U.S. Department of the Interior

U.S. Geological Survey

Ground water is one of Hawaii’s most 

important natural resources. It is used for 

drinking water, irrigation, and domestic, 

commercial, and industrial needs. 

Ground water provides about 99 percent 

of Hawaii’s domestic water and about 50 

percent of all freshwater used in the 

State. Total ground water pumped in 

Hawaii was about 500 million gallons per 

day during 1995, which is less than 3 

percent of the average total rainfall 

(about 21 billion gallons per day) in 

Hawaii. From this perspective, the 

ground-water resource appears ample; 

however, much of the rainfall runs off to 

the ocean in streams or returns to the 

atmosphere by evapotranspiration. 

Furthermore, ground-water resources can 

be limited because of water-quality, 

environmental, or economic concerns.

Water beneath the ground surface 

occurs in two principal zones: the 

unsaturated zone and the saturated zone. 

In the unsaturated zone, the pore spaces 

in rocks contain both air and water, 

whereas in the saturated zone, the pore 

spaces are filled with water. The upper 

surface of the saturated zone is referred to 

as the water table. Water below the water 

table is referred to as ground water. 

Ground-water salinity can range from 

freshwater to that of seawater. 

Freshwater is commonly considered to be 

water with a chloride concentration less 

than 250 mg/L, and this concentration 

represents about 1.3 percent of the 

chloride concentration of seawater 

(19,500 mg/L). Brackish water has a 

chloride concentration between that of 

freshwater (250 mg/L) and saltwater 

(19,500 mg/L). 

GROUND-WATER RECHARGE

The amount of recharge available to 

enter the aquifers is the volume of 

rainfall, fog drip, and irrigation water that 

is not lost to runoff or evapotranspiration 

or stored in the soil. Rainfall is spatially 

variable because of the islands’ 

topography and the persistent 

northeasterly tradewinds. In dry areas, 

annual rainfall is less than 10 inches; in 

wet areas, annual rainfall is greater than 

400 inches. In general, southwestern, 

leeward sides of the islands are driest and 

northeastern, windward sides are wettest. 

Fog drip, which is cloud vapor that is 

intercepted by vegetation and 

subsequently drips to the ground, 

commonly occurs between altitudes of 

2,000 and 6,000 ft. 

Recharge is typically about 10 to 50 

percent of the rainfall, fog drip, and 

irrigation water. Runoff is directly related 

to factors including rainfall, topography, 

soil type, and land use. Runoff is 

typically about 10 to 40 percent of 

rainfall, but is higher where rainfall is 

high and slopes are steep and where rain 

falls on poorly permeable land surfaces. 

Evapotranspiration is the loss of water to 

the atmosphere by the combination of 

transpiration of plants and direct 

evaporation from land and water surfaces 

and can exceed 50 percent of rainfall. 

Water stored in the soil is available for 

plants or can eventually flow downward 

to recharge the aquifer.

Ground Water in Hawaii

U.S. Department of the Interior

U.S. Geological Survey FS 126-00

Large-scale development of ground water in 

Hawaii began after the first successful well was 

drilled in 1879 in southern Oahu. The well 

shown, drilled in 1909 near Pearl Harbor, 

derives its water from a volcanic-rock aquifer 

that is confined by overlying sedimentary 

deposits. This well is an artesian well because it 

taps a confined aquifer, and is free flowing 

because the pressure in the aquifer is sufficient to 

raise the water above the land surface.

PACIFIC OCEAN

Kauai

Niihau

Oahu

Maui

Molokai

Lanai

Kahoolawe

Hawaii

HAWAII
HYDROGEOLOGY

The most extensive and productive 

aquifers in the Hawaiian islands are 

formed by volcanic rocks that erupted 

during the principal building stage of 

each volcano. Lava from this stage, called 

the shield stage, consists of basalts that 

characteristically form thin flows ranging 

in thickness from a few feet to a few tens 

of feet. The shield stage is the most 

voluminous phase of eruptive activity 

during which 95 to 98 percent of the 

volcano is formed. Thousands of lava 

flows erupt from the central caldera of 

the volcano and from two or three rift 

zones that radiate out from the caldera. 

