occur in many areas of Hawai‘i have been 

an important source of water supply to 

rural communities. Although most of these 

perched water bodies appear to be small 

and localized, some regionally extensive 

perched aquifers have been identified by 

examining well-drilling records and the 

locations of springs.

Dike-impounded systems—Volcanic 

“dikes” are tabular sheets of volcanic rock 

that form when magma (molten rock) is 

injected or “intruded” into cracks or faults 

in the Earth’s crust. Dikes typically are 

concentrated along elongate systems of 

vertically oriented fractures, called “rift 

zones,” created when movement related 

to volcanic activity has caused the ground 

to spread apart. Much like a dam, dikes 

can block or slow the flow of water so that 

an elevated water table develops inland 

of the dikes. The hydraulic conductivity 

in areas with many dikes can be low (less 

than 0.1 ft/d), and if the dikes are oriented 

nearly perpendicular to the direction of 

ground-water flow, water levels inland of 

dikes can be many hundreds to several 

thousands of feet above sea level. For 

example, dikes impound water to great 

heights, as much as 3,300 feet above sea 

level on the islands of Maui and Hawai‘i 

and as much as 1,600 feet above sea level 

on O‘ahu. Much of the water collected 

from the windward side of the Island of 

O‘ahu is through tunnels that penetrate 

dikes and harvest the impounded water.

Vertically extensive freshwaterlens systems—Areas of low regional 

permeability may have a vertically 

extensive freshwater-lens system with 

a water table high above sea level. The 

low regional permeability may result 

from a high degree of weathering or be 

an inherent characteristic of the aquifer 

material, such as a sequence of thick 

lava flows that cooled slowly. The low 

permeability of such aquifers generally 

limits the water output of individual wells 

and reduces their usefulness as watersupply sources.

Frictional resistance associated with the 

low-permeability material blocks or slows 

the flow of water in the aquifer enough that 

a thick freshwater body develops. This is 

much like what happens when you steadily 

add water to a potted plant. If the soil in the 

pot is very fine grained (silt or clay; low 

permeability), the water level in the soil 

will remain higher as the water is draining 

through the soil than if the soil is coarse 

High-Level Ground Water 9
10 Ground Water on Tropical Pacific Islands

High-Level Ground-Water Systems

perched ground water

unsaturated zone

unsaturated zone

Pacific Ocean

coastal

spring

spring

water table

brackish water

saltwater

water table

freshwater lens

100

Sea 

level

200

Feet

Ground water may occur at elevations high above sea level on 

some tropical Pacific islands. High-level ground-water bodies can 

form where low-permeability geologic features or structures block 

or slow the movement of ground water, either vertically downward 

or horizontally toward the shoreline. 

Perched system Dike-impounded system

A low-permeability, horizontal layer results in a perched aquifer. Volcanic dikes compartmentalize ground-water bodies.

Sea 

level

1,000

2,000

Pacific Ocean

freshwater

brackish water

saltwater

water table
Freshwater

Brackish water

Saltwater

Water table

Ground-water flow 

Volcanic dikes

Fault movement

Explanation

 High-permeability volcanic rock Low-permeability volcanic rock

unsaturated

saturated

unsaturated

saturated

saltwater brackish water

Sea 

level

1,000

2,000

Feet

water table

Pacific Ocean

freshwater

Sea 

level

1,000

2,000

Feet

impediment

water table

brackish water

freshwater

Pacific Ocean

saltwater

High-Level Ground Water 11

A low-permeability geologic sructure of unknown origin forms an 

impediment that slows the seaward flow of ground water.

Vertically extensive freshwater-lens system System created by low-permeability geologic structure

A regional low-permeability geologic formation results in 

a vertically extensive freshwater lens.
12 Ground Water on Tropical Pacific Islands

grained (sand or gravel; high permeability). 

In both cases, the water will eventually drain 

out the holes at the bottom of the pot at the 

same rate as you are adding water at the top. 

Well-water levels throughout a vertically 

extensive freshwater-lens system are much 

higher than those that would occur in an 

ordinary freshwater-lens system. In recharge 

areas, the elevation or level of the top of the 

water in a well that penetrates deeply into a 

vertically extensive freshwater-lens system 

is significantly lower than that in a well that 

penetrates only the upper part of such an 

aquifer. This decline in well-water level with 

depth occurs because of the predominately 

downward flow of water in such aquifers. As 

the water flows downward through a lowpermeability geologic formation, it loses 

energy because of friction and therefore 

water levels in deeper wells are lower. In 

areas where the ground water is discharging, 

such as near the coast, the situation is 

reversed. In this case, the upward movement 

of water results in water levels in deeper 

wells being higher in elevation than water 

levels in shallower wells. 

Other high-level ground-water systems—

In some areas, a low-permeability geologic 

structure of unknown origin separates a 

high-permeability aquifer with water levels 

high above sea level from a freshwater-lens 

Frictional resistance associated with lowpermeability geologic material can block or 

slow the flow of water in an aquifer enough 

that a thick freshwater body develops. This is 

much like what happens when you steadily 

add water to a potted plant. If the soil in 

the pot is very fine grained (silt or clay; low 

permeability), the water level in the soil will 

remain higher as the water is draining through 

the soil than if the soil is coarse grained (sand 

or gravel; high permeability). In both cases, 

the water will eventually drain out the holes at 

the bottom of the pot at the same rate as you 

are adding water at the top.

Fine soil

water

level

water

level

Coarse soil

system, slowing the flow of water between 

them. One example is on the west side of 

the Island of Hawai‘i, which has an aquifer 

of relatively unweathered flows of layered 

basalt with hydraulic conductivities higher 

than 1,000 ft/d. A buried low-permeability 

impediment oriented nearly parallel to 

the coast appears to slow the movement 

of ground water toward the ocean, so that 

the elevation of the water table is less than 

10 feet on the ocean side and several hundred 

feet on the inland side of the impediment. 

Other examples of areas with high groundwater levels resulting from a structure of 

uncertain origin are the central plateau of 

O‘ahu and the Waimea area on the northern 

part of the Island of Hawai‘i. 

Interactions Between Ground 

Water and Surface Water

Water beneath the ground surface occurs in 

two principal zones—the “unsaturated zone” 

and the “saturated zone.” In the unsaturated 

zone, the pore spaces in rocks contain both air 

and water, whereas in the saturated zone pore 

spaces are only filled with water. The upper 

surface of the saturated zone is the water table. 

Where the water table of an aquifer intersects 

the land surface, ground water discharges to 

a spring or stream even during periods of no 
rain. Streamflow sustained by such discharge 

is known as “base flow.”

In areas where the water table is lower 

than a stream channel, water infiltrates 

downward from the stream to the aquifer, 

and streamflow is reduced as the aquifer is 

recharged. Perennial streamflow (flowing 

continuously throughout the year) at high 

elevations indicates the presence of a highlevel ground-water body with a water table 

above the stream channel. Overall, the baseflow characteristics of streams are controlled 

by the distribution of ground water, which, 

in turn, is controlled by the local geologic 

setting and climatic conditions.

Dike-impounded and perched groundwater bodies are typical sources of perennial 

discharge that sustain streamflow at higher 

elevations. Erosion has exposed volcanic 

dikes in many of the deeply incised valleys on 

the windward sides of the Hawaiian Islands, 

and discharge of the dike-impounded water is 

the source of base flow to many streams.

