HYDROGEOLOGY AND GROUND-WATER RESOURCES OF 

KAHLAP ISLAND, MWOAKILLOA ATOLL, STATE OF 

POHNPEI, FEDERATED STATES OF MICRONESIA

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 91-4184

Prepared in cooperation with the

STATE OF POHNPEI,

DEPARTMENT OF CONSERVATION AND

RESOURCE SURVEILLANCE
HYDROGEOLOGY AND GROUND-WATER RESOURCES OF 

KAHLAP ISLAND, MWOAKILLOA ATOLL, STATE OF 

POHNPEI, FEDERATED STATES OF MICRONESIA

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 91-4184

Prepared in cooperation with the

STATE OF POHNPEI,

DEPARTMENT OF CONSERVATION AND

RESOURCE SURVEILLANCE
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government

For sale by the

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

For additional information write to:

District Chief

U.S. Geological Survey

677 Ala Moana Blvd., Suite 415

Honolulu, HI 96813
CONTENTS

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

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

Purpose and Scope ...................................................................... 1

Background............................................................................ 4

Acknowledgments ...................................................................... 4

Methods of Study.............................................................................. 4

Reconnaissance......................................................................... 5

Installation of Driven-Well Network ........................................................ 5

Surface Geophysical Survey............................................................... 5

Measurement of Rainfall ................................................................. 7

Collection of Water Samples .............................................................. 7

Measurement of Water Levels ............................................................. 7

Slug (Bail) Tests ........................................................................ 7

Hydrogeology................................................................................. 8

General Atoll Features ................................................................... 8

Hydrogeologic Units..................................................................... 8

Conceptual Ground-Water Flow Model..................................................... 11

Hydraulic Characteristics ................................................................ 11

Ground-Water Resources....................................................................... 14

Recharge............................................................................. 14

Occurrence ........................................................................... 17

Storage .............................................................................. 23

Quality .............................................................................. 29

Demand.............................................................................. 30

Sustainable Yield ...................................................................... 30

Development Alternatives ............................................................... 31

Need for additional data ................................................................. 32

Summary ................................................................................... 32

References Cited ............................................................................. 35

FIGURES

1-3. Maps showing:

1. Location of Mwoakilloa Atoll, State of Pohnpei, Federated States of Micronesia ................ 2

2. Mwoakilloa Atoll and location of study area............................................. 3

3. Location of dug wells, driven wells, and lines of section, Kahlap Island. Mwoakilloa Atoll........ 6

4-5. Drawings showing:

4. Schematic cut-away view of an atoll................................................... 9

5. Conceptual hydrogeologic framework of an atoll island................................... 10

6. Drawing showing schematic section of freshwater lens with transition zone, and graph showing

relation of salinity and depth ............................................................ 12

7-10. Graphs showing:

7. Relation between A, tidal efficiency and B, tidal lag with distance from the nearest

shore for selected driven wells, Kahlap Island, Mwoakilloa Atoll........................... 13

Contents ill
8. Variation in tidal-efficiency with depth for selected driven wells, Kahlap Island,

Mwoakilloa Atoll................................................................. 15

9. Variation in hydraulic conductivity with depth for selected driven wells, Kahlap Island,

Mwoakilloa Atoll................................................................. 16

10. Mean monthly rainfall, based on intermittent monthly totals between 1962 and 1977, 

and continuous monthly totals from July 1988 through May 1990, Kahlap Island, 

Mwoakilloa Atoll................................................................. 18

11-12. Maps showing:

11. Mean annual rainfall in the western Pacific ............................................. 19

12. Chloride concentration of ground water at the water table, November 1989, Kahlap

Island, Mwoakilloa Atoll........................................................... 20

13. Graph showing variation in chloride-concentration' with depth at driven-well site B, Kahlap

Island, Mwoakilloa Atoll............................................................... 21

14-15. Diagrams showing:

14. Hydrologic section through the Kahlap freshwater lens, November 1989, Kahlap Island,

Mwoakilloa Atoll................................................................. 22

15. Hydrologic and geophysical sections through the Kahlap freshwater lens, August 1989,

Kahlap Island, Mwoakilloa Atoll..................................................... 24

16-17. Maps showing:

16. Estimated depth to electromagnetic-interpreted interface and thickness of potable freshwater, Kahlap Island, Mwoakilloa Atoll ............................................... 25