Intrusive dikes fed by rising magma 

extend down the rift zones and may erupt 

if they reach the surface. Some volcanoes 

have a postshield-stage during which lava 

flows over the shield-stage basalt. The 

post-shield stage lava flows are marked 

by a change in lava chemistry and 

character that commonly leads to the 

formation of massive lava flows that can 

be many tens of feet thick. After a period 

of volcanic inactivity, lava might issue 

from isolated vents on the volcano during 

a final, rejuvenated stage. 

The permeability of volcanic rocks is 

variable and depends on the mode of 

emplacement, amount of weathering, and 

thickness of the rocks. (Permeability 

describes the ease with which fluid can 

move through rock.) The three main 

groups of volcanic rocks (lava flows, 

intrusive dikes, and pyroclastic deposits) 

are formed by different modes of 

emplacement. Weathering reduces the 

permeability of all types of volcanic 

rocks. The thickness of a lava flow can 

depend on the lava chemistry and the 

topography over which it cooled. 

Thicker flows generally are less 

permeable and form from highly viscous 

lava on flat topography. 

Lava flows are mainly pahoehoe, 

which has a smooth, undulating surface 

with a ropy appearance and aa, which has 

a surface of coarse rubble (clinker) and 

an interior of massive rock. A typical 

sequence of lava flows contains both aa 

and pahoehoe flows. The interconnected 

void spaces in a sequence of pahoehoe 

flows may lead to high permeability. The 

layers of clinker at the top and bottom of 

aa flows also impart high permeability 

(similar to that of coarse-grained gravel) 

to volcanic-rock aquifers. However, the 

lava in the core of an aa flow typically 

cools as a massive body of rock with 

much lower permeability. The most 

productive and most widespread aquifers 

consist of thick sequences of numerous 

thin lava flows. 

Dikes are thin, near-vertical sheets of 

massive, low-permeability rock that 

intrude existing rocks, commonly 

permeable lava flows. Dikes can extend 

vertically and laterally for long distances 

and impede the flow of ground water. 

Dikes can intersect at various angles and 

compartmentalize the more permeable 

rock in which ground water can be 

impounded. The dikes lower overall rock 

porosity and permeability. 

Pyroclastic rocks include ash, cinder, 

spatter, and large blocks. Compaction and 

weathering can reduce the permeability; 

weathered ash beds commonly act as thin 

confining units within lava sequences. 

Volcanic-rock aquifers are found 

throughout the eight major islands and 

are locally overlain by sedimentary 

deposits. Sedimentary deposits of 

alluvium, coralline limestone, and 

cemented beach or dune sand that 

typically are considered to be productive 

aquifers in much of the conterminous 

United States are relatively poor aquifers 

in the Hawaiian islands. Limestone 

deposits are highly permeable in many 

places and usually yield brackish water or 

saltwater because of good hydraulic 

connection between the ocean and the 

limestone and because of low recharge to 

the limestone.

 In some places, weathered volcanic 

rocks or sedimentary deposits form a 

low-permeability confining unit 

overlying high-permeability volcanic 

rocks. In coastal areas, confining units of 

weathered volcanic rocks and 

sedimentary deposits are called caprock, 

which impedes the discharge of 

freshwater to the ocean. Confining units 

are also found in non-coastal areas, 

separating the volcanic rocks of two 

different volcanoes. Valley-filling 

sedimentary deposits generally are also of 

low permeability and can impede the 

seaward and lateral movement of ground 

water.

Hawaii District web site – http://hi.water.usgs.gov

A

A

B

B

C

A typical sequence of lava flows contains aa clinker zones (A) of relatively high permeability that 

occur above and below the massive central cores of aa flows (B), and many thin pahoehoe flows 

(C). The sequence shown is about 50 feet thick. (photo by Scot K. Izuka, USGS).

A

B

A

B

C
WHAT ARE THE GROUND-WATER 

SETTINGS IN HAWAII?

In Hawaii, the major fresh groundwater systems are below the lowest water 

table, and are either freshwater-lens or 

dike-impounded systems. Where 

freshwater-lens and dike-impounded 

systems are adjacent, they form a single, 

hydrologically connected ground-water 

flow system. Minor perched systems can 

exist above the lowest water table.