Perched ground-water bodies may discharge to isolated springs, or to more regionally extensive features that provide base flow 

to several streams. For example, an area on 

the north flank of Haleakalä Volcano (Ha‘ikü, 

Island of Maui) appears to be an extensive 

perched aquifer. Ground water in this area 

occurs in both an upper perched aquifer 

and a lower freshwater-lens system; the two 

aquifers are separated by an unsaturated zone 

several hundred feet thick. The water table 

in the perched aquifer typically is within a 

few hundred feet of the land surface, intersecting the surface in deep valleys. About 10 

percent of the total ground-water recharge 

from precipitation in the area subsequently 

discharges from the upper aquifer and provides base flow to streams. The remaining 

90 percent of the recharge infiltrates past the 

low-permeability layer (beneath the perched 

ground-water body) and recharges the lower 

freshwater-lens system.

An extreme example of ground-water 

discharge to streams is the Lïhu‘e Basin on 

the southeast side of the Island of Kaua‘i. The 

regional hydraulic conductivity for the entire 

Lïhu‘e Basin has been estimated to be a few 

tenths of a foot per day, although localized 

exceptions exist. The low permeability of 

the area, in combination with high recharge, 

results in an aquifer that is saturated many 

hundred feet above sea level, with a water 

table at or near the land surface. This water 

table intersects stream channels into which 

water discharges from the aquifer. As a result, 

the base flow of streams in the basin increases 

in the downstream direction. Discharge to 

streams accounts for 60 to 70 percent of the 

total ground-water recharge in the basin, with 

the rest either discharged directly into the 

ocean or withdrawn by wells.

Freshwater-lens systems generally 

contribute to streamflow only at low elevations 

near the coast because the water table is close 

to sea level. Thus, if no high-level water 

bodies discharge ground water, streams will be 

ephemeral (flowing only as a result of nearby 

and recent precipitation), except where they 

intersect the freshwater-lens system at low 

elevations. Streams that intersect a freshwaterlens system typically start perennial flow near 

the shoreline, and much of the ground-water 

discharge is within the estuarine parts of these 

streams, where freshwater comes into seawater 

and tidal effects occur. 

Ground-Water Quality

On most tropical Pacific islands, the 

availability of ground water from freshwaterlens systems that is suitable for drinking 

is limited primarily by salinity. The salt in 

the ground water comes from the seawater 

that surrounds and underlies the islands, 

and natural processes and pumping mix this 

saltwater with freshwater. Seawater contains 

many dissolved salts, and the concentration 

of dissolved chloride (Cl−

) is typically used 

to indicate the presence of salt from seawater, 

which has a dissolved chloride concentration 

of about 19,500 milligrams per liter (mg/L). 

The U.S. Environmental Protection Agency’s 

Ground-Water Quality 13
14 Ground Water on Tropical Pacific Islands

recommended or “secondary” standard 

for dissolved chloride in drinking water 

is 250 mg/L, or about 1.3 percent of the 

chloride concentration of seawater. What this 

standard means is that freshwater mixed with 

only a small amount of seawater will be of 

poor quality for drinking.

Island aquifers are susceptible to contamination from various human activities. 

Land uses such as agriculture or urban and 

suburban development can alter both the 

quantity and quality of water recharging an 

aquifer. Leaks and inappropriate disposal 

of industrial solvents and petroleum products, such as motor oils, have contaminated 

ground water in many areas. Similarly, 

pesticides and nutrients introduced by agricultural practices have also contaminated 

ground water in some areas.

Improper handling of human sewage can 

result in potentially serious contamination of 

ground water by harmful bacteria and other 

microscopic organisms, particularly where 

the ground water is close to the surface 

or where the soil and aquifer permit fast 

vertical movement. Sewage contamination 

of ground water may come from leaking, 

poorly constructed, or failing cesspools, pit 

latrines, or septic systems, as well as from 

leaking sewer pipes or failing and overflowing sewage-pump stations. For example, 

several years ago failing sewage-pump 

stations in some areas of Guam resulted in 

large spills of untreated wastewater on the 

ground surface. Owing to the high permeability and hydraulic conductivity of the 

soil and underlying limestone there, public 

water-supply wells (some as deep as 300 

feet and some as far away as several miles) 

tested positive for microbial indicators of 

sewage after these spills. Microbial indicators were commonly detected within very 

short time periods, sometimes hours to days, 

after a wastewater spill.

Geochemical and biologic reactions 

in aquifers alter the chemical composition 

of ground water from the time it infiltrates to the time it discharges. Weathering 

of volcanic and carbonate (limestone or 

coral) aquifer materials, primarily driven 

by carbon dioxide (CO2

) in rainwater and 

biologic respiration in soil, adds dissolved 

salts to ground water. Organic matter in 

ground water is biodegradable by the 

bacteria naturally present in an aquifer, a 

process that produces CO2 and may result 

in additional chemical weathering. Ground 

water on the islands of many atolls (islands 

that sit atop ring-shaped coral reefs with 

central lagoons) has high levels of bacterial 

activity, and USGS studies on some atolls 

suggest that contamination from petroleum products is at least partly removed by 

microbial activity in ground water.

Effects of Ground-Water 

Pumping

Pumping ground water from a well 

both lowers the level of the water table and 

removes water from storage in the aquifer, 

causing a decrease in the amount of groundwater discharge equal to the amount of 

pumping. In settings where ground water 

discharges to streams, the lowering of 

water-table levels by pumping can result in 

reduced streamflows. In a freshwater-lens 

system, pumping also shrinks the freshwater 

lens, resulting in upward movement of the 

transition zone. These effects are greatest 

near the source of water withdrawal. Over 

time, the water table and transition zone 

reach a new balance or “equilibrium.”

The effects of pumping may be 

localized or widespread, depending on 

the natural patterns of ground-water 

flow, pumping rate (pumpage), and 

geologic structures that affect flow. For 

example, pumping from a freshwaterlens system does not affect an overlying 

perched aquifer, whereas pumping from a 

perched aquifer can affect the underlying 

freshwater-lens system by decreasing 

the amount of water that recharges the 

lens. Also, buried dikes and other lowpermeability features that block or slow 

ground-water flow may reduce the effects 
Salinity as indicated by dissolved chloride concentration,

in parts per million (ppm)

Depth, in feet below mean sea level

0

200

400

600

800

1,000

0 5,000 10,000

2005

1995

1985

15,000 20,000

midpoint

Profiles of freshwater lenses and transition zones are used to evaluate the 

response of an aquifer to pumpage. They can also be used to estimate the 

sustainable yield of nearby ground-water wells and are helpful in calibrating 

numerical models of ground-water flow. Upward movement in the transition 

zone between freshwater and saltwater, as measured at the “midpoint” (see 

The Ghyben-Herzberg Principle on pages 6-7) is caused by pumping large 

volumes of freshwater from nearby wells. This diagram shows the change in 

the depth of the transition zone over 20 years in the Waiehu Deep Monitoring 

Well on the Island of Maui, Hawai‘i.

Ground-Water Pumping Can Cause the Transition Zone to Rise

of pumping in nearby areas and increase 

the time before such effects are observed. 

If too much ground water is pumped, 

a freshwater lens may shrink enough 

that brackish water from the transition 

zone is drawn into the well. This process, 

known as saltwater intrusion, can result 

in the need to shut down wells and may 

reduce the availability of drinking water. 

Saltwater intrusion can either affect 

individual wells or degrade an entire 

aquifer. One remedy is to reduce pumpage. 

Because of the delay between the start 

of pumping and the rise in the transition 

zone, it may take several years before 

it becomes apparent that a well is being 

overpumped or that a well is recovering 

from the effects of overpumping. The time 

lag for a particular well to reach a new 

equilibrium is determined by, among other 

things, the location and depth of the well, 

pumpage from that well and others in the 

same aquifer, the physical properties of the 

aquifer, and the volume of ground water 

flowing through the aquifer. 

