GEOHYDROLOGY OF THE ISLAND OF OAHU, HAWAII

Kauai

HAWAII

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Geohydrology of the Island of Oahu, Hawaii

By CHARLES D. HUNT, JR.

REGIONAL AQUIFER-SYSTEM ANALYSIS OAHU, HAWAII

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1412-B
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply

endorsement by the U.S. Government

Library of Congress Cataloging in Publications Data

Hunt, Charles D., 

Geohydrology of the island of Oahu, Hawaii / by Charles D. Hunt, Jr.

p. cm. (Regional aquifer-system analysis Oahu, Hawaii) 

(U.S. Geological Survey professional paper ; 1412-B) 

Includes bibliographical references. 

Supt. of Docs, no.: 119.16 :1412-B 

1. Geohydrology Hawaii Oahu. I. Title. II. Series 

III. Series: U.S. Geological Survey professional paper ; 1412-B. 

GB1025.H3H86 1996 

551.49'09969'3-dc20 96-24418

CIP ISBN 0-607-86102-9

For sale by the U.S. Geological Survey, Information Services, Box 25286, 

Federal Center, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program was started 

in 1978 following a congressional mandate to develop quantitative appraisals of the major ground-water systems of the United States. The RASA 

Program represents a systematic effort to study a number of the Nation's 

most important aquifer systems, which in aggregate underlie much of the 

country and which represent an important component of the Nation's total 

water supply. In general, the boundaries of these studies are identified by 

the hydrologic extent of each system and accordingly transcend the political 

subdivisions to which investigations have often arbitrarily been limited in 

the past. The broad objective for each study is to assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the system, and to develop predictive capabilities that will contribute 

to the effective management of the system. The use of computer simulation 

is an important element of the RASA studies, both to develop an understanding of the natural, undisturbed hydrologic system and the changes brought 

about in it by human activities, and to provide a means of predicting the 

regional effects of future pumping or other stresses.

The final interpretive results of the RASA Program are presented in 

a series of U.S. Geological Survey Professional Papers that describe the 

geology, hydrology, and geochemistry of each regional aquifer system. Each 

study within the RASA Program is assigned a single Professional Paper 

number, and where the volume of interpretive material warrants, separate 

topical chapters that consider the principal elements of the investigation 

may be published. The series of RASA interpretive reports begins with 

Professional Paper 1400 and thereafter will continue in numerical sequence 

as the interpretive products of subsequent studies become available.

Gordon P. Eaton 

Director

CONTENTS

Foreword...........................................................................

Abstract............................................................................

Introduction......................................................................

Purpose and Scope......................................................

Study Area..................................................................

Physical Setting..........................................................

Climate........................................................................

Previous Studies.........................................................

Geologic Framework........................................................

Regional Geologic Setting..........................................

Shield-Building Volcanism.........................................

Shield-Stage Volcanic Rocks......................................

Waianae Volcanics................................................

Koolau Basalt........................................................

Modification of the Shields by Secondary Processes. 

Subsidence and Slope Failure ..............................

Weathering and Erosion.......................................

Sedimentation.......................................................

Rejuvenated-Stage Volcanism....................................

Geohydrology of the Water-Bearing Deposits.................

Volcanic Rocks and Deposits......................................

Lava Flows.............................................................

Dikes......................................................................

Pyroclastic Deposits..............................................

Saprolite and Weathered Basalt..........................

Hydraulic Properties of the Volcanic Rocks.........

Hydraulic Conductivity and Transmissivity.. 

Porosity.............................................................

Specific Yield and Storage Coefficient............

Page 

III

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11

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14

14

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16

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24

24

Geohydrology of the Water-Bearing Deposits Continued 

Volcanic Rocks and Deposits Continued

Heterogeneity, Anisotropy, and Aquifer Structure... 

Sedimentary Rocks...........................................................

Modes of Ground-Water Occurrence.....................................

Basal Ground Water.........................................................

Basal Water in the Volcanic Rocks............................

Basal Water in the Sedimentary Rocks.....................

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

Dike-Impounded Water..............................................

Perched Ground Water and Water in the 

Unsaturated Zone.......................................................

High-Level Water in the Schofield Area....................

Saltwater..........................................................................

Regional Aquifer System.......................................................

Principal Volcanic-Rock Aquifers and

Confining Units ...............................................................

Subordinate Aquifers.......................................................

Geohydrologic Barriers and Ground-Water Areas .........

Transient Hydraulic Behavior of the

Aquifer System.................................................................

Ground-Water Flow Systems ................................................

Rift Zones and the Eastern and Western 

Ground-Water Systems....................................................

Central Flow System........................................................

Southeastern Flow System..............................................

Deep Saltwater Flow System...........................................

Summary................................................................................

References..............................................................................

25

25

26

26

27

30

31

31

31

31

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32

32

32

37

41

43

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46

46

49

51

ILLUSTRATIONS

Page 

FIGURES 1. Geologic map of Oahu showing rift zones and calderas of the Waianae and Koolau Volcanoes ................................... B3

2-6. Maps showing:

2. Land use, island of Oahu........................................................................................................................................ 4

3. Topography, island of Oahu.................................................................................................................................... 5

4. Geomorphic provinces, island of Oahu .................................................................................................................. 6

5. Streams, island of Oahu......................................................................................................................................... 7

6. Mean annual precipitation, 1916-83, island of Oahu........................................................................................... 8

7. Diagrams showing relation of primary porosity features and occurrence of dikes to magnitude of

permeability in lava flows................................................................................................................................................. 13

8. Map showing historic lava flows through 1953, island of Hawaii.................................................................................. 15

9. Histograms showing estimates of hydraulic conductivity from specific-capacity tests of volcanic

aquifers on the island of Oahu.......................................................................................................................................... 23

10. Map showing structural contours on top of Koolau Basalt in the Pearl Harbor area, island of

Oahu, and section showing stratified coastal-plain sediments....................................................................................... 28

11. Diagrammatic section showing modes of ground-water occurrence on the island of Oahu.......................................... 30
VI CONTENTS

ILLUSTRATIONS

Page

FIGURES 12-14. Maps showing:

12. Ground-water areas and geohydrologic barriers, island of Oahu................................................................... 33

13. Potentiometric surface in the principal volcanic aquifers of the island of Oahu........................................... 35

14. Effects of valley-fill barriers on water level and chloride concentration in the Koolau aquifer,

Kahuku area, island of Oahu............................................................................................................................ 38

15. Graph showing ground-water levels in central Oahu................................................................................................ 40

16. Map showing ground-water flow systems and the major areas of the central flow system, island of Oahu .......... 42

17-18. Graphs showing:

17. Relation between total measured discharge from the Pearl Harbor springs and freshwater head in the

Koolau aquifer, island of Oahu......................................................................................................................... 47

18. Water levels in well 187 open to freshwater and well T133 open to saltwater in the Pearl Harbor area,

island of Oahu.................................................................................................................................................... 48

TABLES

Page

TABLE 1. Water-bearing properties of the rocks and occurrence of ground water on the island of Oahu........................................... 17

2. Hydraulic properties of the Waianae and Koolau aquifers, island of Oahu ......................................................................... 18

3. Ground-water areas and geohydrologic boundaries within the principal volcanic aquifers of the island of Oahu............ 41

CONVERSION FACTORS

Multiply By To obtain

foot (ft)

foot per day (ft/d)

foot per mile (ft/mi)

square foot per day (ft2/d)

gallon per minute per square foot (gal/min)/ft2

inch (in.)

inch per year (in/yr) 

mile (mi)

mile per hour (mi/h) 

million gallons per day (Mgal/d)

pound per cubic foot (lb/ft3) 

square mile (mi2) 

square mile (mi2)

0.3048 meter

0.3048 meter per day

0.3048 meter per mile

0.09290 square meter per day

630.9 liter per second per square meter

25.4 millimeter

25.4 millimeter per year

1.609 kilometer

1.609 kilometer per hour

0.0438 million cubic meters per second

16.018 kilogram per cubic meter

259.0 hectare

2.590 square kilometer

Temperature: is given in degrees Fahrenheit (°F), which can be converted to degrees 

Celsius (°C) by the following equation:

°C = (temp x °F-32) / 1.8
REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

GEOHYDROLOGY OF THE ISLAND OF OAHU, HAWAII

By Charles D. Hunt, Jr.

ABSTRACT

The island of Oahu, Hawaii, is the eroded remnant of two 

coalesced shield volcanoes, the Waianae Volcano and the Koolau 

Volcano. Shield-building lavas emanated mainly from the rift zones 

of the volcanoes. Subaerial eruptions of the Waianae Volcano 

occurred between 3.9 and 2.5 million years ago, and eruptions of the 

Koolau Volcano occurred between 2.6 and 1.8 million years ago. The 

volcanoes have subsided more then 6,000 feet, and erosion has 

destroyed all but the western rim of the Koolau Volcano and the 

eastern part of the Waianae Volcano, represented by the Koolau and 

Waianae Ranges, respectively.

Hydraulic properties of the volcanic-rock aquifers are determined 

by the distinctive textures and geometry of individual lava flows. 

Individual lava flows are characterized by intergranular, 

fracture, and conduit-type porosity and commonly are highly permeable. The stratified nature of the lava flows imparts a layered 

heterogeneity. The flows are anisotropic in three dimensions, with 

the largest permeability in the longitudinal direction of the lava flow, 

an intermediate permeability in the direction transverse to the flow, 

and the smallest permeability normal to bedding. Averaged over 

several lava-flow thicknesses, lateral hydraulic conductivity of dikefree lava flows is about 500 to 5,000 feet per day, with smaller and 

larger values not uncommon. Systematic areal variations in lavaflow thickness or other properties may impart trends in the 

heterogeneity.

The aquifers of Oahu contain two flow regimes: shallow 

freshwater and deep saltwater. The freshwater floats on underlying 

saltwater in a condition of buoyant displacement, although the 

relation is not necessarily a simple hydrostatic balance everywhere. 

Natural driving mechanisms for freshwater and saltwater flow differ. 

Freshwater moves mainly by simple gravity flow; meteoric water 

flows from inland recharge areas at higher altitudes to discharge 

areas at lower altitudes near the coast. Remnant volcanic heat also 

may drive geothermal convection of freshwater in the rift zones. 

Saltwater flow is driven by changes in freshwater volume and sea 

level and by dispersive and geothermal convection. Freshwater flow 

is much more active velocity is higher and residence time is 

shorter than salt-water flow. Hydrodynamic dispersion produces a 

transition zone of mixed water between the freshwater and the 

underlying saltwater.

The Waianae aquifer in the Waianae Volcanics and the Koolau 

aquifer in the Koolau Basalt are the two principal volcanic-rock 

aquifers on Oahu. The sequences of coastal-plain and valley-fill 

deposits locally form aquifers, but these aquifers are of minor 

importance because of the small volume of water contained in them. 

The two principal volcanic-rock aquifers are composed mainly of 

thick sequences of permeable, thin-bedded lava flows. These aquifers 

combine to form a layered aquifer system throughout central Oahu 

where the Koolau aquifer overlies the Waianae aquifer. They are 

separated by a regional confining unit formed by weathering along 

the Waianae-Koolau unconformity, which marks the eroded and 

weathered surface of the Waianae Volcano buried by younger Koolau 

lava flows.

The areal hydraulic continuity of the aquifers of Oahu is 

interrupted in many places by steeply dipping, stratigraphically 

unconformable, geohydrologic barriers. These low-permeability 

features include eruptive feeder dikes, sedimentary valley fills, and 

former erosional surfaces now buried by younger lava flows or 

sediments. The barriers impede and divert lateral ground-water flow 

and impound ground water to greater heights than would occur in 

the absence of the barriers, causing abrupt stepped discontinuities in 

the potentiometric surface. The largest discontinuities are associated 

with dense concentrations of dikes in the eruptive rift zones of each 

volcano. The dikes in these zones originate from great depths and 

impede flow both in shallow-freshwater and in deep-saltwater 

flow systems. Valleys filled with sedimentary deposits are partly 

penetrating barriers that impede freshwater flow in shallow parts of 

the volcanic-rock aquifers. These barriers tend to cause smaller 

discontinuities in potentiometric surfaces than the rift-zone dikes.

Following earlier classification schemes, seven major groundwater areas are recognized within the Waianae and Koolau aquifers. 

These are the broad areas between prominent barriers in which 

hydraulic continuity is high and the potentiometric surface is 

smoothly continuous for the most part, except in the rift zones where 

dikes cause numerous stepped discontinuities. Several of the major 

ground-water areas are divided into subordinate ground-water areas 

by surficial geohydrologic barriers. A combination of large aquifer 

hydraulic conductivity, high pumping rates, and lateral 

geohydrologic barriers results in bounded-aquifer response within 

many of the ground-water areas.

Bl
B2 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

Four regional fresh ground-water flow systems the eastern, 

western, central, and southeastern Oahu are sustained by meteoric 

recharge. The central Oahu flow system can be further divided into a 

northern and a southern Oahu flow system. Ground-water divides 

within the Waianae and Koolau Ranges divert water into the interior 

flow system in central Oahu and exterior flow systems in western, 

eastern, and southeastern Oahu. Each flow system encompasses one 

or more of the major ground-water areas, with water flowing across 

geohydrologic barriers from areas of higher head to areas of lower 

head. The magnitude of flow across these barriers varies depending 

on head differences across the barrier and the hydraulic conductivity 

of the barrier. Total predevelopment recharge to the freshwater flow 

systems has been estimated at 792 million gallons per day, of which 

543 million gallons per day was recharge to the central Oahu flow 

system, the most heavily developed flow system on Oahu.

INTRODUCTION

Many areas of the United States depend on ground 

water for a large part of their water needs or for a 

reserve supply during droughts. As part of the national 

response to the severe drought of 1976-77 in the continental United States, the 95th Congress provided 

funding to initiate a program to develop quantitative 

appraisals of the major aquifer systems of the United 

States (see Foreword). The U.S. Geological Survey was 

responsible for completing this program.

The Regional Aquifer-System Analysis (RASA) Program encompasses studies of 25 ground-water 

systems nationwide, including the island of Oahu, 

Hawaii (fig. 1). The reports issuing from the Oahu 

RASA have two principal objectives: (1) develop a better understanding of the complex hydrology and 

hydraulics of Oahu's ground-water flow system, and (2) 

provide a framework for future hydrologic studies and 

data-collection activities.

Demands for water have continued to increase on 

Oahu, and ground-water withdrawals have 

approached estimated sustainable yields in some 

areas. The quality of the ground water also is of concern. Potential threats to water quality include saltwater intrusion and contamination by various agricultural and industrial chemical compounds. A 

better understanding of the ground-water hydrology of 

Oahu will aid in ensuring the availability and quality 

of its ground-water resources.

PURPOSE AND SCOPE

This report is one of a series on various aspects of 

Oahu's regional aquifer system. The purpose of this 

report is to provide an overview of the geohydrology of 

Oahu established by previous studies and recent 

efforts of the Oahu RASA program. The report 

describes the geologic features and processes that form 

the geohydrologic framework, the occurrence of ground 

water, the subdivision of Oahu into 15 ground-water 

areas, and the ground-water flow systems of Oahu.

STUDY AREA

The study area encompasses the entire island of 

Oahu (fig. 1), the third largest island in Hawaii, covering 593 mi2 of land area (University of Hawaii Department of Geography, 1983). It is the most heavily 

populated of the islands; the population was 762,565 

(State of Hawaii Department of Planning and 

Economic Development, 1981) in 1980. Oahu is the 

center of commerce, industry, and government in 

Hawaii and site of the State capital, Honolulu. Principal elements of the economy are tourism, agriculture, 

and Federal government expenditures, both civilian 

and military.

Although Oahu is the most populous island in 

Hawaii, much of its land area is forested or cultivated 

(fig. 2). Most of the population is concentrated in urban 

centers near the coast. Gently sloping upland areas are 

planted in sugarcane and pineapple, but urban development has encroached progressively on these areas in 

recent decades. The rugged, mountainous terrain of 

Oahu is mostly forested conservation land, typically 

designated as watershed preserves. Intensive urban 

and agricultural development on Oahu creates high 

demand for water. Most of this demand is supplied by 

ground water.

PHYSICAL SETTING

The island of Oahu is formed by the eroded 

remnants of two elongated shield volcanoes with broad, 

low profiles (figs. 1 and 3). Weathering and erosion 

have modified the original domed surfaces of the volcanoes, leaving the Koolau Range in eastern Oahu and 

the Waianae Range in western Oahu. Much of the subaerial mass of these volcanoes has been removed, 

leaving a landscape of deep valleys and steep interfluvial ridges in the interior highlands. In central 

Oahu, which forms the saddle between the Waianae 

and Koolau Ranges (figs. 3 and 4), erosion has been less 

severe and has modified the original volcanic domes 

only slightly.

A flat coastal plain underlain by sedimentary deposits surrounds much of Oahu (fig. 4). It varies in width 

from a narrow marine terrace to a broad plain several 

miles wide. Where it is extensive, as in southern Oahu, 

its surface is composed mainly of emerged Pleistocene 

reefs and associated sediments.

Streams on Oahu are short, with steep gradients 

and small drainage areas, the largest of which is 

45.7 mi2 in area. Main courses of streams (fig. 5) 

generally follow the consequent drainage pattern 

established on the original domed surfaces of the shield 

volcanoes. Lower-order tributaries branch off from the 

main courses in a dendritic pattern. Steep terrain and
INTRODUCTION B3

158°

160° 158° 156°

FISSURE

 ZONES

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Alters equal area 

projection, standard parallels 21°15' and 2r45', 

central meridian 157°59'

.

Diamond / 

Head EXPLANATION 

| | SEDIMENTARY DEPOSITS (CAPROCK) (HOLOCENE AND PLEISTOCENE)

WAIANAE VOLCANO

^TE j KOLEKOLE VOLCANICS (PLEISTOCENE)

WAIANAE VOLCANICS (PLIOCENE) 

|' j Palehua Member

[_' '-, ..-J Kamaileunu and Lualualei Members, undivided (Kamaileunu Member includes 

Mauna Kuwale Rhyodacite Flow)

KOOLAU VOLCANO

HONOLULU VOLCANICS (HOLOCENE (?) AND PLEISTOCENE)

h'\- I KOOLAU BASALT (PLEISTOCENE (?), AND PLIOCENE, INCLUDES KAILUA MEMBER)

GEOLOGIC CONTACT

~ IZ H GENERALIZED RIFT ZONE

 CALDERA RING FAULT Approximately located

FIGURE 1. Generalized geology, island of Oahu (modified from Stearns and Macdonald, 1940; Stearns, 1946; Sinton, 1986; Langenheim 

and Clague, 1987) showing rift zones and calderas of the Waianae and Koolau Volcanoes (modified from Macdonald, 1972).
B4 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

steep stream gradients cause water to run off rapidly 

following precipitation, and permeable upland soils 

permit rapid infiltration of water to underlying aquifers. As a result, streamflow is characteristically flashy, 

with high flood peaks and little baseflow. Few streams 

are perennial over their entire reach. Perennial 

streamflow occurs at high altitudes where precipitation is persistent; in deeply incised valleys in rift zones, 

where streams intersect the water table; and near sea 

level where streams intercept shallow ground water. 

These conditions virtually preclude surface-water 

development on Oahu and lead to near-total reliance 

on ground water.

CLIMATE

The subtropical climate of Oahu is characterized by 

mild temperatures, moderate to high humidity that 

varies diurnally from about 60 to 90 percent at most

locations, prevailing northeasterly tradewinds that 

average about 9 mi/h in January and about 13 mi/h in 

June, and extreme variation in precipitation over short 

distances. Climate varies spatially with altitude and in 

relation to prevailing and local winds. Mean annual 

temperature is about 76°F in lowland Honolulu, 

decreasing to less than 70°F in the mountainous 

uplands (Blumenstock and Price, 1967). A pronounced 

orographic pattern of cloud cover and precipitation is 

established as moist oceanic air is forced up and 

over the mountainous terrain of Oahu by persistent 

tradewinds. Mean annual precipitation (fig. 6) has a 

steep orographic gradient and varies widely, ranging 

from about 60 in/yr on the windward (northeastern) 

coast of Oahu to about 275 in/yr near the crest of the 

Koolau Range to less than 25 in/yr over the leeward 

(southwestern) lowlands. The Waianae Range lies in 

the tradewind rainshadow of the Koolau Range and

158°

Kahuku Point

EXPLANATION 

LAND USE 

Agriculture

Conservation (forested) 

Urban

21°20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

MAUNALUA BAY

Diamond 

Head 024 KILOMETERS 

FIGURE 2. Land use, island of Oahu (modified from State of Hawaii Department of Agriculture, 1980).
INTRODUCTION B5

receives much less precipitation, with a maximum of 

about 80 in/yr falling on the Waianae summit.

Precipitation on Oahu is markedly seasonal. In 

lowland and coastal areas, the winter months 

October through April receive about 70 percent of the 

total annual precipitation. Mountainous areas receive 

a fairly steady contribution of tradewind precipitation 

that is supplemented by intense, episodic rains from 

hurricanes, winter cold fronts, and convective disturbances associated with low pressure in the upper atmosphere. Lowland areas receive a lesser proportion of 

total rainfall as tradewind rain and a greater proportion of episodic winter rain. Mean annual precipitation 

over the open ocean near Oahu is about 25 in. 

(Blumenstock and Price, 1967).

PREVIOUS STUDIES

The geology and ground-water hydrology of Oahu 

have been studied in detail and are the subject of 

numerous reports. Many previous studies were not 

regional in scope and were concerned primarily with 

the geology and hydrology of local areas. The focus of 

the present report is island wide.

