Prepared in cooperation with the American Samoa Power Authority

Reconnaissance of the Hydrogeology of Ta’u, 

American Samoa

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004-5240

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Reconnaissance of the Hydrogeology of 

Ta’u, American Samoa

By Scot K. Izuka

Prepared in cooperation with the American Samoa Power Authority

Scientific Investigations Report 2004-5240

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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

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

endorsement by the U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Izuka, S.K., 2005, Reconnaissance of the Hydrogeology of Ta’u, American Samoa: U.S. Geological Survey Scientific 

Investigations Report 2004-5240, 20 p. 
iii

Contents

Abstract ................................................................................................... 1

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

Purpose and Scope ................................................................................. 3

Acknowledgments ................................................................................. 3

Setting ...................................................................................................... 3

Ground-Water Hydrology .............................................................................. 6

Wells, Springs, and Sources of Possible Contamination ....................................... 6

Conceptual Model of Ground-Water Occurrence and Flow ................................. 9

Ground Water in Northwestern Ta’u .................................................................. 13

Study Limitations and Additional Data Needs ......................................................... 18

Conclusions ................................................................................................ 19

References Cited .......................................................................................... 19

Figures

 Figure 1. Location map of Ta’u, American Samoa ................................................ 2

 Figure 2. Map showing wells and other sites of hydrologic interest on Ta’u .................. 4

 Figure 3. Map showing major geologic features of Ta’u .......................................... 5

 Figure 4. Graph of water levels in well 209, near the boat harbor on Ta’u, and tidal records 

from Pago Pago Harbor, American Samoa ............................................. 8

 Figure 5. Diagrammatic representation of ground-water occurrence in Ta’u .................. 10

 Figure 6. Diagram of the extent of the numerical ground-water model of Ta’u ............... 12

 Figure 7. Diagram of model-simulated water-table elevations for Ta’u ........................ 14

 Figure 8. Map of existing wells, proposed well sites, and landfill in northwestern Ta’u ...... 15

 Figure 9. Diagram of model-simulated water-table elevations and ground-water flow 

directions in northwestern Ta’u ............................................................ 17

Tables

 Table 1. Characteristics of wells and other important hydrologic sites visited during the 

October 2003 reconnaissance survey of Ta’u, American Samoa ..................... 7

 Table 2. Estimates of hydraulic conductivity for shield-volcano lavas on Ta’u ............... 11

 Table 3. Hydraulic conductivities used in numerical-model simulations of ground water in 

northwestern Ta’u ........................................................................... 16
iv

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

square mile (mi2) 2.590 square kilometer (km2) 

gallon (gal) 3.785 liter (L) 

million gallons (Mgal) 3,785 cubic meter (m3)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8

Elevations are referenced to mean sea level.

Horizontal coordinate information is referenced to World Geodetic System of 1984 (WGS84)

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).
Abstract

Analysis of existing data and information collected on 

a reconnaissance field visit supports a conceptual model 

of ground-water occurrence in Ta'u, American Samoa, in 

which a thin freshwater lens exists in a predominantly highpermeability aquifer that receives high rates of recharge. 

Because the freshwater lens is thin throughout most of the 

island, the productivity of wells, especially those near the 

coast where the lens is the thinnest, is likely to be limited by 

saltwater intrusion.

The landfill in northwestern Ta'u is closer to the north 

coast of the island than to any of the existing or proposed 

well sites. Although this may indicate that ground water 

beneath the landfill would flow away from the existing and 

proposed well sites, this interpretation may change depending 

on the hydraulic properties of a fault and rift zone in the 

area. Of four plausible scenarios tested with a numerical 

ground-water flow model, only one scenario indicated that 

ground water from beneath the landfill would flow toward 

the existing and proposed well sites; the analysis does not, 

however, assess which of the four scenarios is most plausible. 

The analysis also does not consider the change in flow paths 

that will result from ground-water withdrawals, dispersion of 

contaminants during transport by ground water, other plausible 

hydrogeologic scenarios, transport of contaminants by surfacewater flow, or that sources of contamination other than the 

landfill may exist.

Accuracy of the hydrologic interpretations in this study 

is limited by the relatively sparse data available for Ta'u. 

Understanding water resources on Ta'u can be advanced by 

monitoring rainfall, stream-flow, evaporation, ground-water 

withdrawals, and water quality, and with accurate surveys of 

measuring point elevations for all wells and careful testing of 

well-performance. Assessing the potential for contaminants 

in the landfill to reach existing and proposed well sites can 

be improved with additional information on the landfill itself 

(history, construction, contents, water chemistry), surfacewater flow directions, spatial distribution of ground-water 

levels, and the quality of water in nearby wells. Monitoring 

water levels and chemistry in one or more monitoring wells 

between the landfill and existing or proposed wells can 

provide a means to detect movement of contaminants before 

they reach production wells. Steps that can be implemented 

in the short term include analyzing water in the landfill and 

monitoring of water chemistry and water levels in all existing 

and new production wells.

Placing future wells farther inland may mitigate saltwater 

intrusion problems, but the steep topography of Ta'u limits 

the feasibility of this approach. Alternative solutions include 

distributing ground-water withdrawal among several shallowpenetrating, low-yield wells.

Introduction

Ta'u is a small (15 mi2) island in the tropical South 

Pacific island group of American Samoa (fig. 1). The interior 

of the island is steep and densely forested, and most of the 

island's population of 873 (U.S. Bureau of the Census, 2000) 

is concentrated in two coastal areas: (1) the northeast corner of 

the island, which includes the villages of Fitiuta and Leusoalii; 

and (2) the northwest corner of the island, which includes the 

villages of Ta'u, Faleasao, and Luma. In the mid-20th century, 

residents obtained their water from small spring- or stream-fed 

reservoirs, dug wells, and rain catchments (Davis, 1963). By 

the late 1970s, a few deep wells had been drilled, but some 

of these wells yielded salty water (U.S. Army Engineering 

District, Honolulu, 1977). Because wells, like the villages 

they serve, are near the coast, they are susceptible to saltwater 

intrusion if the wells are too deep or pumping rates are high. 

Ta'u currently has two functioning production wells. Well 207 

serves the villages in the northeast, and well 212 near Manua 

High School serves the villages in the northwest. The well 

serving the eastern villages currently produces water having 

relatively low salinity, but water pumped from the well in the 

west currently has such high salinity that it must be processed 

through reverse osmosis to make the water potable. The 

reverse-osmosis unit currently produces about 2,500 gallons 

per day (Wilfredo Carreon, American Samoa Power Authority, 

oral communication, 2004).

The American Samoa Power Authority (ASPA), the 

agency responsible for providing water in American Samoa, 

plans to drill new wells in search of better-quality water, but 

Reconnaissance of the Hydrogeology of Ta’u, American 

Samoa

By Scot K. Izuka
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Figure 1. Location and topography of Ta’u, American Samoa.

2 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
ground-water exploration is hindered because basic hydrologic 

information such as rainfall, streamflow, ground-water levels, 

and water quality are either lacking, sparse, or dispersed in 

unpublished files and documents. Only a few wells have 

been drilled into the volcanic rock of the island (fig. 2). All 

of these wells are near or within coastal villages, thus the 

ground-water hydrology of the island's interior is virtually 

unknown. Also, because the wells are near villages, the 

potential for contamination of existing and proposed wells by 

human activities is a concern. Initial motivation for this study 

came from concern that some of the proposed sites for new 

wells in northwestern Ta'u are close to an existing landfill. To 

address this concern, and to identify needs for the collection 

and analysis of additional data to guide the development 

and management of water resources on the island, the U.S. 