Freshwater-Lens System

A freshwater-lens system includes a 

lens-shaped freshwater body, an 

intermediate transition zone of brackish 

water, and underlying saltwater. The 

transition zone can be quite thick (several 

tens to hundreds of feet) depending on 

the extent of mixing between freshwater 

and saltwater. Freshwater-lens systems 

are found in dike-free volcanic rocks and 

sedimentary deposits under confined or 

unconfined conditions. The most 

important sources of ground water in 

Hawaii are from the freshwater parts of 

these systems in volcanic rocks. 

In general, for a given aquifer 

permeability, low recharge results in low 

water levels and a thin freshwater lens. 

For a given recharge rate, low aquifer 

permeability results in high water levels 

and a thick freshwater lens. In the most 

permeable volcanic rocks, the water table 

is no more than a few feet above sea 

level, and the slope of the water table is 

nearly flat. In some low-permeability 

volcanic-rock aquifers, such as in eastern 

Kauai and northeastern Maui, a vertically 

extensive freshwater-lens system has 

freshwater extending from below sea 

level to the water table that is several 

hundreds or even thousands of feet above 

sea level. Dikes may intrude the lowpermeability rocks, but a vertically 

extensive freshwater-lens system can 

exist in areas that are dike free. 

Meinzer (1930) defined water below 

the lowest water table as basal ground 

water to distinguish it from perched 

water. According to this broad 

definition, ground water in freshwaterlens and in dike-impounded systems both 

can be considered basal ground water. 

Descriptions of ground water in Hawaii 

have generally limited the use of the term 

"basal" to occurrences of ground water 

with a water table near sea level in highpermeability rocks, although Meinzer’s 

definition of basal ground water was not 

so restrictive. Ground water in vertically 

extensive freshwater-lens systems can 

also be considered basal ground water 

using Meinzer’s definition. 

Freshwater-lens systems are 

recharged by direct infiltration of 

precipitation and irrigation water, and by 

inflow from upgradient ground-water 

systems. Discharge from freshwater-lens 

systems in highly permeable rocks is by 

diffuse seepage near the coast and to 

subaerial and submarine coastal springs. 

In a vertically extensive freshwater-lens 

system, much of the fresh ground water 

discharges directly to stream valleys 

above sea level where the ground surface 

intersects the water table. In highly 

Pearl

Harbor

EXPLANATION

Volcanic-rock aquifer—Yellow where overlain by sedimentary deposits 

Axis of rift zone- or caldera boundary—Approximately located

Well with water level less than 50 feet above sea level

Well with water level greater than 50 feet above sea level

160 ̊ 159 ̊ 158 ̊

22 ̊

21 ̊

0 20 40 MILES

0 20 40 KILOMETERS

P a c i f i c

O c e a n

Niihau

Kauai

Oahu
permeable rocks, freshwater flow is 

mainly horizontal and toward the coast, 

but in low-permeability rocks, vertical 

flow can be significant.

Where confining units impede 

discharge of ground water, the freshwater 

lens is thicker than it would be without 

confinement. Caprock confining units 

impede the discharge of ground water to 

the ocean in southern Oahu, central Maui, 

and western Kauai. Salinity of ground 

water in the caprock is variable and this 

water is considered part of the 

freshwater-lens system. In some places, 

weathered volcanic-rock confining units 

impede the flow of ground water between 

a lower volcanic-rock aquifer and an 

upper volcanic-rock aquifer. Confined 

freshwater in the lower aquifer can be 

overlain by brackish water in the upper 

aquifer. This type of layering in a 

freshwater-lens system occurs in eastern 

Hawaii where a confining unit on the 

surface of the older rocks of Mauna Kea 

Volcano underlies the younger rocks of 

Mauna Loa Volcano. 

 The occurrence of fresh ground water in each of the Hawaiian islands can be depicted using water levels measured in wells. Water levels less than 50 feet 

above sea level were arbitrarily chosen to show occurrences of thin freshwater lenses. Water levels greater than 50 feet above sea level were chosen to show 

areas where vertically extensive freshwater-lens systems or dike-impounded water exist. Non-dike-intruded areas containing wells that penetrate below sea 

level and that have high water levels are considered to have vertically extensive freshwater-lens systems. Where high water levels are found in wells that do 

not penetrate below sea level, the possibility of a perched-water system cannot be ruled out. Although many of the ground-water systems of the islands are 

well understood, exploration in others that are not well understood is only just beginning.