17. Estimated depth to base of freshwater nucleus, Kahlap Island, Mwoakilloa Atoll............... 26

18. Graphs showing variation in relative salinity with depth, Kahlap Island, Mwoakilloa Atoll:

a. Driven-well site A................................................................ 27

b. Driven-well site B ................................................................ 27

c. Driven-well site E ................................................................ 27

d. Driven-well site F ................................................................ 28

e. Driven-well site H................................................................ 28

f. Driven-well site I................................................................. 28

19. Graph showing monthly rainfall totals for July 1988 through April 1990, Kahlap Island,

Mwoakilloa Atoll..................................................................... 29

20-21. Diagrams showing schematic of:

20. Vertical-tube well................................................................. 33

21. Horizontal-infiltration well.......................................................... 34

TABLES

1. Driven-well construction information, Kahlap Island, Mwoakilloa Atoll.............................. 5

2. Temperature, specific conductance, and chloride-concentration data from dug wells, Kahlap

Island, Mwoakilloa Atoll.................................................................. 37

3. Temperature, specific conductance, and chloride-concentration data from driven wells, Kahlap

Island, Mwoakilloa Atoll.................................................................. 40

4. Water-quality data from selected dug and driven wells collected November 1989, Kahlap Island,

Mwoakilloa Atoll........................................................................ 44

5. Tidal response data, Kahlap Island, Mwoakilloa Atoll ........................................... 14

6. Monthly and annual rainfall data, Kahlap Island, Mwoakilloa Atoll................................. 18

7. Nitrate concentrations in water from selected dug wells, Kahlap Island, Mwoakilloa Atoll .............. 30

8. Relation between reduction in recharge and thickness of freshwater lens, Kahlap Island,

Mwoakilloa Atoll........................................................................ 31

iv Contents
CONVERSION FACTORS AND ABBREVIATIONS

Multiply By To obtain

acre 4,047

foot (ft) 0.3048

foot per day (ft/d) 0.3048

gallon (gal) 3.785

gallon per minute (gal/min) 0.06308

gallon per day (gal/d) 0.003785

inch (in.) 25.4

inch per year (in/yr) 2.54

mile (mi) 1.609

million gallons (Mgal) 3 735

million gallons per day (Mgal/d) 0.04381

million gallons per day per square mile [(Mgal/d)/mi2] \ 450

ounce (oz) 28.35

square foot (ft) 0.09294

square mile (mi) 2.590

square meter

meter

meter per day

liter

cubic decimeter per minute

cubic meter per day

millimeter

centimeter per year

kilometer

cubic meter

cubic meter per second

cubic meter per day per square kilometer

gram

square meter

square kilometer______________

Abbreviations used: g/cm3 , grams per cubic centimeter; g/d, grams per day; |Limho/m, micromhos per meter; meq/L, 

milliequivalents per liter; mg/L, milligrams per liter; mS/m, millisiemens per meter.

Specific conductance is given in microsiemens per centimeter (|LiS/cm) at 25° Celsius, which is numerically equal to 

micromhos per centimeter (n,mho/cm)

Temperature is given in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by using the 

equation:

1.8x°C + 32 = °F

Contents v
Hydrogeology and Ground-Water Resources of 

Kahlap Island, Mwoakilloa Atoll, State of Pohnpei, 

Federated States of Micronesia

By Stephen S. Anthony 

Abstract

The lens of fresh ground water on Kahlap 

Island contains about 21.3 million gallons of potable water. Recharge to the freshwater lens is estimated to be 125,000 gallons per day on the basis of 

a mean annual rainfall of 120 inches. The longterm average sustainable yield is estimated to be 

about 17,300 gallons per day. The estimated 

demand for water is about 13,500 gallons per day. 

Shallow-vertical-tube wells or horizontal-infiltration wells could be used to develop the freshwater 

lens. The effect of development on the lens can be 

determined by monitoring the chloride concentration of water from a network of shallow-watertable and deep driven wells.

The ground-water resource on Kahlap can be 

used in conjunction with individual rainwatercatchment systems: rainwater can be used for 

drinking and cooking, and ground water can be 

used for sanitary uses. When rainwater-catchment 

systems fail during extended dry periods, ground 

water would be available to meet the total demand.

INTRODUCTION

The demand for water on Mwoakilloa Atoll in the 

State of Pohnpei, Federated States of Micronesia (fig. 1) 

is expected to increase as a result of a desire to construct 

sanitary facilities such as showers, flush toilets, and 

laundry facilities. Water supplies on Mwoakilloa are 

obtained from individual and community rainwatercatchment systems and from shallow dug wells that 

yield fresh to brackish ground water. During extended 

dry periods the demand for potable water commonly 

exceeds the supply.

