Ground-Water 

Reconnaissance of 

American Samoa

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1608-C

Prepared in cooperation with 

the Territory of American Samoa
Ground-Water 

Reconnaissance of 

American Samoa

By DAN A. DAVIS

CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA 

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1608-C

Prepared in cooperation with

the Territory of American Samoa

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C., 20402
CONTENTS

Abstract________________________________________________________ Cl

Introduction. ___________________________________________ 1

Purpose and scope_____________________________________________ 1

Location and extent of area____________________________________ 2

Previous investigations..______________________________________ 2

Acknowledgments. __________________________________________ 3

Geography______________________________________________________ 3

Surface features-_____________________________________________ 3

Climate._____________________________________________________ 4

Population and development--__-___-______-_--_-____--__-_---_- 5

Geology._________________________________________________________ 6

Tutuila_______________________________________________ 6

Aunuu_______________________________________________________ 8

Ofu and Olosega_____________________________________________ 8

Ta'u_________________________________________________________ 8

Ground water_____________________________________________________ 8

Streamflow-__________________________________________________ 9

Tutuila.______________________________________ 10

High-level ground water___________________________-_---_-__ 10

Basal ground water_____________________________________ 12

Development and use of ground water______________________ 13

Potential development of ground water_____________________ 15

Aunuu.______________________________________________________ 16

Ofu and Olosega_____________________________________________ 16

Ta'u____________________________________________________.____ 19

Summary_______________________________________________________ 21

References______________________________________________________ 21

ILLUSTRATIONS

Paso

FIGURE 1. Islands of American Samoa--____________--__---__-__--_ C2

2. Map of Tutuila and Aunuu showing principal springs and wells. 14

3. Geologic conditions at a seepage zone near Fatamafuti, Tutuila- 15

4. Fresh basal ground water in lava flows and beach deposits.__> 17

5. Map of Ofu and Olosega showing principal springs and wells. __ 18

6. Geologic conditions at a small spring near Fagamalo, Ta'u 19

7. Map of Ta'u showing principal springs and wells.___________ 20

in

CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA

GROUND-WATER RECONNAISSANCE OF AMERICAN

SAMOA

By DAN A. DAVIS

ABSTRACT

The principal islands of American Samoa are Tutuila, Aunuu, Ofu, Olosega, 

and Ta'u, which have a total area of about 72 square miles and a population of 

about 20,000. The mean annual rainfall is 150 to 200 inches.

The islands are volcanic in origin and are composed of lava flows, dikes, tuff, 

and breccia, and minor amounts of talus, alluvium, and calcareous sand and 

gravel. Tutuila is a complex island formed of rocks erupted from five volcanoes. 

Aunuu is a tuff cone. Ofu, Olosega, and Ta'u are composed largely of thin-bedded 

lava flows.

Much of the rock of Tutuila has low permeability, and most of the ground water 

is in high-level reservoirs that discharge at numerous small springs and seeps. 

The flow from a few springs and seeps is collected in short tunnels or in basins for 

village supply, but most villages obtain their water from streams. A large 

supply of basal ground water may underlie the Tafuna-Leone plain at about 

sea level in permeable lava flows. Small basal supplies may be in alluvial fill 

at the mouths of large valleys.

Aunuu has small quanties of basal water in beach deposits of calcareous sand 

and gravel. Minor amounts of high-level ground-water flow from springs and 

seeps on Ofu, Olosega, and Ta'u. The generally permeable lava flows in the 

three islands contain substantial amounts of basal ground water that can be 

developed in coastal areas in wells dug to about sea level.

INTRODUCTION

PURPOSE AND SCOPE

The reconnaissance on which this report is based was made in 

cooperation with the Territory of American Samoa to obtain information on water resources that would assist in improving the water 

supplies of villages in the islands of American Samoa. Field work 

was done by Max H. Carson and D. A, Davis and included an examination of water sources in about 65 villages. Streamflow and surfacewater supplies were examined by Carson, and observations on groundwater resources were made by Davis.

01
C2 CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA

The report gives a brief description of streams of the islands. It 

relates the modes of occurrence and methods of development of ground 

water and estimates the availability of ground water in the various 

islands

LOCATION AND EXTENT OP AREA

The islands of American Samoa are a part of the Samoan archipelago, which lies roughly 1,000 miles south of the equator and forms a 

westward-trending chain between long 168° and 173° W. The principal islands of American Samoa are Tutuila, Aunuu, Ofu, Olosega, 

and Ta'u (fig. 1). These form the eastern part of the archipelago

171° 170" 169° 168°

14°

TUTUILA 

.S^j^^AUNUU

SOUTH

0 20 40 60 IV

OFU^QLOSEGA 

fS>TA'U 

MANU'A ISLANDS

PA C 1 F 1 C

1ILES

ROSE 1

O

OCEAN

171° 170° 169"

FIGCEB 1. Map showing Islands of American Samoa.

168°

and extend over a distance of about 100 miles along a line just 

south of lat 14° S. Ofu, Olosega, and Ta'u, which lie about 60 miles 

east of Tutuila, make up the Manu'a Islands. Two small atolls 

Rose Island, about 200 miles east of Tutuila, and Swains Island, 

about 400 miles north of Tutuila which are administratively a part 

of American Samoa, were not included in the study.