Stearns (1939) prepared a detailed geologic map of 

Oahu and published thorough descriptions of Oahu's 

geology and ground-water resources (Stearns and 

Vaksvik, 1935; Stearns, 1940). Other important studies 

of Oahu's geology include those of Hitchcock (1900), 

Palmer (1927, 1946), Wentworth (1926, 1951), 

Winchell (1947), Wentworth and Winchell (1947), Macdonald and others (1983), and Stearns (1985). Sinton 

(1986) presented a revised geologic map and strati158°

Kahuku Point

EXPLANATION

TOPOGRAPHIC DIVIDE Waianae Divide 

to the west and Koolau Divide to the east

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59' 

Contours from U.S. Geological Survey 

digital elevation model data, 1:250,000,1986

024 MILES 

 H ' 

024 KILOMETERS

Contour interval 500 feet 

Datum is mean sea level

MAUNALUA BAY

Diamond 

Head

FIGURE 3. Topography, island of Oahu.
B6 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

graphic nomenclature for the Waianae Volcano, and 

the stratigraphic nomenclature for all of Oahu was 

reviewed and updated by Langenheim and Clague 

(1987). Walker (1986,1987) presented detailed descriptions of the dike complex of the Koolau Volcano. 

Aspects of the geology of Oahu also are described in 

reports of the Hawaii Institute of Geophysics and in 

various doctoral dissertations and master's theses.

The hydrology of Oahu is described in various 

reports by the U.S. Geological Survey, the Honolulu 

Board of Water Supply, the State of Hawaii Department of Land and Natural Resources, the University of 

Hawaii Water Resource Research Center, and in various doctoral dissertations and master's theses. Early 

works of particular importance include those of Palmer 

(1927, 1946), Wentworth (1926, 1951), Stearns and 

Vaksvik (1935), and Stearns (1940). Visher and Mink 

(1964) discussed the ground-water resources of southern Oahu, which includes the Pearl Harbor area. 

Takasaki and others (1969) described the water 

resources and development of dike-impounded ground 

water of most of windward Oahu. (The location of windward Oahu and that of the other areas mentioned in 

the following discussion are shown in fig. 12A). 

Takasaki and Valenciano (1969) discussed the geohydrology and water resources of the Kahuku area. 

Rosenau and others (1971) described the water 

resources of north-central Oahu and estimated groundwater recharge to the area. Ground water in the 

Waianae area, which is the area west of the crest of the 

Waianae Range was described by Takasaki (1971). The 

effects of increased pumpage from the Schofield area 

was evaluated by Dale and Takasaki (1976). Finally, 

the water resources of southeastern Oahu were 

described by Takasaki and Mink (1982). Dikeimpounded ground water and the effects of tunnels

158°

Kahuku Point

4 MILES

21°30

21°20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

FIGURE 4. Geomorphic provinces, island of Oahu (modified from Visher and Mink, 1964, fig. 1).
GEOLOGIC FRAMEWORK B7

used to develop this ground-water source in windward 

Oahu have been described and discussed by Hirashima 

(1971) and Takasaki and Mink (1985).

GEOLOGIC FRAMEWORK

The geology of Oahu (fig. 1) is the end result of varied 

geologic processes, including shield-building volcanism, subsidence, weathering, erosion, sedimentation, 

and rejuvenated volcanism. These processes have 

imposed on the aquifers their respective geometries 

and determined their textural and hydraulic properties. The processes have produced the rocks that constitute the aquifers of Oahu, as well as the geohydrologic 

boundaries that subdivide the regional aquifer system 

into distinct ground-water areas. (The geohydrologic 

boundaries, topographic divides, and ground-water 

areas are shown on fig. 12). The principal aquifers of

Oahu are delineated mainly on the basis of stratigraphy

REGIONAL GEOLOGIC SETTING

Oahu and the other islands in Hawaii are the subaerial peaks of large volcanic mountain ranges, most of 

which lie beneath the sea, that comprise the Hawaiian 

Ridge. The Hawaiian Ridge is thought to have formed 

by the relative drift of the Pacific lithospheric plate 

over a convective plume, or hotspot, in the mantle 

(Wilson, 1963). Continuous volcanism at the hotspot 

and the northwestward relative motion of the Pacific 

plate produced a southeasterly succession of mountain 

building with progressively younger islands to the 

southeast. Present volcanic activity occurs at the 

extreme southeast end of the island chain on the island 

of Hawaii. The submarine parts of the Hawaiian Ridge 

rise about 15,000 ft above the adjacent sea floor before

158°

Kahuku Point

21°30'

21°20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

FIGURE 5. Streams, island of Oahu.
B8 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

protruding above sea level. The highest point on Oahu 

is 4,020 ft above sea level in the Waianae Range.

SHIELD-BUILDING VOLCANISM

The island of Oahu is formed by the remnants of two 

coalesced shield volcanoes, the Koolau Volcano to the 

east and the Waianae Volcano to the west (fig. 1). A 

shield volcano is formed by submarine eruptions of 

very fluid lava that build a dome-shaped structure 

resembling a shield on the ocean floor. Submarine volcanic deposits include pillow lavas, hyaloclastite, and 

flow-foot breccia; vitric ash deposits are formed when 

eruptions occur in shallow water. Eventually, the lavas 

build to sea level and above, and shield building continues with mostly quiescent subaerial eruptions of fluid 

lava. Building of the Waianae and Koolau Volcanoes 

occurred during the Pliocene and Pleistocene epochs.

The Waianae Volcano is older than the Koolau, in 

accordance with the southeasterly volcanic succession 

ascribed to the mechanism of plate motion over a 

hotspot in the mantle. However, both volcanoes likely 

erupted concurrently during at least part of their active 

life spans, as indicated by their large submarine 

masses. Shield-stage lava flows from the two volcanoes 

may interfinger at depth, although this has not been 

observed in surface outcrops or in boreholes. At the relatively shallow depths of such observations, lava flows 

from the Koolau Volcano invariably overlie those from 

the Waianae Volcano unconformably, with soil and 

saprolite developed on the older Waianae surface and 

weathered alluvium separating the two formations in 

some places (Stearns and Vaksvik, 1935).

The shield-building lava flows emanated mainly 

from prominent rift zones of the Waianae and Koolau 

Volcanoes (fig. 1). These elongate zones form the topographic crests of the volcanoes and their submarine

158°

Kahuku Point

EXPLANATION

LINE OF EQUAL MEAN ANNUAL 

PRECIPITATION Interval, in 

inches, is variable

21°30'

21°20

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

Diamond 

Head 024 KILOMETERS 

FIGURE 6. Mean annual precipitation, 1916-83, island of Oahu (modified from Giambelluca and others, 1986).
GEOLOGIC FRAMEWORK B9

extensions. Within each rift zone, a dike complex zone 

and a marginal dike zone have been delineated 

(Stearns and Vaksvik, 1935; Takasaki and Mink, 1985). 

Dikes are most heavily concentrated in the dike complexes, where they commonly number several hundred 

per mile and constitute 10 percent or more of the total 

rock. In the marginal dike zones, dikes commonly number less than 100 per mile and account for less than 

5 percent of the total rock (Wentworth, 1951; Takasaki 

and Mink, 1985). Most of the dikes dip steeply and are 

arranged in a subparallel pattern roughly aligned parallel to the trends of the major rift zones. Away from the 

rift zones, on the flanks of the volcanoes, dikes are 

sparse or absent.

The summits of both volcanoes collapsed and formed 

calderas, probably at numerous times during their 

growth. It was long thought that caldera collapse was a 

single event near the end of the shield-building stage of 

growth (Stearns, 1946), but the recent view is that 

caldera collapse occurs repeatedly during the growth of 

a shield volcano (Peterson and Moore, 1987). Late in 

the life of the Waianae Volcano, the chemical composition of its lava changed progressively from tholeiitic 

basalt to alkalic basalt, which is more 

viscous. Eruptions during this postshield period were 

more violent than the shield-building eruptions and 

produced greater proportions of ash and cinder. Lava 

flows also were thicker, more massive, and shorter than 

earlier flows, resulting in a steeper cap at the summit 

of the more gently sloping shield. The Koolau Volcano 

did not undergo this capping stage of growth. Finally 

after a period of volcanic quiescence, subsidence, and 

erosion a late stage of rejuvenated volcanism 

occurred from middle- to late-Pleistocene time (fig. 1). 

Numerous cinder cones and valley-filling lava flows 

were formed during this stage, but the total volume of 

these products represented only a small addition to the 

Oahu volcanic edifice.

SHIELD-STAGE VOLCANIC ROCKS

The shield-building rocks of the Waianae and Koolau 

Volcanoes are known respectively as the Waianae Volcanics and the Koolau Basalt (Swanson and others, 

1981; Langenheim and Clague, 1987). The Koolau 

Basalt is wholly of basaltic composition. The Waianae 

Volcanics encompass shield and postshield stages of 

activity (Langenheim and Clague, 1987) and are lithologically diverse, therefore, the broader term "volcanics" is applied. Waianae Volcanics and Koolau Basalt 

form the uplands and mountains of western and eastern Oahu, respectively, and extend to great depths 

beneath the island and offshore where they comprise 

the submarine mass of the island.

WAIANAE VOLCANICS

Waianae Volcanics are mainly lavas and feeder dikes 

of tholeiitic and alkalic basalt, with lesser amounts of 

talus breccia, explosion breccia, cinder, and spatter. 

Occasional thin soils and ash beds are intercalated 

with the lava flows. Lava flows of the Waianae Volcanics comprise the principal water-bearing rocks of 

western Oahu (fig. 1).

Stearns subdivided The Waianae Volcanic Series 

into informal lower, middle, and upper members 

(Stearns and Vaksvik, 1935), but did not differentiate 

them as separate units on the geologic map. He recognized faulted and erosional unconformities that separated the lower member from the middle and upper 

members and speculated that the faulting may have 

been related to caldera collapse. He also recognized a 

gradation from the primarily basaltic composition of 

the middle member to a more andesitic upper member 

(Stearns and Vaksvik, 1935, p. 76). Macdonald (1940) 

published a revised geologic map with the upper member mapped separately from the still undifferentiated 

lower and middle members.

More recent field mapping, petrologic study, and 

radiometric dating led Sinton (1986) to revise the 

stratigraphic nomenclature and geologic map of 

the Waianae Volcano. He subdivided the Waianae 

Volcanics into three formal units, the Lualualei, 

Kamaileunu, and Palehua Members, that roughly are 

equivalent to the lower, middle, and upper members of 

Stearns. The Lualualei Member is composed largely of 

thin pahoehoe flows (ropy lava) and fewer aa flows 

(massive lava and clinker), dipping at angles of 4° to 14° 

away from the eruptive center of the volcano. The flows 

range in thickness from 5 to 75 ft and average about 

25 ft (Macdonald, 1940). Lavas of the Kamaileunu 

Member are nearly horizontal and thicker than flows of 

the underlying Lualualei Member, presumably because 

they ponded within a caldera. The flows are predominantly pahoehoe with some aa, and the flows range in 

thickness from 10 to 120 ft and average about 40 ft 

(Macdonald, 1940). The Palehua Member consists 

mainly of aa flows, many of them 50 to 100 ft thick. 

Palehua flows are generally thicker than either 

Lualualei or Kamaileunu flows. The Palehua Member 

probably formed a broad, postshield cap of alkalic 

basalt over the earlier rocks; subsequent erosion has 

severely dissected it and obscured its original extent. 

Radiometric dating of basalt samples from subaerial 

sites yield ages of 2.5 to 3.9 Ma (million years) for 

Waianae Volcanics (Clague and Dalrymple, 1987; Langenheim and Clague, 1987).
BIO REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

KOOLAU BASALT

The Koolau Basalt consists of tholeiitic basalt lavas 

and feeder dikes of tholeiitic basalt, with lesser 

amounts of talus breccia, explosion breccia, cinder, and 

spatter. Occasional thin soils and ash beds are intercalated with the lavas, though more sparingly than in 

the Waianae Volcanics. Lava flows of Koolau Basalt 

comprise the principal water-bearing rocks of central 

and eastern Oahu (fig. 1).

Koolau lavas range in thickness from several feet to 

as much as 80 ft, and average 10 ft or less (Stearns and 

Vaksvik, 1935; Wentworth, 1951). The flows commonly 

dip 3° to 10° away from the eruptive axis of the volcano. 

Radiometric dates of samples from subaerial exposures 

range from 1.8 to 2.6 Ma (Clague and Dalrymple, 1987; 

Langenheim and Clague, 1987).

MODIFICATION OF THE SHIELDS BY SECONDARY 

PROCESSES

The shield volcanoes of Oahu have undergone substantial modification by secondary geologic processes. 

Gravitational loading of the lithospheric plate by the 

massive shields caused downwarping of the plate and 

subsidence of the volcanoes. Slope instability led to 

large-scale slumping and landslides. Chemical weathering of easily decomposed basaltic rocks produced 

erodible soil and thick zones of clay-rich saprolite. 

Streams dissected the shields, eroding material and 

redepositing it in valleys or transporting it to the 

coastal estuaries or the sea. Marine processes 

reworked these terrigenous sediments and redeposited 

them on the submarine flanks of the volcanoes, 

together with calcareous sediments produced by 

marine organisms.

SUBSIDENCE AND SLOPE FAILURE

Subsidence was contemporaneous with shield development and continued long after eruptions ceased. 

Moore and Campbell (1987), in a study of tilted, deeply 

submerged reefs in the Hawaiian islands, concluded 

that subsidence ended about 0.5 Ma after the end of 

shield-building volcanism. Direct evidence of the subsidence of Oahu comes from wells that have penetrated 

alluvium or weathered basalt at depths of 1,100 to 

1,200 ft below sea level (Stearns, 1935; Stearns and 

Chamberlain, 1967). Indirect evidence includes deep 

submarine canyons of probable subaerial origin, submarine terraces thought to mark paleo-sea levels on 

the flanks of the volcanoes, and interpretations of 

crustal structure deduced from geophysical surveys. 

Moore (1987) provides a comprehensive summary of 

this evidence for the Hawaiian islands and, together

with new evidence and analysis, concluded that most of 

the volcanoes have subsided 6,500 to 13,000 ft.

Perhaps the most persuasive evidence of the magnitude of subsidence is the group of V-shaped sub-marine 

canyons on the eastern flank of Oahu (Hamilton, 1957; 

Shepard and Dill, 1966; Andrews and Bainbridge, 

1972). These canyons are aligned with major stream 

valleys on land and can be traced to depths of more 

than 6,600 ft below sea level. Although submarine erosion has been proposed as a possible cause of the canyons, most investigators have favored a subaerial 

origin. Therefore, subsidence of 6,500 ft or more after 

the shield-building stage of volcanism seems reasonable for Oahu. Lavas that originally erupted subaerially and have textural characteristics of subaerial 

lavas have since been carried to great depths by this 

subsidence.

In addition to subsidence, slope failure also modified 

the flanks of Oahu's volcanoes. Landslides of various 

sizes and rates of movement occurred during and after 

the volcanoes were built. Two extremely large submarine landslides have been identified, one off the 

Waianae coast and one off the coast of windward Oahu 

(Moore, 1964; Moore and others, 1989).

WEATHERING AND EROSION

Weathering and fluvial and marine erosion proceeded contemporaneously with subsidence in Oahu's 

posteruptive period and continue to the present. These 

processes also were active to some degree during the 

waning phases of shield and postshield volcanism 

when progressively decreasing eruptive frequency 

would have left large areas of the volcanoes exposed for 

long periods of time.

Fluvial erosion has cut stream valleys several thousand feet deep in the Waianae and Koolau shields. 

Much of the western part of the Waianae shield and 

much of the eastern side of the Koolau shield have been 

removed, leaving remnants of the shields as the 

Waianae and Koolau Ranges (the intensive erosion at 

these areas may have been aided or partly initiated by 

large-scale landsliding). Interstream divides on the 

western slopes of the Waianae Range and the eastern 

slopes of the Koolau Range have been eroded thoroughly, and numerous individual valleys have coalesced into U-shaped composite valleys with broad 

amphitheater heads. In contrast, stream valleys in the 

Honolulu area also were cut to depths of several thousand feet but did not coalesce to the same degree. These 

valleys are narrower, though still with amphitheater 

heads, and have broader, less dissected interfluves. 

Valleys elsewhere on Oahu are narrower still and generally V-shaped. In central Oahu, gentle slopes 

have inhibited stream incision and resulted in narrow,
GEOHYDROLOGY OF THE WATER-BEARING DEPOSITS Bll

shallow gulches separated by broad, little-dissected 

interfluves. Considerable fluvial erosion may have 

occurred very early after the shields were built, perhaps within a few hundred thousand years and at least 

within one-half to one million years.

The chronology of early erosion in central Oahu has 

caused complex stratigraphic and hydrologic relations 

between Waianae and Koolau rocks. During or shortly 

after the final postshield eruptions of the Waianae volcano, its eastern flank was modified by weathering and 

erosion. A composite amphitheater-headed valley was 

formed on the eastern flank, which subsequently was 

filled by late Koolau lavas and Waianae-derived alluvium (Stearns and Vaksvik, 1935). Stearns (1939; 

Stearns and Vaksvik, 1935), mainly on the basis of surface outcrops, postulated that: (1) Waianae-derived 

soil, saprolite, and weathered alluvium separate 

Waianae Volcanics from Koolau Basalt throughout central Oahu; and (2) that the alluvium is intercalated 

with lava flows of Koolau Basalt. Subsequent observations from drill holes and geophysical surveys support the first interpretation, but the second has not 

been verified.

SEDIMENTATION

Sedimentary deposition proceeded concurrently 

with subsidence and continued after submergence was 

complete. Shelves built by coral reefs and sediment 

were submerged and are now found at various depths 

around Oahu. Valleys cut very early in the erosional 

period were filled by marine, estuarine, and fluvial 

sediments as island subsidence raised the base level of 

streams and drowned their lower reaches. Eustatic 

fluctuations of sea level, coinciding with glacial and 

interglacial periods of the Pleistocene and ranging hundreds of feet, were superimposed on these processes 

(Steams, 1935; 1978). Much of Oahu's coastal plain is 

underlain by ancient reefs and other marine sediments 

deposited during various Pleistocene sea stands.

REJUVENATED-STAGE VOLCANISM

After the long period of subsidence and erosion, 

eruptive activity resumed at scattered vents at the 

southern ends of the Koolau and Waianae Ranges 

(fig. 1). This stage of volcanism has been observed on 

other islands of Hawaii and has long been called the 

posterosional stage; more recently, it has come to be 

called the rejuvenated stage. The term "rejuvenated" 

connotes a resumption of eruption after a prolonged 

hiatus in volcanism. It typically implies different rock 

chemistry and magmatic origin than the earlier shieldbuilding and postshield volcanism (Clague and Dalrymple, 1987; Langenheim and Clague, 1987). The volume of material erupted during the rejuvenated stages 

was small in comparison with the volume produced by 

earlier volcanism.

Rocks of the Waianae rejuvenated stage are called 

the Kolekole Volcanics and those of the Koolau rejuvenated stage are called the Honolulu Volcanics (Langenheim and Clague, 1987). Kolekole Volcanics are small 

in areal extent (fig. 1), comprising about six cinder 

cones and associated lava flows and ash, mostly at the 

southern end of the Waianae Range (Sinton, 1986). 

Radiometric ages for these deposits have yet to be published, but recently completed radiometric and 

geochemical studies (T.K. Presley, J.M. Sinton, and 

Malcolm Pringle, Univ. of Hawaii, written commun., 

1996) indicate that the Kolekole Volcanics are posterosional in the geomorphic sense of covering eroded 

topography, but are not rejuvenated in the sense of 

renewed magmatism of markedly different age and 

chemical composition. This would imply a need for 

slight revision of the stratigraphy used in this report, 

which is that of Langenheim and Clague (1987).

Honolulu Volcanics are limited in areal extent 

(fig. 1), occurring only near the southeastern end of the 

Koolau Range. The rocks are strongly alkalic and range 

in composition from alkalic basalt, basanite, and 

nephelite to melilitite (Clague and Dalrymple, 1987, 

p. 51). A large proportion of the rocks are pyroclastic 

products such as ash, cinder, spatter, and tuff. Eruptions inland from the coast left deposits of black ash 

and cinder and produced lavas that flowed down valleys and spread out over the coastal plain. Lava flows 

that ponded in valleys or other depressions in the preexisting erosional topography are commonly 40 to 

100 ft thick and massive. Some units of the Honolulu 

Volcanics are water bearing and others, such as massive flows, overlie and confine ground water in valley 

alluvium or cause perched-water springs.

Potassium-argon ages for the Honolulu Volcanics 

range from 0.9 to 0.03 Ma (Clague and Dalrymple, 

1987) and indicate that eruptions occurred sporadically 

over a period of about one million years. The major subsidence of Oahu appears to have been completed prior 

to Koolau rejuvenated volcanism because Honolulu 

Volcanics lie on or are intercalated with only the uppermost of the sedimentary units on the coastal plain.

GEOHYDROLOGY OF THE WATER-BEARING 

DEPOSITS

The rocks of Oahu vary widely in origin, chemical 

composition, and texture and in their ability to store 

and transmit water. This section outlines the physical 

and hydraulic properties of the rocks as a function of 

their origin and evolution.
B12 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

VOLCANIC ROCKS AND DEPOSITS

Although most volcanic rocks in the islands of 

Hawaii have similar basaltic composition, their modes 

of emplacement caused a variety of physical properties 

that govern their hydraulic properties. For purposes of 

discussion, the volcanic rocks have been divided into 

four groups: (1) lava flows, (2) dikes, (3) pyroclastic 

deposits, and (4) saprolite and weathered basalt. Each 

of these groups of rocks have markedly different physical and hydraulic properties. Much of the following 

discussion of rock textures and of the areal extent of 

lava flows is from Wentworth and Macdonald (1953) 

and Macdonald and others (1983).