On many tropical Pacific islands, 

horizontal wells that skim water from 

near the water table are used to spread the 

effects of pumping over a larger area. Such 

wells can provide an effective water supply 

even in settings with only a thin freshwaterlens system. 

Effects of Ground-Water Pumping 15
16 Ground Water on Tropical Pacific Islands

The effects of overpumping may take some time to become evident. Once ground water is pumped 

from an aquifer, the water table falls, and the zone of brackish water—the “transition zone” (see 

The Ghyben-Herzberg Principle on pages 6 and 7)—beneath the well rises (left). The rise in the 

transition zone is slower than the lowering of the water table. If too much water is pumped, the 

transition zone rises high enough that the salinity of water pumped from the well becomes too high 

for human consumption (right).

The Effects of Overpumping Ground Water

freshwater

saltwater

ocean ocean

Pumping well Pumping well

table

water

saltwater

brackish

water

brackish

water

freshwater

table

water

Freshwater

Brackish water

Saltwater

Water table

Ground-water flow

Explanation
This U.S. Geological Survey hydrologist is using 

a steel tape to measure the water level in a 

monitoring well on the windward side of the 

Hawaiian Island of O‘ahu. Understanding groundwater resources requires accurate information 

that often must be collected with careful field 

measurements. Water level is frequently measured 

with a plastic tape that has an electric sensor that 

shows when water is reached in the well, but steel 

tapes are preferred in some settings because 

they have greater accuracy and durability. (USGS 

photograph by Delwyn Oki.)

The Need for Ground-Water Data

Accurate field data are required to 

determine the effects of ground-water 

pumping. Water-level and salinity data 

are needed to estimate the availability of 

fresh ground water and so are important 

for effective ground-water management. 

Water-level measurements provide 

information about aquifer conditions, 

changes in the amount of recharge, and the 

response of ground water to pumping. In 

general, measurements in wells dedicated 

to monitoring ground water provide more 

accurate data than measurements taken in 

wells that are used for pumping ground water. 

The water level in an actively pumped well 

is lower than the water table immediately 

adjacent to the well because of frictional 

resistance as water enters the well. The 

difference in water level in a pumped well 

and the adjacent aquifer, known as “well 

loss,” is a measure of a well’s efficiency 

in transmitting water; a small difference 

indicates a more efficient well.

In freshwater-lens systems, salinity 

data collected at different depths in an 

aquifer are of great value in monitoring 

the position of the transition zone. Salinity 

profiles created using data from wells that 

penetrate the transition zone have been 

used to provide information about the 

thickness of freshwater-lens systems and 

the mixing characteristics of aquifers in the 

Hawaiian Islands, on the Island of Guam, 

and on the Island of Saipan. Changes in 

the thickness of a freshwater lens over 

time provide important information about 

the effects of pumping; however, care is 

needed in using data from such wells, 

because measurements have shown that 

the vertical flow of water within some 

boreholes can result in a profile that is not 

wholly representative of conditions in the 

adjacent aquifer.

Long-term information about waterlevel, rainfall, salinity, irrigation, and 

pumpage is needed to evaluate the historical 

effects of pumping and climate on groundwater levels. Similarly, data collected 

from wells over an extensive area provide 

a regional description of an aquifer. Also, 

data from aquifer-pumping tests can be used 

to calculate the physical properties of an 

aquifer that control the flow of ground water. 

Ecological Importance of 

Ground Water

The ecological importance of ground 

water on tropical Pacific islands is often 

underestimated. When it discharges to 

streams such that they flow perennially, 

ground water allows biological communities to develop that are specialized for life 

Ecological Importance of Ground Water 17
18 Ground Water on Tropical Pacific Islands

in the typically small and fast-flowing island streams. The animals found in these 

streams have, in nearly all cases, evolved 

from marine ancestors, and biologically 

different species have similar ecological 

niches or roles on different Pacific island 

groups. Most native aquatic species found 

in these streams must return to the ocean 

in the early stages of their lives. These 

instream communities are valuable for 

both their biological diversity and cultural 

importance. For example, shrimp and fish 

from perennial freshwater streams were 

important foods for Native Hawaiians, 

and streamwater was used to irrigate taro, 

the principal staple in the Polynesian diet. 

Ground water from springs was also used 

Aerial view of Keawa Nui Fishpond, on the Island 

of Moloka‘i, Hawai‘i. This centuries-old fishpond 

encloses an area of 72 acres, and although 

vegetation covers the wall at left, this pond is still 

used to cultivate fish. Before European contact, such 

fishponds were important sources of protein for Native 

Hawaiians, and they carefully managed them to 

provide a sustainable supply of food. The productivity 

of fishponds is believed to be enhanced by groundwater discharge, and Native Hawaiians often 

constructed them in areas with coastal springs and 

estuaries sustained by such discharge. (Photograph 

from U.S. Environmental Protection Agency.)

for religious and ceremonial practices, such 

as cleansing and blessing.

The discharge of ground water at 

the coastline can also be ecologically 

important. In settings such as sheltered 

bays or extensive coral reef flats, freshwater 

discharge can lead to estuarine conditions 

that provide important habitat for some 

marine animals, even without input from 

perennial streams. Ground water may 

contain nutrients that increase the biologic 

productivity of coastal areas, and the lower 

salinity may be important to creating 

suitable habitat for some species. There is 

scientific debate as to the importance of 

coastal ground-water discharge in sustaining 

biological productivity, but it is clear that 

many native peoples on tropical Pacific 

islands recognized the importance of such 

discharge. In particular, Native Hawaiians 

built extensive fishponds in some areas 

with enhanced ground-water discharge. 

Because the walls enclosing the ponds were 

assembled with many large rocks moved 

by hand, the construction of a fishpond 

represented a considerable investment in 

The native Hawaiian fish ‘o‘opu 

näkea (Awaous guamensis) in 

‘Ïao Stream, on the Island of 

Maui. Adults like this individual 

live in freshwater streams with 

perennial flow sustained by groundwater discharge. Revealing their 

evolutionary origins, all native 

freshwater fishes found in Hawaiian 

streams must spend the early 

stages of their lives in the ocean 

and thus need a stream that at 

least periodically flows continuously 

along its length to the ocean to 

complete their life cycle. (USGS 

photograph by Reuben Wolff.)
Upstream view of the diversion of ‘Ïao Stream in the central part of the Island of Maui, Hawai‘i. On many 

tropical Pacific islands perennial streams are often diverted because they provide a cheap and reliable source 

of freshwater. Perennial flow in the ‘Ïao Stream is sustained by the discharge of high-level ground water 

impounded by dikes at the back of the valley. Essentially all of the average ground-water discharge of 20 

million gallons per day that supplies the stream is captured by the diversion for use in agricultural irrigation 

and other societal needs, leaving the stream below the diversion dry most of the time. Native fish thrive in the 

stream’s upper reaches, but the diversion restricts the ability of larvae to reach the ocean and complete their 

life cycle to times of high flow from heavy rainfall. (Hawai‘i State Office of Hawaiian Affairs photograph.)

manpower. These fishponds often formed 

extensive complexes that were carefully 

managed as vital sources of food.

The demand for freshwater has led 

to some adverse ecological impacts on 

tropical Pacific islands. Perennial streams 

are often diverted because they provide a 

cheap and reliable source of freshwater. 

Many times, the need for water is so great 

that the diversion is designed to capture 

most of the ground-water discharge, 

leaving the stream channel downstream of 

the diversion dry (except during storms) 

and unfavorable for aquatic life. Perhaps 

more significantly, the dry section of 

stream prevents young native stream 

animals from parts of the stream above 

the diversion from reaching the ocean and 

thus greatly reduces the ability of stream 

animals to maintain their life cycles. 