 Kauai has a large area with high water levels along the eastern side of the island. High water levels in wells that penetrate below sea level outside of any 

known rift zone indicate that a vertically extensive freshwater-lens system is present. Niihau receives little rain and data from existing wells indicate that a 

thin freshwater lens is present.

 A large number of wells on Oahu in nearshore areas around most of the periphery of the island have low water levels. High water levels are found in rift 

zones near the eastern and western sides of the island and low-permeability features create high water levels in the central part. Some small areas of perched 

water in the southern part of Oahu are in alluvial deposits, but the perched water is not a significant source of supply. 

 In the northern part of Molokai, areas of high water levels are found in the northwest rift zone of East Molokai Volcano. Lanai has high water levels in the 

interior of the island near the rift zone and caldera complex. Few wells exist on Kahoolawe but because rainfall is low, the freshwater lens is probably thin. 

 The central isthmus and most of the coastal areas of Maui have low water levels indicative of a thin freshwater lens. High water levels are found in the 

interior of West Maui Volcano where rocks are intruded by dikes. On East Maui Volcano, high water levels are found along the northern flanks of the 

volcano in the high rainfall areas. Where high and low water levels occur together, a perched-water system exists above a freshwater lens. Farther to the 

east, high water levels are found in wells drilled below sea level indicating that a vertically extensive freshwater-lens system is present. 

 The island of Hawaii contains high water levels in the rift zones of Kilauea and Kohala Volcanoes. High 

water levels, possibly associated with a buried rift zone of Hualalai Volcano or fault scarps draped with 

lava flows, also are present along the western coast. Areas of high water levels also are found along 

the northern flank and eastern flanks of Mauna Kea and on the southeastern flank of Mauna Loa. 

These high water levels are not fully understood.

157 ̊ 155 ̊

19 ̊

20 ̊

156 ̊

Molokai

Lanai

Kahoolawe

Maui

Hawaii

East

Molokai

Volcano

East

Maui

Volcano

Hualalai

Volcano

Kohala

Volcano

Kilauea

Volcano

Mauna Kea

Volcano

Mauna Loa Volcano

West

Maui

Volcano
Dike-Impounded System

A dike-impounded system is found in 

the rift zones and caldera of a volcano 

where low-permeability dikes have 

intruded other rocks. The flow system 

includes the freshwater body, and where 

it exists, the underlying brackish water 

and saltwater. Near-vertical dikes tend to 

compartmentalize areas of permeable 

volcanic rocks. Dikes impound water to 

heights as much as 3,300 feet above sea 

level on the islands of Maui and Hawaii 

and as much as 1,600 feet above sea level 

on Oahu. The depth to which freshwater 

extends below sea level within a dikeimpounded system is not known in places 

with high water tables. Where few dikes 

intrude permeable volcanic rocks, the 

water table may be only a few feet to a 

few tens of feet above sea level and 

saltwater has been found below the 

freshwater.

Water table

Water table

Freshwater Freshwater

Saltwater Saltwater

Brackish

water

Brackish water

Pacific

Ocean Sea

level

Pacific

Ocean Sea

level

About

40h

NOT TO SCALE

 h

A B

Water table

Freshwater

Saltwater

Brackish

water

Pacific

Ocean

Sea

level

Ground-water discharge

to stream

Caprock

D

C

Pacific

Ocean Sea

level

Well

Water table

Older

Younger

Weathered volcanic rocks

Freshwater

Saltwater Brackish

water

On oceanic islands, such as the Hawaiian islands, freshwater commonly occurs as a body of water called 

a freshwater lens that floats on saltwater and is separated from the saltwater by a transition zone of 

brackish water. (A) The Ghyben-Herzberg principle is commonly used in Hawaii to estimate the depth 

where the brackish water in the transition zone has a salinity about 50 percent that of seawater. The 