LAVA FLOWS

Lava flows in Oahu, as well as the other islands of 

Hawaii, are mainly of two textural types: (1) pahoehoe 

(ropy lava), which has a smoothly undulating surface 

and contains numerous elongate voids; and (2) aa (clinkery lava), which has a surface of coarse rubble and an 

interior of massive rock. Stratified sequences of thinbedded lava flows form the most productive aquifers in 

Hawaii. At the local scale, the hydraulic properties of 

lava aquifers depend on the number, size, types, and 

distribution of openings or pores. The diverse rock textures encompassed by the two types of lava impart a 

complex porosity distribution to the lavas, one that differs in character from most sedimentary, metamorphic, 

and crystalline igneous rocks. Lava sequences also typically include cinders and ash transported by the wind 

from eruptive vents.

In a layered sequence of lava flows (fig. 7A), several 

types of primary porosity are present:

1. Vesicular small gas vesicles that form in 

molten lava;

2. Fracture joints, cracks, and bedding-plane 

separations;

3. Intergranular fragmental rock, including 

cinders, rubble, and clinkers that are analogous to clean, coarse gravel; and

4. Conduit large openings such as lava tubes 

and interflow voids that take forms similar to 

solution conduits in limestones.

Vesicular porosity is a conspicuous element of bulk 

porosity, but the vesicles are poorly connected and contribute little to effective porosity. Fracture and intergranular porosity form a pervasive network of small 

openings that facilitates diffuse ground-water flow. 

Conduits provide avenues for highly channelized flow 

similar to solution cavities found in carbonate aquifers 

(White, 1969).

Pahoehoe flows are formed by basaltic lavas that are 

fluid, flow rapidly, and tend to spread out. Most pahoehoe flows are thin, contain voids of various sizes, and 

are cracked and collapsed in places. Ponding of pahoehoe lava in depressions or on gentle slopes can result in 

thick accumulations of massive pahoehoe. Interflow 

voids form where the irregular upper surface of a flow 

is not completely filled in by the viscous lava of a subsequent flow. Lava tubes form when the flow surface 

cools and hardens into a crust and the molten lava 

beneath drains out. Most tubes are less than a few feet 

in diameter and are localized, but roofing over of lava 

rivers may result in tubes as large as several tens of 

feet in diameter that extend several miles. Some lava 

tubes covered by subsequent lava flows collapse upon 

burial and compaction. Wentworth (1945) gives a noteworthy description of conduits encountered during a 

tunneling project in Honolulu:

Many small lava tubes and several large ones were 

encountered in the Red Hill-to-Halawa water-transmission tunnel. At one point a large tube, 15 to 30 ft wide 

and up to 8 or 10 ft high above the debris which partly 

filled it, crossed over the tunnel and was explored for 

two or three hundred feet inland.

The mixture of voids and fractures in pahoehoe flows 

imparts high intrinsic permeability similar to that of 

carbonate rocks. Vertical permeability elements in 

pahoehoe include cooling joints; collapse features; skylights or holes in the roofs of large lava tubes; and 

large, open cracks where lava is pushed up into humps 

and pressure ridges. Lateral permeability elements in 

pahoehoe include drained lava tubes and interflow 

voids (small depressions in the irregular upper surface 

of a lava flow that are not filled in by the viscous lava 

of the subsequent flow). Where pahoehoe is massive, 

effective porosity and permeability are comparatively 

low and mainly of the fracture type.

Pahoehoe lava commonly grades into aa lava with 

increasing distance from the eruptive vent. Aa flows 

are typified by a central core of massive rock several 

feet to tens offset thick, with layers of coarse, fragmental rock above and below them. As the flowing aa lava 

cools, degases, and becomes more viscous, the hardened crust on top of the flow breaks up into angular, 

scoriaceous rubble known as aa clinker, or flow-top 

breccia. Beneath the clinker a core of viscous, incandescent lava continues to spread, carrying along the 

sheath of clinker like a tractor tread. As the flow 

advances, clinker and blocks of the massive core spall 

off and cascade down the front of the flow and are overridden by it. Subsequent cooling and volumetric contraction of the core result in well-developed joints. 

Vertical joints are most conspicuous, although lateral 

and oblique joints resulting from shear stresses within 

the flowing core also are common. As with pahoehoe,
GEOHYDROLOGY OF THE WATER-BEARING DEPOSITS B13

Shrinkage joints 

and fractures

Clinker 

bridges

Solid phase of 

aa lava flow

Aa clinker 

beds

 Interflow 

void between 

pahoehoe 

lava flows

Lava tube

in pahoehoe

lava flow

EXPLANATION

Arrow length denotes relative magnitude 

of permeability in direction of arrows

e dir^on

FIGURE 7. Structural features associated with lava flows (modified from Takasaki and Valenciano, 

1969, fig. 5). A, Dike-free lava flows. B, Dike-intruded lava flows.
B14 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

emplacement of aa on gentler slopes results in thicker, 

more massive flows.

The intergranular porosity of aa clinker is analogous 

to the intergranular porosity of coarse, well-sorted 

gravel. Widespread beds of clinker contribute high horizontal permeability to a layered sequence of lava flows 

(fig. 7A). In contrast, the smaller effective porosity of 

massive aa cores suggests permeability several orders 

of magnitude less than aa clinker or thin-bedded 

pahoehoe. The principal permeability of an aa core is 

imparted by cooling joints. In flows thicker than a few 

feet, these fractures may be spaced several feet apart, 

intersecting in plan view to form crude polygons several feet across. The intervening blocks of massive rock 

are virtually impermeable. Marginal clinker at the 

periphery of an aa flow may provide local avenues of 

high vertical permeability in lava sequences. Such 

flow-margin clinker bridges may be more prevalent in 

sequences of thinner flows than in thicker flows.

The areal extent of lava flows is much larger than 

their typical thickness of about 10 ft. Wentworth and 

Macdonald (1953, p. 31) listed measurements for 

22 historical flows on Mauna Loa and Kilauea on the 

island of Hawaii (fig. 8), which presumably are typical 

of flows on Oahu as well. The flows on Hawaii average 

about 15 mi in length and about one-half mile in width. 

Individual flows can be traced and mapped on the surface of young, unweathered volcanoes. Areal correlation of flows in the subsurface requires careful drilling 

and collection of core samples or drill cuttings, detailed 

description and petrographic study of the samples, and 

borehole geophysical logging. These data are not 

collected routinely in Hawaii, and there are few 

instances when data are sufficient for lateral tracing of 

subsurface flows.

DIKES

Dikes are thin, near-vertical sheets of massive, 

intrusive rock that typically contain only fracture 

porosity and permeability. Most dikes are no more than 

several feet thick, but can extend vertically thousands 

of feet and laterally several miles. Where dikes intrude 

lava flows, they inhibit ground-water flow principally 

in the direction normal to the plane of the dike (fig. IB). 

Dike complexes are areas where dikes are more numerous and intersect at various angles, forming small compartments and lowering overall rock porosity and 

permeability (Takasaki and Mink, 1985). Marginal 

dike zones are areas where dikes are subparallel and 

widely scattered. Dikes in these zones impound water 

within large compartments of more permeable lavas 

and tend to channel ground-water flow parallel to the 

general trend of the dikes (Hirashima, 1962; Takasaki, 

1971).

PYROCLASTIC DEPOSITS

Pyroclastic deposits include ash, cinder, spatter, and 

larger blocks. These deposits are essentially granular; 

porosity and permeability are similar to that of granular sediments with similar grain size and degree of 

sorting. Ash, being fine-grained, is less permeable than 

coarse pyroclastic deposits such as cinder and spatter. 

The permeability of ash may be reduced further by 

weathering or by compaction to tuff. Weathered ash 

beds can act as thin confining units within lava 

sequences.

SAPROLITE AND WEATHERED BASALT

The humid, subtropical climate of Oahu fosters 

intense weathering of basaltic rocks, reducing the permeability of the parent rock by altering igneous minerals to clays and oxides. Weathered surficial materials 

mediate the infiltration of water and anthropogenic 

contaminants to deeper aquifers. Exposed weathering 

profiles on Oahu typically include inches to feet of soil 

underlain by several feet to several tens of feet of saprolite, a soft, clay-rich, thoroughly decomposed rock. The 

soils of Oahu, which have been classified and mapped 

by Foote and others (1972), differ in parent material, 

topography, and precipitation. Most of the soil types are 

well-drained, except for some soils that formed on alluvium in lowland areas. The soils generally have high 

proportions of iron and aluminum oxides and of clay 

(typically kaolinitic).

Saprolite is weathered material that has retained 

textural features of the parent rock. In basaltic saprolite, diverse parent textures and a variable degree of 

weathering impart a heterogeneous permeability 

structure with preferred avenues of water movement 

and retention. Miller (1987) measured saturated 

hydraulic conductivities of saprolite core samples and 

obtained values that ranged over five orders of magnitude, from 1x10 3 ft/d to Ixl02ft/d. Miller (1987, p. iv) 

further found that preferential flow occurs in channels 

between macropores and along joints. In outcrop, 

saprolite commonly has a friable texture, with extensive networks of joints forming small, blocky aggregates. It is not clear to what degree this is a general 

characteristic or simply reflects shrinkage from desiccation at the exposed outcrop.

Saprolitic weathering proceeds downward in flat 

areas and laterally inward from steep ridges and 

valley walls. Rocks with a high proportion of pore space 

and surface area, such as ash, cinder, and aa clinker, 

are weathered preferentially; weathering of massive 

rock proceeds more slowly. In the case of massive aa, 

whose principal permeability is vertical cooling joints,
GEOHYDROLOGY OF THE WATER-BEARING DEPOSITS B15

156°

Upolu Point

Honokohau

Bay { Kailua

^1942 

MAUNALOA

VOLCANO '

i 

1949

Base modified from U.S. Geological Survey Ka Lae 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 18°52'30" and 

21°18'30", central meridian 154°56'15"

EXPLANATION 

LAVA FLOW AND DATE OF FLOW

10 MILES

0 5 10 KILOMETERS

Contour interval 1 ,000 feet 

Datum is mean sea level

FIGURE 8. Historic lava flows through 1953, island of Hawaii (modified from Wentworth and Macdonald, 1953, fig. 13).
B16 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

slight weathering and swelling may seal the joints, 

resulting in a layer of low permeability.

Minerals leached from overlying material are redeposited at greater depths in the saprolite to form clayrich zones and sheets and concretions of iron oxides. 

These deposits reduce permeability and may contribute to perching or near-saturation in the unsaturated 

zone (Walker, 1964; Patterson, 1971). Perched water or 

near-saturated conditions have been encountered in 

saprolite at several sites in the Schofield Plateau of 

central Oahu (fig. 4) (Mink, 1981; State of Hawaii, 

Department of Agriculture, 1983) and elsewhere on 

Oahu (Walker, 1964; selected drillers' logs in files of the 

U.S. Geological Survey, Honolulu, Hawaii).

Observations of outcrops and drillers' logs suggest 

weathering intensity and saprolite thickness increase 

with precipitation. Saprolite typically is less than 

100 ft thick in areas where precipitation is less than 

50 in/yr and about 100 to 300 ft thick where precipitation is between about 50 to 80 in/yr. Weathering may 

extend even deeper in mountainous areas where precipitation is from 80 to 275 in/yr. There also is some 

suggestion that bedrock weathering may be more 

intense beneath valleys than beneath adjacent interfluves. Weathered material extends to a depth of nearly 

600 ft beneath a shallow gulch in the wet, eastern part 

of the Schofield area (drilling log for well 3-3059-02 in 

files of the U.S. Geological Survey, Honolulu, Hawaii). 

Although precipitation of about 80 in/yr at this site is 

somewhat greater than at most other sites in central 

Oahu, the depth of weathering is several hundred feet 

thicker than elsewhere and may be due to enhanced 

weathering associated with streambed infiltration.

HYDRAULIC PROPERTIES OF THE VOLCANIC ROCKS

The excellent water-yielding properties of the basaltic lavas of Oahu were known to early Hawaiian settlers, but became especially apparent near the end of 

the 19th century when the first deep wells were drilled. 

Flowing artesian wells commonly produced several 

hundred to several thousand gallons per minute, and 

pumped wells and shafts provided similar yields with 

drawdowns of no more than several feet. Increasing 

exploitation of ground-water resources eventually 

brought about the need for quantitative resource 

appraisal, requiring knowledge of aquifer hydraulic 

properties.

Estimates of aquifer properties on Oahu have been 

made by several methods, including laboratory measurements on rock samples, local aquifer tests in which 

drawdown is measured in a pumped well or in one or 

more observation wells at some distance from the 

pumped well, regional aquifer tests in which one or

more wells are pumped, and with regional 

numerical models of ground-water flow.

The most commonly estimated properties have been 

hydraulic conductivity and transmissivity; fewer estimates are available for storage coefficient, specific 

yield, bulk porosity, and effective porosity. Specific 

storage and aquifer compressibility have been given in 

only a few reports. Generalized water-bearing properties of the volcanic rocks are given in table 1.

HYDRAULIC CONDUCTIVITY AND TRANSMISSIVITY

Most estimates of aquifer hydraulic properties on 

Oahu have been derived from aquifer tests at various 

field scales. Results from these tests, selected mainly 

from published reports, are presented in table 2. Values calculated from a given test are listed in table 2 

rather than the average value or range of values. This 

allows a greater appreciation of the variability of the 

estimates, even for a single test, based on the choice of 

different analytical methods or from the analysis of 

data from multiple observation wells. Variability in 

results from well to well might be attributed to heterogeneity, anisotropy, or other conditions not adequately 

accounted for in the analysis. Where different analytical methods have been applied to the same test, it is not 

clear which method yields the most appropriate result; 

judgments have been made in these cases by the 

original investigators.

The thickness of the volcanic-rock aquifers of Oahu 

is not known, but probably is at least several thousand 

feet. This has important implications for estimates of 

hydraulic conductivity and transmissivity derived from 

aquifer tests because transmissivity is a function of 

hydraulic conductivity and aquifer thickness and is 

given by

T = Kb (1)

where

T is transmissivity, foot squared per day;

K is hydraulic conductivity, foot per day;

b is aquifer thickness, feet;

(the units above are one example; in fact, any consistent set of length and time units can be substituted). 

Hydraulic conductivity likely decreases with depth 

because of compaction and may decrease sharply at 

depth where there is a transition from subaerially 

erupted lavas to submarine lavas.

Numerous estimates of hydraulic conductivity and 

transmissivity have been made for aquifers in Koolau 

Basalt and the Waianae Volcanics (table 2). However, 

the lack of a definable aquifer thickness and the partial 

penetration of wells introduce ambiguity to most of
GEOHYDROLOGY OF THE WATER-BEARING DEPOSITS B17

TABLE 1. Water-bearing properties of the rocks and occurrence of ground water on the island ofOahu

[Modified from Takasaki and Mink, 1982. Basal refers to the deepest or main water table near sea level, typically corresponding to a lens of freshwater floating on

seawater. ft/d, foot per day; <, less than; , no data]

Rock type Permeability

Hydraulic 

conductivity

(ft/d)

Ground-water occurrence

Principal Secondary

Volcanic rocks

Waianae Volcanics 

Lava flows

Dike complex.................... Low to moderate 1 - 500 Dike impounded Perched

Marginal dike zone ............... Moderate to high 100 -1,000 Dike impounded Basal

Dike free. ....................... Moderate to very high 500 - 5,000 Basal

Breccia............................ Low 1 - 100 Perched

Saprolite. .......................... Very low <1 Perched

Koolau Basalt 

Lava flows

Dike complex.................... Low to moderate 1 - 500 Dike impounded Perched

Marginal dike zone ............... Moderate to high 100 -1,000 Dike impounded Basal

Dike free. ....................... Moderate to very high 500 - 5,000 Basal

Breccia............................ Low 1 -100 Perched

Saprolite. .......................... Very low <1 Perched

Honolulu Volcanics

Lava flows ......................... Low to moderate 1 - 500 Perched

Cinders. ........................... Low to moderate 1 - 500 Perched

Tuff. .............................. Low 1 - 100 Perched

Saprolite. .......................... Very low <1 Perched

Sedimentary rocks

Coral

In situ reef limestone ................ Moderate to very high 100 - 20,000 Basal Perched

Reworked coral rubble ............... Moderate to very high 100 -10,000 Basal Perched

Dunes

Consolidated ....................... Low 1 -100 Perched Basal

Unconsolidated ..................... Low to moderate 1 - 500 Basal Perched

Sand

Consolidated ....................... Low to moderate 1 - 500 Basal

Unconsolidated ..................... Moderate to high 100 -1,000 Basal

Lagoonal

Sand.............................. Low to moderate 1 - 500 Basal

Mud .............................. Very low <1 Basal Perched

Alluvium

Younger ........................... Low to moderate 1 - 500 Perched Perched

Older .............................

Consolidated .................... Very low <1 Perched

Unconsolidated .................. Low 1 - 100 Perched Basal
TABLE 2. Hydraulic properties of the Waianae and Koolau aquifers, island ofOahu

[Published estimates from field-scale aquifer test values shown only for properties reported by the original authors; hydraulic conductivity or transmissivity equivalents 

in which only one or the other was reported. Multiple values for the same aquifer test are estimates from multiple observation wells or from multiple analytical m

refers to specific yield when the aquifer is unconfined and refers to storage coefficient when the aquifer is confined, ft, foot; ft/d, foot per day; ft2/d, foot squared per d

Source of

information

Aquifer;

ground-water

area

Aquifer

conditions

during test

Transmissivity,

T

Method of analysis (ft2/d x 106)

Hydraulic

conductivity,

K

(ft/d)

Storage

coefficient,

S

(x 10-2) 

Dike-free lava flows

Dale (1978) Koolau; 

Waialua

Unconfined;

basinwide

drawdown

Thiem (1906) 1,600 10 Boundaries poss

treated. K derived 

lens thickness (a

was 600-ft h

equivalent to a 

penetrates the 

Unconfined stora

equating volume 

cumulative pumpa

Eyre (1983)

Eyre and 

others (1986)

Koolau; 

Pearl 

Harbor

Koolau; 

Waialae

Confined; 

weathered

Unconfined, 

sparse dikes; 

basinwide 

drawdown

Thiem (1906), 

Zangar (1953)

Hemispherical flow 

equation

50

68, 85,100,

120, 340,

400, 590,

930, 960

Specific capacity 

Subtracted well 

Zangar methods.

Boundaries poss

treated. Analyze

Wentworth (1938

flow equation in

equation.

Mink (1980) Koolau;

Pearl

Harbor

Unconfined;

basinwide

recovery

Theis (1935)

Cooper-Jacob (1946)

0.93,1.1, 

1.2, 1.3, 

1.6, 2.7

1.4,1.6, 

2.6,1.5, 

1.5,1.6

1,600

1,600

5.9, 2.6, 

1.4, 3.6, 

4.1, 2.8

8,2, 

3,3, 

8,14

Boundaries poss

treated. Report 

drawdowns are 

drawdowns for s

longer duration. 

values of T and d

flow" (presumed 

recovery during 

pumping (1958 su

Rosenau and Koolau; 

others (1971) Wailua, 

Kawailoa

Not specified

(probably

Theis)

Not specified 3.6 to 8.4 Range from sever

Waianae; 

Mokuleia

2.0 to 3.3 Range from sever
TABLE 2. Hydraulic properties of the Waianae and Koolau aquifers, island of Oahu Continued

Source of 

information

Aquifer; 

ground-water 

area

Aquifer 

conditions 

during test Method of analysis

Hydraulic 

Transmissivity, conductivity, 

T K

(ft2/d x 106) (ft/d)

Storage 

coefficient,

S

(x 10"2) 

Dike-free lava flows Continued

Soroos (1973)

Takasaki and 

others (1969)

Todd and 

Meyer (1971)

Visher and 

Mink (1964)

Wentworth 

(1938)

Williams and 

Soroos (1973)

Williams and 

Soroos (1973)

Various

Koolau; 

Kahuku

Koolau; 

Pearl Harbor

Koolau; 

Pearl Harbor

Koolau; 

Waialae

Koolau; 

Pearl Harbor 

(Kaonohi

site)

Koolau; 

Kahuku 

(Punaluu 

site)

Various

Confined; 

basinwide 

drawdown

Confined; 

basinwide 

drawdown

Unconfined; 

basinwide 

drawdown

Unconfined; 

sparse dikes; 

basinwide 

drawdown

Unconfined; 

or semiconfined

Confined; 

basinwide 

drawdown

Confined

Zangar (1953)

Theis (1935)

Theis (1935)

Theis (1935)

Thiem (1906)

Theis (1935) 

Walton (1962) 

Hantush (1961)

Theis (1935)

Theis (1935) 

Cooper- Jacob (1946) 

Hantush (1961)

Theis (1935)

26 to 85,000 

840 (median) 

3,400 (mean)

0.19, 0.54, 0.54

0.55 1,800

0.57, 0.59, 0.59, 

0.61

1,800, 2,100, 

2,200, 2,700, 

3,500

1.1, 4.7 

2.3, 5.6 

7.8, 16 1,100, 650

4.2

0.62 

0.87, 1.1 

1.8. 0.99 320, 230

0.75, 0.65 

0.65, 0.65

Specific capacity 

applied Zan

distribution of 7

Boundaries po

treated. Values f

Boundaries trea

derived by divid

flow" (well activ

4, 4, Boundaries po

5, 6 treated. Analysi

period of no pum

Boundaries po

treated. K deriv

assumed 200-ft 

half presumed le

0.47, 0.034 Leaky boundar

0.25, 0.029 treats leakage, 

4.3, 0.30 wells pumped.

0.042 Analysis by Hon

0.16 Test 7-14-65. B

0.17, 0.13 time data used. 

0.49, 0.49

0.064, 0.14 Test 2-14-66. B

(duration of test 

Cooper-Jacob (1946) 0.65, 0.74, 0.73 0.076, 0.099,

0.062
Source of 

information

Aquifer; 

ground-water 

area

TABLE 2.