Geological Survey (USGS), in cooperation with ASPA, 

conducted a reconnaissance study of the hydrogeology of Ta'u.

Purpose and Scope

This report describes a conceptual hydrogeologic model 

that will aid in the exploration for and management of water 

resources on Ta'u, with emphasis in the northwestern part of 

the island, where new wells have been proposed and concerns 

about ground-water contamination from a landfill have been 

raised. The model is based on compilation, analysis, and 

hydrologic interpretation of existing data and data collected 

during a reconnaissance survey of the existing fresh-water 

resources on Ta'u. The model is tested with a simplified 

island-wide numerical ground-water model. Four scenarios 

of plausible hydrogeologic conditions in northwestern Ta'u 

were simulated in the numerical model to study possible 

ground-water flow paths beneath the landfill. This report also 

discusses limitations to hydrologic interpretations owing to the 

limited data available, and identifies additional information 

that is needed to improve the conceptual model and its utility 

in ground-water exploration, development and management on 

Ta'u.

Acknowledgments

The author is grateful to the staff of ASPA, its Executive 

Director, Abe Malae, and Wilfredo Carreon, for their 

assistance in this project. ASPA employees Fola Tanielu, 

Aloalii Maui, Pauono Saiana, and Junior Faaitu assisted the 

author during fieldwork on Ta'u. Maatifa Eleasaro and Peter 

Peshut of the American Samoa Environmental Protection 

Agency (ASEPA) also provided background information and 

assistance. This report has benefited from the contributions of 

Charles D. Hunt, Deborah J. Parliman and Luis Menoyo of the 

USGS.

Setting

Ta'u has a broad dome-shape profile and a rectangular 

outline in plan view (fig. 1). Despite its small size, the island 

has high relief, with a maximum elevation of more than 

3,000 ft. Rainfall records for Ta'u are meager, but data from 

the National Climate Data Center (2002) for the villages of 

Luma and Faleasao indicate that the northwestern end of the 

island receives about 150 to 200 in. of rain per year. Longterm rainfall data from the airport on Tutuila (about 70 mi to 

the west of Ta'u) indicate that the periods over which the data 

for Luma (July 1969 through October 1975), and Faleasao 

(November 1975 through December 1986) were collected 

were about average for the American Samoa archipelago. The 

spatial variation of rainfall on Ta'u is not known, but it is likely 

that as on other islands in American Samoa, rainfall on Ta'u is 

higher at higher elevations than it is at lower elevations (Izuka, 

1999).

Previous studies indicate that Ta'u has the geologic 

structure and plate-tectonic setting of a young, basaltic, 

shield-volcano island that has been modified by erosion, 

collapse, and renewed volcanism (Stearns, 1944; Stice, 1966; 

Stice and McCoy, 1968). According to these studies, Ta'u 

was formed by mid-oceanic-plate volcanism. The main shield 

volcano that built Ta'u, the Lata Shield (fig. 3), formed by the 

accumulation of fluid lavas emanating from weakly explosive 

summit and rift-zone eruptions. At some point during shield 

building a caldera was formed, but volcanic activity continued. 

Rift-zone eruptions formed smaller satellite shield volcanoes 

that built out the northwest (Tunoa Shield) and northeast 

(Lautele Shield) coasts of the island (fig. 3). Erosion created 

stream gullies and sea cliffs, some of which were buried by 

subsequent eruptions, and large-scale collapse or slope failure 

resulted in the removal of the southern half of the original 

shield volcano.

Volcanic dikes (near-vertical sheets of dense igneous rock 

that intrude into the lava flows that form the shield volcano) 

are likely to be associated with the caldera and rift zones of 

the Lata Shield, although few dikes are exposed on Ta'u. Stice 

and McCoy (1968) described dike exposures in Laufuti Stream 

on the south-facing cliffs of Ta'u (fig. 1). More dikes probably 

exist within the Ta'u shield, especially along the principal rift 

zones. Although Stice and McCoy (1968) do not delineate rift 

zones in their geologic map of Ta'u, they describe a rift zone 

that extends to the northeast through the Lautele Shield and 

another rift zone that extends northwest through the Tunoa 

Shield. Stice and McCoy (1968) also describe caldera deposits 

in the cliffs of the central south coast of Ta'u (fig. 3). A 

gravity survey by Machesky (1965) shows a Bouguer anomaly 

that is consistent with the caldera interpretation. Stice and 

McCoy (1968) also describe a central depression (the Tunoa 

Depression, fig. 1) on the satellite Tunoa Shield, but this 

depression is smaller than a caldera and whether it has rocks 

similar to those of a caldera is unknown.

Setting 3
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210

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208

 211

 1

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Figure 2. Wells and other sites of hydrologic interest on Ta’u, American Samoa.

4 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
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Figure 3. Major geologic features of Ta’u, American Samoa (modified from Stice and McCoy, 1968). Setting 5
Ground-Water Hydrology

Current understanding of the ground-water hydrology of 

Ta'u is based on scattered and relatively sparse data. The main 

published sources are reports by Davis (1963), the U.S. Army 

Engineering District, Honolulu (1977), and Bentley (1975). 

Additional information is contained in files at the USGS 

office in Honolulu, Hawaii, including field notes, memoranda, 

and correspondence of USGS hydrologists R.H. Dale, K.J. 

Takasaki, and P.R. Eyre who visited the island several times in 

the 1960s –90s.

To supplement the existing published and unpublished 

data, a field reconnaissance was conducted from October 23 

to 25, 2003 as part of this study. The field reconnaissance 

included (1) visits to all existing wells and, where possible, 

springs, former well sites, proposed well sites, and sites of 

potential contamination that may present a threat to groundwater resources (fig. 2); (2) using a global positioning system 

(GPS) instrument to obtain accurate geographic coordinates 

of all sites; (3) measurement of water levels in wells, where 

possible; (4) recording present well pumping rates and spring 

discharge rates; and (5) testing the chloride concentrations of 

the water from springs and wells.

Wells, Springs, and Sources of Possible 

Contamination

Accurate location, using GPS, of wells, springs, and 

sources of possible ground-water contamination, is considered 

one of the more important contributions of the reconnaissance 

field survey conducted for this study. Accurate location 

of wells, springs, potential sources of contamination, and 

geologic structures is essential to the analysis of ground-water 

resources in any area.

Information in the files of the USGS indicates that as 

many as 10 wells have been constructed on Ta'u, but in the 

field reconnaissance for this study, only nine sites of existing 

or former wells were found (table 1). Of these wells, two were 

destroyed, so their locations were only approximated from 

the recollection of island residents; three were plugged; two 

were unused but accessible for water-level measurements; and 

one was being used and could be sampled but had no access 

for water-level measurements. In only one well could both a 

sample and water-level measurement be taken. Of the wells 

visited, only two had surveyed measuring-point elevations. 

Measuring-point elevations were surveyed for three other 

wells during the field reconnaissance.