The water-supply problem on Mwoakilloa was 

accentuated during a drought in 1983. Although rainfall 

was not recorded on Mwoakilloa during 1983, rainfall 

on the island of Pohnpei 88 mi to the northwest was 

only 13 percent of normal for the period January 

through May 1983 (van der Brug, 1986). The subnormal rainfall created a severe shortage because most of 

the potable water on Mwoakilloa comes from rainwater-catchment systems. In addition to strict rationing of 

the water supply, it was necessary to use shallow dug 

wells as a source of drinking water. One way to alleviate 

the chronic water-supply shortage would be to further 

develop ground-water resources for non-potable use so 

that rainwater can be saved for drinking and cooking. 

To address the water-supply concerns of Mwoakilloa 

Atoll, the U.S. Geological Survey (USGS), in cooperation with the State of Pohnpei, Department of Conservation and Resource Surveillance, made a hydrogeologic study to describe the ground-water resources 

of the atoll. Similar studies were made for Pingelap and 

Sapwuahfik Atolls (Anthony, 1996a,b).

Purpose and Scope

The purpose of this report is to describe the hydrogeology and ground-water resources of Mwoakilloa 

Atoll. Kahlap, the only inhabited island at Mwoakilloa, 

is the focus of the report (fig. 2).

This report describes the occurrence, quantity, and 

quality of fresh ground water beneath Kahlap Island. 

Information from an inventory of existing shallow dug 

wells provides a basis for a preliminary assessment of 

the nature of the resource. The thickness and areal 

extent of this fresh ground-water body were determined 

on the basis of a surface geophysical survey and chloride-concentration data collected from a network of 

driven wells installed during the study. The quality of 

ground water was determined on the basis of chemical

Introduction 1
140° E

160° 180°

20°N

Area of inset map

COMMONWEALTH

\

I : jOFTHE NORTHERN: ' 

tSaipan'. IMARIANA ISLANDS I ^_j

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AUSTRALIA

160°W____________

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1 ' - 1^ HAWAII

:-* . SS*.......^--:- - Y"-'":" V :; 

PACIFIC \ OCEAN

AMERICAN 

SAMOA

140° E

160°

20° N

INSET MAP

0 300 MILES i ,-u ,t . .1.

PACIFIC i OCEAN

\

500 KILOMETERS f~ I 

I

\ OF THE NORTHERN

i MARIANA ISLANDS

t

j

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Wake'

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QFTHE 1 

MARSHALL' 

-ISLANDS

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~---._ / STATES OF I^Pohnpei State +' i x 4-j^djf.

 * . MICRONESIA x. ) I:

terK.,.,..,.. .........:..:..:.: .: ...L .- ... A 4 / >'., .,,.......,,.

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 / *."r- ' '? ^^ "*

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Majuro*6

 

Sapwuahf|< ; *Kosrae

i Kosrae | 

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I Naiiru

Figure 1. Location of Mwoakilloa Atoll, State of Pohnpei, Federated States of Micronesia.

2 Hydrology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
159°46'E 159°47'

6°41'N

6°40'

OCEAN

6°39'

Reef flat

Reef flat

Airstrip

Reef flat

PACI/K' 

OC'EAN

Figure 2. Mwoakilloa Atoll and location of study area.

Introduction 3
analyses of water samples collected from selected dug 

wells and driven wells. A conceptual model of the 

hydrogeology of Kahlap Island was developed from 

near-surface observations combined with published 

descriptions of the hydrogeology of other atoll islands. 

Ground-water occurrence is related to assumed variations in lithology and is clarified by observations of 

ground-water quality and tidal fluctuations. This report 

discusses changes in storage of freshwater during a 17- 

month period (July 1988 to November 1989). Estimates 

of water demand and sustainable yield of the groundwater resource, development alternatives, and the need 

for additional data are discussed.

Background

Geography~Mwoaki\\oa Atoll, formerly named 

Mokil Atoll, is located at latitude 6°41' N. and longitude 

159°47' E. (fig. 1). Geographically, Mwoakilloa is part 

of the Caroline Islands archipelago of the western Pacific. The atoll is about 88 mi southeast of Pohnpei and 

about 3,000 mi southwest of Hawaii. Mwoakilloa is 

small, compared with other atolls, 2.5 mi long from 

north to south, and about 1.0 mi wide east to west (fig. 

