Summary of Preliminary Findings in 

Ground-Water Studies of Southern

Oahu, Hawaii

GEOLOGICAL SURVEY 

CIRCULAR 435

TiSMiJ u/TTU TUC CTATC rw MAU/AII rrUAAAISSIONFR OF PUUIC LANDS
Summary of Preliminary Findings in 

Ground-Water Studies of Southern

Oahu, Hawaii

By F. N. Visher and J. F. Mink

Prepared in cooperation with the State of Hawaii Commissioner of Public Lands

Geological Survey Circular 435

Washington, 7960
United States Department of the Interior

FRED A. SEATON, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on application to the U. S. Geological Survey, Washington 2), D. C.
CONTENTS

Page Page

Introduction....______.____... 1 Dynamics of the basal-water

Occurrence of basal ground water ... 3 system Continued

Development of basal ground Effects of pumping.____.__...____. 9

watec.__- __________________ 3 Quantities of water_____ ... __ 11

Dynamics of the basal-water Pearl Harbor area__ _ 11

system ______________.____ 4 Honolulu area ___________ - ___ 15

Zone of transition _____._______ 5 Conclusions _______ _... -_ 15

ILLUSTRATIONS

Page 

Figure 1. Map of Oahu showing area of study.___..___._-____- . .___ 1

2. Map of southern Oahu showing ground-water areas, the distribution of cap rock,

and the distribution of irrigated sugarcane land.___...-.__._._________-_-__ 2

3. Diagram showing the theoretical pattern of movement of water through a

fresh-water lens floating in sea water ____ .. - --- - - 3

4. Graph showing average daily pumpage from Pearl Harbor area, 1901 57 _______ 4

5. Graph showing change in chloride content of water with depth in test wellT-67__ 5

6. Map showing major pumping stations and springs and approximate boundaries

of ground-water areas in southern Oahu ______.__.__.-____--._____ 6

7. Map showing distribution of chloride content of water in Waimano Spring area __ 7

8. Map showing the change in position of the 200- and 1,000- ppm isochlors in the

Pearl Harbor area between 1910 and 1950_____..______.._________ 8

9. Schematic diagram showing effect of pumping on the thickness in the

transition zone ______.._.__._.__.____.__.___..._.____._-._-_-____ 9

10. Graph showing decrease in chloride content of water in well 7-F at the

Honolulu Board of Water Supply Kaimuki pump after a change from intermittent to continuous pumping _______»____.___.___.....___.__..___ 10

11. Graph of chloride content and head at well 153 for the years 1924 through 1958, 

showing rapid rise in chloride associated with on-and-off operation of 

Kalihi shaft ____._-___.____________.___._._.___________ 11

12. Graph showing draft at Oahu Sugar Co. pumps 6 and 6-B and the Navy Waiawa 

shaft, the head at well 190, and the chloride content of the water at Oahu 

Sugar Co. pumps 6 and 6-B____>_________.___..___-__.._._ 12

13. Graph showing draft at Honolulu Plantation Co. pumps 4-E and 4-H, the Navy

Halawa shaft, the Board of Water'Supply Halawa shaft, and Honolulu Plantation 

Co. Aiea Shaft; the head at well 190; and the chloride content of the water at 

well 190, the Navy Aiea pump, and Honolulu Plantation Co. pumps 4-E and 

4-H______________________________________________________ 13

14. Graph showing draft at Ewa shaft, head at Ewa Pump 1, and chloride content of

water at Ewa pumps 1, 3, 4, 7, and 9-D ___-._.__.._ _ . ._ 14

III
Summary of Preliminary Findings in Ground-Water Studies of

Southern Oahu, Hawaii

By F. N. Visher and J. F. Mink

INTRODUCTION

This summary presents the preliminary 

findings reached in a study instituted to determine the amount of additional ground water 

that might be developed, and the geologic and 

hydrologic conditions that control the development and use of water, in southern Oahu.

Large amounts of basal ground water have 

been developed and used in southern Oahu for 

more than half a century, and the continued 

substantial flow from springs on the inland 

shore of Pearl Harbor has suggested that additional water in relatively large quantities is 

available for beneficial use. The development 

of ground water, however, has caused an increase in salinity of water at some previously 

developed sources. In addition to the problem 

of salinity, there has been the possibility that 

the total water pumped from wells and discharged from springs to date has included 

a considerable amount of water withdrawn

PEARL HARBOR

Jf__Y"" L

Figure 1. Map of Oahu showing area of study.

from storage and that the prevailing rates of 

use, therefore, might be exceeding the rate of 

replenishment of the ground water.