Well 207. – Well 207 is the easternmost of a pair of 

wells in Faga (wells 207 and 208, table 1, fig. 2). Well 207 

is synonymous with well 92-1327-01 in the USGS files and 

well 129 in older notes, and because it provides water to 

the villages in northeastern Ta'u, including Fitiuta, this well 

is commonly referred to as the "Fitiuta Well." The well is 

579 ft inland from the northwest coast of Ta'u. Records in 

the USGS files indicate that the well was drilled in 1981 

and report a "well slab elevation" of 28.33 ft. During the 

reconnaissance field visit for this study, a two-tiered concrete 

slab surrounded this well; it is not clear whether the 28.33 ft 

elevation corresponds to the upper or lower tier of the slab. 

The top of the upper tier of the slab surrounding the casing is 

0.60 ft above the lower tier, and the top of the measuring tube 

is 0.33 ft above the top of the upper tier. Assuming that the 

lower tier of the slab has an elevation of 28.33 ft, the elevation 

of the top of the measuring tube is 29.26 ft. Depth-to-water 

measurements in the USGS files indicate that the pre-pumping 

water-level elevation was about 2 to 3 ft. The depth of this well 

is not clear from the data in the USGS files, but it is apparently 

at least 36.67 ft, which was the reported depth of the pump 

used during testing. The USGS files also contain data from 

a pumping test (not dated) which ran for 375 min (6 hours, 

15 minutes). During this test, the pumping rate was variable 

(40 to 150 gal/min), the maximum recorded drawdown was 

5.92 ft, and chloride concentrations of the pumped water 

were less than 200 mg/L. During another test (dated April 14, 

1981), which ran for 238 minutes (about 4 hours) at a variable 

pumping rate of 60 to 100 gal/min, the maximum recorded 

drawdown was 6.25 ft.

During the field reconnaissance for this study (October 

24, 2003), the well was being pumped at a rate of 55 gal/min 

(the duration of pumping prior to the visit is unknown), the 

water-level elevation was -0.3 ft (assuming the elevation of 

the top of the measuring tube is 29.26 ft), and the water had a 

chloride concentration of 57 mg/L.

Well 208. – Well 208 is the westernmost of a pair of wells 

in Faga (table 1, fig. 2). Well 208 is synonymous with well 

92-1327-02 in the USGS files. The USGS files indicate that 

the well was drilled in 1985 at an elevation of about 49 ft (no 

precise surveyed elevation was given) at a location about 700 

ft from the coast, is 61 ft deep with 52 ft of solid casing above 

and 10 ft of slotted casing below, and had a "static water" 

level (probably depth-to-water) of 49.42 ft. At the time of the 

reconnaissance field visit for this study, the well was not being 

used. The well casing was surrounded by a two-tiered concrete 

pad; the lower tier was about 1.10 ft above ground, the second 

tier was 0.5 ft above the first tier, and the top of the casing was 

0.10 ft below the top of the upper tier. The water level at the 

time of the reconnaissance visit (October 24, 2003) was 49.90 

ft below the top of the casing.

Well 209. – Well 209 is the northernmost of a pair of 

wells drilled near the boat harbor on the western side of the 

island of Ta'u (table 1, fig. 2). The well is synonymous with 

well TA-1430-03 in the files of the USGS, and well 130 in 

older notes. According to information in the USGS files, the 

well was drilled in 1981 at an elevation of about 40 ft and 

is 52 ft deep, and produced brackish water. A level survey 

conducted during the field reconnaissance visit for this study 

indicated the elevation of the top of the casing, which was 

flush with the upper tier of the surrounding concrete slab, 

was 40.24 ft (elevation surveyed from the USGS "Tide Gage" 

bench mark south of the boat harbor). The well is 328 ft from 

the coast, according to the well location determined by GPS.

6 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
Table 1. Characteristics of wells and other important hydrologic sites visited during the October 2003 reconnaissance survey of Ta'u, American Samoa 

[latitude and longitude in degrees:minutes:seconds in the WGS 84 datum, determined by hand-held global positioning system (GPS) instrument; all elevations 

are relative to mean sea level; NA, data not available or not applicable; ft, feet; Q, well discharge; gal/min, gallons per minute; ASPA, American Samoa Power 

Authority]

Hydrologic site Latitude and 

longitude

Total depth

(feet)

Water level-measurement

Notes Measuring

point

elevation

(feet)

Depth below 

measuring 

point

(feet)

Date and

time

(Samoa)

Elevation 

relative to mean 

sea level

(feet)

Well 207 14:12:49 S

169:27:13 W

>37 29.26 29.56 10/24/03

16:25

-0.30 Q = 55 gal/min

at time of water-level

measurement 

Well 208 14:12:52 S

169:27:28 W

61 NA 49.90 10/24/03 

16:08

NA Unused. Difficulty getting GPS 

reading at well site

Well 209 14:14:26.1 S

169:30:31.2 W

52 40.24 40.50 10/24/03 

13:30

-0.26 Unused; water level fluctuates 

with tides

Well 210 14:14:27.6 S

169:30:29.9 W

51 NA NA NA NA Plugged at depth of 24.88 ft

Well 211 14:13:42.2 S

169:30:40.5 W

250 NA NA NA NA Plugged at depth of 189 ft 

Well 212 14:13:38.5 S

169:30:42.1 W

223 201.29 NA NA NA Measuring tube blocked, water 

meter not working, ASPA 

technician said pumping rate is 

about 60 gal/min.

Faleasao dug well 14:13:15.8 S

169:30:49.7 W

NA NA NA NA NA Not located (destroyed?)

Fitiuta School well 14:13:00.5 S

169:25:30.2 W

NA NA NA NA NA Plugged to near top of casing

Fitiuta dug well 14:12:49.8 S

169:26:08.6 W

NA NA NA NA NA Not located (destroyed?)

Fagamalo Spring 14:14:44.6 S

169:30:08.5 W

NA NA NA NA NA

Ta’u landfill 14:13:15.3 S

169:30:02.6 W

NA NA NA NA NA

Proposed well site 1 14:13:36.3 S

169:30:17.6 W

NA NA NA NA NA

Proposed well site 2 14:13:51.7 S

169:30:21.5 W

NA NA NA NA NA

Proposed well site 3 14:13:56.9 S

169:30:25.8 W

NA NA NA NA NA

Proposed well site 4 14:13:46.3 S

169:30:38.8 W

NA NA NA NA NA North end of old airfield

Proposed well site 5 14:14:01.3 S

169:30:39.6W

NA NA NA NA NA South end of old airfield

Ground-Water Hydrology 7
Because no pump was installed in the well, an 

opportunity was available to set a pressure transducer and 

data logger to monitor water-level fluctuations over the 

two-day period of the reconnaissance visit. The water-level 

monitoring data shows a semi-diurnal cycle corresponding 

to the fluctuation of ocean tides (fig. 4). The range of the 

water-level fluctuation during the two-day monitoring period 

was about 2.8 ft, which is 75 percent of the concurrent tidal 

range recorded at the tide gage at Pago Pago Harbor on Tutuila 

(National Oceanic and Atmospheric Administration, 2003). 

The relatively strong tidal signal in this well indicates that the 

rock penetrated by the well has high permeability.

Well 210. – Well 210 is the southernmost of a pair 

of wells near the Ta'u boat harbor (table 1, fig. 2). It is 

synonymous with well TA-1430-04, and has also been called 

well 130a, and Ta'u Harbor Well 2. According to the USGS 

files, the well was drilled in 1982 at an elevation of about 40 

ft, is 51 ft deep, and had rapid fluctuations (possibly caused 

by tides) in water level. At the time of the reconnaissance visit 

for this study, the well was not being used and was plugged at 

24.88 ft below the top of the casing.