Ghyben-Herzberg principle is named after two scientists who independently described a freshwatersaltwater relation for conditions in which the two fluids do not mix (no transition zone) and the 

freshwater flow is predominantly horizontal. For these conditions, the freshwater-lens thickness below sea 

level is directly proportional to the height of the top of freshwater above sea level. In principle, at a place 

where the water table stands 1 foot above sea level, for example, 40 feet of freshwater will be below sea 

level, and the freshwater lens will thus be 41 feet thick. This relation exists because seawater is about 

one-fortieth more dense than freshwater. In a real freshwater-lens system, mixing creates a transition zone 

and reduces the amount of freshwater above the transition zone. In some places where freshwater is 

extensively mixed with saltwater, brackish water may exist immediately below the water table. (B) Where 

low-permeability caprock acts as a confining unit near the coast, a thicker freshwater lens develops 

because ground water cannot discharge as readily to the ocean. The Ghyben-Herzberg principle applies where flow is predominantly horizontal. (C) Weathered 

volcanic rocks on the surface of an older volcano form a confining unit between the lower and upper aquifers. The well penetrates freshwater underlain by 

brackish water. Beneath the confining unit, the well penetrates deeper freshwater underlain by brackish water and saltwater. Fresh ground water flows from the 

deeper volcanic-rock aquifer through the confining unit into the upper volcanic-rock aquifer. (D) In systems where low-permeability rocks cause significant 

vertical hydraulic-head gradients, the Ghyben-Herzberg principle does not accurately predict the depth where the brackish water in the transition zone has a 

salinity about 50 percent of seawater. Although the depth to saltwater has not been measured in wells in these systems, where flow is downward, the freshwater 

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

Dike-compartment Spring water table Perched

water body Freshwater- lens

system

water table

Freshwater

Saltwater Saltwater

Honolulu

Brackish water

Brackish water

C B A

D

F

E

NOT TO SCALE

Fresh ground water generally moves from 

topographically high areas towards the 

ocean. Fresh ground-water flow is generally 

downward in the inland areas, upward in the 

coastal areas, and horizontal in between. A 

saltwater circulation system exists beneath 

the freshwater lens. Saltwater flows landward 

in the deeper parts of the aquifer, rises, then 

mixes with fresher water and discharges to 

the ocean. 

A freshwater-lens system underlies much of 

southern Oahu. Well A produces saltwater 

from below the transition zone, well B

produces brackish water from the transition 

zone, and well C produces freshwater. 

Horizontal shaft D (sometimes called a Maui 

shaft) produces large volumes of freshwater 

by skimming water from near the top of the 

freshwater lens. Shaft E (sometimes called a 

Lanai shaft) is dug horizontally into one or 

more of the dike-bounded compartments. 

Location F indicates a perched water body 

containing minor amounts of water.

Pacific

Ocean

Pacific

Ocean Northeast Southwest

Sedimentary deposits (caprock)

Volcanic rocks

Low-permeability rocks

Dike

B

E Shaft

Well 

Direction of ground-water 

movement

 Freshwater

 Saltwater

EXPLANATION
Where erosion has exposed dike 

compartments in stream valleys such as 

in windward Oahu, ground water can 

discharge directly to the streams. In 

other areas, fresh ground water in dikeimpounded systems can discharge to 

downgradient ground-water systems or 

directly to the ocean. 

Perched System

Perched water is found in areas where 

low-permeability rocks impede the 

downward movement of ground water 

sufficiently to allow a perched water 

body to develop within otherwise 

unsaturated rocks. These lowpermeability rocks include massive, 

thick-bedded lava flows, extensive soil 

and weathered ash layers, and 

sedimentary deposits. The areal extent 

and distribution of these rocks is highly 

variable. The height of the water table 

above sea level depends on the altitude of 

the low-permeability rocks and the rate 

and duration of recharge, and the extent 

of the perched system depends on the 

areal extent of the low-permeability 

rocks. Discharge from a perched system 

can be to springs, streams, the ocean, 

downward to a lower water table, or to 

the atmosphere by evaporation.

WHAT LIMITS GROUND-WATER 

AVAILABILITY?

The main factors limiting groundwater availability in the State of Hawaii 

are saltwater intrusion, the reduction of 

discharge to streams and the ocean, and 

lowering of water levels.