Southern Oahu is defined in this report as 

that part of the Island of Oahu lying southwest 

of the crest of the Koolau Range, east of the 

crest of the Waianae Mountains, and south of 

the summit of the Schofield plateau (fig. 1). 

It is composed of the Pearl Harbor area 

(fig. 2, areas 6 and 11) on the west and the 

Honolulu area (fig. 2, areas 1 to 5) on the east.

The investigation was started by the U.S. 

Geological Survey in July 1956 and is carried 

on in cooperation with the Hawaii Division of 

Hydrography. Funds for the work were provided in grants made to the Division of Hydrography by the Hawaii Economic Planning and 

Coordination Authority and the City and 

County of Honolulu, the total of which was 

matched by the Geological Survey in accordance with its authorization for cooperative 

work.

The investigation is an extension of earlier 

geologic and hydrologic work done on Oahu by 

H. S. Palmer, H. T. Stearns, and C.K.Wentworth, who developed the current broad 

knowledge and understanding of basal groundwater systems of the island. This report includes a resume', based largely upon publications by these three geologists, of information 

concerning the occurrence and development 

of the basal ground water.

The authors are indebted to the Honolulu 

Board of Water Supply, the Suburban Water 

System of the City and County of Honolulu, the 

Public Works Office of the. Fourteenth Naval 

District, the Hawaiian Electric Co., the Oahu 

Sugar Co., Ltd., and the Ewa Plantation Co., 

Ltd., for the wholehearted cooperation and 

assistance they provided during the investigation.
SUMMARY OF PRELIMINARY FINDINGS IN GROUND-WATER STUDIES OF SOUTHERN OAHU. HAWAII

I ; / / ' ! <D M v \s

y
INTRODUCTION

OCCURRENCE OF BASAL GROUND WATER

Rainfall in the Koolau Range is the principal source of the fresh ground water in southern Oahu. Some water, mainly that falling in 

heavy rainstorms, flows quickly to the sea in 

streams; some evaporates or is transpired 

by plants; and the remainder moves downward through the soil and rocks to become 

ground water. Some of this ground water is 

stored temporarily in the "high-level" groundwater reservoirs in the Koolau Range or the 

Schof ield plateau; most of it eventually enters 

the large basal Honolulu-Pearl Harbor 

ground-water reservoir, and moves through 

it toward the sea. This basal ground-water 

reservoir supplies practically all the fresh 

water used in the vicinity of Honolulu.

The fresh basal water floats on the slightly 

heavier sea water that saturates the rocks of 

the island below sea level. It forms a lens 

which extends below sea level about 40 times 

the height that the free fresh-water surface 

stands above sea level. An idealized sketch 

(fig. 3, not drawn to scale) shows the approximate shape of a fresh-water lens and the 

paths of movement of the fresh water. A zone 

of mixing, which exists between the salt and 

fresh waters even under natural conditions, 

leaves a thinner body of truly fresh water 

than that suggested by the sketch.

The basaltic lava flows of which the island 

of Oahu is built are highly permeable when 

fresh, but upon weathering they form clayey 

residual soils, slope wash, and alluvium of 

very low permeability. Weathered lava and 

alluvium form a caprock under the coastal 

plain of southern Oahu which, by retarding the 

escape of fresh water, causes higher water 

levels, or heads, and thicker fresh-water 

bodies than would exist if the caprock were 

absent. The water beneath the caprock is 

artesian that is, it is confined under pressure. In the lowlands it is the source of water 

for flowing wells.

Weathered lava and alluvial fill in ancient 

valleys that cut the permeable aquifer, form 

barriers which extend several hundred feet 

below sea level and divide the basal aquifer 

into compartments. Figure 2 shows the extent 

of the caprock and the approximate boundaries of the basal-water compartments. Water levels in the various compartments in the 

Honolulu area (fig. 2, areas 1 to 5) stand 9 to 

29 feet above sea level, but the water-level

ALT WATER

Figure 3. Diagram showing the theoretical pattern of movement 

of water through a fresh-water lens floating in sea water. The 

cross section shown is half of a hypothetical island receiving 

continuous rainfall distributed uniformly over the island. The 

height of the water table above sea level is greatly exaggerated.

gradient within each compartment apparently 

is very low. In the Pearl Harbor area the 

water-level gradients are appreciable andean 

be shown by contour lines (fig. 2, areas 6 and 

11).