Ground-water pumping can also reduce 

streamflow. Although it is unusual for 

ground-water pumping to cause a section 

of a stream to go dry, it has been observed 

in some settings. Less understood is the 

impact of reducing the amount of ground 

water discharging at coastlines. More 

research is needed to evaluate the impacts 

of ground-water discharge on the ecology 

of nearshore biological communities, and 

the consequences of reduced discharge 

resulting from pumping or other causes.

Ecological Importance of Ground Water 19
20 Ground Water on Tropical Pacific Islands

Ground-Water Management

To ensure a continued supply of fresh 

ground water, this vital resource must 

be carefully managed. “Ground water 

sustainability” is the development and use 

of ground-water resources in a way that can 

be continued indefinitely without causing 

unacceptable environmental, economic, or 

social consequences. Simply put, an aquifer 

that is pumped too much will not provide a 

reliable long-term source of water. Managing 

ground water on the basis of this definition 

of sustainability calls for resource managers 

to recognize the relation between water 

quality and water availability, the connection 

between ground water and surface water, 

and the economic and social importance of a 

reliable source of drinking water.

Management of ground-water resources 

on tropical Pacific islands largely focuses 

on maintaining a supply of water with 

acceptable salinity, because most water is 

pumped from freshwater-lens systems. In 

other words, the volume of water available 

from a freshwater-lens system is mainly 

limited by salinity. To reduce the potential 

for pumping brackish water, wells in a 

freshwater-lens system are ideally located 

where the freshwater lens is thickest. 

In addition to location, how a water well 

is constructed is also vital to ensuring that 

it provides freshwater. The intake or bottom 

of a well must not be so deep in the aquifer 

that brackish water from the transition 

zone is pumped. Similarly, both the depth 

of the well and the desired pumpage must 

be considered when installing a pump. As 

noted earlier, overpumping a well can result 

in saltwater intrusion or excessive lowering 

of the water table.

Ground-water resources are managed 

by several methods. The simplest method 

is to measure the salinity of the pumped 

water and reduce the pumpage if the salinity 

reaches unacceptable levels. A somewhat 

more proactive method is to monitor the 

elevation of the water table or the transitionzone salinity some distance from the 

pumped well, generally between the pumped 

well and the coast. Neither of these methods 

predicts or provides information about 

the overall sustainability of the freshwater 

supply in an aquifer. In the Hawaiian Islands 

and on the Island of Guam, an algebraic 

model has been used to estimate regional 

aquifer sustainability and to manage the use 

of ground-water resources. The model is 

based on straightforward calculations and 

could be readily applied to aquifers on other 

tropical Pacific islands, but its accuracy is 

limited by theoretical shortcomings and its 

inability to account for the complexity of 

most aquifers. 

The most rigorous and powerful 

approach to ground-water management is 

the use of a calibrated numerical model 

to simulate freshwater flow through an 

aquifer. A numerical model is the best 

available method to represent groundwater-flow patterns, assess the effects of 

pumping, and evaluate the sustainability 

of alternative scenarios for groundwater development. USGS scientists use 

numerical models to synthesize waterbudget data, aquifer properties, and 

ground-water data into a mathematical 

representation of water flow through an 

aquifer. Although constructing and using 

a numerical model that provides useful 

information for ground-water management 

can be time consuming and costly, this 

expense may be offset by the value of 

understanding (1) variations in both 

the timing and the three-dimensional 

distribution of ground-water flow, (2) the 

effects of proposed ground-water pumping 

on natural discharge and water-supply 

wells, and (3) the overall sustainability of 

ground-water resources.
Numerical Models of Ground-Water Flow

Ground-Water Management 21

Honolulu

22°N

20°N

160°W 158°W 156°W

HAWAIØI

Pacific Ocean

KauaØi

NiØihau

OØahu

Maui

MolokaÔi

LanaÔi

KahoÔolawe

HawaiØi 0

0 75 150 km

45 90 mi

OÔahu

Honolulu

N

0

-1,000

-2,000

-3,000

-4,000

-5,000

-6,000

Depth, in feet 

below sea level 0 5,000 10,000 15,000 20,00 Distance 0

, in feet

Pearl 

Harbor

Pearl 

Harbor

Numerical models that simulate the flow of freshwater and saltwater through 

an aquifer are the best tools for understanding and managing ground-water 

resources. Such models incorporate known information about the geology 

and hydrology of an aquifer so that issues such as aquifer sustainability and 

the effects of pumping and climate change can be assessed. This computergenerated image shows a representation of salinity in the Pearl Harbor 

Aquifer, on the Hawaiian Island of O‘ahu, with saltwater shown in red and 

freshwater shown in blue. The transition zone is seen as bands of yellow and 

green. The thick transition zone in the middle of the figure shows the effects 

of many years of sustained pumping, which has caused the freshwater lens 

to thin and the transition zone to rise. The use of models such as this allows 

water resources to be developed in a way that helps ensure that they will 

continue to be available for future generations.
22 Ground Water on Tropical Pacific Islands

Examples of Ground-Water Use 

on Tropical Pacific Islands

Although many tropical Pacific islands 

are similar in their climate, geology, and 

culture and how these influence their 

ground-water or “hydrologic” systems, 

each island is unique. The following 

examples illustrate only a few of the many 

issues in ground-water use on tropical 

Pacific islands. These examples come from 

the research of the U.S. Geological Survey 

scientists who work to understand groundwater resources on these islands and help 

ensure their availability.

Laura Area of Majuro Atoll, Republic of the Marshall Islands—

A Freshwater-Lens System Provides Relief During Drought

On many small tropical Pacific islands, 

rain is the preferred source of freshwater, 

and ground water is used only incidentally, 

because it commonly contains salt and 

bacteria. However, ground water may provide 

an important and underutilized resource that 

can partially reduce the hardships caused 

by drought. Ground water in the Laura area 

on Majuro Atoll, Republic of the Marshall 

Islands, is one example. 

Majuro Atoll receives an average of about 

130 inches of rainfall annually. Although 

rainfall collected in catchment systems has 

.

. . .

.

.

.

.

. . . . . . .

. . .

.

.

.

.

.

.

. . . . . .. . .

. .

.

. . .

.

.

. .

.

.

...

..

.

...

..

.

.

.

. . .

. .

.

.

.

.

. .

.. . . . .

.

. . . .

.

. . ..

. . .

. . . . . . .

.

.

.

.

.

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.

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.

.

.

. .

HawaiÔi

Indonesia

New Zealand

American

Samoa

Federated States

of Micronesia

Papua

New Guinea

New Caledonia

Fiji

Tonga

French

Polynesia

Australia

Majuro Atoll

Tutuila

Guam

Saipan

Republic

of Palau

Commonwealth of the

Northern Mariana Islands

Republic of the 

Marshall Islands

P acific Ocea n

1,000 miles

1,500 kilometers

20° N

0°

20° S

140° E 160° 180° 160° W

of the total rainfall. The aquifer consists of 

unconsolidated permeable sand- and cobblesize coral-reef debris, underlain by highly 

permeable marine limestone.

During droughts, rainfall decreases by 

about a third or less of average amounts, and 

during the 1998-99 drought most domestic 

rain-catchment systems were depleted. In 

response, ground-water pumpage from the 

Laura area was increased from 191,000 to 

286,000 gallons of freshwater per day. The 

reduced recharge and increased pumpage 

resulted in thinning of the freshwater lens. 