When water is withdrawn from a 

freshwater lens, the freshwater lens 

shrinks and saltwater or brackish water 

will intrude upward and landward into 

parts of the aquifer that formerly 

contained freshwater. The degree of 

saltwater intrusion depends on several 

factors, which include the hydraulic 

properties of the rocks, recharge rate, 

pumping rate, and well location. The 

effect of intrusion on a particular well 

depends on the vertical and lateral 

distance between the well and the 

transition zone. Wells completed in the 

freshwater lens near the coast are 

particularly likely to induce brackish 

water or saltwater movement into the 

well as pumping continues. Saltwaterintrusion problems can be minimized by 

appropriately locating wells and by 

controlling withdrawal rates. 

In the Honolulu area of Oahu, some 

wells that originally produced fresh 

ground water were later abandoned 

because of increased salinity due to 

saltwater intrusion. Pumping from a well 

can cause the freshwater-saltwater 

transition zone to rise into the pumped 

well. Many wells in Hawaii that are 

pumped at high rates or drilled too deeply 

are affected by this process, resulting in 

increased sodium and chloride 

concentrations in pumped water. 

Ground-water withdrawal ultimately 

reduces the amount of discharge to 

springs, streams, or the ocean by the 

amount that is withdrawn. Reduction of 

springflow and streamflow is a concern 

for several reasons, including loss of 

habitat for native aquatic species, reduced 

water supply for agricultural diversions, 

aesthetics, and recreational use. Reduced 

flow to the ocean may affect marine 

habitat and aquaculture practices in 

coastal fishponds.

Ground-water withdrawal also lowers 

water levels around the pumped well. 

Nearby wetlands and ponds may shrink 

or dry up causing loss of habitat for 

aquatic species. Discharge from nearby 

flowing wells may decrease or stop if 

water levels are lowered sufficiently.

Additional Reading:

This fact sheet is based on the 

publication Oki, D.S., Gingerich, S.B., 

and Whitehead, R.L., 1999, Hawaii in

Ground Water Atlas of the United States, 

Segment 13, Alaska, Hawaii, Puerto 

Rico, and the U.S. Virgin Islands: U.S. 

Geological Survey Hydrologic 

Investigations Atlas 730-N, p. N12–N22, 

N36. 

http://sr6capp.er.usgs.gov//gwa/gwa.html 

REFERENCES

Hunt, C.D., Jr., 1996, Geohydrology of the 

island of Oahu, Hawaii: U.S. Geological 

Survey Professional Paper 1412-B, 54 p.

Izuka, S.K., and Gingerich, S.B., 1998, 

Ground water in the southern Lihue 

Basin, Kauai, Hawaii: U.S. Geological 

Survey Water-Resources Investigations 

Report 98-4031, 71 p.

Macdonald, G.A., Abbott, A.T., and Peterson, 

F.L., 1983, Volcanoes in the sea: 

The geology of Hawaii (2d ed.): 

Honolulu, Hawaii, University of Hawaii 

Press, 517 p.

Meinzer, O.E., 1930, Ground water in the 

Hawaiian Islands in Geology and water 

resources of the Kau District, Hawaii: 

U.S. Geological Survey Water-Supply 

Paper 616, p. 1-28.

Takasaki, K.J., and Mink J.F., 1985, 

Evaluation of major dike-impounded 

ground-water reservoirs, island of Oahu: 

U.S. Geological Survey Water-Supply 

Paper 2217, 77 p.

Thomas, D.M., Paillet, F.L., and Conrad, 

M.E., 1996, Hydrogeology of the Hawaii 

scientific drilling project borehole KP-1- 

2. Groundwater geochemistry and 

regional flow patterns: Journal of 

Geophysical Research, v. 101, no. B5, p. 

11,683-11,694.

–By Stephen B. Gingerich and Delwyn S. Oki

For further information contact:

District Chief

U.S. Geological Survey, WRD

677 Ala Moana Blvd., Suite 415

Honolulu, HI 96813

2000

Pacific

Ocean

Pumped well

A

B

Freshwater

Saltwater

Water table

Br ackish water

Br ackish water

Pacific

Ocean

NOT TO SCALE

Pumped well

Freshwater

Saltwater

Water table

Saltwater intrusion is a potential problem near 

the coast. (A) Diagram of a well completed in a 

volcanic-rock aquifer in which withdrawal is 

small. Only limited saltwater intrusion has taken 

place. (B) Diagram of the same well under 

conditions of large ground-water withdrawal. 

Pumping has lowered the water table and caused 

the freshwater lens to thin. Brackish water has 

reached the well.