The natural mode of discharge of ground 

water from the compartments in the Honolulu 

area is by lateral leakage from areas of higher 

head to those of lower head and by leakage into 

the ocean through or under the caprock. In 

the Pearl Harbor area, where the inland edge 

of the caprock is relatively low, the natural 

discharge takes place principally through 

springs from which a large amount of water 

flows into the harbor.

(DEVELOPMENT OF BASAL GROUND WATER

In the first two decades (1879-1900) of development of ground water in southern Oahu, 

the main use of water was in the lowlands, 

where the needs could be supplied by flowing 

artesian wells drilled through the caprock 

into the basal aquifer. Beginning about 1900, 

batteries of deep wells equipped with largecapacity stream-driven suction pumps were 

installed in the coastal plain and at low altitudes in the bottoms of valleys, and the cultivation and irrigation of sugarcane in the Pearl 

Harbor area expanded to higher lands. Because of the thickness of the caprock and the 

depth to which the impermeable zones extend 

below the valley floors, many of these wells 

entered permeable rock hundreds of feet below sea level, in the zone of transition between 

fresh and salt waters. Irrigation water from 

the deep wells in the Pearl Harbor area is 

augmented by water pumped from large 

springs on the shore of Pearl Harbor and, 

since 1913, by high-level water transported 

through the Waiahole tunnel (fig. 2) from windward Oahu.
SUMMARY OF PRELIMINARY FINDINGS IN GROUND-WATER STUDIES OF SOUTHERN OAHU, HAWAII

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Figure 4. Graph showing average daily pumpage from Pearl Harbor area (areas 6 and 11, fig. 3), 1901-57. The flow from springs and wells

is not included.

Beginning in the 1930's, several shafts or 

basal tunnels have been constructed to supply 

water for irrigation and for municipal and 

military use. From points as high as 300 feet 

above sea level and as far as 3 miles inland, 

vertical or inclined shafts were sunk nearly 

to sea level, and horizontal tunnels driven as 

much as 1,200 feet at about sea level. The 

shafts are more advantageous than the older 

deep-well stations in that they are farther inland and are located so as to tap only the 

upper part of the fresh-water lens. Their 

construction was expensive eveninthe 1930's, 

and a large yield had to be presupposed to 

justify this initial cost. Several of the shafts 

are equipped with pumps that discharge 10 to 

30 mgd (million gallons per day). During this 

period of latter development, the owners and 

operators of large wells have worked together 

for skillful management of the reservoir on 

the basis of scientific studies and technical 

advice.

As shown by figure 4, total pumpage in the 

Pearl Harbor area increased from less than 

20 mgd in 1901 to 165 mgd in 1912, which is 

close to the average rate of pumping during 

the 45 subsequent years. The introduction and 

development of large-capacity shafts in the

past 20 years did not result in an overall increase in pumpage.

DYNAMICS OF THE BASAL-WATER SYSTEM

The present investigation required thorough consideration of the physical principles 

and theory of a fresh-water lens floating upon 

salt water: Under steady-state conditions of 

uniform inflow and outflow, as depicted in 

figure 3, the contact between fresh and salt 

water should be sharp and well defined. This 

is not true in the Pearl Harbor area, where 

many wells and springs yield water having a 

chloride concentration of less than 200 ppm, 

but many others yield water in which some 

sea water is apparently included. A mixture 

containing more than 10 percent of sea water 

is sufficiently salty to necessitate abandonment of a well for irrigation, and even 2 percent of sea water makes an undesirable mixture for municipal use.

How does sea water get into the fresh-water 

lens, and how is it lifted so that it is discharged from spring orifices several feet 

above sea level? The studies have led to concepts of the dynamics of a fresh-water lens
DYNAMICS OF THE BASAL-WATER SYSTEM

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CHLORIDE, IN THOUSANDS OF 

PARTS PER MILLION

Figure 5. Graph showing change in chloride content of water 

with depth in test well T-67.

which will be included in a forthcoming comprehensive report. Some of the tentative 

findings, as set forth below, are of value in the 

effective management of the ground-water 

reservoir.