Aquifer 

conditions 

during test

 Hydraulic properties of the Waianae and Koolau aquifers, island of Oahu Continued 

Method of analysis 

Transmissivity, 

T

(ft2/d x 106)

Hydraulic 

conductivity, 

K

(ft/d)

Storage 

coefficient, 

S 

(x 10'2) 

Dike-free lava flows Continued

Williams and 

Soroos (1973)

Koolau; 

Kahuku

Confined Hantush (1961) l.l, 0.86, 1.3, 

1.3

180, 200, 420, 

470

0.27, 0.35, 

0.60, 0.55

(Punaluu 

site)

Zangar(1953) 91, 220

320

Specific capacit

Subtracted well 

method.

Williams and Koolau;

Soroos (1973) Beretania 

(Wilder 

Ave. site)

Confined Thiem (1906) 0.60, 0.96, 

0.27, 0.37

Used paired obse

four tests.

Thiem (1906)

Theis (1935) 

Zangar(1953)

0.045, 0.063, 

0.068, 0.099, 

0.10, 0.095, 

0.094, 0.10, 

0.10, 0.092, 

0.16, 0.16, 0.16

0.12, 0.19, 0.21

250, 900, 930,

1,500

Used pumped-w

pairs. Data from 

pairs for each test

Used recovery dat

Specific capacity 

well losses, applie

Summary 0.27 2,700 Authors judge thi

divided by "avera

to estimate K.

Williams and Koolau;

Soroos (1973) Pearl 

Harbor 

(Kalauao 

site)

Unconfined Theis (1935) 0.25, 0.28, 

0.28, 0.35, 0.42, 

0.45, 0.47

2.2, 2.4, 1.1, 

2.4, 2.8, 1.0, 

0.78

Data from aquif

dates, although th
TABLE 2. Hydraulic properties of the Waianae and Koolau aquifers, island of Oahu Continued

Source of

information

Aquifer;

ground-water

area

Hydraulic

Aquifer Transmissivity, conductivity,

conditions T K

during test Method of analysis (ft2/dxl06) (ft/d)

Storage

coefficient,

S

(x lO'2) 

Dike-free lava flows Continued

Williams and

Soroos (1973)

Koolau;

Pearl

Harbor

(Kalauao

site)

Unconfined Cooper-Jacob (1946) 0.35,0.26,

0.30, 0.39,

0.49, 0.51,

0.50

Hantush (1961) 0.63, 0.43, 820, 560,

0.76, 0.83, 360, 710,

0.64, 0.90 240, 340

1.4, 1.7, 0.96,

2.0, 2.0,

0.85, 0.77

11, 17,

5.0, 19,

9.6, 4.7

Dike-intruded lava flows

Takasaki and

others (1969)

Williams and

Soroos (1973)

Koolau;

Koolau rift

zone

Koolau;

Koolau rift

zone

(Waihee

site)

Confined and Theis (1935) 0.0027,

unconfined 0.00033,

0.00040,

0.0011, 0.0017

Semiconfined Cooper-Jacob (1946) 0.034,0.034,

0.039, 0.044,

0.030, 0.048

Walton (1962) 0.025, 0.022,

0.029, 0.043,

0.030, 0.046

Hantush (1961) 0.062, 0.068, 56, 58

0.097, 0.054, 42, 110

0.076 44, 82

Zangar (1953) 27, 110, 37

12, 3.9,

0.084, 6.6,

0.19, 2.2

16, 5.3, 0.28,

7.0, 0.20, 2.4

42,

1.8, 5.0,

1.3, 6.1

...

Data from aq

Aquifer describ

Leaky boundar

treats leakage, 

tests.

Specific capacit

well losses, app
B22 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

these estimates. Transmissivity commonly is determined using the non-leaky analyses of Thiem (1906) or 

Theis (1935). However, without correction for partial 

penetration, the reliability of the estimates is open to 

question. Further, given a transmissivity estimate, 

ambiguity remains as to the implicit aquifer thickness 

that corresponds to that transmissivity.

A number of investigators have equated aquifer 

thickness to the thickness of freshwater flow or to the 

length of a well open to the aquifer. Although one or 

more of these approximations may be meaningful, 

their validity generally has not been supported by 

analysis and, more importantly, they have not been 

applied consistently. Dividing a transmissivity estimate by the length of the uncased or screened interval 

of a well, which in many cases is only 100 ft or so, would 

yield a hydraulic conductivity as much as an order of 

magnitude greater than if the same transmissivity 

were divided by the freshwater lens thickness (see section on occurrence of ground water), which may be as 

much as 1,000 ft.

For steady horizontal flow in a freshwater lens, it 

can be reasoned that aquifer thickness theoretically 

corresponds to the thickness of the freshwater lens 

because freshwater head gradients induce only freshwater flow and not flow in the underlying saltwater. 

Therefore, an estimate of transmissivity using the 

Thiem, non-leaky, steady-state method is valid if: (1) a 

steady state has been sufficiently approximated, (2) the 

aquifer is isotropic, and (3) a correction has been made 

for partial penetration if the radial distance to the 

observation well is less than 1.5 times the aquifer 

thickness.

The validity of equating aquifer thickness to freshwater lens thickness is less certain under transient 

conditions where analysis by the Theis (1935) method 

is indicated. During the transient period, some saltwater flow is induced by upconing of the freshwatersaltwater interface, and saltwater may be released 

from storage by decompression of deeper parts of the 

aquifer.

If the ratio of horizontal to vertical hydraulic conductivity is large, a transmissivity estimate is likely to 

be more representative of the pumped interval than of 

the transmissivity that corresponds to lens thickness. 

Aquifer-test results also are more likely to reflect lateral rather than vertical hydraulic conductivity, simply 

because the typical geometric arrangement is for observation wells to penetrate the same water-bearing zone 

as the pumping well. Many of the ambiguities 

described above can be circumvented by using methods 

that approximate the actual field conditions of partial 

penetration and anisotropy; for example, Hantush, 

1961 (partial penetration), Hantush, 1966 (horizontal

anisotropy), Weeks, 1969 (vertical anisotropy), Neuman, 1974 (vertical anisotropy, partial penetration in 

unconfined aquifers), Hsieh and Neuman, 1985 

(3-dimensional anisotropy).

Estimates of hydraulic conductivity for dike-free 

lavas, based mostly on the solution of the Theis (1935) 

and Thiem (1906) equations, tend to fall within about 

one order of magnitude, from several hundred to several thousand feet per day. Mink and Lau (1980) suggest that a regional value for hydraulic conductivity of 

unweathered, flank lavas is in the range of 1,000 to 

5,000 ft/d with the most probable values centering 

around 2,000 ft/d. Liu and others (1983) stated that 

hydraulic conductivity for basalt in the Pearl Harbor 

area is between 1,000 and 2,000 ft/d. Soroos (1973) estimated hydraulic conductivity from 79 specific-capacity 

tests on Oahu and characterized the statistical 

distribution of his results (fig. 9) as follows:

The values range from 26 to 85,000 ft/d *** the mean 

is 3,413 ft/d and the median is 840 ft/d *** the mode of 

the grouped values is between 300 and 400 ft/d *** it is 

obviously a highly skewed distribution *** this distribution is probably more representative of the Pearl Harbor 

regional aquifer than Oahu aquifers in general because 

of the concentration of sites above Pearl Harbor from 

which data was taken.

The skew referred to by Soroos corresponds to findings by other investigators that hydraulic conductivity 

commonly conforms to a log-normal distribution.

A small range of hydraulic conductivity has been 

applied to numerical models of ground-water flow on 

Oahu, most of which have been two-dimensional areal 

representations of horizontal freshwater flow. Areal 

models of southern and southeastern Oahu were calibrated using hydraulic conductivity values of 500 to 

1,500 ft/d (Liu and others, 1983; Eyre and others, 1986; 

Eyre and Nichols, (in press). These values were determined by model calibration using repetitive, trial-anderror forward modeling. Souza and Voss (1987) simulated miscible, density-dependent, freshwatersaltwater flow and solute transport in vertical 

cross sections in southern Oahu. They used values 

of 1,500 ft/d for horizontal hydraulic conductivity and 

7.5 ft/d for vertical hydraulic conductivity as 

determined by model calibration.

Layered and channeled lavas indicate a hydraulic 

conductivity tensor that is anisotropic in three dimensions, with its greatest principal axis in the longitudinal lava-flow direction, intermediate principal axis in 

the transverse areal direction, and least principal axis 

normal to bedding, or essentially near-vertical (fig. 8). 

Anisotropy has not been measured directly in Hawaiian lavas. Wentworth (1938) suggested that the permeability of Koolau lavas probably was greater in the 

horizontal dimension than in the vertical. Only a few
GEOHYDROLOGY OF THE WATER-BEARING DEPOSITS B23

60

50

40

30

20

10

CO

CO 

HI

HI 

0

Z 

LLJ 

O 

DC 

LLJ 

Q_

111 II

14

12

10

10 "" -- -20--.___30 40 50 60 70 80 

HYDRAULIC CONDUCTIVITY, IN FEET PER DAY x 103

90 100

2345678 

HYDRAULIC CONDUCTIVITY, IN FEET PER DAY x 103

10

FIGURE 9. Estimates of hydraulic conductivity from specific-capacity tests of volcanic aquifers on the island of Oahu (modified 

from Soroos, 1973, fig. 10). Histogram frequency (y-axis) is expressed as a percentage of the total number of tests, N, where 

N=79. In the bottom graph, the x-axis is expanded by an order of magnitude to show additional detail in the lower part of the 

range. The histogram class width (bar width) is also expanded, being 100 feet per day in the lower graph and 1,000 feet per
B24 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

estimates of anisotropy have been made for Hawaiian 

lava aquifers. Eyre and Nichols (in press) used an 

anisotropic transmissivity ratio of 3:1 to reproduce the 

observed areal head distribution in a simulation of 

ground-water flow in southern Oahu. The maximum 

principal axis was aligned parallel to the longitudinal 

direction of lava flows. Burnham and others (1977, 

p. 17) presumed horizontal:vertical anisotropy ratios of 

10:1 and 100:1 in a three-dimensional numerical simulation of wastewater injection on the island of Maui, 

but suggested that a regional ratio may be closer to 5:1. 

Souza and Voss (1987) estimated a 200:1 horizontal to 

vertical anisotropy in a cross-sectional model used to 

simulate flow in the Pearl Harbor ground-water area.

POROSITY

Total porosity is the bulk volume of material occupied by voids, whether isolated or connected. Estimates 

of the total porosity of various volcanic rocks on Oahu 

have been made by laboratory analysis of rock samples, 

borehole photographic logging, and gravity profiling in 

underground tunnels. Wentworth (1938) made laboratory determinations of total porosity in six hand specimens and cores of Hawaiian basalt. For Koolau Basalt, 

he found porosities ranging from 5 to 51 percent, with 

a median value of about 43 percent. However, Wentworth noted that "the greater part of the space in the 

vesicles of these flows is not interconnected, and only to 

a slight extent does it contribute to the effective storage 

and movement of water in the formation" (Wentworth, 

1938, p. 175). Wentworth also reported porosities of 

5 and 10 percent for two samples of massive Koolau 

dike rock, and that porosities of massive basalt of the 

Honolulu Volcanics were 16 percent or less, typically 

less than 10 percent. Ishizaki and others (1967) 

reported laboratory values of porosity of about 8 to 

10 percent for massive aa core (Koolau Basalt) and 

about 50 percent for aa clinker.

Peterson and Sehgal (1974, p. 8), using borehole 

photographic logs, estimated intergranular and secondary porosity of basaltic lavas from less than 

5 percent in dense flows to as much as 50 percent in 

some aa clinker zones, especially if unweathered. 

Huber and Adams (1971, p. 19) estimated from gravity 

surveys to a depth of 550 ft in Schofield Shaft (a waterdevelopment tunnel in the Schofield area of central 

Oahu, fig. 4), that total porosity decreased from 42 percent at the surface to 18 percent at a depth of 340 ft and 

then remained constant at 18 percent throughout the 

extent of the shaft. The porosity-depth profile reflects 

the gradation from shallow, high-porosity, clayey 

saprolite to deeper, unweathered lavas.

All the estimates described above are of total porosity, which includes the unconnected vesicular component of porosity. Values of effective porosity may be 

lower by as much as a factor of 10.

SPECIFIC YIELD AND STORAGE COEFFICIENT

Specific yield represents the volume of water an 

unconfined aquifer releases from storage per unit surface area of aquifer per unit decline in the unconfined 

water level. It represents an actual dewatering of the 

pores of the aquifer material. The storage coefficient of 

a confined aquifer is the volume of water released from 

storage per unit surface area of the aquifer per unit 

decline in hydraulic head and is given by

S = (2)

where

S is the storage coefficient, dimensionless;

Ss is specific storage, in I/feet; and

b is aquifer thickness, in feet;

(the units above are an example; other length units 

could be substituted). Water released from storage in a 

confined aquifer represents water released by the 

expansion of water and the compaction of the aquifer 

when hydraulic head in the aquifer is reduced. Estimating a storage coefficient for confined basalt aquifers 

is difficult, as is estimating hydraulic conductivity, 

because a value for aquifer thickness is required.

Estimates of specific yield range from about 1 to 

20 percent with the exception of one value of 

42 percent (table 2). Most of the values lie within a 

narrow range of about 5 to 10 percent. Estimates of 

confined storage coefficient given in table 2 range from 

about 3 x 10-4 to 1 x 10'3 . Investigators commonly 

interpreted the larger values to indicate leaky or 

semiconfined, conditions.

Eyre and others (1986) examined the effect of 

specific yield on transient ground-water flow using a 

vertically averaged, two-dimensional, areal model of 

ground-water flow in southeast Oahu. Values of 0.02 

and 0.05 were found to reproduce observed transient 

water levels. However, it was recognized that transient 

simulation might violate the equilibrium assumptions 

inherent in the model formulation, and that some error 

could be associated with these values. A subsequent 

modeling study of the same problem was described 

by Essaid (1986) using a vertically averaged, twodimensional model of coupled freshwater-saltwater 

flow. Essaid reproduced the observed transient water 

levels with specific yields of 0.05 and 0.10 and found 

that the model was not sensitive to specific yield in this 

range. Souza and Voss (1987) calibrated a vertical 

cross-sectional model of southern Oahu using a value of 

0.04 for the specific yield of the Koolau aquifer.
GEOHYDROLOGY OF THE WATER-BEARING DEPOSITS B25

HETEROGENEITY, ANISOTROPY, AND AQUIFER STRUCTURE

Lavas in the islands of Hawaii differ markedly from 

continental flood basalts of the Columbia Plateau and 

Snake River Plain in Washington, Oregon, and Idaho. 

Lavas in Hawaii are much thinner and more vertically 

repetitious, resulting in a greater prevalence of highly 

permeable elements such as lava tubes, interflow voids, 

and flow-top breccia (clinker). The smaller areal extent 

of the lavas in the islands of Hawaii, especially the 

smaller transverse areal dimension, results in smaller 

lateral correlation scales and a lesser importance of 

discrete, regionally extensive permeable zones.

The hydraulic properties of the volcanic-rock aquifers are determined by the distinctive textures and 

characteristic dimensions of lava flows. Most hydrologic analyses of Hawaiian lava aquifers have been 

deterministic because of a lack of detailed spatial information. For these analyses, the aquifer is represented 

as a homogeneous, porous medium. A homogeneous 

representation may be appropriate for the generalized 

predictive purposes of many analyses, but heterogeneity at various scales may introduce unforeseen shortcomings to the results. It is important, then, that the 

approach to parameter estimation be consistent with 

the scale and purpose of the analysis.

For regional analyses of ground-water flow, estimation of representative or average values requires field 

experiments of sufficiently large scale or the averaging 

of many local-scale measurements. Rigorous analysis 

of some problems at the local scale may negate the 

presumption of homogeneity and require detailed drilling, borehole logging, and geologic interpretation sufficient to define important conductive or confining zones. 

This is especially true of solute transport problems in 

which the arrival and concentration of solutes, being 

determined by travel times along particular flowpaths, 

may depend more on the spatial distribution of conductive zones than on any average aquifer properties that 

may be assumed.

The spatial distribution of permeability within a volcanic-rock aquifer can be visualized at various scales. 

At the bedding scale encompassing dimensions of feet 

to tens of feet, permeability varies over many orders of 

magnitude, from very low within massive aa blocks to 

extremely high within lava tubes. Massive layers, even 

though jointed, possess mainly fracture porosity and 

are much less permeable than lavas in general. Aa clinker probably has high intergranular hydraulic conductivity similar to coarse gravel, ranging from about 

1,000 to as much as 100,000 ft/d (U.S. Bureau of 

Reclamation, 1977).

At the well-field scale encompassing hundreds to a 

few thousands of feet laterally, hydraulic behavior may

be influenced strongly by discrete layers. In a stratified 

sequence of lavas, the repetitive alternation of massive 

layers with highly porous clinker layers and void zones 

imparts a distinctly ordered heterogeneity similar to 

bedding, termed "layered heterogeneity" by Freeze and 

Cherry (1979, p. 30). Permeability contrast is large 

between massive layers and porous layers. Wateryielding zones in excavations and wells commonly are 

clinker layers or voids, with much less water entering 

from massive rock layers. Averaged over several lavaflow thicknesses, hydraulic conductivity at the wellfield scale ranges from about 100 to 100,000 ft/d, with 

field aquifer tests yielding values most commonly 

between 500 and 5,000 ft/d for dike-free lava flows (see 

table 2).

At well-field and larger scales, the areal dimensions 

of the lava flows introduce a second type of ordered 

heterogeneity corresponding to the elongate tongue, 

or lobe, morphology described by Peterson and 

Moore (1987). Although stratification is pronounced 

and some conductive and confining zones may extend 

laterally for large distances, others may not because of 

lateral lava-flow termination or change in lava-flow 

morphology.

At the regional scale, which encompasses thousands 

of feet to miles laterally, systematic spatial variation in 

lava-flow thickness may impart trending heterogeneity, a systematic variation in locally averaged hydraulic 

conductivity (Freeze and Cherry, 1979, p. 30). Similarly, variation in the areal dimensions of flows may 

cause trends in anisotropy ratios by varying the proportional relations among bedding thickness, length, 

and width. Thicker massive flows tend to diminish 

overall permeability, but probably more so in the vertical dimension, increasing the horizontal to vertical 

anisotropy. Wider flows would give greater transverse 

extent to permeable interflow zones and clinker 

and would diminish longitudinal to transverse areal 

anisotropy.

SEDIMENTARY ROCKS

Sedimentary rocks on Oahu are of both marine and 

terrestrial origin and have wide ranges in composition, 

grain size, and degree of induration. Marine sedimentary rocks are mostly calcareous and include coralalgal reefs, coralline rubble and sands, and lagoonal 

sands and marls. Terrestrial sedimentary deposits 

include deposits of talus, colluvium, and alluvium 

ranging in size from estuarine muds to dune sands to 

boulders. Sedimentary deposits commonly are less permeable than the basaltic lavas and in some areas 

confine ground water in the volcanic-rock aquifers.

Marine sedimentary deposits are areally widespread, occupying the coastal plain and the lower
B26 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

reaches of coastal valleys. The largest volume of sediment occurs in a stratified sequence beneath the 

coastal plain (fig. 10 is an example for southern Oahu). 

This wedge of calcareous and volcanogenic sediment is 

referred to locally as caprock because it overlies and 

confines ground water in the underlying volcanic-rock 

aquifers. The confining property of the coastal plain 

deposits stems mainly from the presence of finegrained muds and marls and from weathering of 

basalt-derived material. Despite the generally low permeability of the coastal plain deposits, shallow limestone aquifers in the coastal plain contain water that is 

fresh to brackish; deeper coastal plain aquifers generally contain saline water.

Alluvial deposits, generally restricted to long, 

narrow valleys, are commonly a poorly sorted mixture 

of boulders, cobbles, gravel, and sand. Alluvium also 

forms deltas of silt and sand in estuaries at the mouths 

of streams. The alluvial deposits can be divided into 

younger alluvium, which is unconsolidated and 

unweathered, and older alluvium, which is wholly or 

partly consolidated and, in general, highly weathered 

(Stearns and Vaksvik, 1935; Wentworth, 1951). 

Younger alluvium is far less voluminous than older 

alluvium and occurs mainly as thin streambed deposits 

laid down during the Holocene. Older alluvium is more 

extensive and forms broad fans and the thick valley 

fills found in the lower reaches of most large valleys.

It is convenient to divide the sedimentary deposits 

into calcareous and noncalcareous deposits for a discussion of hydraulic properties. The permeability of 

calcareous rocks commonly is moderate to very high 

and results from primary depositional textures, as well 

as from development of secondary porosity by solution. 

The most important water-bearing calcareous rocks 

are the uppermost units of the coastal sediments consisting of reef limestone, coralline rubble, and calcareous sand. These units are productive aquifers, and they 

supply significant amounts of fresh and brackish water 

to wells. Cavernous limestone and coarse coralline rubble are among the most permeable rocks on Oahu, commonly more permeable than the basaltic lavas. 

Hydraulic conductivities of several thousand feet per 

day are believed to be representative, and many estimates from aquifer tests and tidal analyses exceed 

IxlO4 ft/d (Dale, 1974; Williams and Liu, 1975; Khan, 

1981; Oberdorfer and Peterson, 1985). The high permeability of many of the limestone and coralline rubble 

deposits is due in part to secondary solution permeability. Sinkholes are common features of the coastal plain, 

and solution caverns and voids are seen in excavations 

and are encountered during drilling.

The noncalcareous deposits tend to be poorly permeable, mainly because their basaltic composition results

in rapid and thorough weathering that increases clay 

content. The most common noncalcareous sedimentary 

deposit on Oahu is alluvium, and its principal hydrologic significance is its generally low permeability. 

Some water is developed from alluvial aquifers locally, 

particularly where alluvium is underlain by massive 

lava of the Honolulu Volcanic Series in valleys. Most 

younger alluvium is moderately permeable. Most older 

alluvium is poorly permeable except in the drier areas 

of Oahu where it is less-weathered and yields small 

quantities of water to wells and tunnels.