Well 211. – Well 211, at the back of the campus of Manua 

High School, was the first of two wells drilled near the school 

(table 1, fig. 2). This well is synonymous with well TA-1430-

02 in the files of the USGS, and well M-1430-02 described 

in Bentley (1975). A level survey conducted during the 

reconnaissance visit of this study indicates that the top of the 

raised concrete slab that is flush with the top of the casing is 

at an elevation of 195.19 ft. This differs slightly from Bentley 

(1975) who reports that the top of the well casing is at an 

elevation of 194.9 ft. Bentley also reported that this well had 

an initial water-level elevation of 2 ft and a transmissivity of 

"about 50,000" ft2/d. Notes in the files of the USGS indicate 

that the well is 250 ft deep, has a 6-in. solid casing to a depth 

of 200 ft, 40 ft of perforated casing, and 10 ft of open hole 

below that. Assuming horizontal flow and dividing Bentley's 

(1975) transmissivity by the 50 ft total of perforated casing 

and open hole yields an estimate of hydraulic conductivity of 

about 1,000 ft/d, which is comparable to the high hydraulic 

conductivities characteristic of the principal aquifers of 

Hawaiian shield volcanoes (Hunt, 1996). The files also 

indicate that in the 1980s, the well produced about 50 to 300 

gal/min of water with chloride concentrations of about 140 

to 300 mg/L. The well was abandoned in the 1980s when 

a pump was accidentally dropped into it. At the time of the 

reconnaissance visit of this study, the well was plugged at a 

depth of 189 ft below the top of the raised concrete pad.

Well 212. – Well 212 is the second of two wells near 

the Manua High School campus in northwestern Ta'u. This 

well is synonymous with well TA-1330-02 in the files of 

the USGS; ASPA employees commonly refer to this well as 

the "High School Well." The well is on the south side of the 

road that passes in front of the school, and is about 1,760 ft 

from the coastline (table 1, fig. 2). This well provides water 

to a reverse-osmosis unit in Faleasao Village. During the 

field reconnaissance for this study (October 24, 2003), the 

well was being pumped but the water level and pumping rate 

could not be determined because the water meter was not 

working and the access for the water-level measuring device 

was plugged. Technicians from ASPA indicated that the 

Figure 4. Water levels in well 209, near the boat harbor on Ta’u, American Samoa, and tidal records 

from Pago Pago Harbor, Tutuila, American Samoa (data for Pago Pago from the National Oceanic and 

Atmospheric Administration, 2003).

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8 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
well is usually pumped at about 60 gal/min. A water sample 

from the well had a chloride concentration of 429 mg/L. A 

level survey was conducted during the reconnaissance visit 

of this study. With a starting reference elevation of 199.22 ft 

for the concrete base slab (reference elevation obtained from 

McConnell Dowell, the construction company that built the 

nearby road), the elevation of the top of the raised concrete 

slab surrounding the well casing was determined to be 200.70 

ft, the top of the metal plate covering the well was determined 

to be 201.00 ft, and top of the existing (plugged) measuring 

tube was determined to be 201.29 ft. Information in the files 

of the USGS indicate that the well was drilled in 1986 and 

stopped at a depth of 223 ft when chloride concentrations of 

the water in the hole rose to 111 mg/L. The files also indicate 

that the pumping rate of this well was increased when well 

211 became unusable.

Faleasao Dug Well. – This well was not located during 

the reconnaissance visit for this study, and was apparently 

destroyed. The well's precise location is not known; the 

coordinates in table 1 and the location shown in figure 2 

are based on recollection of an island resident. This well is 

probably synonymous with TA-1330-01 (also known as Ta'u 

Dug Well 2 in some old notes) in the USGS files, and M1330-

1 in Bentley (1975).

Fitiuta School Well. – During the reconnaissance field 

work for this study, an abandoned well was visited on the 

Fitiuta school grounds (table 1, fig. 2). Its location near (on the 

seaward side of) the main road, and the fact that it is plugged 

near the surface, indicates that this well is probably the same 

as the well that P. Eyre, in his 1982 field notes (in files at the 

USGS), called "well 128", which is synonymous with well 

TA-1325-03 in the files of the USGS. Notes and letters in the 

USGS files report that although the well reached water, the 

yield was not sufficient to develop.

Fitiuta Dug Well. – Bentley (1975) lists a dug well 

(number M1325-1 in his report) near Fitiuta. Island residents 

questioned during the reconnaissance visit of this study 

recall a well on the north coast, but the precise location is not 

clear. The coordinates listed for this well in table 1, and the 

location shown in figure 2, are approximations based on the 

recollection of island residents.

Fagamalo Spring. – Fagamalo Spring issues from a 

cliff on the western side of Ta'u (table 1, fig. 2). Davis (1963) 

reported that at the time of his reconnaissance study, which 

was before deep wells were drilled on Ta'u, Fagamalo Spring 

was used extensively for water supply in western Ta'u. At 

the time of the reconnaissance visit of this study, water was 

observed in a 17-ft long by 11-ft wide concrete catchment, out 

of which distribution pipes led. No flow from the catchment 

was observed, but water in the catchment was 5.3 ft deep, 

and a water sample taken from the catchment had a chloride 

concentration of 6.7 mg/L.

Ta'u landfill. – The Ta'u landfill is on the eastern edge of 

the Tunoa depression in northwestern Ta'u (table 1, fig. 2), and 

is the primary site for disposal of solid waste on the island. 

Staff at the ASEPA and ASPA indicated that the landfill is not 

lined. The proximity of this landfill to existing and proposed 

wells was the primary motivation for this study.

Proposed sites for new wells. – ASPA is proposing 

to drill a new well to serve northwestern Ta'u. During the 

reconnaissance visit for this study, resident ASPA employees 

indicated five possible well locations (table 1, fig. 2). At the 

time of this study, ASPA had plans to drill only one well.

Large metal drums. – A stack of large metal drums 

was observed along the road between the landfill and Manua 

High School (table 1, fig. 2). The content of the drums was 

not determined, but island residents indicated that they once 

contained gasoline and that they are now empty. This study 

does not address whether these constitute a potential source of 

contamination.

Conceptual Model of Ground-Water Occurrence 

and Flow

Specific ground-water data for Ta'u is sparse, but 

consistent with the hypothesis that most of the ground water 

in Ta'u, as in other shield-volcano islands, forms a freshwater 

lens that floats on saltwater in an aquifer formed principally 

by permeable basaltic lava flows (fig. 5). The existence of a 

sizable freshwater lens in Ta'u was postulated by Davis (1963) 

and Bentley (1975). In the simplest conceptualization of a 

freshwater-lens system, the thickness of the freshwater lens 

below sea level is approximately 40 times the thickness of 

the lens above sea level. This relation, known as the GhybenHerzberg relation, is commonly used to estimate the thickness 

of the freshwater lens when the height of the water table 

above sea level is known. The Ghyben-Herzberg relation, 

however, has only limited accuracy in this regard because (1) 

the relation does not consider the thickness of the mixing zone 

between the freshwater lens and the underlying saltwater, and 

(2) the dynamics of the freshwater lens invalidates some of the 

assumptions of the relation.