PREVIOUS INVESTIGATIONS

In 1941 and 1943, Harold T. Stearns, of the U.S. Geological Survey, 

made geologic and water-supply investigations in American Samoa 

for the U.S. Navy. Hydrologic work by Stearns consisted largely 

of investigations of water supplies for the Naval Station at Pago 

Pago Harbor and for a few small Naval and Marine Corps installa-
GROUND-WATER RECONNAISSANCE OF AMERICAN SOMOA C3

tions on other parts of Tutuila. The results of the geologic investigations are given in a published description of the geology of the Samoan 

Islands and a geologic map of Tutuila (Stearns, 1944). The petrography of Samoa, based on specimens collected by Stearns, has been 

described by G. A. Macdonald (1944).

AKNOWLEDGMENTS

The Government of American Samoa provided transportation on 

Tutuila and between Tutuila and the other islands and extended 

numerous other courtesies during the investigation. Lei'ato, Governor of the Eastern District; Letuli, Governor of the Western District; 

and Lefiti, Governor of the Manu'a District, manifested great interest 

in the investigation and assisted in its progress in many ways. Appreciation is due particularly to the chiefs, the "pulenu'u" (administrative 

officers), and the people of the counties and villages who gave great 

material assistance in field work on all islands. Mr. Louis F. Tuu 

gave valuable service during the work as driver on Tutuila and interpreter on all the islands.

GEOGRAPHY

SURFACE FEATURES

The islands of American Samoa are volcanic in origin and exhibit 

topographic and drainage features that commonly are found in Pacific 

islands of this type. The area and maximum altitude of the islands 

covered in this report are given in the following tabulation.

Island 

Tutuila _______ ___ _

Aunuu __ _ ____ _

Ofu_ ______ ___ .

Olosega _ __

Ta'u .,.. .. ......_ _ ...........

Area Maximum 

(square altitude 

miles) (feet)

__ __ _ __ __ . 53 2,140

__ ___ ____ _ . .6 300

._ __ __ ____ __. 2 1,590

__ __ __ ___ . 1.5 2,090

______ _ __ _ . 15 3,050

Tutuila (fig. 2) is long and narrow and is dominated by a high 

irregular ridge extending throughout its length. Except on the relatively low Tafuna-Leone plain, which forms a bulge on the south side 

of the western half of the island, closely spaced streams have eroded 

the island into generally steep slopes. On parts of the island, streams 

have radial patterns around centers of former volcanic activity. The 

streams have steep gradients, except near the shore where widening 

of valley floors results in small coastal flats contained between the 

seaward ends of steep intervalley ridges. The coast between the 

mouths of the valleys is rocky, and the land rises in cliffs which 

range from a few tens of feet to several hundred feet in height. 

Drowned valleys form several embayments, the most striking of which
C4 CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA

is Pago Pago Harbor. The Tafuna-Leone plain, which is a young 

surface formed by late volcanic activity, has no streams and is altered 

.only slightly by erosion.

Aunuu, off the east end of Tutuila, is a small volcanic cone that 

is dominated by a slightly dissected crater whose rim rises 200 to 

300 feet above sea level. The floor of the crater is occupied by a 

marsh and lake. Marine erosion has developed steep cliffs on the 

east and south sides of the cone; on the west and north sides the 

cone is bordered by a low coastal flat.

The islands of Ofu and Olosega form a doublet separated by a 

shallow narrow channel. Both islands rise from the sea in spectacularly steep slopes that culminate in sharp summit ridges, and no flat 

land is to be found on either island except for narrow coastal flats 

along some shores. There are no significant valleys on the islands.

Ta'u, like Ofu and Olosega, rises abruptly from the sea; but, 

in contrast with the smaller islands, it is dome shaped. Streams 

are few and have cut only poorly defined shallow valleys. Narrow 

coastal flats occupy parts of the island.

CLIMATE

The climate of Samoa is tropical, characterized by uniformly warm 

temperatures and high humidity. The islands lie in the belt of the 

southeast trade winds, which tend to be strongest in the winter months, 

June to August, and weakest in the summer, December to February, 

when the intertropical front moves southward.

Eainfall records in American Samoa are insufficient for more than 

speculation on the total rainfall and the geographic distribution. 

During the period 1900 through 1940, a record of rainfall was kept at 

the U.S. Naval Station on Tutuila; but the location of the gage is 

not know definitely, and it probably was moved a considerable distance 

at one or more times during the period of record. As it stands, the 

41-year record indicates a mean annual rainfall of about 200 inches. 

Based on rainfall records kept by Mr. Max Halleck, the mean annual 

rainfall at Pago Pago during the period from 1946 through 1956 was 

about 164 inches. Monthly means from the Pago Pago record during 

the 11-year period are shown in the following tabulation:

Mean rainfall 

Month (inches)

January __________ 17. 3

February _________ 16. 4

March ___________ 17. 2

April ____________ 11. 6

May ______________ 14.1

June __ _____ 10. 4

Mean rainfall 

Month (inches)

July _____________ 8.2

August ___________ 9.2

September ________ 7.1

October __________ 16.2

November ____ _ 14.9

December __ 21. 5
GROUND-WATER RECONNAISSANCE OF AMERICAN SAMOA C5 

POPULATION AND DEVELOPMENT

The distribution of population of American Samoa, as indicated by 

the 1960 census, is shown in the following tabulation.