MODES OF GROUND-WATER 

OCCURRENCE

The occurrence of ground water on Oahu is determined by the type and character of the rocks and by the 

presence of geologic features such as dikes, valley fills, 

and caprock. The primary modes of freshwater occurrence in Oahu are as a basal lens of fresh ground water 

floating on saltwater, as dike-impounded freshwater, 

and as perched ground water (fig. 11). Saltwater occurs 

at depth throughout the island.

BASAL GROUND WATER

The most extensive bodies of freshwater on Oahu 

occur as basal ground water where the freshwater head 

in the aquifer is near sea level (fig. 11). This type of 

freshwater occurrence was recognized early in Hawaii 

by Lindgren (1903). The term "basal ground water" was 

defined by Meinzer (1930) during an early investigation of the island of Hawaii. Meinzer coined the term to 

describe water lying beneath "a general or main water 

table below which all the permeable rocks are saturated" (Meinzer, 1930, p. 10) and to distinguish this 

type of occurrence "from the perched bodies of ground 

water" that were known to exist at the time (all ground 

water at high altitudes was thought to be perched; the 

concept of dike-impounded water was not yet known). 

The term is used locally in Hawaii and has been used 

in studies of other islands, but it is not in general use 

by ground-water hydrologists elsewhere. Synonymous 

terms for basal water include "freshwater lens," "basal 

lens," and "Ghyben-Herzberg lens," after the supposed 

lenticular shape of such bodies of water.

Basal ground water occurs in volcanic-rock aquifers 

and aquifers in the sedimentary deposits on Oahu 

under confined and unconfined conditions. The thickness of the freshwater lens depends on recharge rates, 

aquifer permeability, and the presence or absence of 

confinement. Recharge to a given body of basal water 

may occur by direct infiltration of precipitation and by 

ground-water inflow from upgradient ground water. 

The direct infiltration component typically is more
MODES OF GKOUND-WATEK OCCUKKENCE B27

variable than the ground-water inflow component 

because of the episodic and seasonal distribution of 

precipitation.

The water table, or potentiometric surface, of a body 

of basal water in Hawaii typically is rather flat and 

only several feet to several tens of feet above sea level. 

However, the largest volume of freshwater lies below 

sea level in the same fashion that an iceberg floats in 

the sea with most of its mass submerged. This situation 

results from the buoyant displacement of the underlying saltwater by freshwater and is governed by the difference in their specific gravities. The principle of 

hydrostatic balance between freshwater and saltwater, often called the Ghyben-Herzberg principle, was 

put forth by Du Commun (1828), Badon Ghyben (1889), 

and Herzberg (1901). The principle can be summarized 

by the equation

z = '- z , s (Ps -Pf) »' (3)

where

Zs is thickness of freshwater below sea level,

in feet; 

Zw is freshwater head above sea level, in

feet; 

Pf is density of freshwater, in pounds per

cubic foot; and 

ps is density of saltwater, in pounds per

cubic foot;

(the units above are an example; other units could be 

substituted). Values of 62.4 and 63.96 lb/ft3 commonly 

are taken as the density of freshwater and saltwater, 

respectively, reducing the above equation to Zs = 4QZW; 

that is, the thickness of freshwater below sea level is 

40 times the height of the freshwater head above sea 

level. Wentworth (1939) reported determinations of 

freshwater and saltwater density in and around Oahu 

and proposed that a ratio closer to 38:1 would be more 

appropriate.

Key assumptions of the Ghyben-Herzberg principle 

include the following:

1. A sharp interface separates freshwater from 

saltwater;

2. A hydrostatic pressure gradient exists in the 

freshwater, implying that freshwater is static 

or that the Dupuit assumption of horizontal 

flow is satisfied (Dupuit, 1863; most modern 

ground-water textbooks contain an explanation of the Dupuit assumption, for example: 

Freeze and Cherry, 1979; Todd 1980); and

3. Saltwater is static, implying that saltwater 

head is zero with respect to sea level everywhere in the aquifer.

In most field situations, a sharp interface does not 

exist and freshwater grades into saltwater across a 

transition zone of mixed salinity. In a system that is 

tidally disturbed or episodically recharged, neither the 

freshwater nor saltwater is static; however, measurements of freshwater head and thickness have shown 

the Ghyben-Herzberg principle to be a reasonable 

approximation. Cooper (1959) demonstrated the presence of circulatory saltwater flow in coastal aquifers 

driven by dispersion of saltwater in the freshwater flow 

field, which causes saltwater head to be somewhat less 

than zero. The magnitude of the negative saltwater 

head and the departure from the Ghyben-Herzberg 

hydrostatic balance depend on the permeability of the 

aquifer and any confining unit that restricts saltwater 

entry into the aquifer, as well as on the amount of dispersive mixing by lateral flow and short-term tidal fluctuations. Hubbert (1940) demonstrated that vertical 

components of freshwater velocity will cause systematic departures from hydrostatic buoyancy in the 

coastal discharge zone. Similar departures will occur 

near pumping wells. Transient stresses such as pumpage from wells or recharge also will cause nonconformance to hydrostatic buoyancy based on freshwater 

head alone.

BASAL WATER IN THE VOLCANIC ROCKS

Basal water in the thick volcanic-rock sequences of 

Oahu exists as free-floating lenses underlain by saltwater. At the base of the lens, the transition zone of 

mixed freshwater and saltwater may range in thickness from tens of feet to hundreds of feet. Basal water 

in the volcanic rocks of Oahu is mainly unconfined, 

except near the coast where the aquifer may be overlain and confined by sedimentary caprock (fig. 11). In 

the absence of a confining unit near the coast that 

impedes discharge of water to the sea, a comparatively 

thin basal-water body forms. The water table is a foot 

or so above sea level at the shore, rising inland at about 

1 to 2 ft/mi to an altitude of about 10 ft above sea level 

several miles inland. A freshwater lens with this configuration is no more than about 400 ft thick, and the 

freshwater discharges to the sea by diffuse sea-bottom 

seepage and from distinct submarine and shoreface 

springs. The diffuse discharge from these aquifers is 

virtually impossible to measure.

Where the volcanic-rock aquifer is overlain by 

caprock along the coast, ground water in the aquifer is 

confined and a thicker basal-water body forms. 

Although there is confinement along the coastal zone,
B28 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

21°25'

158° 157°55'

21°20'

 '""- '.V'V/,- ;/.!-V'v'i96v.y-~,'/^V-v.'/.'V'V/,' ,

* '-' - ..- > ' - -- '-' '- 'i mrv 197 -' '."SJ--'- -' ' x -;-

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Alters equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

2 MILES

2 KILOMETERS

EXPLANATION

Pacific 

Ocean

COASTAL-PLAIN DEPOSITS (CAPROCK) 

KOOLAU BASALT 

GEOLOGIC CONTACT

-250 STRUCTURE CONTOUR Shows altitude of top of 

Koolau basalt. Interval, in feet, is variable. Dashed 

where uncertain. Datum is mean sea level

A A' TRACE OF GEOLOGIC SECTION

165®

748

888

DRILLED WELL Top number is well number, middle 

number is casing depth, and bottom number is well 

depth. Long dash indicates no data. Depths are in feet 

below mean sea level

FIGURE 10. Structural contours on top of Koolau Basalt in the Pearl Harbor area, island of Oahu

(modified from Wentworth, 1951, fig. 21) and section showing stratified coastal-plain sediments

(modified from Resig, 1969, fig. 6 and Stearns and Chamberlain, 1967, fig. 2).
MODES OF GROUND-WATER OCCURRENCE B29

LEVEL

i i i i i i i i i i i

1 /2 MILE

0 .5 KILOMETER

VERTICAL EXAGGERATION x 5

EXPLANATION

COASTAL-PLAIN DEPOSITS (CAPROCK)

i i [ 

I | Reef limestone

:=. ^1 Lagoonal mud and marl

Non-marine sediments: soil, brown silt or clay 

(probably weathered, fine-grained alluvium), 

black or green mud and associated lignite 

(probably marsh deposits)

Basaltic sediments: sand and silt, gravel and 

boulders

Lignite

KOOLAU BASALT 

GEOLOGIC CONTACT

FIGURE 10. Continued.
B30 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

Recharge (decreases 

down slope)

Springs, 1

discharge ^^\- '- Knsaturatedzone '

<"-'/,'V'_- '1^.','? iv Basal water-."'^ -\-V/V/i'>. IN-,

Dike complex with dikeimpounded freshwater

ground water ,\'/_'._>\

-1,500

VERTICAL EXAGGERA1ION INCREASED X3 BELOW SEA LEVEL

EXPLANATION

COASTAL-PLAIN DEPOSITS 

(CAPROCK) Consists of 

saprolite and overlying 

coastal-plain sediments

KOOLAU BASALT _V_ WATER TABLE

< GENERALIZED DIRECTION

OF GROUND-WATER FLOW

FIGURE 11. Modes of ground-water occurrence on the island of Oahu.

the inland part of the aquifer is unconfined. The 

hydraulic head in the aquifer is higher than otherwise 

would be reached in the absence of the impounding 

effect of coastal confinement. Beneath the confining 

unit in the coastal zone, the freshwater would be properly referred to as confined basal water. In an 

impounded basal-water body, heads in the confined 

coastal zone are several feet to several tens of feet 

above sea level. In the unconfined part of the aquifer, 

the water table rises inland at about 1 to 2 ft/mi. Maximum inland freshwater heads in impounded basal 

lenses on Oahu typically are about 10 to 40 ft above sea 

level, with lens thicknesses about 41 times the head 

above sea level. The highest basal-water head 

measured on Oahu was 43.5 ft above sea level in the 

Honolulu area (Stearns and Vaksvik, 1938); this 

implies that the freshwater lens was nearly 1,800 ft 

thick. Coastal discharge from an impounded lens is by 

measurable flow from prominent lowland springs near 

the inland edge of the sedimentary confining unit and 

by diffuse, unmeasurable flow through the confining 

unit. Much of this unmeasurable discharge may leak

into overlying permeable layers of limestone and sediment near the inland edge of the sedimentary caprock.

BASAL WATER IN THE SEDIMENTARY ROCKS

Basal water in sedimentary rocks and deposits commonly is no more than 100 ft thick, with water levels 

not more than several feet above sea level. Most basal 

water in sedimentary rocks is unconfined and resides 

in the uppermost aquifers of the coastal-plain sediments. Confining beds of clay or soil are common at the 

base of these aquifers, and freshwater may occupy the 

entire thickness of the aquifer in some areas, although 

these areas are laterally contiguous with more shoreward areas of freely floating freshwater. This type of 

freshwater occurrence has been called "parabasal" by 

Mink (1976), with the connotation that the fully fresh 

areas could change to a free-floating lens given lateral 

intrusion of saltwater and consequent development of 

a free-floating lens. This occurrence of ground water is 

of minor significance on Oahu.
REGIONAL AQUIFER SYSTEM B31

HIGH-LEVEL GROUND WATER

Ground water occurs at high altitudes above sea 

level under two well-known conditions: dike 

impounded and perched. Dike-impounded, high-level 

ground water occurs dominantly in the rift zones of the 

Waianae and Koolau Volcanoes. Perched ground water 

occurs in the unsaturated zone. A large body of highlevel water in the Schofield area of central Oahu differs 

in occurrence from either dike-impounded or perched 

ground water.

DIKE-IMPOUNDED WATER

Steeply dipping volcanic dikes impound freshwater 

to great heights within the rift zones of the Waianae 

and Koolau Volcanoes. The dikes impede lateral flow of 

ground water and impound water in compartments, or 

reservoirs, of permeable lavas (as in fig. 11). The term 

"dike-impounded water" seems to have evolved during 

the 1960's, replacing the earlier terms "dike-confined 

water" and "dike water." The term "dike-impounded 

water" is used to characterize any water that is 

enclosed or restrained laterally by dikes. Low-head, 

dike-impounded freshwater occurs in dry areas where 

recharge is small and in coastal discharge zones of 

dike-intruded aquifers. Wherever dikes provide appreciable impoundment, even at low head, the head is 

higher than it otherwise would be in the absence of the 

dikes, and the aquifer will respond to stress differently 

than if the dikes were not present.

Freshwater in dike-impounded reservoirs reaches 

altitudes of nearly 2,000 ft above sea level in the rift 

zones of Oahu and commonly is higher than several 

hundred feet. Freshwater with such high hydraulic 

head probably is not supported strictly by hydrostatic 

balance with saltwater, and freshwater should not be 

expected to extend as deep as the Ghyben-Herzberg 

principle would predict because of vertical head loss 

associated with downward flow. The rocks beneath dike 

reservoirs probably are progressively less porous and 

permeable with depth because of increasing numbers 

of dikes (Takasaki and Mink, 1985). Dike-impounded 

water contributes to streamflow where valleys intersect the impounded water table; the impounded water 

also moves downgradient into basal-water bodies. Flow 

from dike compartments is thought to be through fractures and gaps in the dikes and by overflow where 

impounded water overtops dikes (as shown in fig. 11).

PERCHED GROUND WATER AND WATER IN THE 

UNSATURATED ZONE

The unsaturated zone is several hundred feet thick 

throughout much of Oahu and more than 1,000 ft thick 

in places. This zone includes the layers of soil, saprolite, sediment, and rock that lie above the deepest 

water table. Within the unsaturated zone, perched 

water occurs where low-permeability material impedes 

downward percolation enough to cause localized saturation. Perched ground water occurs in far less volume 

than basal or dike-impounded water on Oahu, and it 

responds more readily to extremes in climate such as 

droughts and episodic recharge events. It is found in 

valley-fill deposits, in saprolite, and to a lesser extent 

where ash beds and soils are intercalated with the 

lavas. Some of the largest bodies of perched water are 

associated with alluvium and rocks of the Honolulu 

Volcanics in valleys of the Honolulu area.

HIGH-LEVEL WATER IN THE SCHOFIELD AREA

A large body of high-level water occurs beneath the 

Schofield Plateau of central Oahu, and its occurrence 

and behavior do not resemble dike-impounded or 

perched types of high-level water. The Schofield highlevel water is impounded to a height of about 280 ft 

above sea level by regional subsurface, geologic 

structures, probably dikes or weathered, buried volcanic ridges, or a combination of these. The water occurs 

mainly in permeable Koolau lavas that do not seem to 

be divided into small dike compartments, except 

perhaps within the impounding structures themselves. 

Based on ground-water levels in wells, the potentiometric surface in the Koolau aquifer is continuous and 

very flat beneath much of the plateau.

SALTWATER

Rocks beneath the fresh ground water of Oahu are 

saturated with saltwater, which resides in permeable 

subaerial lavas to depths of 6,000 to 10,000 ft below sea 

level (the probable submergence of former subaerial 

lavas) and in less permeable submarine lavas at 

greater depths. The thickest known basal fresh-water 

on Oahu extends to about 1,700 ft below sea level. The 

maximum depth of dike-impounded fresh-water is not 

known; however, based on observed heads of a thousand feet or more in the rift zones, it is 

probably considerably more than the thickest basal 

water, even if Ghyben-Herzberg conditions of simple 

hydrostatic buoyancy do not strictly apply in the rift 

zones.

REGIONAL AQUIFER SYSTEM

Aquifer nomenclature for Oahu evolved historically 

without a consistent approach, resulting in a mixture 

of named and unnamed aquifer references. Examples 

of unnamed references are "the highly permeable 

volcanic-rock aquifer" and "the basal aquifers of the
B32 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

Honolulu area." References to named aquifers have 

implied correspondence to rock-stratigraphic units, 

such as the "Waianae aquifer" and "Koolau aquifer," or 

have invoked other locality names, such as "the Pearl 

Harbor aquifer." In addition to aquifers, other geohydrologic entities have been delineated on the basis of 

structural boundaries and modes of ground-water 

occurrence. Various workers have delineated "artesian 

areas," "isopiestic areas," and "ground-water bodies," 

for example the "Honolulu isopiestic areas" or the 

"Waialua basal-water body." Mink and Sumida (1984), 

noting the lack of an orderly procedure for naming 

aquifers in Hawaii, proposed a systematic aquifer classification for the major islands of Hawaii (later revised 

for Oahu by Mink and Lau (1990). Their classification 

is useful for many purposes, but some elements of it do 

not correspond to identifiable geohydrologic features.

This study defines a regional aquifer system for 

Oahu based on stratigraphy. Gently dipping lava flows 

and sedimentary rocks combine to form the regional 

aquifer system. Within this system rift-zone intrusives 

and weathered valley-fill sediments cut across bedded 

lavas at high angles, impeding lateral ground-water 

flow and causing abrupt lateral discontinuities in 

potentiometric surfaces. These steeply dipping geohydrologic barriers (fig. 12C) delineate distinct groundwater areas underlain by the principal aquifers. The 

result is a regional aquifer system subdivided into distinct compartments or areas in plan view (fig. 12A and 

B). This classification provides a workable conceptual framework for most hydrologic applications.

PRINCIPAL VOLCANIC-ROCK AQUIFERS AND CONFINING 

UNITS

The Waianae aquifer in the Waianae Volcanics and 

the Koolau aquifer in the Koolau Basalt are the two 

principal volcanic-rock aquifers on Oahu (fig. 1). The 

sequences of coastal-plain and valley-fill deposits 

locally form aquifers, but these are of minor 

importance because of the small volumes of water 

involved. The two principal volcanic-rock aquifers are 

composed mainly of thick sequences of permeable, 

thin-bedded lavas. These aquifers combine to form a 

layered aquifer system throughout central Oahu, 

where the Koolau aquifer overlies the Waianae aquifer. 

They are separated by a regional confining unit, the 

Waianae confining bed, formed along the WaianaeKoolau unconformity. The unconformity marks the 

eroded and weathered surface of the Waianae Volcano 

that was buried by younger Koolau lavas. This surface 

dips generally east at about 5° to 15° Figure 12C shows 

the approximate sea-level structure contour of the confining bed.

Associated with the unconformity are Waianaederived paleosols, saprolite, weathered volcanic ash, 

and weathered alluvium. These low-permeability 

materials form a leaky confining unit of regional extent 

between the Waianae and Koolau aquifers, causing discontinuities in freshwater heads of several feet 

between the aquifers. In this report, this confining unit 

is referred to as the Waianae confining bed, after the 

aquifer it overlies (following the guidelines in Laney 

and Davidson, 1986).

No other major confining units at depth are known, 

and permeable lavas in each aquifer are thought to 

extend several thousand feet below sea level. Minor 

confining units are intercalated within the permeable 

lavas, but have no more than local hydraulic influence. 

In most coastal areas, sediments overlie and confine 

the Waianae and Koolau aquifers, impeding freshwater 

discharge and impounding basal water to great thicknesses. The sediments are referred to as caprock 

because they confine, or cap, the underlying volcanicrock aquifers. Coastal-plain sediments also fill the 

mouths of coastal valleys. The structure and composition of the coastal plain sediments near Pearl Harbor 

are shown in figure 10.

Beneath the caprock, the weathered surface of the 

volcanic formations also contributes to the confinement 

of water in the underlying volcanic-rock aquifers. 

Clay-rich saprolite was reported in many wells just 

above the depth where artesian heads exist in underlying unweathered or less weathered basalt. Some 

investigators (McCombs, 1927; Stearns and Vaksvik, 

1935) have suggested the saprolite plays a key role in 

confining the principal volcanic-rock aquifers, but 

others (Wentworth, 1951) have thought its role to be 

less certain.

SUBORDINATE AQUIFERS

Numerous subordinate and local aquifers on Oahu 

are composed of marine deposits, alluvial 

sediments, and rocks of the Honolulu Volcanics (fig. 1). 

Although these materials confine water in the 

volcanic-rock aquifers in a larger sense, they also 

comprise a system of aquifers and confining units in 

and of themselves.

GEOHYDROLOGIC BARRIERS AND GROUND-WATER AREAS

The areal hydraulic continuity of the aquifers of 

Oahu is interrupted in many places by steeply dipping 

geohydrologic barriers (fig. 12C) that cut across the layered lavas at high angles. Two types of barriers are 

recognized: (1) barriers of structural and lithologic origin that are mainly intrusive rocks, but also include 

other types of massive rock such as ponded lavas; and
REGIONAL AQUIFER SYSTEM B33

EXPLANATION

MAJOR GEOHYDROLOGIC BOUNDARY 

TOPOGRAPHIC DIVIDE

WAIALUA-WAHIAWA DISTRICT 

BOUNDARY

EXPLANATION

MAJOR GEOHYDROLOGIC BOUNDARY

 SUBORDINATE GEOHYDROLOGIC

BOUNDARY Queried where uncertain

B

Beretania

Kaimuki Waialae

/ Wailupe-Hawaii 

-~ KaT

Kaukonahua valley fill and 

Waianae confining bed i

 c-^r V^

f-^'b fi

Kalihi valley fill N 

Nuuanu valley fill r 

Manoa valley fill

EXPLANATION

MAJOR GEOHYDROLOGIC BOUNDARY

 SUBORDINATE GEOHYDROLOGIC

BOUNDARY Queried where uncertain

FIGURE 12. Ground-water areas and geohydrologic barriers, island of Oahu. A, Major ground-water areas. 

6, Subordinate ground-water areas within the major areas. C, Geohydrologic barriers.
B34 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

(2) barriers of surficial origin, mainly valley fill deposits, but also including weathered geologic material. 

These barriers impede and divert ground-water flow 

and, in some instances, such as in the Schofield Plateau and in the rift zones, impound ground water to 

much greater heights than would be reached in the 

absence of the barriers (figs. 12 and 13).

Structural barriers are deep-rooted features formed 

during the shield-building phases of the volcanoes. 