2). Three islets, composed mostly of coralline sand, are 

scattered along a reef that encloses a lagoon having a 

surface area of 2.6 mi2. The total land area of the atoll 

islets is 0.5 mi2, and the maximum altitude is less than 

25 ft above sea level. Kahlap, located on the northeast 

and windward side of the atoll, is the only inhabited 

island at Mwoakilloa and has a total land area of 0.23

fj

mi . A population of approximately 270 people live in 

a village that is strung along the lagoon-side of Kahlap 

Island (Ashby, 1987).

C/i>wate--Mwoakilloa, located near the equator, 

has the characteristic climatic features of high temperature, cloudiness, and high humidity. Precipitation is 

heavy, averaging about 120 in/yr; however, droughts 

are common. Wind direction and strength rather than 

rainfall or temperature distinguish one season from the 

other. Northeasterly trade winds dominate from about 

November to June. By about April the trades begin to 

diminish in strength, and by July they have given way 

to the lighter more variable winds of the Intertropical 

Convergence Zone (ITCZ). Between July and November the climate of the island is frequently under the 

influence of the ITCZ which has moved northward. 

This is the season when moist southerly winds and tropical disturbances are most frequent.

Although Mwoakilloa is located within the spawning grounds of typhoons, the major typhoon tracks of 

the western Pacific lie well to the north and west. However, typhoons have caused extensive damage to crops 

and homes on the island on several occasions. The most 

recent typhoon, Lola, struck Mwoakilloa in 1986. 

Typhoons of recent years, however, have not been as 

severe as the one in 1905 that required the evacuation of 

many Mwoakillese to Pohnpei.

Political feistory Mwoakilloa Atoll is politically 

part of the State of Pohnpei, Federated States of Micronesia. The modern history of the Mwoakillese people 

stems from their contact with whaling ships and traders. 

The first recorded contact was in 1815 by Captain G. 

Bethan aboard the vessel Marquis of Wellington. 

Between 1886 and 1986 several foreign flags were 

flown in the islands; the Spanish flag from 1886 to 

1899, the German flag from 1899 to 1914, the Japanese 

flag from 1914 to 1945, and the American flag from 

1945 to 1986. While under a Trusteeship Agreement 

between the United States and the United Nations Security Council, Pohnpei (Ponape) joined in a union with 

Yap, Chuuk (Truk), and Kosrae (Kusaie) to form the 

Federated States of Micronesia in 1979. A Compact of 

Free Association between the Federated States of 

Micronesia and the United States was approved by the 

United Nations Trusteeship Council in 1986.

Acknowledgments

Interest in and support for the project by the former 

Governor of Pohnpei, the Honorable Resio Moses, is 

gratefully acknowledged. Mr. Antonio Actouka, Chief 

of the Energy Division, Department of Conservation 

and Resource Surveillance, provided assistance that 

facilitated the successful completion of the project. Wilson Ben of Mwoakilloa and Don "Arnie" Arnold, a 

USGS employee, assisted in all aspects of the field 

work.

METHODS OF STUDY

The methods used to describe the ground-water 

resources of Kahlap Island, Mwoakilloa Atoll were 

constrained by available transportation to and from the 

atoll, as well as on the atoll itself. To reach the isolated 

atoll of Mwoakilloa, equipment and supplies must be 

shipped by the interisland (field trip) shipping service

4 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
that calls on the island only once every four to six 

weeks. All equipment and supplies must be portable 

enough to be hand-carried because there are no docking 

facilities or vehicles on the island.

Reconnaissance

A reconnaissance of Mwoakilloa Atoll was done in 

December 1986 to collect baseline information needed 

for the identification of a potential fresh ground-water 

resource, and to begin the assessment of ground water 

as an alternative water supply. Twenty-one existing dug 

wells on Kahlap Island were located, described, and 

sampled for temperature, specific conductance, and 

chloride concentration (these wells are included in figure 3). Five of the 21 dug wells were sampled to determine major constituent ion, nitrate, and phosphate 

concentrations.

Installation of Driven-Well Network

A network of 31 driven wells, comprising 13 clusters, was installed in July 1988 to determine and monitor the thickness of fresh ground water beneath Kahlap 

Island (fig. 3). The wells were installed, generally in 

clusters of three, at depths bracketing the lower limit of 

potable water. A nearly ubiquitous hard layer at or near 

the water table restricted the location of driven-well 

clusters to within existing dug wells or excavations dug 

during the study that penetrated the hard layer. The 

driven wells consist of 2-in.-diameter galvanized steel 

pipe and well points with 24-in.-long screens. Pipe was 

added in 5- or 6-ft sections by threaded couplings and 

driven by a 100-pound drop hammer until the screen 

reached the depth desired or the deepest depth attainable. A surge block was used to develop the wells. Construction information for the driven wells, including 

depth and screened interval is provided in table 1.