Number of 

Island milages Population

Tutuila____________________________. 66 16, 814

Aunuu____________________________. 1 436

Ofu______________________________ 1 605

Olosega____________________________. 2 429

Ta'u______________________________. 5 1, 661

Total_________________________. 75 19, 945

All the people live in villages, and most of the villages are on lowlands near the shore.

A road system connects the villages on the south side of Tutuila, 

extending from Tula on the extreme east end to Amanave on the west 

end. Fagasa and Aoa, on the north coast, are connected to the south 

side by passable roads, but communication with other villages on the 

north side is by boat or trail only. Electric power, telephone service, 

and public water are available in the administration area at Pago Pago 

Harbor, but these facilities do not extend beyond the harbor area, 

Transocean and interisland radio communication is provided through 

a station at Pago Pago.

There are no roads on Aunuu, Ta'u, Ofu, and Olosega, but the 

villages are connected by well-kept trails. Small electric-power plants 

are operated at dispensaries on Ta'u and Ofu, and radio communication is maintained between the dispensaries and the station at Pago 

Pago.

Agriculture consists mainly of growing coconuts, breadfruit, bananas, cacao, taro, and yams. Corpra from coconuts is the principal 

agricultural export. Some sugarcane is grown, chiefly for its leaves 

for thatch, and pandanus is grown for mats. Crops are grown without irrigation, mostly on steep hill sides. A few swine, poultry, and 

cattle are raised for food.

The principal industry is a fish cannery on the north shore of 

Pago Pago Harbor, which is supplied with fish caught by a Japanese 

fleet operating out of Pago Pago.

Most of the working population is engaged in subsistence agriculture. Smaller numbers are employed by the government, the fish 

cannery, or in commercial and service activities.

686158 63 2
C6 CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA

GEOLOGY

The geology of the Samoan Islands has been described by Stearns 

(1944). The following discussion, which is based largely on Stearns' 

more elaborate description, is limited in general to features that are 

pertinent to the occurrence of water in the islands.

TUTUILA

Tutuila is made up chiefly of basaltic rock that was erupted from 

five volcanoes arranged in a rough east-west line. The major activity, 

which occurred at four centers in Pliocene or early Pleistocene time, 

produced shield-shaped masses that merged into a single elongate 

island, longer and broader than the present Tutuila. After activity at 

the four centers ceased, deep valleys were cut by streams, and the shoreline retreated under the attack of waves. Toward the end of Pleistocene time, the island had been reduced to approximately its present size 

and form. More recently, the fifth volcano became active and built 

the Tafuna-Leone plain, and a small submarine eruption off the east 

end of Tutuila built the tuff cone that is now Aunuu. During the 

subsequent erosional period, sedimentary deposits were formed at the 

foot of steep slopes, on valley floors, and along the coast. The general character of the rocks on the island and their relative ages are 

shown in the following stratigraphic section.

The large number of eruptive centers in a small area, the overlapping of rocks erupted from different volcanoes, and the presence 

of dikes and interbedded tuff, cinder, and breccia among the lava 

flows have produced complex geologic conditions on Tutuila. The 

bulk of Tutuila is formed by aa and pahoehoe lava flows that range 

from thin-bedded flows with high permeability to thick-bedded dense 

flows with low permeability. The late flow rock on the TafunaLeone plain is mostly thin bedded, but on older parts of the island, 

particularly along the coast, the flows are mostly thick and dense.

Dikes are dense and impermeable, and where their spacing is close, 

they may reduce greatly the permeability of the rock that they cut. 

Unweathered tuff may have moderate permeability, but weathered 

tuff is almost impermeable. The breccia is mostly dense and has low 

permeability. The permeability of fresh cinder is high, but it may 

be greatly decreased by weathering because of consolidation that occurs 

during chemical decomposition. Lava flows, although they far exceed 

the other types of rock in total volume, are divided by dikes and 

interbedded tuff and breccia into units which are irregular in size and 

shape. The, total vertical and horizontal extent of any unit of flows 

is small as compared to the size of the island.
GROUND-WATER RECONNAISSANCE OF AMERICAN SAMOA C7

Stratigraphic section of Tutuila (after Stearns, 1944) and water-bearing 

characteristics af the rocks

Geologic age

Pliocene and 

earliest 

| PleistoFormation

Sedimentary 

rocks

Leone Volcanics

Aunuu Tuff

Trachyte

Taputapu 

Volcanics

Pago Volcanic 

Series (extracaldera volcanics)

Pago Volcanic 

Series (intracaldera volcanics)

Alofau 

Volcanics

Olomoana 

Volcanics

Masefau dike 

complex

Thickness 

(feet)

200+

200+

200+

2, 140+

1,639+

1, 609+

2, 141+

963+

1, 074+

200+

General character

Alluvium on valley floors; 

talus at foot of steep slopes; 

calcareous sand and coralline gravel along coast.