They include rift-zone intrusives, stray dikes that lie 

somewhat apart from well-defined rift zones, and two 

extensive impounding structures in the Schofield Plateau of central Oahu. Structural barriers commonly 

cause large and abrupt head discontinuities ranging 

from tens offset to hundreds of feet. Regional contrasts 

in the principal mode of freshwater occurrence commonly coincide with their locations (for example, the 

transition from basal water to dike-impounded water). 

The largest head discontinuities on Oahu are associated with dense concentrations of dikes in the rift 

zones of the Waianae and Koolau Volcanoes (fig. 13).

Surficial barriers result from processes of erosion, 

sedimentation, and weathering at the surfaces of the 

volcanic formations. The surficial barriers appear to be 

more hydraulically conductive than the deep-seated 

barriers and cause smaller head discontinuities, typically no more than several feet. Hydraulic stresses are 

transmitted either through, beneath, or around these 

barriers. The most common surficial barriers are sedimentary valley-fill deposits and underlying saprolite. 

The valley-fill barriers are superficial, or partially penetrating, barriers that impede freshwater flow in 

shallow parts of the volcanic-rock aquifers.

The Schofield Plateau in central Oahu is bounded on 

the north and south by geohydrologic barriers of an 

undetermined type (fig.!2C), referred to as groundwater dams by Dale and Takasaki (1976). They may be 

zones of intrusive rock, but they also appear to be associated with an erosional surface on Waianae Volcanics 

now buried by Koolau Basalt. These barriers may be a 

composite of dike-intruded rock, a buried erosional surface, and massive lavas. They are discussed in more 

detail later in this section of this report.

The island of Oahu has been divided into seven 

major ground-water areas (fig. 12A) that are delineated by deep-seated, structural, geohydrologic barriers. These areas are southern Oahu, southeastern 

Oahu, the Koolau rift zone and the Kahuku area of 

windward Oahu, north-central Oahu, the Waianae rift 

zone, and the Schofield area. Hydraulic continuity 

within these seven areas generally is high and the 

potentiometric surface is smoothly continuous except 

in rift zones where dikes cause numerous local discontinuities. Some of these areas, notably southern Oahu

and north-central Oahu, are further subdivided by subordinate surficial geohydrologic barriers. Delineation 

of the ground-water areas follows the convention of 

earlier investigators with modifications, and provides a 

useful context for discussions of regional flow and comparisons to earlier studies. The locality names assigned 

to the ground-water areas are ones that have been cited 

most frequently in previous reports.

The Koolau rift zone along the eastern or windward 

side of the island and the Waianae rift zone to the west 

constitute two of the major ground-water areas. Both of 

these areas contain dike-impounded ground water. The 

boundaries of the rift zones are slightly modified from 

those of Dale and Takasaki (1976) and Takasaki and 

Mink (1985). The absence of outcrop exposures or 

water-level measurements in the vicinity of these 

boundaries precludes an accurate delineation of the rift 

zones. The boundaries described for the two rift zones 

in this report are intended to include all areas likely to 

contain significant numbers of dikes.

The north-central Oahu ground-water area 

(fig. 12A), which contains basal lenses of freshwater 

with some confinement along the coast, is bounded on 

the east by the Koolau rift zone, on the south by the 

north Schofield ground-water barrier, on the west by 

the Waianae rift zone, and on the north by the sea. It is 

divided into three subordinate ground-water areas 

the Mokuleia, Waialua, and Kawailoa areas each of 

which contains a basal freshwater lens. The Waianae 

confining unit was considered by Dale (1978) to be the 

principal geohydrologic barrier separating the 

Mokuleia and Waialua areas, but valley-fill sediments 

in Kaukonahua Valley (fig. 12C) also may inhibit the 

lateral flow of ground water between the two areas. 

Valley-fill sediments in Anahulu Gulch (fig. 12C) form 

a barrier between the Waialua area and the Kawailoa 

area (Dale, 1978). The effect of this valley-fill barrier 

can be expected to diminish inland with increasing altitude and decreasing aquifer penetration by the valley. 

At some distance inland from the coast, the potentiometric surfaces and freshwater lenses of the Waialua 

and Kawailoa areas probably are continuous.

The Schofield ground-water area (fig. 12A) encompasses much of the Schofield Plateau of central Oahu. 

It is bounded by the Koolau and Waianae rift zones on 

the east and west, and by the north and south Schofield 

ground-water barriers. Within the Schofield area, 

ground water is impounded to altitudes of about 280 ft 

above sea level (fig. 13). Ground water in this area is 

considered to be high level, but it does not appear to be 

divided into numerous small compartments like the 

dike impounded water in the rift zones.

Schofield high-level water was discovered in 

1936 during construction of Schofield Shaft, a water-
REGIONAL AQUIFER SYSTEM B35

158°

Kahuku Point

4 MILES

21°30'

21°20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

EXPLANATION

Ewa SUBORDINATE GROUND-WATER 

AREA Annotated with mixed case

 MAJOR GEOHYDROLOGIC BOUNDARY

- SUBORDINATE GEOHYDROLOGIC

BOUNDARY Queried where uncertain

 24 WATER-LEVEL CONTOUR Shows altitude 

of water level. Contour interval in feet, is 

variable. Datum is mean sea level

(165) POINT OBSERVATION OF ALTITUDE OF 

WATER LEVEL, IN FEET ABOVE MEAN 

SEA LEVEL

(24) REPRESENTATIVE WATER LEVEL WHERE 

WATER TABLE IS TOO FLAT TO BE 

CONTOURED, IN FEET ABOVE MEAN 

SEA LEVEL

FIGURE 13. Ground-water areas and potentiometric surface in the principal volcanic aquifers of the island of Oahu 

(compiled from previous studies by the U.S. Geological Survey).
B36 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

development tunnel intended to tap thick basal water 

thought to lie beneath the site. The areal extent of the 

high-level water was inferred (Swartz, 1939, 1940) by 

mapping electrically conductive saltwater beneath 

freshwater and delineating the apparent absence of 

saltwater within the depth range of the soundings, over 

much of the Schofield Plateau. Swartz suggested two 

lines, or boundaries, that defined the northern and 

southern limits of high-level water. Later, wells confirmed the presence of high-level water throughout the 

Schofield area with a potentiometric level of about 280 

ft above mean sea level and interannual fluctuations of 

about 15 ft.

Wells drilled in the area of the barriers have groundwater levels that are transitional between the typical 

Schofield level of about 280 ft above mean sea level and 

the basal-water levels of about 20 ft above sea level to 

the north and about 30 ft above sea level to the south. 

Several wells in the western Schofield area appear to 

have been drilled into the ground-water barriers. In 

the northern barrier, water levels in two wells have 

been measured at 130 and 165 ft above sea level (fig. 

13), values that are transitional between Schofield 

high-level water and basal water to the north. The 

wells in the transitional area also have low unit specific 

capacities of 0.26 and 0.61 (gal/min)/ft, which are 

lower, by a factor of 2 to 10, than the specific capacities 

of the nearest wells that tap high-level water just south 

of the northern ground-water barrier. These specific 

capacities also are lower, by a factor of 10 to 20, than 

the minimum value cited as characteristic of dike free 

lavas by Takasaki and Mink (1982). Drillers' logs 

for these wells do not mention dikes or weathered 

material.

Transitional water levels also have been observed at 

the western end of the southern ground-water barrier 

(fig. 12C, fig. 13) in seven wells that span a distance of 

less than 0.4 mi normal to the trend of the barrier. The 

near-surface bedrock at the site has been mapped as 

Koolau Basalt, but a prominent ridge of dike-intruded 

Waianae Volcanics lies just to the west and plunges 

eastward in the subsurface. Water levels in the two 

northernmost wells are close to the Schofield high-level 

head of about 280 ft above sea level. However, water 

levels in the more southerly wells, about 1,000 ft away, 

are about 190 to 210 ft above sea level and appear to be 

stepped, suggesting separate compartments. Measurements of the water level in one well showed a temporal 

change of 60 ft (Visher, no date), greater than is typical 

for wells that tap the Koolau aquifer in the Schofield 

area. Drillers' logs and descriptions of core samples and 

cuttings from these wells indicate weathered rock at 

various depths in several of the wells. On the basis of 

rock-sample examinations and by projecting the slope

of the nearby Waianae ridge in the subsurface, J.F. 

Mink (U.S. Geological Survey, written commun., 1959) 

concluded that several of the wells with transitional 

water levels had likely penetrated Waianae Volcanics 

at depth. Thus, weathered and dike-intruded Waianae 

Volcanics may be associated with at least the western 

part of the southern ground-water barrier.

These observations prompted Dale and Takasaki 

(1976) to revise the boundaries of the area, adding further detail in delineating the two barriers in the areas 

of transitional water levels. Dale and Takasaki (1976) 

also suggested that the low-permeability rocks of the 

barriers consisted of either volcanic dikes or weathered 

bedrock, probably the latter.

Additional indirect information has been provided 

by surface geophysical surveys, primarily electrical 

resistivity soundings and profiles (Zohdy and Jackson, 

1969; Kauahikaua and Shettigara, 1984; Shettigara, 

1985; Shettigara and Peterson, 1985; Shettigara and 

Adams, 1989). The geophysical interpretations differ 

somewhat from the representation of Dale and 

Takasaki (1976), but probably do not warrant substantial revision.

The southern Oahu ground-water area is bounded 

on the east by the Koolau and Kaau rift zones, on the 

north by the south Schofield ground-water barrier, on 

the west by the Waianae rift zone, and on the south by 

the sea. It has been subdivided into six subordinate 

ground-water areas (fig. 12B), mostly by valley-fill type 

barriers. Each of the areas contains a basal freshwater 

lens. The Ewa area on the west is underlain by the 

Waianae Volcanics and originally was identified as a 

separate major ground-water area based on differences 

in ground-water levels with the adjacent Pearl Harbor 

area. Stearns and Vaksvik (1935) recognized that this 

head difference was caused by the Waianae confining 

unit along the Waianae-Koolau unconformity. Without 

further explanation, Visher and Mink (1964) placed the 

boundary between the Ewa area (area 11 of Visher and 

Mink, 1964) and the Pearl Harbor area (area 6 of 

Visher and Mink, 1964) at the approximate location of 

the sea-level structure contour of the Waianae confining unit. The location of the boundary in the present 

study has been revised slightly following Eyre (PR. 

Eyre, U.S. Geological Survey, written commun., 1991) 

to better reflect the presumed constructional-dome 

shape of the now-buried Waianae Volcano.

The Pearl Harbor, Moanalua, Kalihi, Beretania, and 

Kaimuki ground-water areas of southern Oahu 

(fig. 12B) are separated by valley-fill geohydrologic barriers (fig. 12C). As in north-central Oahu, the effectiveness of the barriers may diminish inland from the coast 

with increasing altitude and decreasing penetration of 

the valleys into the underlying basalt. The boundary
REGIONAL AQUIFER SYSTEM B37

between the Pearl Harbor and Moanalua areas has 

been revised from South Halawa Valley to North Halawa Valley (fig. 5) following Eyre (P.R. Eyre, U.S. Geological Survey, written commun., 1991). Eyre reasoned 

that North Halawa Valley is deeper than South Halawa 

Valley and, therefore, is a more appropriate boundary 

between the two areas. This interpretation is consistent with Wentworth (1951). The curved trend of this 

barrier reflects an assumed paleodrainage pattern in 

southern Oahu before island subsidence and sediment 

infilling of the valleys. The Kaimuki area is bounded on 

the east by the Kaau rift zone and, perhaps, by valley 

fill in Palolo Valley (fig. 5).

The southeastern Oahu ground-water area is 

divided into two subordinate ground-water areas, both 

of which contain a basal freshwater lens (fig. 12B). The 

major area is bounded on the north by the Koolau rift 

zone and on the west by the Kaau rift zone (fig. 12C). 

The subordinate Waialae and Wailupe-Hawaii Kai 

ground-water areas are separated by an ill-defined 

zone of northeast-trending dikes (Takasaki and Mink, 

1982). Numerous dikes within the Kaau rift zone 

create an effective hydraulic separation between the 

Waialae area and the Kaimuki area of southern Oahu 

to the west (Takasaki and Mink, 1982), although the 

separation of these two areas previously was 

considered to be an intervening valley fill (Wentworth, 

1951).

Along the northeast coast of windward Oahu is the 

Kahuku ground-water area. Ground water occurs here 

as a basal freshwater lens. Valleys in the Kahuku area 

are not as deep as those of the Honolulu area and are 

more comparable to the shallower gulches of the Pearl 

Harbor area and, perhaps, to Anahulu Gulch. The 

depth and width of valleys increases progressively from 

narrow V-shaped gulches to broader U-shaped valleys 

toward the south. The deeper, southernmost valleys 

likely compartmentalize the Koolau aquifer considerably. However, there are too few wells to allow separate 

ground-water areas to be distinguished as they have 

elsewhere on Oahu. The shallower and more northern 

valleys (fig. 14) cause only slight reentrants in mapped 

water-level contours. This suggests that the northern 

valley fills penetrate the freshwater zone of the Koolau 

aquifer only partially, causing only slight interruption 

of freshwater continuity and flow. The more pronounced reentrants in the lines of chloride concentration (fig. 14) probably are more a result of heavy 

pumping at particular wells than they are a function of 

the depth of valley incision.

There is generally a high degree of hydraulic continuity and a smoothly continuous potentiometric 

surface in the volcanic-rock aquifers in each groundwater area. In the rift zones, however, dikes divide the

volcanic-rock aquifers into numerous compartments 

and cause stepped discontinuities in the potentiometric 

surface. The degree of hydraulic continuity among contiguous areas depends on the nature of the geohydrologic barrier that separates them. The structural 

barriers separating major ground-water areas are 

highly effective impediments to flow; steep hydraulic 

gradients are required to transmit water through the 

barriers, and hydraulic stresses in a given groundwater area are not transmitted readily to adjacent 

areas. For example, in the Koolau rift zone, water is 

impounded by dikes, and ground water levels are as 

much as 1,000 ft higher than in adjacent central Oahu 

and Kahuku areas. Movement of water from the rift 

zones to adjacent ground-water areas occurs as flow 

through fractures in the dikes and overflow at the top 

of dike compartments.

A greater degree of hydraulic connection exists 

among the subordinate ground-water areas delineated 

on the basis of surficial barriers than exists among 

those areas delineated by dikes. Valley-fill deposits and 

saprolitic zones have low permeability compared to the 

volcanic rocks. Where the valleys are sufficiently shallow, considerable amounts of freshwater may flow 

beneath or inland of the valley-fill barriers from one 

area to another. The Waianae confining bed also 

appears to be a comparatively leaky barrier, based on 

freshwater head differences of only several feet 

between the Waianae and Koolau aquifers. If the small 

head difference is a true indication of hydraulic conductivity, then conditions in either aquifer are highly 

dependent on conditions in the other, and their 

hydraulic behavior is best analyzed jointly.

TRANSIENT HYDRAULIC BEHAVIOR OF THE AQUIFER 

SYSTEM

Hydraulic responses within the Waianae and Koolau 

aquifers depend on the nature and magnitude of 

hydraulic stresses and on characteristics of the aquifers, including aquifer thickness and areal extent, 

aquifer hydraulic conductivity and specific yield, and 

the geometries and hydraulic conductivities of confining units and lateral barriers. In most parts of the aquifer system of Oahu, the combination of large hydraulic 

stresses, high permeability, and limited aquifer 

extent result in bounded-aquifer behavior rather than 

infinite-aquifer behavior. High rates of freshwater 

withdrawal result in declines in freshwater head that 

extend rapidly to lateral aquifer boundaries. Declines 

in freshwater head are accompanied by lens thinning, 

reduced freshwater discharge from head-dependent 

discharge boundaries along the coast, and possibly 

additional induced inflow across barriers from upgradient areas of higher head. Changes in freshwater head
B38

158° 

21°45'

REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

158° 157°55'

T

21°40'

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

2 MILES

J

2 KILOMETERS

Pacific 

Ocean

 12 

--50--

EXPLANATION 

COSTAL-PLAIN DEPOSITS (CAPROCK)

KOOLAU BASALT 

GEOLOGIC CONTACT

WATER-LEVEL CONTOUR Shows altitude 

of water level in 1963. Contour interval, 1 foot. 

Datum is mean sea level

LINE OF EQUAL CHLORIDE CONCENTRATION Interval, in milligrams per liter is 

variable

HEAVILY PUMPED WELL 

LIGHTLY PUMPED WELL

FIGURE 14. Effects of valley-fill barriers on water level and chloride concentration in the Koolau aquifer, 

Kahuku area, island of Oahu (modified from Takasaki and Valenciano, 1969, fig. 27).
REGIONAL AQUIFER SYSTEM B39

and lens volume establish saltwater head gradients 

that induce inflow or outflow of saltwater. Saltwater 

flow impeded by low-permeability materials at aquifer 

boundaries contributes to bounded-aquifer response.

Mink (1980) provided an example of boundedaquifer response in the Koolau aquifer by analyzing 

transient drawdowns that accompany seasonal pumping in the Pearl Harbor area of southern Oahu. Mink 

used image-well superposition to compute departures 

from infinite-aquifer transient response. Using 

assumed boundaries on four sides of the aquifer, he 

found that, after a pumping season of about 200 days, 

drawdowns attributable to boundaries were roughly 

equal to drawdowns caused by the pumping wells 

themselves. Bounded responses may be even more pronounced in smaller ground-water areas such as the 

Kalihi, Beretania, and Kaimuki areas of southern 

Oahu (fig. 12B) because of their intervening valley-fill 

barriers. Fluctuations in freshwater head also influence flow at head-dependent boundaries, such as 

spring discharge and seepage through coastal confining 

units.

Transient water-level fluctuations are strikingly 

similar in both character and magnitude in some of 

Oahu's ground-water areas (fig. 15), but are dissimilar 

in others. Similarity in fluctuations may indicate good 

hydraulic connection between contiguous areas or 

could arise by coincidence if the areas have similar 

recharge or pumping stresses. On the other hand, dissimilarities in water-level fluctuations likely indicate 

that an effective hydraulic barrier separates the two 

areas and that hydraulic stresses or boundary conditions are dissimilar. Long-term water-level variations 

are similar in each of the ground-water areas (fig. 15), 

although the amplitude of the variations is subdued in 

the Waialua area of north-central Oahu, the Ewa area 

of southern Oahu, and the Waialae area of southeastern Oahu, and is more pronounced in the Schofield area 

and the Beretania and Pearl Harbor areas of southern 

Oahu. The subdued seasonal fluctuations of water levels in the Schofield area are presumed to indicate the 

seasonal occurrence of recharge because these water 

levels are influenced mainly by natural recharge and 

discharge and are influenced little by pumping. Waterlevel changes observed in the Schofield area are 

evident in the other areas, although the amplitude is 

smaller elsewhere.

Seasonal water-level fluctuations also occur in most 

of the areas. Seasonal fluctuations in the 

Beretania, Pearl Harbor, and Ewa areas of southern 

Oahu are large in amplitude and about 6 months out of 

phase from water-level fluctuations in the Schofield 

area; the variations in these areas primarily reflect the 

magnitude and timing of large, seasonal, domestic and

agricultural pumping (Stearns and Vaksvik, 1935). 

Pumping-induced drawdown in these areas recovers 

rapidly when pumping is curtailed. In heavily pumped 

areas, the immediate recovery response precedes and 

obscures the water-level rise caused by seasonal 

recharge, which, in turn, probably lags precipitation by 

several months. Rapid drawdown and recovery 

responses to pumping have been reproduced in numerical simulations of the Pearl Harbor area by Souza and 

Voss (1987).

In a given ground-water area, the magnitude of seasonal and annual water-level fluctuations reflects the 

magnitude of transient variations in recharge and discharge from average values and, perhaps, the degree of 

hydraulic isolation by lateral barriers and coastal confining units. For example, water-level fluctuations in 

the Ewa area of southern Oahu and the Waialae area of 

southeastern Oahu are much smaller than those in the 

adjacent Pearl Harbor and Beretania areas of southern 

Oahu. This demonstrates the hydraulic effectiveness of 

the intervening geohydrologic barriers that separate 

these areas. The Waianae confining bed apparently is 

sufficiently low in hydraulic conductivity that 

pronounced water-level fluctuations in the Koolau 

aquifer in the Pearl Harbor area are not fully transmitted to the underlying Waianae aquifer in the Ewa area. 

The smaller seasonal fluctuations in the Waianae aquifer could represent a damped response to the Koolau 

fluctuations, a response to local pumping in the Ewa 

area, or more probably a combination of both. The 

recharge area of the Waianae aquifer in the Ewa area 

also is much drier than the recharge area of the Koolau 

aquifer, likely limiting the magnitude of rechargeinduced seasonal and interannual fluctuations. The 

lack of similarity of fluctuations of water levels in the 

Waialae area of southeastern Oahu to those in southern Oahu demonstrates the hydraulic effectiveness of 

the Kaau rift-zone barrier.

The water-level record (fig. 15) of the Waialua area 

in north-central Oahu (fig. 125) also shows a much 

smaller range of seasonal and interannual variation 

than the Schofield area or the Pearl Harbor and 

Beretania areas of southern Oahu (fig. 12B). The 

smaller seasonal variation may reflect a smaller ratio 

of pumpage to ground-water recharge, although agricultural pumpage in the Waialua area is large and as 

strongly seasonal as in the Pearl Harbor area. The 

smaller variation may reflect less variability in 

recharge and, perhaps, less effective hydraulic boundaries in the Koolau aquifer in the Waialua area than in 

southern Oahu. Lesser confinement by coastal sediments in north-central Oahu may impart a leakier 

coastal boundary condition than occurs in southern 

Oahu, requiring smaller head differentials to effect
B40 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

14

10

285

I | | I I I | | I I I

 U WAIALUA

I i i I I I i i i

I I ' ' ' I I

i i i I I i i i I 

SCHOFIELD

280

275

270

269

34

LU

>

LU

LU 

CO

LU 

>

o

DO

LU 

LU 

LL

LU 

>

LU

DC 

LU

30

25

20

18

i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i

EASTERN

PEARL

HARBOR

FIGURE 15. Ground-water levels in central Oahu.
GROUND-WATER FLOW SYSTEMS B41

transient adjustments in recharge and discharge of 

ground water.