The use of driven wells with well points allows the 

determination of water level and water quality at specific depths without disrupting the natural chlorideconcentration distribution in the lens-shaped freshwater 

body. Use of continuously perforated wells on Enewetak (Buddemeier and Holladay, 1977) and Kwajalein 

(Hunt and Peterson, 1980) Atolls in the Marshall 

Islands allowed the movement of underlying saltwater 

into the freshwater section of the well by means of tideinduced mixing thereby obscuring the natural chlorideconcentration distribution in the freshwater lens.

Table 1. Driven-well construction information, Kahlap Island, 

Mwoakilloa Atoll

Well no.: Letter is cluster-site designation; number is depth of well 

below land surface.

Well no.

A-15

A-20

A-24

B-14

B-19

B-24

B-50

C-9

C-10 

C-12

Screened interval 

(depth in feet below 

land surface)

12.5 - 14.5

17.5 - 19.5

21.5 - 23.5

11.5-13.5

16.5 - 18.5

21.5 - 23.5

47.5 - 49.5

6.5 - 8.5

7.5 - 9.5 

9.5- 11.5

Comments

broken well point 

broken well point

D-18.75

E-15 

E-19 

E-22.5

F-10 

F-14 

F-19

F-24

G-11.5 

G-16

H-14 

H-19 

H-24

I-10 

1-15 

1-20

J-12.5

L-9.5 

L-11.5

M-10.5 

N-8.0

16.3 - 18.3

12.5 - 14.5 

16.5-18.5 

20.0-22.0

8.5 - 9.5 

11.5-13.5 

16.5-18.5 

21.5-23.5

9.0-11.0 

13.5-15.5

11.5-13.5 

16.5 - 18.5 

21.5-23.5

7.5 - 9.5 

12.5 - 14.5 

17.5 - 19.5

10.0- 12.0

7.0-9.0 

9.0-11.0

8.0 - 10.0 

5.5 - 7.5

bent well point, pumped sand 

hand pump installed

1-1/4 inch well point 

hand pump installed

pumped sand

bent well point, pumped sand

1-1/4 inch well point 

1-1/4 inch well point

1-1/4 inch well point

1-1/4 inch well point_____

Surface Geophysical Survey

Electromagnetic profiling was done in August 

1989 to interpolate freshwater thickness data between 

driven-well clusters and ultimately to areally map the 

thickness of freshwater. The electromagnetic-profiling

Methods of Study 5
a 

3. o

0)a O

3

ca

I

159°47'

6°41'

6°40'50"

6°40'40"

PACIFIC

159°47'10"

He&fOat

159°47'20"

EXPLANATION

A' LINE OF SECTION-Sections 

shown in figures 14 and 15

DUG WELL AND NUMBER

DRIVEN WELL AND SITE 

DESIGNATION

Fteefflat

PACIFIC 

OCEAN

Figure 3. Location of dug wells, driven wells, and lines of section, Kahlap Island, Mwoakilloa Atoll.
procedure is based on a well-established surface geophysical method (McNeill, 1980a; Stewart, 1982; Stewart, 1988; Anthony, 1992). In this method a magnetic 

field applied at the surface induces electrical currents in 

the earth. These currents generate a secondary magnetic 

field that is measured and used to obtain terrain-conductivity readings. Factors that affect terrain conductivity 

are (1) porosity, (2) pore fluid conductivity, (3) pore 

surface area, (4) degree of saturation, (5) temperature, 

and (6) presence of clays with moderate to high cation 

exchange capacity.

The instrument used in this study was the dual loop 

Geonics EM 34-3XL terrain-conductivity meter. Survey transect lines of about 20 stations were established 

along 10 foot paths that cross the width of the island. 

Four apparent conductivity readings consisting of two 

coil spacings (10 and 20 meters) and two coil orientations (horizontal and vertical coplanar) were made at 

each station providing four different exploration depths. 

In the vicinity of driven-well clusters, the electromagnetic-interpreted interface depth was calibrated with 

chloride-concentration data derived from the drivenwell clusters. The depth to the electromagnetic-interpreted interface was calculated from the field data using 

the computer program EMIX 34 (Interpex, 1988). 

EMIX 34 is a forward and inverse modeling program 

designed to obtain a quantitative interpretation of terrain-conductivity data in terms of a layered-earth solution.