Olivine basalt in pahoehoelava flows; tuff in cones and 

veneer over lava flows; ash 

and cinder in cones.

Tuff forming Aunuu island.

Trachytic dikes, plugs, and 

crater fill younger than most 

of the Pliocene volcanics.

Thin-bedded olivine basalt 

with interbedded tuff beds 

and cinder cones.

Lower member of thin-bedded 

olivine basalt; includes thin 

dikes and thin tuff beds. 

Upper member of massive 

basaltic and andesite flows; 

interbedded tuff and cinder 

deposits.

Massive basaltic and andesitic 

lava flows and some interbedded tuff and breccia and 

cinder cones; few thin lenses 

of gravel.

Thin-bedded olivine basalt 

with numerous dikes.

Olivine basalt and andesitic 

basalt; interbedded cinder 

cones and tuff beds.

Thin basaltic flows cut by 

many dikes.

Water-bearing characteristics

Alluvium in mouths of valleys 

is moderately permeable and 

contains fresh to brackish 

water. Talus locally yields 

water in seeps and small 

springs. Sand and gravel 

contain small supplies of fresh 

water.

Lava flows under TafunaLeone plain have moderate 

to high permeability and may 

contain considerable fresh 

water at about sea level. 

Tuff and ash have low permeability and yield little 

water. A few small springs 

flow from cinder.

Low permeability and yields 

little water.

Low permeability. Yields 

water in high-level seeps and 

small springs.

Moderate permeability. 

Yields water at many scattered high-level seeps and 

small springs.

Moderate to low permeability. 

Yields water at many scattered high-level seeps and 

small springs.

Mostly low permeability. 

Yields water at scattered 

high-level seeps and small 

springs.

Lava flows between dikes have 

moderate to high permeability. Rocks yield water at 

many high-level seeps and 

small springs.

Moderate permeability. 

Yield water at many highlevel seeps and small springs.

Low permeability. Yield 

little water.

The sedimentary rocks of Recent age consist of talus, alluvium, 

calcareous sand, and coralline gravel. The talus is poorly sorted 

material, in which large blocks are prominent, accumulated at the 

bottoms of steep slopes along the coast and in steep-walled valleys; 

the permeability of the talus is variable. Thick deposits of alluvium, 

comprising poorly sorted mixtures of clay, sand, gravel, and boulders 

derived from volcanic rock, underlie the lower ends of large valleys; 

the alluvial fill extends below sea level at the mouths of some valleys
C8 CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA

and has a permeability ranging from low to high. The calcareous 

sand and gravel are composed of material originating in the reefs 

that fringe a considerable part of the island and are most abundant 

in beaches at the mouths of valleys; the calcareous deposits generally 

are more permeable than the alluvium.

Beneath a part of the late volcanic rock underlying the TafunaLeone plain are sediments that were deposited before the eruption 

of the Leone volcanics. The nature and extent of these deposits are 

not known, but they probably consist partly of talus and mainly of 

calcareous beach deposits and alluvium.

AUNUU

The bulk of Aunuu is an indurated bedded tuff produced by submarine volcanic explosions. The tuff, in general, is relatively fine 

grained and has been altered sufficiently by weathering to make the 

material nearly impermeable. The coastal flat on the west and northwest sides of the island is underlain by material washed down from 

the cone and by permeable deposits of calcareous sand and gravel.

OFU AND OLOSEGA

The islands of Ofu and Olosega are remnants of a larger island 

built by a single volcano. Erosion by streams and by the sea has cut 

deeply into the original island and has reduced it to two small steep 

islands separated by a narrow channel. The islands are composed 

mainly of thin-bedded basaltic lava flows whose permeability is high. 

Numerous dikes cut the lava flows on the east end of Ofu and the west 

end of Olosega. Talus is common around the coast, and deposits of 

calcareous sand and gravel mantle considerable parts of the shore of 

each island.

TA'U

Ta'u is the remnant of a single balsaltic dome that has been reduced 

in size and modified in profile by marine erosion and, possibly, by 

faulting. The island consists mainly of thin-bedded lava flows, except 

a small sector in the northwestern part, which is formed by several 

tuff cones. Narrow flats around the shore are underlain by beach 

deposits of calcareous sand and gravel.

GROUND WATER

The source of all fresh water in the islands is rainfall. A part of 

the rainfall escapes rapidly as direct runoff into the sea, and a part 

escapes by evaporation and transpiration. The remainder moves 

downward through the soil and rock and enters storage in groundwater reservoirs.
GROUND-WATER RECONNAISSANCE OF AMERICAN SAMOA C9

Some of the ground water is held above sea level or separated from 

sea water by barriers or impermeable beds such as dikes or beds of tuff 

and breccia similar to those shown in figures 3 and 6. The name "highlevel ground water" is applied to the water impounded in the rock 

above sea level. In Samoa it generally is in small reservoirs that 

discharge underground at low points in the impounding members or 

at the surface of the ground where the impermeable structures intersect erosion surfaces. Discharge at the ground surface is through 

springs and seeps that maintain the flow of streams during periods of 

low rainfall.