Ground water constantly flows through the freshwater 

lens. The ground water originates as recharge from rainfall, 

and flows from high recharge areas in the interior of the island 

toward discharge sites (springs and seeps) at the coast (fig. 5). 

Only a few springs are shown on the topographic map of the 

Manua Islands (USGS, 1963, fig. 1), but historical accounts of 

spring use by villagers (Davis, 1963; U.S. Army Engineering 

District, Honolulu, 1977) indicate that ground-water discharge 

along the coast is widespread. There are also numerous 

references to coastal springs in unpublished notes in the well 

files of the USGS. Large springs that discharge at relatively 

high elevations have historically been used for drinking water, 

whereas springs at the coast (including those that are exposed 

only at low tide) have been used for bathing, washing, and in 

times of drought, drinking water (Davis, 1963).

The freshwater lens is probably thickest in the interior 

of the island and thinnest at the coast where discharge occurs, 

but the overall thickness of the freshwater lens is a function of 

the rate of freshwater flow and the permeability of the rocks. 

In general, the freshwater lens will be thicker in an island 

Ground-Water Hydrology 9
with lower permeability rocks or with a higher island-wide 

recharge. Because Ta'u has few wells, and even fewer wells for 

which accurate water-table elevations relative to sea level are 

known, the thickness of the freshwater lens in the island is for 

the most part unknown.

Volcanic dikes, which are common in shield volcanoes, 

tend to reduce the permeability of the lava-flow aquifers 

into which they intrude. This may result in local thickening 

of the freshwater lens or discharge of ground water at sites 

other than the coast (fig. 5). Laufuti Stream, which flows 

over a dike complex exposed in the benches and cliffs near 

the southeastern coast of the island (fig. 1), is perennial in its 

upper reaches down to an elevation of about 1,422 ft, and in 

its lower reaches below 121 ft elevation (Davis, 1963; Stice 

and McCoy, 1968; Cook 2001). The high-elevation perennial 

flow in Laufuti Stream indicates that the ground-water table 

is high in this area, despite its proximity to the coast, possibly 

as a consequence of the ground-water impounding effects of 

low-permeability geologic structures such as dikes or lowpermeability caldera deposits.

Dikes can act as barriers to ground-water flow and thus 

limit the yield of nearby wells, but dikes also can create 

compartments in which large volumes of ground water are 

stored and water-table elevations are hundreds of feet above 

sea level. On Oahu, Hawaii, such high-level, dike-impounded 

ground-water bodies have been developed for water supply 

(Takasaki and Mink, 1985). Dikes are likely to be present in 

the rift zones of Ta'u (fig. 3), and Bentley (1975) postulated 

that Ta'u may also have high-level ground-water resources.

Rocks formed in calderas in Hawaii generally have 

lower permeability than the lava flows on the flank of the 

shield because in the caldera region, dikes are abundant, lava 

flows are generally thicker due to ponding, and hydrothermal 

alteration of the rocks reduces primary porosity. Analogous 

deposits from the caldera of the main Lata Shield of Ta'u (fig. 

3) are also likely to have low permeability. Whether the rocks 

in the central depression of the satellite Tunoa Shield have the 

low-permeability characteristics of caldera rocks is, however, 

unknown.

Despite having similarities with other mid-ocean basaltic 

shield volcanoes such as Hawaii, Ta'u differs in certain 

hydrologically significant ways. Ta'u has an area above sea 

level that is only about a third of the area of the smallest of 

the major Hawaiian Islands. Such a small island likely offers 

only a small amount of ground-water storage, especially if 

the permeability of the rocks is high. Ta'u also has little or 

no coastal plain, which indicates that there is little to impede 

the discharge of ground water from the freshwater lens to the 

ocean. On Oahu, Hawaii, an extensive coastal plain of lowpermeability sediments (locally known as caprock) impedes 

ground-water discharge from the major basalt aquifers to the 

ocean, thereby substantially increasing the thickness of the 

freshwater lens in the basalt even though the aquifer has high 

permeability. On Ta'u, where aquifer permeability is high but 

no caprock exists, the freshwater lens is likely to be thin even 

though recharge may be high.

An estimate of aquifer permeability. – The relatively 

low pre-pumping water levels (relative to sea level) in well 

207 in northeastern Ta'u and well 211 in northwestern Ta'u, 

as well as the strong tidal signal at well 209, indicate that the 

aquifers penetrated by these wells have high permeability. The 

hydraulic conductivity of a coastal aquifer (a measure of its 

permeability) can be estimated by using equations developed 

by Glover (1964) for the steady-state shape of a freshwater 

lens discharging at a coast. Glover's equations describe 

the shape of the freshwater lens in terms of flow rate and 

Figure 5. Diagrammatic representation of ground-water occurrence in Ta’u, American Samoa. 

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10 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
aquifer hydraulic conductivity; the equations can be solved 

for hydraulic conductivity if freshwater flow rate and certain 

aspects of the shape of the freshwater lens are known. Glover's 

equation that approximates the water table height above sea 

level is:

h = (2γQx/K)0.5, (eq. 1)

where:

h = water table elevation at distance x from the shore [L],

γ = density difference between freshwater and saltwater

 divided by the density of freshwater, which for most

 oceanic islands is about 0.025 [dimensionless],

Q = freshwater flow per unit length of coast [L2/T],

x = distance from shore where h is measured [L], and

K = hydraulic conductivity [L/T].

Solving for K gives:

K = (2γQx)/h2. (eq. 2)

Values for h and x can be obtained from records for well 

211 (fig. 2), which indicate that the water level (h) at that 

location prior to pumping was about 2 ft above sea level. The 

distance (x) from well 211 to the nearest point on the coast is 

about 1,960 ft. An accurate value of Q is not available, but a 

reasonable range of estimates may be obtained by assuming 

that some proportion of rainfall becomes ground-water 

recharge, and dividing the recharge estimates by the 19.5-

mi perimeter of the island. Although rainfall data for Ta'u 

are scarce, the records that do exist indicate that the island 

receives, on average, more than 100 in. of rain per year. For 

this study, a range of Q was computed using rainfall values 

of 100, 150, and 200 in. per year and assuming that 33 to 

50 percent of the rainfall becomes ground-water recharge. 

Computations of K indicate that the hydraulic conductivity of 

the aquifer near well 211 could range from several hundreds to 

a few thousands of feet per day (table 2). Such high hydraulic 

conductivities are consistent with those measured from other 

shield volcanoes such as the Hawaiian Islands (Hunt, 1996), 

and consistent with the high transmissivity (which corresponds 

to a hydraulic conductivity of about 1,000 ft/d) reported for 

well 211 by Bentley (1975).

Numerical simulation of ground water – To test the 

validity of the conceptual model of ground-water occurrence 

and flow on Ta'u, a steady-state numerical model was 

constructed using the finite-difference modeling program 

SHARP (Essaid, 1990). Because SHARP allows the 

simulation of coupled freshwater and saltwater flow, it is 

ideal for simulating ground-water systems in oceanic islands 

where a freshwater lens overlies saltwater. The accuracy of the 

numerical model's representation of the ground-water system 

of Ta'u is limited by the general paucity of hydrologic data for 

Ta'u. Even so, the numerical model provides a means for (1) 

illustrating concepts of how ground-water flow and aquifer 

permeability are related to the size, shape, and geology of Ta'u, 

and (2) assessing whether the assumptions of the conceptual 

model are quantitatively consistent with the equations of 

ground-water flow.