Measurement of Rainfall

Daily rainfall measurements were made with a 4- 

in. diameter rain gage by a local USGS observer from 

July 1988 through May 1990 (fig. 3). These measurements were used to identify seasonal variations in rainfall and to relate changes in rainfall with changes in 

storage of freshwater within the lens.

Collection of Water Samples

Samples were collected for analysis of chlorideconcentration from all wells in July 1988, August 1989, 

and November 1989. Samples collected from driven 

wells were used to develop relative-salinity depth profiles needed to estimate the thickness of freshwater. 

Samples collected from shallow dug wells were used to 

describe the chloride-concentration distribution in

ground water at the water table as a means of defining 

the lateral extent of the fresh ground-water body.

To determine the chemical character of water at 

each site, one sample from each well was analyzed for 

major constituent cations and anions, and for inorganic 

nutrients. The samples were collected in November 

1989. Samples from driven wells were collected using 

an all-plastic, hand-operated positive displacement 

pump to avoid aeration-induced concentration changes 

of the major ionic constituents. The wells were pumped 

until the electrical conductivity of the pumped water 

stabilized, indicating that water in the casing had been 

completely purged, before collecting the water sample. 

Dip samples were collected from the dug wells. Temperature, pH, total alkalinity, and specific conductance 

were determined in the field. Samples for major ion and 

nutrient analysis were preserved for laboratory determinations at the University of Hawaii. Results of these 

analyses are shown in tables 2,3, and 4 at the end of the 

report.

Measurement of Water Levels

Continuous water levels were obtained from 

selected dug and driven wells with a chart recorder used 

in conjunction with an electric water-level sensor. Measurements were made for 2 days to relate ground-water 

responses to tidal fluctuations. A continuous recording 

tide gage was installed on the lagoon shore of Kahlap 

Island for 7 days to relate Kahlap tide data with tide data 

from a gage on Pohnpei that is maintained by the Tropical Ocean Global Atmosphere Sea Level Center at the 

University of Hawaii. The location of the lagoon tide 

gage is shown in figure 3.

Slug (Bail) Tests

Slug (bail) tests are a quick and inexpensive field 

method for obtaining localized, horizontal hydraulic 

conductivity values of aquifer material with a single 

well. Slug (bail) tests were made in November 1989 by 

lowering the water level in driven wells by short-duration pumping with a hand pump and attached suction 

hose that was rapidly removed from the well after the 

water level had lowered 10 to 20 ft. The recovery of the 

water level was timed with a stopwatch and measured 

with a hand-held electric measuring tape.

Methods of Study 7
HYDROGEOLOGY

The following sections describe the general features of atolls and the hydrogeologic units that form 

atoll islands. The last two sections describe a conceptual 

ground-water flow model for atoll islands, and the 

hydraulic characteristics of the sediments that comprise 

Kahlap Island.

General Atoll Features

Atolls are sub-circular reefs composed of a resistant framework of calcareous skeletons enclosing a 

lagoon from the open sea. The reef structure is constantly scoured and planed off by wave action and dissected 

by seaward-trending surge channels. The reef framework commonly is highly porous. For some atolls estimates of as much as 50 percent porosity have been cited 

(Selley, 1970). Islets are composed primarily of unconsolidated carbonate sediments and sit atop the reef 

structure. A schematic diagram of an atoll is shown in 

figure 4.

An atoll is derived from a fringing reef around a 

volcanic island. As the island sinks, the reef organisms 

build upward in an attempt to keep pace with the relative rise in sea level. The volcanic island is eventually 

submerged, leaving only the reef and the enclosed 

lagoon, forming an atoll. As part of the environmental 

studies made in the Marshall Islands in connection with 

atomic-bomb testing, the U.S. Navy drilled a series of 

deep test holes on Enewetak Atoll. Two of the test holes 

went through a 3,900-ft cap of shallow-water reef limestone and bottomed in basalt. The age of fossils in the 

deepest limestone is Eocene, indicating that Enewetak 

Atoll is the top of a coralline accumulation that began 

growing upward about 60 million years ago (Schlanger, 

1963).