Ground water pumped from the Laura aquifer 

provided about 60 percent of the municipal 

water supply during the drought, in contrast to 

about 25 percent before the drought. During 

the drought, monitoring wells were used to 

assess the status of the freshwater lens and 

manage pumpage to avoid saltwater intrusion 

into water-supply wells. Continued monitoring 

of these wells during dry as well as wet 

periods will provide better information on the 

sustainability of ground-water resources in 

the area and help in the planning of mitigation 

efforts for future droughts. (See Hamlin and 

Anthony, 1987, and Presley, 2005, for more 

information.)

historically been the primary 

source of drinking water for 

many households, ground 

water is becoming increasingly 

important. The freshwater 

lens in the Laura area has a 

maximum thickness of about 

50 feet. Water levels are only 

a few feet above sea level and 

are affected by tidal changes in 

the ocean. Because the island is 

nearly flat, there is little surface 

runoff from precipitation, and 

ground-water recharge has 

been estimated at about half 

22 Ground Water on Tropical Pacific Islands
Explanation

Approximate extent of potable freshwater (chloride 

concentration less than 500 mg/L) in June 1998.

Carbonate sand and coral reef debris

 Unsaturated

 Saturated

Water table

Well

Wells

Vertical scale greatly exaggerated

Majuro Lagoon Pacific Ocean

0

10

20

30

40

50

Depth, in feet below water table

Before pumping

September 1984

Pumping and drought

June 1998

freshwater

saltwater Before pumping

September 1984 0 1,000 feet

Approximate line of 500 mg/L chloride concentration

for date shown; dashed where inferred.

Monitoring-well site. Squares represent the

depth of each sampling point.

Examples of Ground-Water Use 23

This diagram is a generalized 

cross section of the Laura 

aquifer on Majuro Atoll, 

showing the estimated extent 

of freshwater both before 

ground-water pumping in the 

area began in September 1984 

and during a period of both 

pumping and drought in June 

1998. “Freshwater” in this case 

is defined as water containing 

less than 500 milligrams (mg) 

of dissolved chloride (Cl−) per 

liter (L). As a comparison, the 

U.S. Environmental Protection 

Agency’s recommended standard 

for dissolved chloride in drinking 

water is 250 mg/L, or about 

1.3 percent of the chloride 

concentration of seawater.

This aerial photograph shows part 

of Majuro Atoll. The Laura area 

is the wide, middle part of the 

island. The substantial width of 

this area promotes the formation 

of a ground-water lens that is an 

important source of freshwater, 

especially during droughts. (USGS 

photograph by Jeff Perreault.)
24 Ground Water on Tropical Pacific Islands

The growing community of Wailuku overlies part of the ‘Ïao aquifer on 

the Island of Maui, Hawai‘i. Water from this area is the largest source of 

municipal water supply on the island. (USGS photograph by Gordon Tribble.)

The ‘Ïao aquifer on the Hawaiian island of 

Maui is formed by highly permeable layered 

volcanic rocks. Low-permeability coastal 

deposits, which are a mix of eroded volcanic 

material, coral-reef debris, and marine sediments, 

block or slow the discharge of freshwater into 

the ocean, resulting in the formation of a thick 

freshwater lens. Average annual rainfall in the 

area ranges from less than 30 to more than 350 

inches, and recharge has been estimated to be 

about 42 percent of rainfall and fog drip. 

Ground-water pumpage from the ‘Ïao aquifer 

increased from less than 2 million gallons per 

day (Mgal/d) in 1950 to more than 20 Mgal/d 

in 1995. As of 2000, the aquifer provided more 

than half of the municipal water supply on Maui. 

In response to this sustained pumping, watertable levels declined to as much as half of their 

estimated predevelopment levels in parts of the 

aquifer. This decline in water-table levels has 

been mirrored by a shrinking of the freshwater 

lens and increasing salinity in several freshwatersupply wells. The average pumpage during 2002 

was about 17 Mgal/d. During this time, watertable levels appeared to be stabilizing in response 

to decreased pumping, although the transition 

zone was still moving upward.

Several estimates have been made of the 

sustainable yield of the ‘Ïao aquifer. As of 2005, 

the most current estimate is that the aquifer 

‘Ïao Aquifer, Maui, Hawai‘i—A Freshwater-Lens System Stressed by Rising Demand

has a sustainable yield of 20 Mgal/d. This 

estimate was made using an algebraic method 

that calculates the sustainable yield as a fraction 

of total recharge. Based on the limitations of 

this method and the historical response of the 

aquifer to pumping, this estimate is probably 

too high for the current distribution of wells. 

Construction of a numerical model of groundwater flow can provide a more precise analysis 

of aquifer sustainability and can be used to 

revise sustainability estimates as aquifer 

conditions change and additional data 

become available. A numerical model can 

also be used to test alternative distributions 

of pumped wells, as well as the effect of 

changing land-use practices on the extent 

of ground-water recharge. (See Meyer 

and Presley, 2001, Oki and Meyer, 2001, 

and Engott and Vana, 2007, for more 

information.)
Graphs of ground-water levels in the ‘Ïao aquifer from 1985 

to 2005. The graph at top shows the rate of ground-water 

pumping from the aquifer (purple line) along with corresponding 

changes in the water level observed in a monitoring well (Test 

Hole B) (blue line). Overall, as the rate of pumping increased, 

water levels in the well have decreased. The graph at bottom 

shows the rise of the zone of brackish ground water, called the 

“transition zone” (see The Ghyben-Herzberg Principle on pages 

6 and 7), in response to the pumping, as measured in the 

Waiehu Deep Monitoring Well. In this case, the transition zone 

is represented by the area between the depth of water with a 

salinity of about 2 percent (limit of potability) and 50 percent 

(the “midpoint”) of seawater.

10

15

20

25 20

15

10

5

0

Year

Water level, in feet

Pumping, in million gallons per day Depth, in feet below sea level

850

800

750

700

650

600

1986 1990 1994 1998 2002 2006

Pumping

Water level

2% salinity (limit of potability)

50% salinity (midpoint)

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HawaiÔi

Indonesia

New Zealand

American

Samoa

Federated States

of Micronesia

Papua

New Guinea

New Caledonia

Fiji

Tonga

French

Polynesia

Australia

Majuro Atoll

Tutuila

Guam

Saipan

Republic

of Palau

Commonwealth of the

Northern Mariana Islands

Republic of the 

Marshall Islands

P acific Ocea n

1,000 miles

1,500 kilometers

20° N

0°

20° S

140° E 160° 180° 160° W

Examples of Ground-Water Use 25
26 Ground Water on Tropical Pacific Islands

This photograph is an aerial view 

looking eastward of the Island of 

Saipan, identifying important geologic 

and geographic features. (Copyrighted 

photograph courtesy of Del Benson.)

Saipan, Mariana Islands—Challenges of Pumping From a Thin Freshwater Lens

The Island of Saipan is formed of highly 

permeable limestone that overlies lowpermeability weathered volcanic rocks. The 

high permeability of the limestone allows the 

freshwater lens to react quickly to changes 

and also allows freshwater to readily discharge 

to the ocean. As a result, water levels in the 

freshwater lens throughout much of the island are 

5 feet or less above sea level. Because the high 

permeability of the limestone facilitates mixing 

between freshwater and saltwater, ground water 

in many areas of the island is brackish, even 

in the absence of pumping. This situation has 

limited the areas where water-supply wells can be 

located on Saipan.

Wells in the highly permeable limestone 

are capable of yielding large volumes of water. 