ZONE OF TRANSITION

Wells penetrating deeply into the basal 

aquifers long have indicated the presence of 

a zone of mixing or transition between the 

fresh-water lens and the sea water on which 

the lens floats, but the data from numerous 

wells having assorted locations, depths, and 

pumping regimens were inconclusive as to 

whether the variations in chemical content 

might result from varied permeabilities of 

zones that were accessible respectively to the 

ocean and to the fresh water coming from the 

mountains. Changes in the salinity of water 

from individual wells indicate that changes

take place in the thickness or position of the 

transition zone.

Measurement of the salinity in a deep test 

hole (T-67, fig. 5), which was drilled on the 

shore of Pearl Harbor as a part of the investigation, shows that all the basal water under 

the caprock just seaward of Kalauao Spring is 

in the zone of transition, and that the proportion of sea water increases progressively 

with depth. As shown in figure 5, the freshest 

water in the hole, which occurs immediately 

under the caprock at a depth of about 140 feet, 

has a chloride content of about 800 ppm; in 

the water below adepth of 1,100 feet the chloride content exceeds 17,000 ppm. Thus, within 

about 1,000 feet vertically the mixtures range 

from 4 to 90 percent of sea water.

The dimensions of the transition zone are 

indicated also by the quality of water discharged by the springs of the Pearl Harbor 

area, which is the principal area of natural 

discharge of the basal water. The Waimano 

spring area is typical; figure 7 shows the 

horizontal distribution of chloride content in 

the zone of transition. (See fig. 6 for locations 

of springs and pumping stations.) The water 

from inland orifices has low salinity and presumably comes from relatively shallow and 

fresh parts of the lens. The orifices that are 

lower in elevation and closer to the shore discharge water of greater salinity, evidently 

from progressively deeper or seaward parts 

of the transition zone.

The water from many springs and from most 

of the old wells in the Pearl Harbor area is 

now more saline than it was 50 years ago. This 

is shown by the change in position of the 200- 

ppm and 1,000-ppm isochlors (fig. 8), representing respectively the lower and upper limits of the mixed water that is usable. Although 

the diagram shows only the change from 1910 

to 1950, most of this change occurred prior to 

1925. The inland movement of the zone of 

transition, shown by figure 8, necessarily 

would involve also an upward movement into 

the zone initially occupied by fresh water.

The greatest changes in position of the 

isochlors in the Pearl Harbor area occurred 

during the first years of heavy pumping from 

wells for irrigation. Pumping is thus indicated as one factor, possibly the principal 

factor, in the development and maintenance of 

a transition zone (fig. 9).
SUMMARY OF PRELIMINARY FINDINGS IN GROUND-WATER STUDIES OF SOUTHERN OAHU. HAWAII

STX.
DYNAMICS OF THE BASAL-WATER SYSTEM

ATERCRES

INDUSTRIAL 

(H.E.CO.)

400 WATERCRESS

INDUSTRIAL

" 6A0H.E.CO.)

CONTOURS IN FEET-DATUM IS SEA LEVEL

no 

o~

Spring

Number indicates 

chloride content, 

in parts per 

million

EXPLANATION

153

Drilled well 

Number indicates 

chloride content, 

in parts per 

mi/lion

 1200 

Isochlor

Number indicates 

chloride content, 

in parts per 

mi/lion

Figure 7. Map showing distribution of chloride content of water, in parts per million, in Waimano Spring area.
SUMMARY OF PRELIMINARY FINDINGS IN GROUND-WATER STUDIES OF SOUTHERN OAHU, HAWAII

Figure 8. Map showing the change in position of the 200- and 1,000-ppm isochlcns in the Pearl Harbor area between 1910 and 1950.
DYNAMICS CF THE BASAL-WATER SYSTEM 9

No pumping

Steadily pumping 

Well

On-and-off-pumping 

well

Figure 9. Schematic diagram showing effect of pumping on the 

thickness of the transition zone.

liEFFECTS OF PUMPING

Referring to the idealized lens of figure 3, 

the effect of pumping from a well would be to 

divert some of the water toward the well, 

modify the flow lines accordingly, and cause 

some upward warping of the fresh-salt-water 

contact beneath the well. Mixing of fresh and 

salt water might be expected as a result of 

changes such as those created by intermittent 

pumping. Through the cooperation of the 

Honolulu Board of Water Supply, the effect of 

intermittent pumping was studied at the 

Board'sKaimuki and Kalihi pumping stations. 

The Kaimuki pump (fig. 2, area 1) had been 

operated for years on an on-and-off schedule 

designed to meet peak demands for water, and 

the Kalihi shaft (area 4) had been pumped 

continuously at a nearly constant rate.