Sea level rises during interglacial periods and falls 

during glacial periods. The cycle of sea-level rise and 

fall has been repeated several times in the past million 

years. These fluctuations affected carbonate depositional sequences on oceanic islands worldwide. During the 

Pleistocene epoch, atolls were affected by four or more 

such fluctuations. With each drop in sea level, as much 

as 300 vertical feet of reef and lagoonal sediments were 

exposed to subaerial weathering and erosion. The subsequent sea-level rise caused accumulation of new reef 

and lagoonal sediments over each preceding erosional

unconformity (Schlanger, 1963). Pleistocene unconformities have been reported in six atolls of the central 

Pacific Ocean: Bikini, Enewetak, Majuro, Mururoa, 

Midway, and Tawara (Emery and others, 1954; Ladd 

and Schlanger, 1960; Anthony and others, 1989; Lalou 

and others, 1966; Ladd and others, 1970; and Marshall 

and Jacobson, 1985, respectively). The unconformities 

were caused by alternation between growth during 

interglacial high stands of sea level and erosion during 

glacial lowering of sea level. Holocene sediments above 

the 120,000-year unconformity at depths of 26 to 32 ft 

at Enewetak and Bikini are generally little more than 

6,000 years old (Tracey and Ladd, 1974).

Hydrogeologic Units

The hydrogeologic framework of an atoll island 

can be considered to consist of four units (fig. 5). The 

first unit is the island itself. The island intercepts rainfall; a fraction of the rainfall is lost to evapotranspiration, and the remainder infiltrates the island's sediments 

to form a lens of freshwater. Because of the high permeability of the soils, there is no surface runoff.

The second unit is the reef-flat plate. The reef-flat 

plate is extremely well-indurated and forms a stable 

foundation on which the island's sediments accumulate. 

It probably is no more than 10 to 15 ft thick and thins 

from the reef front lagoonward, pinching out at some 

distance beneath the island. The permeability of the 

reef-flat plate is less than that of the underlying unconsolidated sediments. The reef-flat plate at Kahlap 

extends beneath much of the island and acts as a confining layer, impeding recharge. Elsewhere, the lens is 

unconfined.

The third unit consists of unconsolidated deposits 

of Holocene age. The deposits are composed of silt-togravel-sized fragments of foraminifera, Halimeda and 

coral. Layering and lateral gradation of these back-reef 

and marginal-lagoon deposits affect the occurrence and 

flow of freshwater in the Holocene deposits. Analyses 

of the rate at which tidal fluctuations propagate into the 

ground-water system and analyses of sediment grainsizes indicate a general areal variation in permeability 

across an atoll island: relatively high values for sediments bordering the ocean, and lower values for sediments adjacent to the lagoon (Ayers and Vacher, 1986; 

Marshall and Jacobson, 1985; Anthony and others, 

1989). The presence of lower permeability sediments

8 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
Island Island

Sea level

< Pinnacle reefs Inter-island passes 

Sand bar or spit

Central 

vv % depression

Boulder ramparts 

Beach rock

Coralline algal ridge 

Relict Pleistocene terrace

1,000 J____I 2,000 FEET __I I I T I I

0 200 400 METERS

Approximate vertical exaggeration x 5

O 

CO 

(D O. O 

CO Figure 4. Schematic cut-away view of an atoll (modified from Underwood, 1990).
I

o

10 

Q) Q. OOC

a

I

33 (DID O

IIDaQ) :rQ)o

Q) JT

6"

Q)

FEET

10 -i

Sea level Lagoon^^'U^-r^^-^;r'-;^T^-^.^.^^^^e^^^^iJ,^?-,

'& xff Set.-'^fV"^ i "'&''^* c FRESHWATER

Datum is sea level

EXPLANATION

HOLOCENE DEPOSITS 

FineCoarse 

REEF-FLAT PLATE

REEFPLEISTOCENE DEPOSITS

Not to scale

Figure 5. Conceptual hydrogeologic framework of an atoll island (modified from Ayers and Vacher, 1986).
adjacent to the lagoon results in a freshwater lens that is 

thicker on the lagoon-side of an atoll island.

The fourth unit consists of highly permeable 

deposits of Pleistocene age that underlie the Holocene 

deposits. The primary skeletal material of the Pleistocene deposits is similar to that of the Holocene deposits.The contact between the Holocene and Pleistocene 

deposits is typically at depths of 50 to 80 ft below sea 

level and represents a pronounced permeability contrast 

in which the Pleistocene deposits are more permeable 

than the Holocene deposits (Tracey and Ladd, 1974; 

Marshall and Jacobson, 1985; Anthony and others, 

1989). The permeability contrast between the two units 

has been estimated to be at least an order of magnitude 

and has been attributed to the development of secondary 

porosity when the Pleistocene deposits were emergent 

during low sea-level stands. The highly permeable 

Pleistocene deposits at Majuro were found to contain 

seawater and thin the transition zone beneath the part of 

the freshwater lens that is sufficiently thick to extend 

down to the Pleistocene deposits (Anthony and others, 

1989).