Although some wells on Saipan produce 

designs can improve the situation. For example, 

the Isley well field was expanded beginning in 

the late 1970s. Wells there were typically drilled 

to a depth of 15 to 45 feet below the water table 

and pumped at rates of 50 to 120 gallons per 

minute (gal/min). The chloride concentration of 

water from this well field averages about 850 

mg/L. In contrast, the nearby Obyan well field 

was developed in the late 1990s with wells that 

penetrated only 6 to 10 feet below the water table 

and pumped at rates of 35 to 50 gal/min. By 

using lower pumping rates and shallower wells, 

an average chloride concentration of about 300 

mg/L was being maintained by 2003.

Some interior parts of Saipan have water 

tables in limestone that are several hundred feet 

above sea level because the limestone overlies 

low-permeability volcanic rocks that extend 

above sea level. Wells in this setting are less 

likely to be affected by saltwater intrusion and 

may yield significant volumes of freshwater. 

However, water levels may decline substantially 

in response to pumping. Wells drilled directly 

into the volcanic rocks yield only small volumes 

of water, owing to the low permeability of the 

material. (See van der Brug, 1985, and Carruth, 

2003, for more information.)

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. .

HawaiÔi

Indonesia

New Zealand

American

Samoa

Federated States

of Micronesia

Papua

New Guinea

New Caledonia

Fiji

Tonga

French

Polynesia

Australia

Majuro Atoll

Tutuila

Guam

Saipan

Republic

of Palau

Commonwealth of the

Northern Mariana Islands

Republic of the 

Marshall Islands

P acific Ocea n

1,000 miles

1,500 kilometers

20° N

0°

20° S

140° E 160° 180° 160° W

water with a dissolved chloride (Cl−) 

concentration less than the 250 milligrams 

per liter (mg/L) recommended by the U.S. 

Environmental Protection Agency as an 

upper limit for chloride in drinking water 

(an indicator of the presence of seawater), 

other wells that have been drilled too deep 

or pumped at high rates are subject to 

saltwater intrusion and commonly produce 

brackish water. Total pumpage increased 

from 2.4 million gallons per day (Mgal/d) 

in 1978 to more than 11 Mgal/d in 2000, 

and the average chloride concentration of 

pumped ground water rose from 690 to 

1,100 mg/L during that same period.

Although most of Saipan has only a 

thin freshwater lens and most wells produce 

brackish water, alternative well-field 

North

Limestone upland and coastal plain Volcanic peak Limestone upland and

coastal plain

South
West East

Freshwater

Brackish water

Saltwater

Water table

Ground-water flow

Explanation

High-permeability marine limestone

 unsaturated saturated

Low-permeability volcanic rock

 unsaturated saturated

Lines of equal chloride concentration, in 

milligrams per liter (mg/L); dashed where 

approximate or inferred, interval is variable; 

 purple equals 250 mg/L (2% salinity or 

 limit of potability); 

 magenta equals 10,000 mg/L (50% 

 salinity or “midpoint”).

Potable part of 

freshwater lens 

pinches out before

reaching shoreline

-200

Sea level

400

800

1,200

Vertical scale greatly exaggerated 0 5,000 feet

Elevation, in feet

spring

water table

saltwater

freshwater

Examples of Ground-Water Use 27

This diagram is a generalized cross section showing geology and ground-water 

occurrence on the Island of Saipan. It shows both freshwater-lens systems and 

high-level ground water elevated by underlying low-permeability volcanic rocks.
28 Ground Water on Tropical Pacific Islands

Tutuila, American Samoa—Contrasts in Aquifer Permeability Affect Well Performance

The Island of Tutuila, American Samoa, 

has two large areas of contrasting aquifer 

permeability. Most of the island, particularly the 

rugged mountains in the north, is composed of 

relatively old, low-permeability volcanic rocks, 

whereas the southwest, an area known as the 

Tafuna-Leone Plain, is composed of younger, 

high-permeability volcanic rocks. Geologic 

differences between the two areas result in 

different responses to ground-water pumping, 

illustrating the importance of understanding how 

ground water interacts with local geology when 

developing ground-water resources.

Ground-water levels in the low-permeability 

area may attain elevations of tens to hundreds of 

feet above sea level; much of this ground water 

discharges from elevated springs to sustain the 

perennial flow of streams on the island. With few 

exceptions, most wells in this area yield only 

modest volumes of water, yet water levels may 

decline many tens of feet as a result of pumping. 

In some wells, pumping has caused water levels 

to decline below sea level, resulting in minor 

saltwater intrusion and an increase in dissolved 

chloride (Cl−

) concentration—an indicator of the 

presence of seawater—from which the wells are 

slow to recover despite reduced pumpage. 

In contrast, ground-water levels in the 

high-permeability area are typically less than 

10 feet above sea level, and the area has no 

This photograph shows the city and harbor of 

Pago Pago and the older, rugged low-permeability 

volcanic interior of the Island of Tutuila, American 

Samoa. (USGS photograph by Scot Izuka.)

This photograph shows the Tafuna-Leone Plain 

on the Island of Tutuila, American Samoa. This 

plain is constructed of relatively young, highpermeability volcanic rocks. (Photograph courtesy 

of Troy Curry.)

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HawaiÔi

Indonesia

New Zealand

American

Samoa

Federated States

of Micronesia

Papua

New Guinea

New Caledonia

Fiji

Tonga

French

Polynesia

Australia

Majuro Atoll

Tutuila

Guam

Saipan

Republic

of Palau

Commonwealth of the

Northern Mariana Islands

Republic of the 

Marshall Islands

P acific Ocea n

1,000 miles

1,500 kilometers

20° N

0°

20° S

140° E 160° 180° 160° W

streams with perennial flow. Because 

of the high permeability and ease of 

access, wells in this area produce about 

70 percent of the ground water pumped 

on the island. Occasional increases in 

chloride concentration in wells occur 

during extended periods of low rainfall, 

but chloride concentrations return to 

low levels soon after normal rainfall 

resumes or pumping is reduced. The 

rapid response of chloride concentration 

to rainfall is consistent with the high 

permeability of the aquifer and the short 

distance between the ground surface and 

the water table that is characteristic of this 

part of Tutuila. (See Izuka, 1999, for more 

information.)
Examples of Ground-Water Use 29

Generalized water-table map of the Island of Tutuila, with graphs showing the 

relation between (1) ground-water pumping; (2) dissolved chloride (Cl− ) concentration (in milligrams per liter; mg/L), which is used to indicate the presence 

of salt from seawater; and (3) water levels in wells in low- and high-permeability 

areas on the island. The U.S. Environmental Protection Agency’s recommended 

standard for dissolved chloride in drinking water is 250 mg/L, or about 1.3 percent of the chloride concentration of seawater.