The graph in figure 10 shows that a change 

in 19 58 from intermittent to continuous ope ration of the Kaimuki pump was followed by a 

small but significant drop in the chloride content of the water. Throughout more than 10 

years of intermittent operation of the well, 

the average chloride content had increased 

gradually but persistently. This rise appears 

to be unrelated either to the average draft or 

to the average head in area 1, but instead to 

reflect a decrease in recharge following years 

of less than average rainfall (a lag of approximately 5 years is indicated). At any rate, the 

trend was reversed, in that chloride content 

decreased when the pumping schedule was 

changed to a steady operation.

To meet peak demands in the municipal 

system during the test at the Kaimuki pump, 

the operation at the Kalihi shaft was changed 

in 1958 from relatively steady to intermittent 

pumping. Coincident with the increased onand-off pumping at the Kalihi shaft there was 

a sharp rise in the chloride content of the 

water in well 153 (fig. 11). This rise is attributed to a thickening of the transition zone, 

caused by the intermittent pumping at the 

shaft.

In several of the heavily pumped, deep irrigation wells the head in adjacent wells decreases, and the chloride content of the water 

pumped increases, with increasing draft. This 

is clearly shown in figure 12 by comparison 

of records for years of least average pumping 

(1935-39, 1948-49, 1955) with those for years 

of greatest pumping (1929, 1931, 1941, 1944, 

1953). However, although the pumping from 

the Oahu Sugar Co. wells in 19 53 was less than 

inl&29, 1941, and 1944, the maximum chloride 

content was significantly greater. This increase in recent years is attributed to pumping 

from the Navy's Waiawa shaft, nearly a mile 

inland from the irrigation well and the only 

significant modifier of the previous withdrawal pattern.

Similarly in figure 13, the chloride content 

of water pumped by the Honolulu Plantation 

Co. from wells 4-E and 4-H has increased 

significantly since 1941, although the draft 

from those wells has been-generally less than 

in years prior to 1941. This increase occurred 

after the Aiea shaft, Navy's Halawa shaft, and 

Honolulu's Halawa shaft were placed in service, 1 to 2 miles southeast of the pumped 

wells. In well 190, shoreward from these
10 SUMMARY OF PRELIMINARY FINDINGS IN GROUND-WATER STUDIES OF SOUTHERN OAHU, HAWAII

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Figure 10. Graph showing decrease in chloride content of water in well 7-F at the Honolulu Board of Water Supply Kaimuki pump after a 

change from intermittent to continuous pumping. Shown also-are the .average daily draft and the average head in area 1 in the Honolulu 

area and the 3-year moving average of the cumulative departure from normal rainfall at Manoa Tunnel No. 2. There appears to be a 5- 

y ear lag between the amount of rainfall and an improvement or deterioration in water quality, so the time scale is shifted for the last graph.
QUANTITIES OF WATER 11

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DRAFT AT KALIHI

'45 '50 '55 1959

YEAR

Figure 11. Graph of chloride content and head at well 153 for the years 1924 through 1958, showing rapid rise in chloride associated with

on-and-off operation of Kalihi shaft.

shafts, the chloride content increased, but 

there was no progressive change in head after 

withdrawals from these shafts began.

The graphs of figure 14 show how an additional withdrawal has changed the shape of 

the transition zone, forced abandonment of 

some producing wells, and eventually resulted 

in merely a change in point of withdrawal with 

no increase in total water produced. Shortly 

after the Ewa jshaft reached full production, 

pump 9 (3 miles seaward) was abandoned, and 

pump 1 (2 miles seaward) was abandoned 

several years later. The chloride content in

other wells seaward from the shaft also increased measurably. The net effect of the 

development of Ewa shaft has been no increase 

in total pumpage from the Ewa Plantation 

wells, but an improvement in quality of total 

water supplies by withdrawal from an inland 

source in lieu of wells tapping the transition 

zone near the edge of the fresh-water lens.