Ground water that is not held up by impermeable barriers moves 

down to about sea level where it accumulates generally as lenslike 

bodies floating on the slightly heavier sea water that saturates the 

rock below sea level. (See figure 4.) The ground water near sea 

level, including the fresh-water lens and the sea water in which it 

floats, is called "basal ground water." It generally is the source of 

the springs that discharge along the shore and that are particularly 

evident along some sandy beaches at low tide.

The principal water-bearing rocks in Samoa are the basaltic lava 

flows. The occurrence and availability of the ground water are controlled by the permeability of the lava flows, the relation of the flows 

to dikes and other rock that are relatively impermeable, and the extent 

to which the lava terranes have been dissected by erosion. Locally, 

tuff, cinder, talus, and calcareous beach sand and gravel yield water, 

but the volume of these materials is small in comparison to that of the 

lava flows.

STBEAMFLOW

Nearly 200 perennial and ephemeral streams flow from the mountains of Tutuila into the sea or onto the Tafuna-Leone plain. Drainage areas of the streams range from less than 0.1 square mile to about 

2 square miles and average about 0.25 square mile. Many of the 

small ephemeral streams flow down precipitous slopes into the sea. 

The gradients of the larger streams are from 500 to 1,000 feet to the 

mile, except near the shore where the gradients are gentle for short 

distances on valley floors of alluvial fill.

The base flow of the perennial streams comes from numerous small 

springs and seeps discharging from high-level ground-water reservoirs. Many of the streams have trickles of a few hundred to a few 

thousand gallons per day during dry weather. The largest streams 

seem to have average flows of a few million gallons per day and 

dry-weather flows of a few hundred thousand gallons per day.

Gaging of streams on Tutuila was started in 1958 by the Geological 

Survey in cooperation with the Government of American Samoa. The
CIO CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA

program includes continuous measurements of flow on about 6 streams 

and occasional measurements on about 50 streams. Records of the 

gaging are compiled for publication in Water-Supply Papers issued 

by the Geological Survey.

The water used in most of the villages of Tutuila is obtained from 

reservoirs or pools in streams. The reservoirs generally are formed 

by low masonry or concrete dams across small streams. Some reservoirs have capacities of several thousand gallons, but most hold only 

a few hundred gallons and are little more than diversion structures for 

pipelines that lead to villages. The supply in some villages is dipped 

from pools in streams; the upstream pools are used for drinking 

water and the downstream pools are used for washing and bathing. 

Occasional water shortages affect many of the villages because of 

deficient streamflow or inadequate storage.

A reservoir formed by a relatively large dam across the Fagaalu 

Stream supplies a large part of the water used in the administration 

area at Pago Pago. The dam impounds flow from a large spring 

and runoff from the upper part of the Fagaalu Valley.

The combined discharge of the major streams on Tutuila probably 

is sufficient to meet all demands for domestic water on the island. 

Collection and storage of water from the scattered sources and distribution to areas of need would involve considerable construction.

Aunuu has no streams. A few springs on the slopes of Ofu and 

Olosega may feed streams that flow short distances, but the water 

soon sinks into the permeable rock.

Charts of Ta'u (fig. 7) show a stream called Laufiti River on the 

south side of the island, but it is in a remote area and no information 

is available on it. A small apparently perennial stream on the north 

slope of Ta'u near the village of Fitiuta has a flow of a few thousand 

gallons a day. This supply is piped to Fitiuta, Maia, and Leusoalii. 

Other similar streams may flow on the upper north slope of the island, 

but none reaches the shore. A small rain catchment and reservoir 

has been used at Siufaga for water supply.

TTTTTTILA 

HIGH-LEVEL GROTJND WATER

High-level ground water is hydrologically important on Tutuila 

because it is the source that maintains the base flow of the streams. 

Discharge to the streams occurs in numerous springs and seeps, some 

of which are large enough to be easily seen but many of which are so 

small or diffuse that their additions to the streams are almost imperceptible. Because of the heavy cover of soil and vegetation, conditions
GROUND-WATER RECONNAISSANCE OF AMERICAN SAMOA Oil

that control the occurrence and discharge of high-level ground water 

usually are not readily apparent.

At many places on Tutuila, permeable lava flows lie on relatively 

impermeable layers of tuff, breccia, or old soil. Water moving downward through the lava is intercepted and accumulates on, or moves 

along, the surface of such an impermeable layer. The thickness of 

the water body will depend on the areal extent and the dip of the 

surface of the barrier, the permeability of the aquifer, and the rate 

at which water is added to it from above. The thickest water bodies 

tend to form on widespread flat-lying impermeable layers. If an 

impermeable layer does not intersect the surface of the ground, water 

will flow over the edges of the layer and move on down through the 

rocks. Where the layer is cut by a cliff or a valley slope, the water will 

discharge at the surface in springs and seeps from which it may flow 

into streams or be absorbed again in the rocks. Discharge of ground 

water in this manner tends to be in long narrow zones, and, on Tutuila, 

the rate of flow from the aquifer commonly is so low that the discharge 

along seepage zones or lines of small springs may be obscure unless 

the cover of soil and vegetation is thin or is cleared away.