GROUND-WATER FLOW SYSTEMS

The aquifers of Oahu contain two fundamental 

ground-water systems: shallow freshwater and deep 

saltwater. Fresh ground water floats on underlying 

saltwater in a condition of buoyant displacement. The 

natural driving mechanisms for freshwater and saltwater flow differ. Most freshwater flow is driven by 

gravity from inland recharge areas at higher altitudes 

to discharge areas at lower altitudes near the coast. 

Remnant volcanic heat may drive geothermal convection of freshwater at great depths in the rift zones. 

Saltwater flow is driven by changes in freshwater volume and in sea level, and by dispersive and geothermal 

convection.

Four separate fresh ground-water flow systems are 

defined and described by this study: (1) the eastern 

Oahu flow system, (2) the central Oahu flow system, (3)

the western Oahu flow system, and (4) the southeastern Oahu flow system (fig. 16). The central Oahu 

ground-water flow system is further divided into 

northern and southern parts. The northern part of the 

central Oahu flow system encompasses the northcentral Oahu ground-water area and the northern part 

of the Schofield area. The southern part of the central 

Oahu flow system encompasses the southern part of 

the Schofield area and the southern Oahu groundwater area (fig. 12A and B, table 3). Meteoric freshwater flow diverges from ground-water divides that lie 

somewhere within the Waianae and Koolau rift zones, 

forming an interior flow system in central Oahu, exterior flow systems in western (Waianae area) and 

eastern (windward) Oahu, and a small flow system in 

southeastern Oahu. The exact location of the groundwater divides is not known but, following a common 

arbitrary convention, they are shown as coinciding 

with topographic divides for diagrammatic purposes. 

Despite this convention, ground-water divides need not 

coincide with the topographic divides, and their

TABLE 3. Ground-water areas and geohydrologic boundaries within the principal volcanic aquifers of the island of Oahu

dikes and valleys cause numerous interruptions). 

Principal aquifer: Aquifer (Waianae or Koolau) having greatest areal extent within depths of freshwater occurrence. Principal freshwater occurrence: Basal, 

dike impounded, or high level.

Geohydrologic boundary: Marks a major contrast in mode of freshwater occurrence, or a barrier that impedes flow and causes an abrupt potentiometric discontinuity.

Ground-water 

area

Principal 

aquifer

Principal freshwater occurrence Geohydrologic boundaries

Eastern Oahu ground-water flow system

Koolau rift zone

Kahuku area

Koolau

Koolau

Dike impounded Rift zone boundaries. The southern part or the area is 

deeply dissected, leaving isolated volcanic interfluves

Basal Valley fills probably impede flow

Western Oahu ground-water flow system

Waianae rift zone

Schofield area

Southern 

Ewa area 

Pearl Harbor area 

Moanalua area 

Kalihi area1 

Beretania area1 

Kaimuki area1

Waianae Dike impounded Kilt zone boundaries. The western part of the area is 

deeply dissected, leaving isolated volcanic interfluves

Central Oahu ground-water flow system

North-central 

Mokuleia area 

Waialua area 

Kawailoa area

Waianae 

Koolau 

Koolau

Basal 

Basal 

Basal

Kaukonahua valley fill and Waianae confining unit 

Anahulu valley fill 

Northern Schofield ground- water dam

Koolau

Waianae 

Koolau 

Koolau 

Koolau 

Koolau 

Koolau

High level Southern Schofield ground-water dam

Basal Waianae confining unit

Basal North Halawa valley fill

Basal Kalihi valley

Basal Nuuanu valley fill

Basal Manoa valley fill

Basal Kaau rift zone, Palolo valley fill (?)

Southeastern Oahu ground-water flow system

Waialae area 

Wailupe-Hawaii Kai area

Koolau Basal Northeast-trending dikes

Koolau Basal Valley fills likely subdivide the area further, though waterlevel measurements are too sparse to define subdivisions 

xThe Kalihi, Beretania, and Kaimuki areas also are known collectively as the Honolulu area.
B42 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

158°

Kahuku Point

024 MILES 

I , 

024 KILOMETERS

21°30'

WESTERN OAHU

Waianae> GROUND-WATER

21°20'

/ ^ - __ ~> Makapuu 

yOUTHEASTERNOAHU} Point

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59' 

Contours from U.S. Geological Survey 

digital elevation model data, 1:250,000,1986

EXPLANATION

CENTRAL OAHU GROUND-WATER 

FLOW SYSTEM

MAJOR GEOHYDROLOGIC BOUNDARY

SUBORDINATE GEOHYDROLOGIC 

BOUNDARY Queried where uncertain

DISTRICT BOUNDARY

ARROW INDICATES GENERALIZED 

DIRECTION OF GROUND-WATER FLOW

FIGURE 16. Ground-water flow systems and the major areas of the central flow system, island of Oahu.

The locations of ground-water divides are not actually known but are here as coinciding with topographic

divides and other surficial boundaries for diagrammatic purposes.
GROUND-WATER FLOW SYSTEMS B43

location may shift in response to transient hydraulic 

stresses.

Each flow system encompasses one or more groundwater areas within the principal volcanic-rock aquifers, 

as well as flow through sediments on the coastal plain. 

Water flows across geohydrologic barriers from areas of 

higher head to areas of lower head in the volcanic-rock 

aquifers (fig. 16) then into overlying sediments, where 

present, and ultimately to the sea. The magnitude of 

the cross-boundary flow varies around average values 

of recharge and discharge in response to associated 

changes in head gradients.

Ground-water recharge for all of Oahu has been estimated by Shade and Nichols (1996). For the flow systems shown in figure 16, predevelopment ground-water 

recharge, in million gallons per day, is estimated as follows:

Flow system

Recharge,

(Mgal/d)

Western Oahu flow system,

Central Oahu flow system, which includes the

northern Oahu flow system, and

the southern Oahu flow system, 

Eastern Oahu flow system, 

Southeastern Oahu flow system,

Total predevelopment ground-water recharge

32

184

359

198

19

792

These estimates represent all ground-water 

recharge, including 22 Mgal/d to caprock areas, which 

does not enter the underlying volcanic-rock aquifers. 

The significance of the central Oahu flow system is 

clearly indicated by its estimated ground-water 

recharge of more than 540 Mgal/d, about 68 percent of 

the total recharge to Oahu.

RIFT ZONES AND THE EASTERN AND WESTERN GROUNDWATER FLOW SYSTEMS

Dike-impounded ground water in the high-rainfall 

areas of the mountains in the Koolau and Waianae rift 

zones is the origin of the regional freshwater flow 

systems of Oahu. The topographic divide in the Koolau 

rift zone is shown as the boundary between the eastern 

Oahu flow system, which includes all of windward 

Oahu, and the flow systems of central and southeastern Oahu. Similarly, the topographic divide along the 

crest of the Waianae Range in the Waianae rift zone is 

shown as the boundary between the western Oahu 

ground-water flow system, which encompasses the 

Waianae area, and the central Oahu flow system. As 

stated before, the actual locations of the ground-water 

divides are not truly known.

The orographic effect of the mountainous terrain 

causes high rates of precipitation, and hence recharge, 

in the rift zones. Ground water is impounded by dikes

to altitudes as high as 1,600 ft above sea level in the 

Waianae rift zone and to nearly 1,000 ft in the Koolau 

rift zone. Both rift zones are deeply dissected by 

erosion, and much of their area is occupied by broad, 

U-shaped, amphitheater-headed valleys filled with 

alluvium and marine sediments. Ground-water flows 

from the high, dike-impounded reservoirs of the 

rift zones to downgradient parts of the volcanic-rock 

aquifers (fig. 13).

Ground water in the eastern Oahu flow system, on 

the windward side of the Koolau Range, moves downgradient in the rift zone from near the crest of the 

range to the sea. The land surface of the entire breadth 

of the northern part of the rift zone is above the potentiometric surface resulting in a recharge area. In the 

central and southern parts of the flow system where 

the rift zone is more dissected by erosion (fig. 3), the 

potentiometric surface is at or near land surface 

(except near the crest of the range), causing groundwater discharge, high evapotranspiration, and rejection of recharge. Dike-impounded ground-water levels 

in the mountains range from 600 to 1,000 ft above sea 

level (fig. 13; Takasaki and Mink, 1985). As valleys 

erode deeper into the rift zone, successive dikes are 

breached and ground water flows parallel to the dikes, 

toward the breaching valley. A valley that penetrates 

deeply into the mountains cuts more dikes and diverts 

more ground water from behind the dikes not yet cut by 

less deeply penetrating valleys (Takasaki and Mink, 

1985).

In the northern part of the eastern Oahu flow system, ground water from the rift zone flows into the 

freshwater lens of the Kahuku area. A narrow band of 

caprock along the northeast Oahu coast confines the 

lens, and freshwater heads rise inland to about 22 ft 

above sea level in the southern part of the Kahuku area 

and about 10 ft above sea level in the northern part of 

the area (fig. 13). If flow is normal to the head contours, 

water flows generally toward the coast at a somewhat 

oblique angle, having an alongshore directional component. This suggests a component of alongshore groundwater flow to the north, where coastal sediments may 

be particularly thin or permeable. However, there probably is considerable discharge of freshwater all along 

this coast and not just at the northern end of the area.

Shade and Nichols (1996) estimated the mean 

annual recharge rate to the eastern Oahu groundwater flow system of windward Oahu to be about 

198 Mgal/d. Of this, nearly 4 Mgal/d is recharge 

directly to the Kahuku area; the remaining 98 Mgal/d 

(P.J. Shade, U.S. Geological Survey, written commun., 

1991) is to the eastern part of the rift zone inland from 

the Kahuku area (from the eastern rift-zone boundary 

to the topographic divide). Some of the recharge flows
B44 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

parallel to the rift zone and discharges to the sea on the 

north, some flows south toward deeper valleys that 

breached the impounding dikes, but much of it is presumed to flow downgradient to the freshwater lens of 

the Kahuku area.

The western Oahu ground-water flow system is in 

the Waianae rift zone, west of the crest of the Waianae 

Range, and is coincident with the Waianae area of previous investigations (Zones, 1963; Takasaki, 1971). 

Ground water in this system flows generally west from 

near the crest of the Waianae Range to the sea. The 

west slope of the Waianae Range is deeply dissected; 

within the shallow depths penetrated by wells, the 

Waianae aquifer is present only as narrow interfluvial 

ridges separated by valley-fill sediments in intervening 

valleys, which may be incised to at least 1,200 ft below 

sea level. The combination of dikes in the rift zone and 

thick valley fills results in a flow system in which the 

volcanic-rock aquifer has little regional hydraulic continuity. Ground-water recharge to the western Oahu 

flow system (Waianae area) is estimated to be about 

32 Mgal/d of which about 6 Mgal/d is transpired by 

phreatophytes in lowland areas and about 26 Mgal/d 

discharges to the sea (Shade and Nichols, 1996).

CENTRAL FLOW SYSTEM

Because the determination of ground-water flow and 

availability in central Oahu is of considerable hydrologic and economic significance, the present understanding of the movement of ground water through this 

area is discussed in some detail. The northern and 

southern parts of the central Oahu ground-water flow 

system (fig. 16) contain large quantities of basal freshwater in the Waianae and Koolau aquifers. Early 

ground-water development in both areas was primarily 

from free-flowing artesian wells open to confined parts 

of the aquifers near the coast, and from springs near 

the surficial basalt-caprock contact. Pumpage historically has been large in southern Oahu (fig. 12A) and in 

the Waialua area of north-central Oahu (fig. 12S), with 

agricultural pumpage constituting a large part of total 

pumpage and imparting strong seasonality to the 

draft. Pumpage in the Mokuleia and Kawailoa areas of 

north-central Oahu (fig. 12.B) has been small in comparison to that in southern Oahu and Waialua. Southern Oahu is the most heavily developed area of Oahu 

and serves as the island's principal source of freshwater. It is the most intensely studied area of the Hawaiian islands, with the Honolulu and Pearl Harbor areas 

(fig. 12.B) receiving particularly detailed attention 

since the early 1900's. Much of the present knowledge 

of ground-water hydrology in Hawaii has come from 

the study of southern Oahu.

The central Oahu flow system encompasses the 

greatest extent of the island. Flow originates in the 

high-rainfall areas of the Waianae and Koolau rift 

zones and continues downgradient to the ground-water 

areas of the central Oahu plateau, diverging to the 

north and south somewhere within the Schofield area. 

Predevelopment ground-water recharge to the central 

Oahu flow system was estimated to be about 

543 Mgal/d; recharge to noncaprock areas of the flow 

system was estimated to be about 521 Mgal/d, about 

68 percent of the total predevelopment recharge to noncaprock areas of the island of Oahu (Shade and Nichols, 

1996). Of this amount, about 199 Mgal/d, or 38 percent, 

was recharge as infiltration of precipitation and 

ground-water underflow from the rift zones to the 

Schofield area (fig. 12A).

Much of the inland part of the central Oahu groundwater flow system is dominated by the high-level 

ground water of the Schofield area (figs. 12A, 16). The 

Schofield area is bounded on the east and west by the 

Koolau and Waianae rift zones, where dike-impounded 

water stands several hundred feet higher than 

Schofield high-level water. Ground-water flow is presumably as underflow from the rift zones into the 

Schofield area and then out of the Schofield area to the 

north and south, with flow diverging from a poorly 

defined ground-water divide somewhere within the 

Schofield area (although the location of the divide is not 

known, it is shown for diagrammatic purposes in figure 

16 as coinciding with the Waialua-Wahiawa district 

boundary which is near the northern boundary of the 

Schofield plateau). Underflow out of the Schofield area 

is controlled by the north and south Schofield groundwater barriers.

Precipitation and recharge are high in the Schofield 

area, and even higher in the upgradient Koolau rift 

zone. Dale and Takasaki (1976) estimated a total inflow 

of 126 Mgal/d to the Schofield area, of which 74 Mgal/d 

was direct infiltration from precipitation, and 

52 Mgal/d was ground-water underflow from the adjacent Koolau rift zone. They assumed that the part of 

the rift zone west of the Koolau topographic divide was 

tributary to the Schofield area. These estimates are 

based on a water budget approach where recharge is 

the residual of precipitation minus runoff minus evapotranspiration. Runoff was estimated as a linear function of precipitation based on a study by Hirashima 

(1971), and evapotranspiration was estimated as a 

function of precipitation using an equation developed 

by Takasaki and others (1969) for windward Oahu. 

Applying the same tributary assumption to the areal 

distribution of recharge computed by Shade and 

Nichols (1996), total recharge to the Schofield area is 

estimated to be about 199 Mgal/d, considerably more
GROUND-WATER FLOW SYSTEMS B45

than previous estimates. About 79 Mgal/d of the 

recharge is from direct infiltration of precipitation and 

about 120 Mgal/d is underflow from the Koolau and 

Waianae rift zones.

Estimates of north-south ground-water underflow 

from Schofield have varied. Dale and Takasaki (1976), 

by comparing estimated discharge from the downgradient areas to their estimate of water surplus for 

Schofield, inferred that 115 Mgal/d of Schofield highlevel water flows south to the Pearl Harbor area and 

18 Mgal/d flows north to the Waialua area. The underflow out of the Schofield area estimated by Dale and 

Takasaki (1976) exceeded their estimate of recharge by 

7 Mgal/d. This difference was attributed to data errors 

or faulty assumptions (Dale and Takasaki, 1976). 

Using a lumped-parameter model to estimate groundwater levels of southern Oahu, Mink and others (1988) 

estimated underflow of 76 Mgal/d from the Schofield 

area to the southern Oahu area. A numerical simulation of ground-water flow in the southern Oahu area 

(Eyre and Nichols, 1996), which included the southern 

part of the Schofield area, estimated underflow from 

the Schofield area to be about 88 Mgal/d for predevelopment conditions and about 92 Mgal/d for 1950's landuse conditions. The areally distributed recharge estimate of Shade and Nichols (1996) suggests about 103 

Mgal/d of underflow from the Schofield area to southern Oahu for predevelopment conditions. Their estimates also suggest about 96 Mgal/d as underflow 

northward from the Schofield area to the north-central 

Oahu area. This is a significantly higher rate than estimated by previous studies.

Southern and north-central Oahu also are recharged 

by direct infiltration of precipitation, as well as the 

underflow from the adjacent rift zones. For the noncaprock areas of the northern and southern Oahu flow 

systems, the following recharge rates were estimated 

for predevelopment conditions (Shade and Nichols, 

1996):

Recharge

Recharge Component Total

(Mgal/d) (Mgal/d)

Northern Oahu Flow System 

Direct infiltration to

North-central Oahu 

Northern part of Schofield area 

North Schofield barrier

Underflow from

Koolau Rift Zone 

Waianae Rift Zone

39

16

108

9

63

117

Recharge

Recharge Component Total 

(Mgal/d) (Mgal/d)

Southern Oahu Flow System

Direct infiltration to

Pearl Harbor and Ewa areas 142

Moanalua, Kalihi, Beretania, and 71 

Kaimuki areas ' * 

Southern part of Schofield area 45 

South Schofield barrier 10

Underflow from

Koolau Rift Zone 62 

Waianae Rift Zone H

Total to noncaprock area of central Oahu 

ground-water flow system

268

73

521

Extensive caprock along the coast of both southern 

and north-central Oahu strongly affects ground-water 

flow. By confining water in the Waianae and Koolau 

aquifers, the caprock impounds basal freshwater to 

thicknesses of 1,000 ft or more in southern Oahu and 

the Mokuleia area of north-central Oahu. As a result, 

the ground-water resource potential is greater in comparison to areas with thinner caprock or no caprock 

and, thus, thinner freshwater lenses. Throughout 

southern Oahu, the coastal confining unit reaches 

thicknesses of 1,000 ft near the coast and just offshore 

(Gregory, 1980). In comparison, the caprock in northcentral Oahu is thinner, less extensive, and probably 

contains a greater proportion of permeable sediments. 

It reaches thicknesses of 500 ft or more in the Mokuleia 

area, 200 ft or more in the Waialua area, and is thin or 

absent throughout most of the Kawailoa area (Dale, 

1978). Natural ground-water discharge from the central Oahu system is by springflow and diffuse seepage 

at the coast where caprock is absent, by diffuse seepage 

through the caprock, and by springflow near the inland 

margin of the caprock where it is thinnest. Much of the 

spring discharge at the caprock margin may be, in 

effect, overflow at the top of the caprock. The Pearl 

Harbor springs (fig. 17) appear to be largely of this 

nature (Visher and Mink, 1964).

Discharge from the central Oahu flow system is difficult to quantify because much of it is by diffuse seepage and minor springflow, both subaerial and 

submarine. In southern Oahu, much of the freshwater 

flow in the Koolau aquifer converges to large subaerial 

springs around Pearl Harbor, which can be measured. 

Measurements of flow at these springs have enabled a 

relation to be established between springflow and
B46 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

freshwater head in nearby wells (Stearns and Vaksvik, 

1935; Visher and Mink, 1964). Soroos and Ewart (1979) 

developed a head-discharge relation for total discharge 

from the Pearl Harbor springs and estimated historical 

springflow as a function of historical head, for which 

more continuous measurements are available. A 

revised version of their graph for several prominent 

springs is presented (fig. 17); containing about twice 

the number of measurements as their original. In addition to measurable springflow, however, there are also 

large amounts of diffuse, unmeasurable seepage 

through the southern Oahu caprock.

Discharge also is difficult to quantify in north-central Oahu. The coastal confining unit diverts groundwater flow nearly parallel to the shoreline, thus water 

discharges at the east and west ends of the confining 

unit where coastal sediments are thin or absent. Along 

the flowpaths that parallel the coast, freshwater probably leaks into the sediments and then to the sea. 

However, available water-level observations are not 

sufficient to quantify this leakage.

SOUTHEASTERN FLOW SYSTEM

The smallest regional freshwater flow system on 

Oahu lies at the southeastern end of the Koolau Range 

where basal water occurs in the Koolau aquifer 

(fig. 16). Inflow to the Waialae and Wailupe-Hawaii Kai 

areas of southeastern Oahu is by direct infiltration of 

precipitation and by underflow of dike-impounded 

water from the upgradient Koolau rift zone. Infiltration of precipitation to the Koolau aquifer was estimated to be about 19 Mgal/d (Shade and Nichols, 

1996), of which about 8 Mgal/d was in the Waialae area 

and about 11 Mgal/d was in the Wailupe-Hawaii Kai 

area.

Coastal sediments overlie and confine the Koolau 

aquifer along the coast and extend inland into the 

valleys. In the Waialae area, the combination of riftzone dikes and the thick caprock causes moderate 

impoundment of freshwater, with inland freshwater 

heads ranging from 8 to 15 ft above sea level (Eyre and 

others, 1986). Ground water discharges along the coast 

predominantly by subaerial and submarine springflow 

and diffuse seepage; it is unmeasurable for practical 

purposes, except at a few distinct subaerial springs.

The Koolau aquifer in southeastern Oahu is 

intruded sparsely by dikes. Few wells have been drilled 

in the area; therefore, water levels are poorly defined. 

Other than along the Kaau rift zone, only one discontinuity in the potentiometric surface resulting from dike 

impoundment has been recognized. However, scattered 

dikes appear to impede flow, based on the relatively low 

regional hydraulic conductivity of 400 ft/d estimated by 

Eyre and others (1986). This estimate was based on

data from an aquifer test of regional extent in the 

Waialae area by Wentworth (1938) and is lower than 

most estimates from tests of dike-free lavas.

DEEP SALTWATER FLOW SYSTEM

Little is known about the saltwater flow system that 

lies beneath the freshwater flow systems of Oahu. Several mechanisms of saltwater flow, however, can be postulated from basic principles or from examples known 

elsewhere as follows:

1. Saltwater inflow or outflow may be induced by 

volumetric changes in the floating freshwater 

lens that depressurize or pressurize underlying saltwater.