0 1 2 3 miles

0 1 2 3 kilometers

Cape

Mataula

Cape

Fogausa

Cape

Taputapu

Pago Pago

International

Airport TAFUNA-LEONE PLAIN

Base modified from U.S. Geological Survey, Tutuila Island, 1:24,000, 1963

Shaded relief from Atlas of American Samoa, University of Hawaii Cartographic Laboratory, 1981

1200

1000

200

5

5 10

200

200

200

200

1000

800

600

600

600

600

600

1200 1200 1000

1000

400

1200

1000

200

5

5 10

200

200

200

200

1000

800

600

600

600

600

600

1200 1200 1000

1000

400 Elevation of water table in feet; dashed where inferred

Location of known water level 

Explanation

Fagaitua

Bay

Aoa

Bay

Masefau

Bay

Pago Pago

 Harbor

Leone Bay

Fagatele

 Bay

Larsen

Bay

 Pala 

 Lagoon

Fagasa

Bay

Malaeimi Valley

Pumpage, in million

 gallons per day

0

400

200

600

800

-20

0

-10

10

20

85 86 87 88 89 90

Year

91 92 93 94 95

Chloride concentration,

in milligrams per liter

Water level, in

feet above sea level

0

0.4

0.2

0.6

0.8

96

Well in High-Permeability Aquifer

0

10

30

20

40

50

0

0.2

0.6

0.4

0.8

1.0

-100

0

-50

50

100

85 86 87 88 89 90 91 92 93 94 95 96

Well in Low-Permeability Aquifer

Chloride concentration,

in milligrams per liter

Water level, in

feet above sea level

Pumpage, in million

 gallons per day

Year
30 Ground Water on Tropical Pacific Islands

Acknowledgments

The author is deeply indebted to the many 

scientists who have contributed to our current 

understanding of ground water on tropical 

Pacific islands. Special thanks are due to the 

following individuals who reviewed earlier 

drafts of this report: Jim Anthony, Robert 

Carruth, Devin Galloway, Stephen Gingerich, 

Roy Hardy, Scot Izuka, William Meyer, 

Jill Nishimura, Delwyn Oki, Jeff Perreault, 

Rachel Ross, Jonathan Scheuer, and Jane 

Schoonmaker. Editorial, graphic, and layout 

assistance was graciously provided by George 

Havach, Keith Kirk, Luis Menoyo, Twyla 

Thomas, Jeanne DiLeo, Susan Mayfield, 

Peter Stauffer, and especially Jim Hendley. 

Finally, the author thanks Michael Dworsky, 

who asked the question “Your USGS reports 

are really complete and full of science, but do 

you guys ever write anything that someone 

without a Ph.D. can understand?” His concern 

provided the motivation for this Circular, 

which I hope now gives an affirmative answer 

to his question.

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119 p. [http://pubs.er.usgs.gov/usgspubs/wsp/

wsp1894].

Tribble, G.W., 1997, Ground-water geochemistry 

of Kwajalein Island, Republic of the Marshall 

Islands, 1991: U.S. Geological Survey WaterResources Investigations Report 97-4184, 47 p. 

[http://pubs.er.usgs.gov/usgspubs/wri/wri974184].

Tribble, G.W., 2001, Ground-water geochemistry at 

the south ramp jet fuel leak, Diego Garcia atoll, 

1993–94: U.S. Geological Survey Water-Resources 

Investigations Report 01-4090, 46 p.

U.S. Environmental Protection Agency, 2002, 

National primary and secondary drinking water 

regulations: Publication 816-F-02-013, 7 p. [http://

www.epa.gov/safewater/contaminants/index.html 

(last accessed Dec. 18, 2006)].

U.S. Environmental Protection Agency, 2004, Report 

to Congress—Impacts and Control of CSOs and 

SSOs: [http://cfpub.epa.gov/npdes/cso/cpolicy_

report2004.cfm (last accessed Feb. 22, 2006)].

Vacher, H.L., and Quinn, T.M., eds., 1997, Geology 

and hydrogeology of carbonate islands: Amsterdam, Elsevier, Developments in Sedimentology, v. 

54, 933 p.

van der Brug, Otto, 1985, Compilation of water 

resources development and hydrologic data of 

Saipan, Mariana Islands: U.S. Geological Survey 

Water-Resources Investigations Report 84-4121, 

578 p. [http://pubs.er.usgs.gov/usgspubs/wri/

wri844121].

Visher, F.N., and Mink, J.F., 1964, Ground-water 

resources in southern Oahu, Hawaii: U.S. Geological Survey Water-Supply Paper 1778, 133 p. 

[http://pubs.er.usgs.gov/usgspubs/wsp/wsp1778].

Winter, T.C., Harvey, J.W., Franke, O.L., and Alley, 

W.M., 1998, Ground water and surface water; a 

single resource: U.S. Geological Survey Circular 

1139, 79 p. [http://pubs.usgs.gov/circ/circ1139/].

Additional Information 31
32 Ground Water on Tropical Pacific Islands

Glossary

Aquifer—A water-bearing geologic 

formation or layer that will yield water in 

usable quantities to wells or springs.

Atoll—A coral reef appearing from a plan 

(or top) view as roughly circular (sometimes 

elliptical or horseshoe-shaped) and largely 

or completely enclosing a shallow (typically 

less than 300 feet deep) lagoon. Low coral 

islands typically lie atop some parts of the 

reef and are generally no more than 1 to 

2 miles long, less than 1⁄2 mile wide, and 

extend less than 15 feet above sea level; land 

of a non-reef origin is absent or rare. Atolls 

develop from reefs fringing volcanic islands. 

As first hypothesized by Charles Darwin, 

and confirmed by ocean drilling done 

by British scientists a century ago, reefs 

fringing volcanic islands build vertically to 

sea level, forming steep-walled barrier reefs. 

As a volcanic island subsides, or sinks, 

with time, the growing reef keeps pace 

with the rising water level. When the island 

eventually submerges, the ring-shaped reef 

forms an atoll with a central lagoon.

Basalt—A general term for dark-colored 

extrusive igneous rocks of relatively low 

silica (SiO2

) content; basalt is the most 

common rock erupted by oceanic volcanoes.

Chloride (Cl−

)—An “anion” of chlorine 

typically found dissolved in seawater at a 

concentration of about 19,500 milligrams 

per liter (mg/L). Common table salt, 

sodium chloride (NaCl), is the most wellknown chloride compound. Chloride 

concentration is commonly used to measure 

the amount of saltwater from the ocean 

that is mixed with freshwater in an aquifer. 

The U.S. Environmental Protection Agency 

recommends that drinking water not exceed 

a chloride concentration of 250 mg/L.

Coral reef—A chemically cemented, waveresistant mound or ridge created by colonies 

of “hard corals,” their accumulated skeletal 

fragments, and the remains of calcareous 

algae, mollusks, and other shallow-water 

tropical marine organisms that form 

calcareous skeletons.

Dike-impounded system—A groundwater system in which the flow of water is 

blocked or slowed by a volcanic dike. Dikeimpounded systems typically have a water 

table elevated hundreds or more feet above 

sea level.

El Niño-Southern Oscillation (ENSO)—A 

global event arising from large-scale 

interaction between the ocean and the 

atmosphere. When the waters of the 

eastern Pacific are abnormally warm (an 

El Niño event), there is a weakening of 

the low-latitude easterly trade winds. This 

is accompanied by a drop in sea-surface 

atmospheric pressure in the eastern Pacific 

and a rise in sea-surface atmospheric 

pressure in the western Pacific. El Niño 

events typically result in below-average 

rainfall and drought conditions on many 

tropical Pacific Islands. El Niño is a term 

originally used by fishermen along the coast 

of Ecuador and Peru to refer to a warm, 

nutrient-poor, ocean current that typically 

first appears in affected years around 

Christmastime (El Niño means Christ 

child in Spanish) and lasts several months. 

The Southern Oscillation, a more recent 

discovery, refers to an oscillation in the 

sea-surface atmospheric pressure between 

the southeastern tropical Pacific and the 

Australian-Indonesian region, as measured 

between Tahiti and Darwin, Australia.

Ephemeral—A stream, river, or other 

surface-water body that flows only in direct 

response to rainfall.

Estuary—The seaward end of a stream or 

river where freshwater comes in contact and 

mixes with seawater and where tidal effects 

are evident.

Freshwater-lens system—A lens-shaped 

body of fresh ground water that overlies 
Perennial—A stream, river, or other surfacewater body that flows continuously over 

time. On tropical Pacific islands, the water in 

perennial streams is sustained by the discharge 

of ground water from springs and seeps.