The numerical model encompasses the entire island of 

Ta'u as well as some of the surrounding sea floor so that the 

simulated freshwater lens is not artificially restricted by model 

boundaries (fig. 6). The model has a single layer, with 66 

cells in the east-west direction and 67 cells in the north-south 

direction. In the northwest corner, the model cells were made 

smaller (277 ft by 277 ft) to increase resolution in the area 

where ASPA proposes to drill new wells (to be discussed in a 

later section).

All model cells representing the main shield volcano of 

Ta'u as well as all offshore areas were assigned a hydraulic 

conductivity of 1,200 ft/d, which is consistent with values 

estimated in the foregoing analysis using Glover's (1964) 

equation, as well as values reported for Ta'u (Bentley, 1975) 

and for Hawaii (Hunt, 1996). Using this value also results in a 

simulated water level of 2 ft at the location of well 212, which 

is the pre-pumping water level reported in nearby well 211. 

Rift zones, caldera deposits, and the tuff cone in the northwest, 

all of which are expected to have lower permeability than 

the main shield, were simulated with model cells having a 

hydraulic conductivity of 0.1 ft/d. The positions of the modelsimulated tuff and caldera are based on the geologic maps and 

descriptions of Stice and McCoy (1968). No rift zones are 

delineated in the geologic map of Stice and McCoy (1968), but 

in the text of their paper, they describe northwest and northeast 

trending rift zones; these descriptions form the basis for the 

position for the simulated rift zones in the numerical groundwater model constructed for this study (fig. 6). Recharge for 

most of the model cells was set at a uniform 50 in/yr, which 

corresponds to an infiltration of 1/3 of a rainfall rate of 

Table 2. Estimates of hydraulic conductivity for shield-volcano lavas 

on Ta'u, American Samoa, using the equation in Glover (1964) over a 

range reflecting uncertainty in rainfall and recharge.

[in/yr, inches per year; %, percent; ft3/s, cubic feet per second; ft/d feet 

per day; Q, ground-water flow per unit length of coastline; K, hydraulic 

conductivity]

Rainfall

(in/yr)

Estimated Recharge Q

(ft2/s)

K

(ft/d) (% of rainfall) (ft3/s)

100 33 36.47 0.000354 750 

150 33 54.70 0.000531 1,125 

200 33 72.93 0.000708 1,499 

100 50 55.25 0.000537 1,136 

150 50 82.88 0.000805 1,704 

200 50 110.50 0.001073 2,272 

Constants:

Island area = 15 square miles

Island coast = 19.5 miles

Distance of well from coast = 1960 feet

Water-table elevation = 2 feet

Ground-Water Hydrology 11
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Figure 6. Extent of the numerical ground-water model of Ta’u, American Samoa.

12 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
150 in/yr. Recharge at the tuff cone, caldera deposits, and rift 

zones was set at 7.5 in/yr because runoff from these lowpermeability rocks and structures is likely to be high.

The model-simulated water table through most of the 

island (except the rift zones, caldera, and tuff cone) has a 

gentle convex slope, consistent with the shape of the upper 

surface of a freshwater lens (fig. 7). The low elevation of 

most of the water table indicates that the freshwater lens is 

thin in most areas. The continuity of the simulated water 

table is interrupted by the low-permeability rift zones that 

radiate to the northwest and northeast from the caldera. Water 

levels in the low-permeability caldera are higher than in the 

adjacent freshwater lens, but the geologic structure of this 

area is not known well enough to allow precise simulation of 

the high-elevation ground-water discharge to Laufuti Stream. 

These results support a conceptual model of ground-water 

occurrence in Ta'u, in which (1) recharge is high; (2) aquifer 

permeability is high except in the rift zones, caldera, and tuff 

cone; (3) the freshwater lens is thin throughout most of the 

island; and (4) water may exist at high elevations in areas such 

as calderas and rift zones where low-permeability geologic 

structures intrude into the otherwise high permeability rocks 

of the island.

Because the freshwater lens is thin throughout most of 

the island, the productivity of most wells is likely to be limited 

by saltwater intrusion. The freshwater lens is thinnest near the 

coast, therefore wells located near the coast are more likely 

to encounter production limitations due to salinity than wells 

located farther inland, where the freshwater lens is thicker. The 

history of high salinity and low water levels at wells 211 and 

212 indicate that those wells were drilled too deep given the 

thin freshwater lens in that area. Using the Ghyben-Herzberg 

relation and the reported pre-pumping water-table elevation 

in well 211 gives a freshwater thickness of only 82 ft, part of 

which is brackish.

Ground Water in Northwestern Ta’u

To relieve the high demand on well 212 and the 

associated reverse-osmosis system that currently provides 

all the municipal drinking water for western Ta'u, ASPA has 

proposed the construction of a new well in the area inland 

from Ta'u and Siufaga Villages. Five potential locations for 

this new well were visited during the field reconnaissance 

phase of this study (fig. 8). Concern has been raised, however, 

that these proposed sites are so close to an active landfill 

that contamination from the landfill may threaten the quality 

of water from wells drilled at the proposed sites. In this 

section, the details of the hydrogeology of northwestern Ta'u 

are discussed, particularly as it bears on the existing well, 

proposed well sites, and the possibility of contamination from 

the landfill.

The geology of this area of Ta'u is dominated by the 

Tunoa Shield, a small shield volcano that was built on 

the northwestern flank of the main Ta'u shield volcano 

(fig. 3). The Tunoa Shield was formed by eruption of lava 

from the northwest rift zone of the main Ta'u shield, and is 

predominantly composed of thin-bedded pahoehoe lava flows. 

In the center of the Tunoa Shield is a large, fault-bounded, 

arc-shaped depression, known as the Tunoa Depression 

(fig. 2), which was partly filled by later lava flows and other 

volcanic rock. The Tunoa Shield, including part of its central 

depression, is truncated by a former sea cliff that was later 

partly covered by a complex of consolidated tuff. Remnants of 

the former sea cliff are still exposed behind Ta'u Village, but 

the present-day coastline is several hundred feet west of the 

cliff and is separated from it by a marsh and beach (Stice and 

McCoy, 1968).

Plots of the GPS data obtained during the field 

reconnaissance indicate that well 212, the proposed well 

sites, and the landfill all lie in the Tunoa Depression (fig. 8). 

Although the initial motivation for this study was a concern 

over the proximity of proposed new well sites to the landfill, 

the GPS data indicate that well 212 is closer to the landfill 

than some of the proposed new wells sites. The landfill also 

lies closer to the north coast of the island than to any of the 

existing or proposed well sites. Without consideration of 

geologic structures or the manner in which pumping wells 

can alter ground-water flow, the location of the landfill near 

the north coast suggests that ground-water from beneath the 

landfill would flow toward the north, away from the existing 

well and the proposed well sites. The landfill lies at the 

northern boundary of the Tunoa depression, however, and 

very close to the escarpment that Stice and McCoy (1968) 

interpreted to be a fault (fig. 8). The landfill is, in fact, so close 

to the fault that it is difficult to determine on which side of the 

fault the landfill lies. Depending on the hydraulic properties 

of the fault and a nearby rift zone relative to the surrounding 

rock, these structures may have a significant effect on groundwater flow directions in this area.