Dikes are vertical or have steep dips, and where they occur in groups, 

the country rock is divided into segments bounded by walls of dense 

dike rock. These segments are variable in size and shape according 

to the spacing and orientation of the dikes. A series of parallel dikes 

will produce a corresponding sequence of slices of country rock. A 

second series transverse to the first will divide the rock into cells that 

have well-defined lateral boundaries and may have considerable vertical extent. Generally, the spacing of intersecting dikes is uneven and 

the parallelism is crude; consequently, the cells of country rock have 

a great range in size and shape. Kocks with night permeability that 

lie between widely spaced intersecting dikes are potentially highly 

productive aquifers, but as the distance between dikes decreases, the 

overall permeability of the rock is lowered because of the increasing 

proportion of dense dike rock. Discharge of water from dike compartments usually takes place over the dikes at low points where they 

are crossed by streams. Dikes are numerous in some parts of Tutuila, 

but, in general, they are so closely spaced that the yield of water, in the 

form of springs, from the dike compartments apparently is small.

The simple conditions described above may be seen in a few places on 

Tutuila, but most of the high-level ground water apparently is controlled by complex combinations of the simple relations. For example, 

dikes or faults, or both, may complicate the simple combination of 

permeable lava lying on impermeable tuff or of erosional unconformities and overlapping of rocks from different eruptive centers:
C12 CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA

when combined with other structures, these combinations may produce 

intricate patterns of permeability. As a result, most of the high-level 

reservoirs are small.

Some high-level water accumulates in talus and appears as seeps 

and small springs, generally near the foot of the talus slopes.

High-level ground water in cinder is the source of small springs 

in a few places. An example of this occurrence is on the upper slopes 

of the cinder cone Olotele, where interbedded lava flows and weathered 

layers less permeable than the fresh cinder hold up small bodies of 

perched water.

The total amount of high-level water discharged daily in all of 

Tutuila is large, but, with one known exception, it is widely dispersed 

in many small springs and seeps. An exceptional spring near the 

head of Fagaalu Valley has a reported flow of about 500,000 gpd 

(gallons per day). This spring is behind a dam built across the 

Fagaalu Stream, and the resulting reservoir is used as a principal 

source of water for the supply system in the Pago Pago Harbor area.

BASAL GROUND WATER

Fresh ground water accumulating at sea level and floating on the 

heavier sea water forms lenses that vary in size and quality with 

the character of the rock, the amount of fresh-water recharge, and 

the tidal movement in the ground water. If the permeability of the 

rock and the rate of recharge are favorable, the lens will remain 

perennially fresh. Movement of the ground water in reaction to the 

tide promotes mixing of the fresh water and sea water, and if the 

mixing is not damped within the rock or its effect is not balanced 

by sufficient recharge of fresh water, the lens will become salty.

The coast of the mountainous part of Tutuila has not been explored 

for basal ground water with any thoroughness; however, the dense and 

massive appearance of the lava flows near the shore suggests that 

the permeability of the rock is too low for large basal supplies. 

Springs in beaches and along rocky shores, which are the most common 

evidence of basal water discharge, are rare in this part of the island.

In an effort to develop basal ground water, a well was dug near the 

head of Pago Pago Harbor in 1943. The well has 325 feet of horizontal infiltration tunnels at the bottom of a shaft 76 feet deep. At 

the time of construction, the static water level was 24 feet above sea 

level, and the yield was about 200 gpm (gallons per minute). The 

rocks cut by the tunnels are lava flows, breccia, and dikes. Two 

faults were crossed by the tunnels. The high water level and the 

complex geology exposed in the. tunnels indicate that the ground water
GROUND-WATER RECONNAISSANCE OF AMERICAN SAMOA CIS

at the site of the well is impounded and separated from sea water by 

dikes and other structures. Hydrogeologic conditions in many parts 

of Tutuila probably are similar to those at the Pago Pago well.

The rocks of a large part of Tafuna-Leone plain appear to be 

favorable for a considerable supply of basal water. Tuff is found in 

the area, but most of the plain is underlain by thin-bedded pahoehoe 

lava flows which appear to form a highly permeable unit. Some 

springs, several of which discharge a considerable volume of water, 

are reported to flow along the shore of the plain. Although some are 

brackish, the springs indicate that a large amount of water moves 

through the rock under the plain, and reasonably fresh water probably 

saturates the rock more than half a mile inland. The sedimentary 

rocks that underlie the basalt along the inland edge of the plain may 

affect the quality and quantity of the water in the basalt, but their 

effect cannot be determined without subsurface exploration.

Alluvium underlying the lower ends of large valleys probably contains water that is recharged by streams.

DEVEI/OPMENT AND USE OF GROUND WATER

Ground water from the spring in the reservoir in Fagaalu Valley 

and from the well at Pago Pago supplies a large part of the needs in 

the Government area on the shore of Pago Pago Harbor. Outside 

the harbor area, however, only a few ground-water sources are developed. A few villages channel water from high-level seeps and springs 

into basins or small reservoirs, from which the water flows through 

pipelines to one or more outlets in the villages. Most of the springs 

are in dense lava flows or tuff and flow only a few gallons per minute. 