2. Saltwater inflow or outflow may be induced by 

sea-level variations.

3. Circulatory flow may occur when saltwater is 

entrained and discharged by freshwater flow, 

thereby inducing a compensating inflow of seawater (Cooper, 1959).

4. Circulatory flow may occur when thermal gradients induce density-driven, buoyant convection. Geothermal gradients may arise within 

the island mass from remnant geothermal 

heat associated with former magma reservoirs 

or from pervasive conduction from deep within 

the lithosphere. Thermal gradients also 

will be established by the temperature-depth 

profile in surrounding seawater and the 

contrast between seawater and terrestrial 

temperatures.

Each of these mechanisms involves exchange of water 

between the aquifer and sea.

Few measurements of saltwater head have been 

made on Oahu. Water-level records for well T133, 

which is open to saltwater at a depth of 1,100 ft, and 

well 187, which is open only to freshwater, are shown 

in figure 18. The casing of well T133 apparently has 

one or more breaks that have allowed brackish water 

from sediments higher in the stratigraphic section to 

enter the well and dilute the saltwater; this lowers the 

fluid density of the saltwater and raises the head 

correspondingly. Alternatively, the well bottom may be 

open to the lower part of the transition zone in the 

Koolau aquifer and thereby not tap 100-percent saltwater. The water-level record for the saltwater well has 

not been corrected for this dilution, which may vary 

over time; correction for dilution would shift the record 

downward slightly Regardless of these shortcomings, 

the well is open in or near the deep saltwater zone of 

the volcanic-rock aquifer, and it is presumed here that 

the water-level fluctuations closely reflect fluctuations 

in saltwater head.
GROUND-WATER FLOW SYSTEMS B47

21=24'

158° 157°56'

21°20'

MAMALA BAY

EXPLANATION

0

W187

v^O

WELL 187

SPRING

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

2 MILES

2 KILOMETERS

OAHU

Pacific 

Ocean

cc

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 \2 13 14 15 16 17 18 19 2

WATER LEVEL IN WELL 187, IN FEET ABOVE SEA LEVE

FIGURE 17. Relation between total measured discharge from the Pearl Harbor springs 

and freshwater head in the Koolau aquifer, island of Oahu.
B48 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

Water-level fluctuations in the freshwater well, 187, 

reflect transient pumping and recharge stresses and 

are similar to fluctuations observed elsewhere in 

southern Oahu. It can be seen in figure 18 that the 

water-level fluctuations in the saltwater well are virtually the same as those in the freshwater well, except for 

a slightly smaller range of variation. The similarity in 

water-level fluctuations indicates that the volumetric 

addition and withdrawal of freshwater causes fluctuations in freshwater and saltwater head of almost equal 

magnitude and with little discernible lag. Hydraulic 

stresses wholly within the freshwater zone appear to 

be transmitted to the underlying saltwater zone with 

little apparent attenuation, at least in the vicinity of 

these two wells. In the freshwater zone, freshwater 

head fluctuates about an average value that is positive 

with respect to sea level. In the saltwater zone, head 

should fluctuate about some average saltwater head 

that is not zero, but in theory must be slightly negative 

in order to maintain dispersion-driven saltwater 

circulation (Cooper, 1959).

The saltwater circulation is not steady, but is 

perturbed by transient fluctuations. Recharge deficit 

or discharge of freshwater by pumping will tend to 

lower freshwater heads from their average values; 

more strongly negative saltwater heads will induce 

salt-water inflow to the aquifer and allow upward and 

landward displacement of the freshwater-saltwater 

interface. Episodically, the addition of recharge will 

increase heads and expand the freshwater lens; positive saltwater heads will force saltwater to flow 

through the confining unit and out of the aquifer, allowing downward and seaward interface displacement. 

These effects are also mediated by adjustment of other 

system boundary conditions, for example the headdependent discharge of freshwater at springs and diffuse leakage through the confining unit. Visher and 

Mink (1964) discussed these aspects of transient 

hydraulics, though without consideration of the effects 

of the confining unit.

A temperature profile in an uncased test well (Visher 

and Mink, 1964; in the confined Koolau aquifer in 

southern Oahu, revealed an isothermal freshwater lens 

underlain by saltwater at 1,300 ft below sea level that 

is almost 27°F warmer than ocean water. They attributed the higher temperature to slight geothermal heating of the relatively immobile saltwater within the 

aquifer. They further speculated that the isothermal 

part of the profile was the result of a high rate of freshwater flow that prohibited a geothermal temperature 

gradient from becoming established in the freshwater 

lens. These concepts are supported by Souza and Voss 

(1987), who simulated the regional freshwater-saltwater flow system in the same area with a numerical 

model that incorporated density-dependent flow and 

solute transport.

The saltwater flow system responded distinctly to 

the early development of Oahu aquifers after about 

1880. This development caused a rapid lowering of 

freshwater heads and a corresponding shrinkage of 

basal freshwater bodies, particularly in southern 

Oahu. Records of salinity for these early wells (Stearns, 

1938) indicate a clear pattern of progressive saltwater 

encroachment, both lateral and vertical, in response to

25

LU 20

<s

15

:-s 10

LU O

> -I

LU LU

_l CD 5

DC DC 

LU

 * O

i i i i i i i i i i i i i i

NOTE: Well locations are shown on figure 10

FRESHWATER 

WELL 187

1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 

FIGURE 18. Water levels in well 187 open to freshwater and well T133 open to saltwater in the Pearl Harbor area, island of Oahu.
SUMMARY B49

hydraulic stress. Visher and Mink (1964) presented a 

map of chloride concentrations for the Pearl Harbor 

area that showed progressive inland movement of lines 

of 200- and 1,000-mg/L (milligrams per liter) chloride 

concentration between 1910 and 1950. The 1,000-mg/L 

line shifted about 1.5 mi inland over this time.

SUMMARY

The island of Oahu is formed by the eroded remnants of the Koolau and Waianae Volcanoes; two elongated shield volcanoes with broad, low profiles. 

Weathering and erosion have modified the original 

domed surfaces of the volcanoes leaving the Koolau 

Range in eastern Oahu and the Waianae Range in 

western Oahu. Much of the original mass of these volcanoes has been removed, leaving a landscape of deep 

valleys and steep interfluvial ridges in the interior 

highlands. A flat coastal plain underlain by sedimentary deposits surrounds much of Oahu. The coastal 

plain varies in width from a narrow marine terrace to 

a broad plain several miles wide. Where it is extensive, 

such as in southern Oahu, its surface is composed 

mainly of emerged Pleistocene reefs and associated 

sediments.

The shield-building rocks of the Waianae and Koolau 

Volcanoes are known respectively as the Waianae Volcanics and the Koolau Basalt. The Waianae Volcanics 

encompass both shield and postshield stages of activity 

and a chemically diverse lithology. The Koolau Basalt 

is wholly of basaltic composition. Waianae Volcanics 

and Koolau Basalt form the uplands and mountains of 

western and eastern Oahu, respectively, and extend to 

great depths beneath the island and offshore where 

they constitute the submarine mass of the island. Subsidence of the volcanoes was contemporaneous with 

shield development and continued long after eruptions 

ceased. Lavas that originally erupted subaerially and 

have textural characteristics of subaerial lavas have 

since been carried to depths of 6,000 ft or more below 

sea level by this subsidence. Lava flows of the Waianae 

Volcanics comprise the principal water-bearing rocks of 

western Oahu. Radiometric dating of basalt samples 

from subaerial sites yield ages of 2.5 to 3.9 Ma for 

Waianae Volcanics. Lava flows of Koolau Basalt comprise the principal water-bearing rocks of central and 

eastern Oahu. Radiometric dates of samples from subaerial exposures of the Koolau Basalt range from 1.8 to 

2.6 Ma. After a long period of subsidence and erosion, a 

rejuvenated stage of volcanic activity resumed at 

scattered vents at the southern ends of the Koolau and 

Waianae Ranges (recent results that have yet to be 

published, however, suggest that these Waianae vents

erupted late in the postshield stage and do not represent a rejuvenated stage of volcanism).

The rocks of Oahu vary widely in origin, chemical 

composition, texture, and in their ability to store and 

transmit water. Although most volcanic rocks of the 

islands of Hawaii, including those of Oahu, have 

similar basaltic chemical and mineralogical composition, their modes of emplacement resulted in a variety 

of physical properties that govern their hydraulic properties. These rocks can be divided into four groups: 

(1) lava flows, (2) dikes, (3) pyroclastic deposits, and 

(4) saprolite and weathered basalt. Each of these 

groups of rocks has markedly different physical and 

hydraulic properties. Lava flows are mainly of two textural types: (1) pahoehoe, which has a smoothly 

undulating surface and contains numerous elongate 

voids; and (2) aa, which has a surface of coarse rubble 

and an interior of massive rock. The diverse rock textures encompassed by the two types of lava impart a 

complex porosity distribution to the lavas, one that 

differs in character from most sedimentary, metamorphic, and crystalline igneous rocks. Stratified 

sequences of such lava flows form the most productive 

aquifers in Hawaii. Dikes are thin, near-vertical sheets 

of massive intrusive rock that typically contain only 

fracture permeability. Where dikes intrude lava flows, 

they impede ground-water flow principally in the direction normal to the plane of the dike. Dike complexes are 

areas where dikes are more numerous and intersect at 

various angles, forming small compartments and 

lowering overall rock porosity and permeability. Pyroclastic deposits are essentially granular, with porosity 

and permeability similar to that of granular sediments 

of similar grain size and degree of sorting. Saprolite is 

formed by intense weathering of basaltic rocks, which 

alters silicate minerals to clay and reduces the 

permeability of the parent rock.

Sedimentary rocks on Oahu are of both marine and 

terrestrial origin and have wide ranges in composition, 

grain size, and degree of induration. Marine sedimentary rocks are mostly calcareous and include coralalgal reefs, coralline rubble and sands, and lagoonal 

sands and marls. Terrigenous sedimentary deposits 

include deposits of talus, colluvium, and alluvium. 

The largest volume of sediments occurs in stratified 

sequences beneath the coastal plain. This wedge of 

calcareous and volcanogenic sediments is known 

locally as caprock because it overlies and confines 

ground water in the underlying volcanic-rock aquifers.

Estimates of aquifer properties on Oahu have been 

made by several methods, including laboratory measurements on rock samples, local aquifer tests in which 

drawdown is measured in a pumped well or in one or 

more observation wells at some distance from the
B50 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

pumped well, regional aquifer tests in which one or 

more wells are pumped, and regional numerical models 

of ground-water flow. The most commonly estimated 

properties have been hydraulic conductivity and transmissivity; fewer estimates are available for storage 

coefficient, specific yield, bulk porosity, or effective 

porosity. The thickness of the volcanic-rock aquifers of 

Oahu is not known, but probably is at least several 

thousand feet. Aquifer thickness has important 

implications for estimates of hydraulic conductivity 

and transmissivity derived from aquifer tests because 

transmissivity is a function of hydraulic conductivity 

and aquifer thickness. Numerous estimates of hydraulic conductivity and transmissivity have been made for 

the Waianae Volcanics and Koolau Basalt, but the lack 

of a definable aquifer thickness and the partial 

penetration of wells introduce ambiguity to these 

estimates. Most estimates of hydraulic conductivity for 

unweathered dike-free lava flows range from 500 to 

5,000 ft/d with an average of from 1,500 to 2,000 ft/d. 

Estimates of the ratio of horizontal to vertical hydraulic conductivity (anisotropy ratio) range from 5:1 to 

200:1. Estimates of the total porosity of various 

volcanic rocks on Oahu range from 5 to 51 percent, 

with a median value of about 43 percent. Values of 

effective porosity may be lower by as much as a factor 

of 10. Estimates of specific yield range from 1 to 20 percent; most of the values lie within a narrow range of 

about 5 to 10 percent. Estimates of confined storage 

coefficient range from about 3 x 10'4 to 1 x 10-2 suggesting semiconfined conditions for the larger values.

The occurrence of ground water in Oahu is 

determined by the character of the rocks and by the 

presence of geohydrologic barriers. The primary modes 

of freshwater occurrence are as a basal lens of fresh 

ground water floating on saltwater, as dike-impounded 

ground water, and as perched ground water. Saltwater 

occurs at depth throughout the island. The most extensive bodies of freshwater on Oahu occur as basal 

ground water. Basal ground water occurs in volcanicrock aquifers and aquifers in the sedimentary deposits 

on Oahu under confined and unconfined conditions. 

The thickness of the freshwater lens depends on 

recharge, aquifer permeability, and the presence or 

absence of confinement. The water table, or potentiometric surface, of a Hawaiian basal-water body is typically rather flat and commonly is only several feet to 

several tens of feet above sea level. However, the largest volume of freshwater lies below sea level; the thickness of freshwater below sea level is about 40 times the 

height of the freshwater head above sea level. A sharp 

interface does not exist between the fresh-water in the 

aquifer and the underlying salt-water; freshwater

grades into the saltwater across a transition zone of 

mixed salinity.

Basal water occurs in the thick volcanic-rock 

sequences of Oahu as free-floating lenses underlain 

everywhere by saltwater. At the base of the lens, the 

transition zone of mixed freshwater and saltwater may 

range in thickness from tens of feet to hundreds of feet. 

Basal water in the volcanic rocks is mostly unconfined, 

except near the coast where the aquifer may be overlain and confined by sedimentary caprock. In the 

absence of a confining unit near the coast, which 

impedes discharge of water to the sea, a comparatively 

thin basal-water body, no more than about 400 ft thick, 

exists.Where the volcanic-rock aquifer is overlain by 

caprock along the coast, ground water in the aquifer is 

confined and there is a thicker, impounded basal-water 

body. In the unconfined part of the aquifer, the water 

table rises inland at about 1 to 2 ft/mi. Maximum 

inland freshwater heads in impounded basal lenses on 

Oahu typically are about 10 to 40 ft above sea level, 

with lens thicknesses about 41 times the head above 

sea level. The highest basal-water head measured 

on Oahu was 43.5 ft above sea level in the Honolulu 

area; this implies that the freshwater lens was nearly 

1,800 ft thick.

Ground water occurs under two well-understood 

conditions at high altitudes above sea level, dikeimpounded and perched. Dike-impounded ground 

water occurs dominantly in the rift zones of the 

Waianae and Koolau Volcanoes. Perched ground water 

may occur in the unsaturated zone. A large body of 

high-level water occurs in the Schofield area of central 

Oahu that is different from either dike-impounded or 

perched ground water.

Nearly flat-lying lava flows and sedimentary rocks 

combine to form the regional aquifer system. Rift-zone 

intrusives and weathered valley-fill sediments cut 

across bedded lavas at high angles, impeding lateral 

ground-water flow and causing abrupt lateral discontinuities in potentiometric surfaces. Steeply dipping geohydrologic barriers delineate distinct ground-water 

areas underlain by the principal volcanic-rock aquifers. 

The result is a regional aquifer system subdivided into 

distinct compartments or areas. This classification provides a workable conceptual framework for most hydrologic applications. The Waianae aquifer in the Waianae 

Volcanics and the Koolau aquifer in the Koolau Basalt 

are the two principal volcanic-rock aquifers on Oahu. 

The two volcanic-rock aquifers combine to form a layered aquifer system throughout central Oahu, where 

the Koolau aquifer overlies the Waianae aquifer. They 

are separated by a regional confining unit, formed 

along the Waianae-Koolau unconformity, that consists 

of Waianae-derived paleosols, saprolite, weathered
REFERENCES B51

volcanic ash, and weathered alluvium. In most coastal 

areas, sediments overlie and confine the Waianae and 

Koolau aquifers, impeding freshwater discharge and 

impounding basal freshwater to great thicknesses. 

Beneath the caprock, the weathered surface of the volcanic formations also contributes to the confinement of 

water in the underlying volcanic-rock aquifers. The 

sequences of coastal-plain and valley-fill deposits 

locally form aquifers, but are of minor importance 

because of the small quantities of water involved.

The areal hydraulic continuity of the aquifers of 

Oahu is interrupted in many places by steeply dipping, 

stratigraphically unconformable, geohydrologic barriers that cut across the layered lavas at high angles. 

Two types of barriers are recognized: those of structural and lithologic origin, mainly intrusive rocks, but 

also other types of massive rock such as ponded lavas; 

and those of surficial origin, mainly valley-fill deposits, 

but also weathered geologic material. These barriers 

impede and divert ground-water flow and, in some 

instances such as in the Schofield Plateau and in the 

rift zones, impound ground water to much greater 

heights than would be reached in the absence of the 

barriers. Structural barriers are deep-rooted features 

formed during the shield-building phases of volcanoes 

and include rift-zone intrusives, stray dikes that lie 

somewhat apart from well-defined rift zones, and two 

extensive impounding structures in the Schofield 

Plateau of central Oahu. Surficial barriers result from 

processes of erosion, sedimentation, and weathering at 

the surface of the volcanoes. The most numerous 

barriers of this type are sedimentary valley-fill 

deposits and underlying saprolite.

The island of Oahu has been divided into seven 

major ground-water areas that are delineated by deepseated structural geohydrologic barriers. These areas 

are southern Oahu, southeastern Oahu, the Koolau 

Rift Zone, the Kahuku area, north-central Oahu, the 

Waianae Rift Zone, and the Schofield area. Hydraulic 

continuity within these seven areas generally is high, 

and the potentiometric surface is for the most part 

smoothly continuous, except in rift zones where dikes 

cause numerous local discontinuities. Some of these 

areas, notably southern Oahu and north-central Oahu, 

are further subdivided by subordinate surficial geohydrologic barriers.

Four separate fresh ground-water flow systems 

western, central, eastern and southeastern are 

defined and described by this study The central Oahu 

ground-water flow system is further divided into 

northern and southern parts. The northern Oahu flow 

system encompasses the north-central Oahu groundwater area and the northern part of the Schofield area. 

The southern Oahu flow system encompasses the

southern part of the Schofield area and the southern 

Oahu ground-water area. Meteoric freshwater flow 

diverges from ground-water divides that lie somewhere 

within the Waianae and Koolau rift zones, forming an 

interior flow system in central Oahu, exterior flow systems in western (Waianae area) and eastern (windward) Oahu, and a small flow system in southeastern 

Oahu. Predevelopment ground-water recharge, in 

million gallons per day, for each of the flow systems of 

Oahu has been estimated as follows:

Flow system

Recharge 

(Mgal/d)1

Western Oahu flow system ..................................... 32

Central Oahu flow system....................................... 543

Eastern Oahu flow system...................................... 198

Southeastern Oahu flow system............................. 19

Total predevelopment ground-water recharge....... 792

1These estimates include all ground-water recharge, including 22 Mgal/d 

to caprock areas, which does not enter the underlying volcanic-rock aquifers.

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Walker, J.L., 1964, Pedogenesis of some highly ferruginous 

formations in Hawaii: Honolulu, University of Hawaii, Hawaii 

Institute of Geophysics HIG-64-10, 406 p.

Weeks, E.P., 1969, Determining the ratio of horizontal to vertical permeability by aquifer-test analysis: Water Resources Research, 

v. 5, no. 1, p. 196-214.

Wentworth, C.K., 1926, Pyroclastic geology of Oahu: B.P. Bishop 

Museum Bulletin 30, 121 p.

 1938, Geology and ground water resources of the PaloloWaialae District: Unpublished manuscript in files of the Board 

of Water Supply, City and County of Honolulu, 274 p.

 1939, The specific gravity of sea water and the GhybenHerzberg ratio at Honolulu: University of Hawaii Bulletin, v. 18, 

no. 8, 24 p.

 1945, Geology and ground-water resources of the Pearl Harbor 

District: Unpublished manuscript in files of the Board of Water 

Supply, City and County of Honolulu, 156 p. plus appendices.

 1951, Geology and ground-water resources of Honolulu-Pearl 

Harbor area, Oahu, Hawaii: Board of Water Supply, City and 

County of Honolulu, 111 p.

Wentworth, C.K., and Macdonald, G.A., 1953, Structures and forms 

of basaltic rocks in Hawaii: U.S. Geological Survey Bulletin 994, 

98 p.

Wentworth, C.K., and Winchell, Horace, 1947, Koolau basalt series, 

Oahu, Hawaii: Geological Society of America Bulletin, v. 58, 

p. 49-78.

White, W.B., 1969, Conceptual models for carbonate aquifers: 

Ground Water, v. 7, no 3, p. 15-21.

Williams, J.A., and Liu, T.C., 1975, Response to tides of coastal 

aquifers: Analog simulation vs. field observation: University of 

Hawaii, Water Resources Research Center Technical Report 86, 

67 p.

Williams, J.A., and Soroos, R.L., 1973, Evaluation of methods 

of pumping test analyses for application to Hawaiian 

aquifers: University of Hawaii, Water Resources Research 

Center Technical Report 70,159 p.

Winchell, Horace, 1947, Honolulu series, Oahu, Hawaii: Geological 

Society of America Bulletin, v. 58, p. 1-48.

Wilson, J.T., 1963, A possible origin of the Hawaiian islands: 

Canadian Journal of Physics, v. 41, p. 863-870.

Zangar, C.N., 1953, Theory and problems of water percolation: U.S. 

Bureau of Reclamation Engineering Monograph 8, 76 p.

Zohdy, A.A.R., and Jackson, D.B., 1969, Application of deep electrical 

soundings for groundwater exploration in Hawaii: Geophysics, 

v. 40, p. 584-600.

Zones, C.P, 1963, Preliminary report on the ground-water resources 

of the Waianae area, Oahu, Hawaii: State of Hawaii, Department of Land and Natural Resources Circular C16.
SELECTED SERIES OF US. GEOLOGICAL SURVEY PUBLICATIONS

Periodical

Preliminary Determination of Epicenters (issued monthly).

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