As discussed previously, the low water-table elevation at 

well 212 and the strong tidal signal at well 209 indicate that 

the permeability of the rocks in those areas is high. This is 

consistent with observations of Stice and McCoy (1968) that 

the Tunoa Shield is composed primarily of thin-bedded lava 

flows, as are the high-permeability aquifers in Hawaii (Hunt, 

1996). The consolidated tuff, if analogous to similar deposits 

in Hawaii, is likely to have low permeability (Hunt, 1996). The 

hydraulic properties of faults in basaltic islands are virtually 

unknown. Whether the fault bounding the Tunoa Depression 

has a lower, equal, or higher permeability than the surrounding 

rocks can affect ground-water flow directions and hence 

whether the contaminants from the landfill, if they leach down 

to the water table, will flow toward the existing production 

well 212 and the proposed well sites, or away toward the north 

coast.

Which of these conditions is most likely to exist in 

northwestern Ta'u cannot be determined with the current 

knowledge of Ta'u geology and the hydraulic properties 

of faults in general, but a limited number of scenarios is 

Ground Water in Northwestern Ta’u 13
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Figure 7. Model-simulated water-table elevations, in feet relative to mean sea level, for Ta’u, American Samoa.

14 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
Figure 8. Existing wells, proposed well sites, and landfill in northwestern Ta’u, American Samoa (presence of the fault is based 

on Stice and McCoy, 1968; location of the fault as depicted here is based on topographic features).

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Ground Water in Northwestern Ta’u 15
plausible. These scenarios can be simulated with the numerical 

model to determine which, if any, present a situation in 

which water quality from wells in western Ta'u would be 

threatened by contaminants from the landfill. Four possible 

hydrogeologic scenarios were modeled: (A) the permeability 

of neither the fault nor rift zone is substantially different 

from that of the surrounding rock, (B) the rift zone has a 

lower permeability but the fault has a permeability that is 

the same as that of the surrounding rock, (C) the fault has 

a lower permeability but the rift zone has a permeability 

that is the same as that of the surrounding rock, and (D) 

both the fault and the rift zone have permeabilities that are 

substantially lower than that of the surrounding rock. In all of 

these scenarios, it is assumed that the landfill is on the same 

side of the fault as the well sites. This represents a higher 

probable effect (on the existing and proposed wells) than if the 

landfill and well sites were on opposite sides of the fault. The 

hydraulic conductivities of the simulated rock units for each 

scenario are shown in table 3.

In the two scenarios in which the permeability of the fault 

is not substantially different from the surrounding rock (fig. 9, 

A and B), the water table beneath the landfill slopes toward the 

north coast. Assuming horizontal aquifer homogeneity, ground 

water would flow in a direction perpendicular to the contour 

lines of the water table, as shown by the arrows in figure 9. 

Thus, results of the numerical simulation indicate that in these 

scenarios, ground water beneath the landfill would flow away 

from production well 212 and the proposed new well sites. 

If the rift zone has a lower permeability than the surrounding 

rock as shown in figure 9B, it may constitute an additional 

ground-water separation between the landfill and the existing 

and proposed well sites.

In the simulation in which the fault has a lower 

permeability but the rift zone has a permeability that is the 

same as the surrounding rock (fig. 9C), the water table beneath 

the landfill slopes toward the west, and the natural flow of 

ground water beneath the landfill will likewise be toward the 

west. The simulation results indicate that if this is true, the 

well most closely aligned with ground-water flow from the 

landfill is the existing production well 212. The proposed new 

well sites in the Tunoa Basin are not as closely aligned on a 

flow line from the landfill.

In the simulation in which both the fault and the rift 

zone have permeabilities that are substantially lower than the 

surrounding rock, the ground-water table between the fault 

and the rift zone reaches a substantially higher elevation than 

the surrounding area (fig. 9D). The low-permeability fault, 

rift zone, and tuff cone form a compartment in which ground 

water accumulates to form a mound. The landfill is on the 

northern side of the mound, indicating that ground water from 

beneath the landfill will probably flow to the north, away from 

well 212 and the proposed well sites.

The model simulations indicate that of the four scenarios 

tested, only one (fig. 9C) would result in ground-water flow 

paths that would bring water from beneath the landfill toward 

the wells. In all four scenarios tested, the proposed well sites 

2, 3 and 5 are farthest from any ground-water flow lines 

originating at the landfill and are therefore least likely to be 

affected by contamination from the landfill. The available data 

are insufficient, however, to assess which of these scenarios is 

most likely, and these simulations do not consider the change 

in flow paths that could result from the withdrawal of water 

from any of the well sites, nor do the simulations consider 

other hydrogeologic conditions that may exist in this area.

Because at least one of the simulated possible scenarios 

indicates that water leaching from the landfill may reach the 

existing production well 212, testing of the water quality of 

this well and comparing it to the quality of water from the 

landfill may provide information on the movement through 

ground water of contaminants from the landfill. Analysis 

of a water sample collected from well 207 by the ASEPA 

in 2003 (data provided by Peter Peshut, ASEPA, written 

communication, 2004) indicates that except for sodium, 

concentrations of all inorganic constituents analyzed were 

below the maximum contaminant levels (MCLs) or secondary 

maximum contaminant levels (SMCLs) established by the 

U.S. Environmental Protection Agency (2003), and none of 

the organic constituents analyzed were above the detection 

limit of the analytical method. The high concentration (240 

mg/L) of sodium in the water sample may reflect the known 

Table 3. Hydraulic conductivities applied in numerical-model simulations of ground water in northwestern Ta'u, 

American Samoa.

Aquifer Hydraulic conductivity (feet per day)

Scenario 1 Scenario 2 Scenario 3 Scenario 4

Main shield volcano 1000 1200 1350 1250

Tuff cone 0.1 0.1 0.1 0.1

Fault 1000 1200 0.1 0.1

Rift zones and caldera 1000 0.1 1350 0.1

Offshore areas 1000 1200 1350 1250

16 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
Figure 9. Model-simulated water-table elevations, in feet above sea level, and ground-water flow directions in northwestern Ta’u, American Samoa: (A) 

scenario in which the permeability of neither the fault nor rift zone is substantially different from that of the surrounding rock, (B) scenario in which the rift 

zone has a lower permeability but the fault has a permeability that is the same as that of the surrounding rock, (C) scenario in which the fault has a lower 

permeability but the rift zone has a permeability that is the same as that of the surrounding rock, and (D) scenario in which both the fault and the rift zone 

have permeabilities that are substantially lower than that of the surrounding rock.

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Ground Water in Northwestern Ta’u 17
relatively high salinity of water from this well. If all the 

sodium comes from sea salt (NaCl), a sodium concentration 

of 240 mg/L would correspond to a chloride concentration 

of 370 mg/L, which is consistent with the high chloride 

concentration of water sampled during the reconnaissance visit 

of this study (429 mg/L). The absence of any definitively high 

concentrations of any constituents other than sodium could 

indicate that (1) water from the landfill has not yet reached 

or will not reach the well, (2) the constituents are removed 

or diluted as they travel from the landfill to the well, or (3) 

water from the landfill does not carry any of the constituents 

analyzed. The analysis of water quality at well 212 or any 

other well that may be potentially affected by the landfill 

would be made more meaningful if the contents of the landfill 

or the water leaching from it were known.

Study Limitations and Additional Data 

Needs

Limitations of the island-wide conceptual and 

numerical models – The accuracy of the conceptual and 

numerical ground-water models presented in this study, and 

the hydrologic interpretations made from them, are limited by 

the relatively sparse hydrologic data that is available for Ta'u. 