Two springs in cinder on the slope of Olotele have a combined discharge of about 20 gpm, and this flow is piped to the villages of Iliili, 

Futiga, and Vaitogi. (See figure 2.)

The water supply at Tula, on the east end of the island, comes from 

a sealed tunnel driven into a high-level aquifer concealed by soil and 

vegetation. A tunnel and spring in talus near the shore of Fagasa 

Bay supply a few gallons a minute to Fagasa village. Figure 3 shows 

the geologic conditions at a seepage zone at Fatamaf uti, where a tunnel 

in lava overlying breccia and tuff develops about 5 gpm. The water 

collected by tunnels and other improvements at springs commonly is 

equaled or exceeded by the amount that escapes.

No basal water lias been developed in the communities of Tutuila. 

However, water flowing from basal springs along the shore of the 

Tafuna-Leone plain is used occasionally.
C14 CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA
GROUND-WATER RECONNAISSANCE OF AMERICAN SAMOA C15

FIGURE 3. Sketch showing geologic conditions at a high-level seepage zone and waterdevelopment tunnel near Fatamafuti, Tutuila.

POTENTIAL DEVELOPMENT OP GROUND WATER

Geologic conditions in most of Tutuila appear unfavorable for the 

development of ground water in large quantities at single installations. However, high-level supplies in the mountains probably could 

be found to meet modest demands in many of the villages. These 

sources could be developed by means of tunnels or other structures 

designed to intercept effectively the flow from springs and seepage 

zones. Selection of most sites for development would require careful 

examination of the geology and the occurrence of water. Construction work necessary to develop the supplies would be relatively simple, 

and the work would be within the capabilities of the labor available 

in most of the villages. Hydrologic guidance and engineering advice, 

however, would be necessary in most development work. Potential 

capacities of individual water sources are variable; the normal range 

probably is 5 to 10 gpm, but a few sources may yield as much as 20 gpm.

High-level water supplies of a few hundred gallons per minute 

possibly could be developed in some places by elaborate wells consisting of long infiltration tunnels draining into pump sumps. The 

well at Pago Pago, supplying a part of the water in the Harbor area, 

is of this type. This manner of development is costly, however, and 

normally suitable sites are difficult to find without detailed and expensive exploration.

There is no evidence of basal ground water in large quantities in the 

volcanic rock along the shore of the mountainous part of Tutuila; 

however, evaluation of this potential source can be made only after a 

careful study of the coast of the island. Small basal supplies in lava 

flows probably are available in many places, but their development 

would require the digging of wells from which the water would have
C16 CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA

to be pumped. Capacities of 5 to 10 gpm could be expected in most 

dug wells. Depths of the wells, which would be sunk at least to sea 

level, would depend on the altitude of the locations. An increase in 

the salt content of the water in basal wells is always a possibility, 

especially if pumping rates are high.

Alluvium underlying the lower ends of some large valleys may 

contain considerable amounts of basal water that could be developed 

in dug wells. The most favorable places to explore for this water are 

the two valleys entering Pago Pago Harbor at Aua and at Pago Pago.

Large quantities of fresh basal water may be available in lava flows 

underlying the Tafuna-Leone plain, but exploratory drilling at several 

sites would be necessary to find the best well sites. Information 

needed for selection of well sites includes the quality of the ground 

water, the altitude of the water table, and the water-bearing character 

of the rock at and below sea level.

AtTNUtT

High-level ground water on Aunuu is insignificant, but a basal 

ground-water body of appreciable volume exists in the calcareous 

sand and gravel that underlie the coastal flat on the west and northwest sides of the island. Four or five shallow dug wells in the calcareous deposits supply the water used in Aunuu village. According 

to reports, the wells are a dependable source of supply for the village, 

and the salinity does not rise excessively during dry periods. A freshwater lake and swamp occupies the floor of the crater of Aunuu cone, 

and ponds and swamps lie on the inland edge of the coastal flat.

OFtT AND OLOSEGA

Ofu and Olosega are composed largely of thin-bedded lava flows 

that have high and generally uniform permeability and do not appear 

favorable for the existence of large reservoirs of high-level ground 

water. If such water bodies exist, they are deep within the masses of 

the islands and do not discharge at the surface. Locally, dense lava 

beds perch small bodies of water, some of which produce small springs 

having flows of 1 or 2 gpm. Seeps and a small spring discharge along 

dikes at the west end of Olosega.

The thin-bedded lava flows at sea level contain large volumes of 

fresh basal ground water. The fresh water flows from the lavas 

into beach deposits of sand and gravel and discharges into the sea 

at springs along the shore (fig. 4).

Although the flow from high-level springs in Ofu is small, these 

sources are used for public water supply. The water from two 

springs, each discharging about 1 gpm, is piped to outlets in the 

village of Ofu. (See figure 5.) The reservoirs into which the springs 

flow are often empty, and the village frequently is short of water.
GROUND-WATER RECONNAISSANCE OP AMERICAN SOMOA C17

Permeable lava flows

Talus

Water table

Beach deposits 

I___

 ui .jj ^ p-^n

bi_J__Jl-.L_iiN (i i. I .