QUANTITIES OF WATER

PEARL HARBOR AREA

Since 1919 the pumpage from wells in the 

Pearl Harbor area (fig. 2, areas 6 and 11) has 

ranged from 140 to 190 mgd and averaged 

about 160 mgd (fig. 4). The zone of transition 

moved inland and thickened, and the freshwater lens shrank during this period of heavy 

withdrawal. The amount of fresh water now 

stored in the reservoir is appreciably less 

than before any wells were drilled. But the 

continued heavy pumping for several decades, 

with only minor fluctuations in the shape and 

size of the fresh-water lens, is a strong indication that under the prevailing conditions of 

development and use the draft does not exceed 

the sustained yield of the system. _QiT the 

other hand, the installation of new largecapacity wells has not resulted in an appreciable increase in the overall draft in the area.
12 SUMMARY OF PRELIMINARY FINDINGS IN GROUND-WATER STUDIES OF SOUTHERN OAHU, HAWAII

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NAVY WAIAWA SHAFT

OAHU SUGAR CO. PUMPS 6 AND 6-B

\B27'28'29'30'3\ '3 2 '33 '34 '3 5 '3 6 '37 '38 '394041 '42 '43 '4445 '46 '47 '48 '49 '

YEAR

/52 /53'54 /55 /56 /57

Figure 12. Graph showing draft at Oahu Sugar Co. pumps 6 and 6^B and the Navy Waiawa shaft, the head at well 190, and the chloride content of the water at Oahu Sugar Co. pumps 6 and 6-B. Compare the peak levels of chloride content of water at pumps 6 and 6-B during 

years of equal draft before and after the installation of the shaft.
QUANTITIES OF WATER 13

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HONOLULU PLANTATION CO

AIEA SHAFT

BOARD OF WATER SUPPLY 

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NAVY HA

HO

LAWA SHAFT

NOLULU PLANTATION CO. PUMPS

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-E AND 4-H

77

1922 25 '55 '57

Figure 13. Graph showing draft at Honolulu Plantation Co. pumps 4-E and 4-H, the Nayy Halawa shaft, the Board of Water Supply Halawa 

shaft, and Honolulu Plantation Co. Aiea shaft; the head at well 190; and die chloride content of the water at well 190, the Navy Aiea 

pump, and Honolulu Plantation Co. pumps 4-E and 4-H.
14 SUMMARY OF PRELIMINARY FINDINGS IN GROUND-WATER STUDIES OF SOUTHERN OAHU, HAWAII

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EWA SHAFT

i927'28'29'30'3l '32 "33 *34'35'36'37'38'39 40'41 '42'43 '4445'46'47 48 49'50'5I '52'53'54'5 5'56'57

YEAR

Figure 14. Graph showing draft at Ewa shaft, head at Ewa pump 1, and chloride content of water at Ewa pumps 1, 3, 4, 7, and 9-D. The 

draft at each of the pumps changes very little from year to year and therefore is not shown. The combined draft from all sources was higher 

during the period when both pump 1 and the shaft were operating but returned to former levels when pump 1 was abandoned.
CONCLUSIONS 15

It appears, therefore, that the basal-water 

system in the Pearl Harbor area is virtually 

in equilibrium.

The average discharge of water of low 

salinity from the Pearl Harbor springs is 

about 82mgd. About 17 mgd of this is pumped 

to sugarcane fields, and the remaining 65 mgd 

discharges into the sea after flowing through 

watercress or other wetland crops, or through 

heat exchangers at the Waiau power stations 

of the Hawaiian Electric Co. In addition to the 

water from the springs, nearshore wells discharge about 15 mgd into the harbor through 

wetland crops and about 10 mgd through the 

power station. Thus the discharge of basal 

water of good quality by springs and flowing 

wells averages about 107 mgd, and the total 

from all wells and springs approximates 

270 mgd.

However, during periods of maximum demand and pumping for sugarcane irrigation, 

the flow into the harbor is materially lower. 

During the summers of 1953 and 1954, for 

example, when pumping for irrigation from 

wells and springs was at an average rate of 

260 mgd, the flow from the springs into the 

harbor was 32 mgd, the flow from wells at the 

powerplant was 8 mgd, and the flow from wells 

watering wetland crops was about 10 mgd a 

total of 50 mgd from springs and flowing 

wells.

Of the total average pumpage of about 160 

mgd from basal ground water, about 130 mgd 

is used for the irrigation of sugarcane in the 

Pearl Harbor area. In addition, about 17 mgd 

is pumped from the Pearl Harbor springs and 

about 30 mgd-of high-level water is diverted 

from windward O'ahu through the Waiahole 

ditch for sugarcane irrigation. About onefourth of the total irrigation water, nearly 180 

mgd, is used on fields lying on the caprgck, but 

the remainder goes to the higher fields where 

much of the water in excess of the needs of 

the sugarcane can move down to the basal 

ground-water reservoir.