Rainfall data are available only for the northwestern corner 

of the island, and does not cover a long period of time. There 

are currently no data to show how rainfall varies spatially on 

Ta'u, nor are there any long-term streamflow or evaporation 

data. Because these data are lacking, the recharge estimates 

used in the analytical computations and model simulations in 

this study are, at best, gross approximations. Precise waterlevel elevation data for Ta'u are difficult to obtain, in part 

because not many wells have been drilled, especially in the 

interior of the island, but also because accurate surveys of well 

elevations and measuring points are not available for most 

wells. Understanding of the water resources of Ta'u could be 

significantly advanced by (1) monitoring rainfall, streamflow, 

and evaporation data; (2) monitoring withdrawal, pumping 

schedules, pumping rates, and water quality at all wells; (3) 

establishing accurately surveyed measuring points for all 

wells, both used and unused, that are still open to the water 

table; and (4) testing for aquifer properties (i.e. pump tests or 

aquifer tests).

Because of the objectives of this study, the numerical 

model has greater detail in northwestern Ta'u than anywhere 

else on the island. All aspects of hydrology, including geology, 

recharge, and hydraulic properties for areas except the 

northwest are only grossly simulated. The numerical model is 

not intended for applications beyond the scope of this study.

Limitations of the analysis of northwestern Ta’u – This 

study has focused on the movement of ground water, and 

has not addressed the possibility that contaminants from 

the landfill may be carried by surface water to areas beyond 

the present-day limits of the landfill. If surface transport is 

significant, the potential for contaminants reaching wells 

may be greater than identified in this study. The study also 

has not considered the influence of well withdrawals, which 

can alter the direction of ground-water flow, or the possibility 

that contaminants will be dispersed as they are transported by 

ground water. This study has focused on the relation between 

the existing landfill, existing wells, and proposed sites for new 

wells as indicated by ASPA. It was beyond the scope of this 

reconnaissance study to identify other potential sources of 

contamination that may affect drinking-water wells. A survey 

of all potential sources of contamination, including sources 

from within villages and farms, would enable a more complete 

assessment of the steps needed to protect ground-water 

resources.

As with the island-wide conceptual and numerical 

models, the more focused analysis of the hydrogeology of 

northwestern Ta'u, and the assessment of the potential that 

contamination from the landfill may reach existing and 

proposed well sites, are limited by a paucity of data. The few 

water levels available for the area are insufficient to determine 

hydraulic gradients or the slope of the water table, which have 

a direct bearing on the movement of contaminants that may be 

carried in the ground water. There is little or no information 

on the historical dimensions, construction details, or contents 

of the landfill, nor the chemistry of the water running off the 

landfill.

In addition to the general hydrologic data needs discussed 

above, information that would improve assessment of the 

potential that contaminants in the landfill may reach existing 

and proposed well sites includes (1) history, construction 

details, and contents of the landfill; (2) surface-water flow 

directions in the vicinity of the landfill; (3) chemistry of 

the water flowing from the landfill; (4) spatial distribution 

of water levels in the area; and (5) the quality of water 

in wells that may be affected by the landfill. Because the 

landfill is likely to continue to be a concern for ground-water 

development, these unknowns must be addressed to allow 

a better assessment of the potential threat to both existing 

and proposed wells. Construction and operation (monitoring 

water levels and chemistry) of one or more monitoring wells 

between the landfill and existing or proposed wells would 

provide a means to detect movement of contaminants before 

they reach production wells. Steps that could be implemented 

in the short term include monitoring of water chemistry 

and water levels in all existing and new production wells. 

Determining the chemistry of water percolating through the 

landfill could help identify what contaminants to look for in 

the monitoring of the production wells.

For the long-term future of development of ground-water 

resources on Ta’u, drilling wells farther inland, away from the 

coast and up gradient from existing sources of contamination 

in and near villages, may solve some of the problems posed by 

high-salinity water and contamination from human activities. 

The feasibility of this strategy for Ta'u may be limited, 

however. Toward the interior of the island, the land surface 

rises steeply whereas the water table rises gently. Thus, the 

18 Reconnaissance of the Hydrogeology of Ta’u, American Samoa
farther inland the wells are placed, the deeper the wells would 

have to be, yet the freshwater lens may be only a little thicker 

than it is at the coast. An alternative strategy to reduce the 

problem of saltwater intrusion would be to distribute the 

ground-water withdrawal among several small, low-yield 

wells, constructed so that they do not penetrate far into the 

freshwater lens.

Conclusions

Ta'u probably receives high rates of ground-water 

recharge, but the freshwater lens is thin throughout most 

of the island because aquifer permeability is high in most 

areas. Because the freshwater lens is thin, the productivity of 

most wells is likely to be limited by saltwater intrusion. The 

freshwater lens is thinnest near the coast, therefore wells near 

the coast are more likely to encounter high salinity than wells 

located farther inland, where the freshwater lens is thicker. 

High salinity and low water levels at wells 211 and 212 

indicate that those wells were drilled too deeply into the thin 

freshwater lens in that area. Limited areas of high-level ground 

water may exist in areas such as calderas and rift zones where 

low-permeability geologic structures exist. 

The landfill in northwestern Ta'u is closer to the north 

coast of the island than to any of the existing or proposed 

well sites. This suggests, if other hydrogeologic factors are 

not considered, that ground water beneath the landfill would 

flow away from existing and proposed well sites, but flow 

directions may differ depending on the hydraulic properties 

of a fault and rift zone in the area. Of four plausible scenarios 

tested with a numerical ground-water flow model, only one 

indicated that ground water from beneath the landfill would 

flow toward the existing and proposed well sites. There is, 

however, not enough data to assess which of the scenarios 

tested is most plausible, and the model simulations do not 

consider the influence of well withdrawals on ground-water 

flow directions, surface-water transport of contaminants, 

dispersion of contaminants as they are transported, or other 

sources of contamination besides the landfill.

Accuracy of the analyses and models in this study, 

and the hydrologic interpretations made from them, are 

limited by the relatively sparse data that is available for 

Ta'u. Understanding of the water resources of Ta'u could be 

significantly advanced by monitoring climate, streamflow, and 

well withdrawals, water levels and water quality. Hydrologic 

interpretations can also be improved with accurate surveys of 

measuring-point elevations of all wells and careful testing of 

well performance.

Assessing the potential for the movement of contaminants 

from the landfill to existing and proposed wells can be 

improved with better information on the landfill (history, 

construction, contents, water chemistry) and the surrounding 

area (surface-water flow, ground-water levels, water chemistry 

in wells near the landfill). Monitoring water levels and 

chemistry in wells between the landfill and production wells 

can provide a means to detect movement of contaminants 

before water supplies are affected. Steps that could be 

implemented in the short term include analyzing water in the 

landfill and continuous monitoring of water chemistry and 

water levels in all existing and new production wells.

For the long-term future development of ground-water 

resources on Ta’u, drilling wells farther inland may solve 

some of the problems posed by high-salinity water and 

contamination from human activities, but drilling very far 

inland may not be feasible given the steep topography of Ta'u. 

Alternatively, to mitigate the problem posed by saltwater, 

ground-water withdrawal could be distributed among several 

low-yield wells, constructed so that they do not penetrate far 

into the freshwater lens.

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20 Reconnaissance of the Hydrogeology of Ta’u, American Samoa