 _ " " 'j "I 

i I, if i. / i

±j^3pf"fM z o n e .\ o t i/i,__|J,_^_

FIGOBB 4. Sketch showing the occurrence of fresh basal ground water in lava flows and

beach deposits.

A spring at 790 feet above sea level on the west slope of the island 

supplies water for the dispensary in the village., When flow from 

these sources is insufficient, water for drinking is obtained from 

beach springs that are uncovered at low tide.

Sili village, on Olosega, uses water from a small spring flowing 

from breccia cut by dikes. This source is reported to fail occasionally, 

and the installation around the spring is in danger of damage from 

flood debris in the intermittent stream that flows over it. Supplementary supplies are provided by a shallow well dug in beach deposits 

and a beach spring. A small spring high on the east slope of Olosega 

supplies water for Olosega village. Additional water for the village 

is obtained from several shallow dug wells in the sand and gravel 

underlying the flat on which the village is located.

Large supplies of high-level ground water probably cannot be 

developed on Ofu and Olosega, but ample supplies are present in the 

basal aquifers along the coast. Some water used on Olosega is dipped 

from shallow wells tapping basal water in beach deposits. Supplies 

could be obtained on both islands from wells dug in lava rock at the 

inland edge of the coastal flats. However, because of the excavation 

in hard lava, the cost of this development would be high, and these 

wells would be farther from the villages than the beach wells.
I4el0'

OFU

NUU I

NUU ->* Ofu 

SlLAELAElV

Contour interval 500 

Base and topography f

U S Hydrogrophic Office C

O

EXPLANATION

 ~ High-level spring

o Shallow dug well tap

ground water in beac

°~ Beach spring dischar

ground water

 -- pipeline

 House I

169*40'

FIGURE 5. Map of Ofu and Olosega showing principal sprin
GROUND-WATER RECONNAISSANCE OF AMERICAN SOMOA C19

TA'U

The central part of Ta'u may contain considerable quantities of highlevel ground water, but erosion has not cut deep enough into the volcanic dome to expose any large reservoirs that exist. A few seeps 

and springs in cliffs at the coast flow from the tops of dense lava flows 

or from lava lying on tuff beds (fig. 6); in the northwest corner of the

) )' j< pi- ' '

id

-

( i I, I } J(

11 _ 11 __ J\ __ li _____ 1

FIGUBE 6. Sketch showing geologic conditions at a small spring near Fagamalo, Ta'u.

island, some water flows from joints in tuff beds. As on Ofu and 

Olosega, springs along the shore indicate the presence of fresh basal 

ground water.

Water piped to outlets in Luma, Siuf aga, and Fagamalo villages 

flows from a small spring in aa lava lying on tuff. (See figure 7.) 

This source is inadequate for the villages, and most washing and 

bathing is done in beach springs that flow at low tide in front of the 

villages. Faleasau obtains part of its water from small springs flowing from tuff and part from wells dug in beach deposits. A small 

surface-water supply at Fitiuta, Maia, and Leusoalii is supplemented 

by a few dug wells tapping basal water in beach deposits.

The basal ground-water supply on Ta'u probably is sufficient to meet 

all needs on the island. This water can be developed most readily in 

wells dug to about sea level into lava flows at the inland edge of coastal 

flats. Sites for wells should be selected carefully. Wells should be 

in permeable lava flows and not in tuff or other relatively impermeable 

rock tha t yields water to wells only at low rates.
C20 CONTRIBUTIONS TO THE HYDROLOGY OF ASIA AND OCEANIA
GROUND-WATER RECONNAISSANCE OF AMERICAN SOMOA C21 

SUMMARY

Ground water in the mountainous part of Tutuila is largely highlevel water contained in many small reservoirs that discharge at small 

springs and seeps. The water at a few springs and seeps is developed 

in short tunnels or channeled into basins for village supplies, but 

streams are the source of water used in most villages. Additional 

village supplies of as much as 10 gpm could be developed by means 

of short tunnels or other improvements at numerous spring and 

seepage zones. Larger supplies of high-level water, as much as a 

few hundred gallons per minute, might be obtained in elaborate 

infiltration-tunnel systems.

Basal ground water in large quantities apparently is not available 

at most places on Tutuila because of the low permeability of the 

rock at sea level. Substantial supplies of basal water may underlie 

the Tafuna-Leone plain, but exploratory drilling would be needed 

to find the best sites for wells.

Beach deposits on Aunuu contain small quantities of fresh water, 

which is tapped in shallow dug wells.

The islands of Ofu, Olosega, and Ta'u have only minor high-level 

ground-water resources, but the basal water supplies are large. A 

few high-level springs and several shallow wells dug to basal water 

in beach deposits are the sources of water for the villages. Good 

supplies can be obtained in wells dug to about sea level into permeable 

lavas at the inland edge of many of the coastal flats.

REFERENCES

Macdonald, G. A., 1944, Petrography of the Samoan Islands: Geol. Soc. America

Bull. v. 55, p. 1333-1362. 

Stearns, H. T., 1944, Geology of the Samoan Islands: Geol. Soc. America

Bull. v. 55, p. 1279-1332.

O