Permeability—A measure of the ability of a 

material to transmit a fluid; it is a measure of 

the relative ease of fluid flow under pressure. 

For a rock or aquifer that is transmitting 

water, permeability is usually expressed as 

hydraulic conductivity.

Pleistocene Epoch—The unit of geologic 

time preceding the Holocene Epoch. It 

began about 1.8 million years ago and 

was characterized by repeated cycles of 

continental glaciation and warm periods 

similar to the Earth’s current climate. At 

times during the Pleistocene, sea level was 

lowered by more than 300 feet as a result of 

the volume of water frozen as glacial ice.

Porosity—A measure of the void (empty) 

spaces in a material, usually expressed 

as a fraction (between 0 and 1) or as a 

percentage. “Effective porosity” refers to 

the fraction of the total volume in which 

fluid flow can effectively take place. 

Effective porosity excludes dead-end pores 

or unconnected cavities and is a more useful 

term in discussing the flow of water through 

an aquifer.

as the rate of flow (in gallons per day) 

through a cross section of one square foot 

under a standard driving force or “unit 

hydraulic gradient.”

Hydrologic systems—An assemblage 

of geologic, climatic, ecologic, and 

socioeconomic factors that influence the 

flow of water through a given area, and how 

that water is used as a natural resource.

Limestone—A sedimentary rock composed 

of calcium carbonate minerals. On tropical 

Pacific islands, most limestone is precipitated 

by coral-reef organisms as the minerals 

aragonite and magnesium calcite. Over time, 

chemical reactions convert the minerals 

to calcite and dolomite and cement the 

individual particles into a solid framework. 

Midpoint—In a freshwater-lens system, the 

midpoint is the depth in the aquifer, usually 

given relative to “mean sea level,” where 

the water is an equal mix of freshwater and 

seawater.

Perched systems—Perched systems include 

a saturated body of high-level ground water 

separated from an underlying freshwaterlens system by a zone of unsaturated rock. 

Perched systems typically occur where lowpermeability rocks significantly impede the 

downward movement of freshwater. 

saltwater. A zone of brackish water with 

transitional salinity typically separates the 

freshwater lens and saltwater.

Ground water—Water located beneath the 

ground surface in pore spaces, fractures, and 

other voids in geologic formations. Water 

enters the ground as rain that percolates 

through the soil and root zone. On islands, 

ground water flows towards the ocean, but 

along the way it may be intercepted by streams 

or withdrawn by wells for human uses.

Ground-water sustainability—The 

development and use of ground-water 

resources in a manner that can be continued 

indefinitely without causing unacceptable 

environmental, economic, or social 

consequences.

High-level ground water—The occurrence 

of ground water at high elevations. In such 

situations the water table is many tens to 

several thousand of feet above sea level.

Holocene Epoch—The most recent 

geologic time period, which started about 

11,000 years ago. Sea level rose rapidly 

during the early part of the Holocene in 

response to the melting of glacial ice from 

the most recent ice age.

Hydraulic conductivity—The capacity of 

a rock to transmit water, typically expressed 

Glossary 33
34 Ground Water on Tropical Pacific Islands

Potable—Water that is safe to drink. Water 

can be made nonpotable by excess salinity, 

the presence of pathogens, or contamination 

from pollutants.

Rain-catchment system—A mechanical 

system used to collect and store rainwater 

for future use. Rain-catchment systems can 

range from a simple barrel that collects 

runoff from a roof to large paved areas (such 

as an airport runway) designed to drain into 

a collection trench that is connected to a 

water-treatment plant. 

Recharge—The movement of water from 

the land surface, past the zone where it can 

be taken up by plants, to an underlying 

aquifer. Also refers to the rate at which new 

water reaches an aquifer.

Saltwater intrusion—The landward 

and upward movement of saltwater in a 

freshwater-lens system, typically caused by 

overpumping of ground water or reduced 

recharge. The amount of water drawn 

from freshwater-lens systems needs to be 

carefully managed to prevent degradation 

of the quality of the pumped water by 

saltwater intrusion.

Saturated zone—The zone below the water 

table, in which the voids and interstices 

are filled with water (ground water) under 

pressure greater than that of the atmosphere. 

Also known as the “phreatic zone.”

Sea level—Sea level, or more precisely “mean 

sea level,” is the average or the mean level of 

the ocean’s surface halfway between high and 

low tide as measured at a given place over 

a 19-year period. It is used as the reference 

elevation for geographic and hydrologic 

features and sea depths. Sea level varies over 

geologic time periods (millennia) as a result of 

climatic cycles such as ice ages.

Sedimentary deposits—Rock and (or) 

unconsolidated material derived from the 

mechanically formed fragments of older 

rocks (for example, sandstone and sand) 

or by chemical or organic processes (for 

example, limestone deposits formed by the 

precipitation of calcium carbonate minerals 

by coral-reef organisms). 

Transition zone—The zone of mixing in a 

freshwater-lens system, where the salinity 

ranges from nearly fresh in the upper part of 

the zone to a salinity close to seawater in the 

bottom part of the zone.

Unsaturated zone—The zone between the 

land surface and the water table. Water in 

the unsaturated zone has a pressure less than 

atmospheric pressure. Also known as the 

“vadose zone.”

Vertically extensive freshwater-lens 

system—A ground-water-flow system that 

is saturated with freshwater from below 

sea level to elevations hundreds or more 

feet above sea level. Vertically extensive 

freshwater-lens systems are found in areas 

with low regional hydraulic conductivity.

Volcanic ash—Finely pulverized rock, 

mineral, and volcanic-glass fragments that 

are ejected from a volcano into the air. These 

fragments often settle in a layer over a wide 

area that may be subsequently covered by lava 

flows or other material. Over time, weathering 

processes can cause buried ash layers to have 

a low hydraulic conductivity. Such ash layers 

have been associated with perched groundwater bodies on tropical Pacific islands.

Volcanic dike—An intrusive sheet of 

volcanic rock that cuts across other rock 

layers. Volcanic dikes are more common 

near the center of a volcano and along 

“volcanic rift zones.” Because of their 

orientation and low permeability, steep or 

vertical volcanic dikes can block or slow 

the flow of ground water and result in the 

development of significant bodies of fresh 

water at elevations high above sea level.

Volcanic rock—Rock formed by volcanic 

processes. On tropical Pacific islands, basalt 

is the most common type of volcanic rock. 
casings that are constructed with a powerful 

drill rig. Similarly, the pumps used to 

withdraw water from wells can range from 

simple hand-operated devices to complex 

designs that deliver large volumes of water 

from significant depths. On some tropical 

Pacific islands, horizontal trenches that 

penetrate just below the water table have 

been used to skim the freshest water from 

the top of a freshwater-lens system. On the 

Hawaiian island of Maui (and elsewhere), 

Water table—The level in an aquifer at which 

the water pressure is equal to the atmospheric 

pressure. Geologic materials above the water 

table are considered “unsaturated” by water, 

whereas materials below the water table are 

“saturated” by water.

Well—A constructed hole in the ground 

from which water is withdrawn. Wells can 

range from shallow open pits dug with 

simple tools to deep holes lined with metal 

wells are large tunnels, excavated from the 

land surface to the water table, that obtain 

water from horizontal “galleries.” 

Well loss—The difference between the 

water level inside a well used for pumping 

water and the water level in the aquifer 

immediately adjacent to the well. Well loss 

is caused by turbulence as water flows to 

the well and is used by engineers as one 

measure of a well’s efficiency in pumping.

Glossary 35
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Tribble—Ground Water on Tropical Pacific Islands—Understanding a Vital Resource

—Circular 1312