Studies in Maui by the Hawaiian Sugar 

Planters' Association indicate that the consumptive use of water by sugarcane is about 

the same as the evaporation from a U.S. 

Weather Bureau class A evaporation pan in 

the same area. On this basis, the calculated 

consumptive use of sugarcane in the Pearl 

Harbor area is about one-third of water 

pumped or diverted to the fields. If all the

remaining two-thirds of the water moves 

downward to the water table by deep seepage, 

the average recharge to the basal aquifer 

(underlying about three-fourths of the area 

irrigated) from water applied to the fields in 

the Pearl Harbor area would amount to about 

90 mgd. The return of irrigation water to the 

basal-water table is not directly measurable 

at the present, but the amount undoubtedly is 

substantial. It has been an important element 

in the replenishment of the basal system during the long period of heavy pumping in the 

Pearl Harbor area.

HONOLULU AREA

Analysis of rainfall-evapotranspiration relations during the present study indicates that 

the recharge to basal ground water from rainfall in the Honolulu area (fig. 2, areas 1 to 5) 

may be at least 65 mgd, which is about twice 

the average pumpage in the area. Some of .the 

excess of recharge over pumpage probably 

mbves northwestward into the Pearl Harbor 

area and contributes to the flow of springs 

and wells in that area. The remainder flows 

through or under the caprock into the ocean. 

Additional investigation, including some drilling of test wells, will be necessary to ascertain the amount of water that is now discharged 

to the ocean, and the potentialities and limitations of salvaging it for beneficial use by increasing the rate of pumping in the Honolulu 

area. However, increased draft in the Honolulu area probably would result in a reduction 

of the supply now believed to be reaching the 

Pearl Harbor area.

CONCLUSIONS

1. The basal ground water in the Honolulu 

and Pearl Harbor areas is in a single but compartmented reservoir in very permeable rock, 

floating upon se . water in equally permeable 

rock. The only factors.that tend to keep these 

two miscible liquids segregated are the natural flow lines of the fresh water and the slightly greater density of the .salt water. Thus 

there is a delicate balance between fresh water 

and the underlying sea water: a well will pump 

water of increasing or of high salinity if it is 

too deep, if it is too close to the shore, if it is 

pumped too heavily, if it is pumped intermittently, or if a well farther inland is pumped 

too heavily especially if that pumping is intermittent. The established equilibrium could
16 SUMMARY OF PRELIMINARY FINDINGS IN GROUND-WATER STUDIES OF SOUTHERN OAHU, HAWAII

be upset by unwise development, with the result that municipal and other supplies would 

be contaminated.

2. Nevertheless, under existing patterns of 

development, use, and draft of basal water, 

there has been no overall deterioration in 

water quality; although local increases in salinity have occurred, and some wells have 

been abandoned because of increasing salinity.

3. The recharge of water to the basal aquifer from overlying sugarcane fields, including 

the recharge from high-level water diverted 

from windward Oahu and water pumped from 

springs, is an important component in the 

present equilibrium of the basal ground-water 

system.

4. During periods of minimum flow and 

maximum demand, about 50 mgdof basal water 

of good chemical quality discharges into Pearl 

Harbor from springs and nearshore wells. 

This water might be applied to more beneficial use by (a) collecting the water downstream 

from the sites of industrial use and wetland 

irrigation and pumping it to areas of need, 

which would not disturb existing conditions in 

the basal-water system; and (b) pumping the

water from wells, constructed inland from the 

springs.

5. A further increase in the net draft on the 

aquifer by pumping from wells farther inland 

would salvage some of the water now discharged to the ocean, but would also upset the 

equilibrium existing under the present pattern 

of development, draft, and use. Development 

of the water now flowing from the Pearl Harbor springs and nearshore wells by means of 

wells inland from the springs would decrease 

the flow of the springs and wells and cause an 

increase in the salinity of the water flowing 

from them, and also would cause higher salinities at existing pumping stations that have 

wells extending into the transition zone. In 

any additional development, the location, depth, 

capacity, and pumping schedules of new wells 

should be designed so as to cause least effect 

upon the salt water and upon the mixed water 

in the transition zone.

6. Studies show that there is an excess of 

recharge over pumpage in the Honolulu area. 

Determination of the maximum quantity of 

water that can be pumped from the basal 

aquifer in the area will require additional 

investigation.

INT.DUP..D.C.60- tt)S16