A conceptual model of the hydrogeology of Kahlap Island was developed from near-surface observations combined with published descriptions of the 

hydrogeology of other atoll islands in the western Pacific (fig. 5). This model incorporates a dual-aquifer system that consists of surficial Holocene deposits 

overlying more permeable Pleistocene deposits (Buddemeier and Holladay, 1977; Ayers and Vacher, 1986; 

Anthony and others, 1989; Underwood, 1990). Layering and lateral gradation of back-reef and marginallagoon deposits affect the occurrence and flow of freshwater within the Holocene deposits. Core drilling was 

not done at Mwoakilloa Atoll; however, Mwoakilloa 

Atoll has probably undergone about the same depositional and tectonic histories and sea-level changes as 

other atolls in the western Pacific. Therefore a generalized conceptual hydrogeologic model of atoll islands 

should apply to the island of Kahlap.

Conceptual Ground-Water Flow Model

Atoll islands are composed of permeable sediments that are readily infiltrated by rainfall. If the infiltration from rainfall is sufficiently great, a lens of 

freshwater that floats on saline ground water forms, 

somewhat like an iceberg floating in the ocean (fig. 6). 

Freshwater moves downward and then radially outward, toward the coastal margins of the island to discharge into the sea. Some of the freshwater mixes with 

underlying saltwater to form a transition zone of mixed, 

or brackish water. The freshwater, which has a

O

density of 1.000 g/cm , displaces the underlying saltwater, which has a density of about 1.025 g/cm . The 

depth of this lens is about 40 times the elevation of the 

water table above sea level. This 40:1 ratio is known as 

the Ghyben-Herzberg relation. The actual thickness of 

freshwater is influenced by the recharge and discharge 

rates, size and shape of the island, and the hydraulic 

characteristics of the hydrogeologic units. Variations in 

the thickness of the transition zone are affected by mixing induced by tidal fluctuations, variations in recharge 

and pumping rate, and the rate and direction of groundwater flow.

In most atoll islands, the hydraulics of the groundwater system are characterized by long-term horizontal 

flow that is driven by recharge, and short-term vertical 

fluctuations that are driven by semi-diurnal tides. Tidal 

fluctuations enhance mixing within the transition zone 

and ultimately reduce the potable part of the freshwater 

lens. In general, the freshwater lens is thicker on the 

lagoon-side of an atoll island because of the lower permeability of lagoon-side sediments relative to oceanside sediments (Cox 1951; Hamlin and Anthony 1987). 

Numerical computer simulation of this conceptual 

ground-water flow model has been done by Herman and 

others (1986), Hogan (1988), Griggs (1989), and 

Underwood (1990).

Hydraulic Characteristics

The hydraulic characteristics of the Holocene 

deposits at Mwoakilloa were estimated by analyzing 

tidal responses and slug (bail) tests. Tidal response analysis was used to qualitatively assess the permeability of 

aquifer material between a given well and the ocean. 

Slug (bail) test analysis was used to quantitatively 

determine localized horizontal hydraulic-conductivity 

values of aquifer material in the vicinity of a single well. 

Because of the difference in scale measured by tidal 

response and slug (bail) tests, it is not possible to correlate the results from these analyses.

Tidal efficiency is expressed as the ratio of waterlevel fluctuation in a well to the tidal fluctuation in the 

ocean. Similarly, tidal lag is the time difference 

between ocean tide and corresponding fluctuation in 

ground-water level. The results from tidal response and

Hydrogeology 11
Not to scale

Figure 6. Schematic section of freshwater lens with transition zone, and graph showing relation 

of salinity and depth (modified from Vacher, 1974).

slug (bail) tests can be affected by the clogging of the 

well screen with fine grained sediments during the construction of a driven well. All driven wells were developed by surging to remove fine-grained sediments and 

ensure that water levels in wells represent water levels 

in the aquifer. It is not possible to remove all of the finegrained sediments. The reduction in well efficiency due 

to the clogging of a well screen will have a greater effect 

on slug (bail)-test results than on tidal response data 

because the rate of change in water level is much larger 

during a slug (bail) test than during a tidal-response test.

Tidal lag and tidal efficiency data from Bikini, 

Enewetak, Kwajalein, and Majuro Atolls have been 

analyzed in terms of a dual-aquifer system consisting of 

surficial Holocene deposits overlying more permeable 

Pleistocene deposits (Underwood, 1990). The tidal lag 

and tidal efficiency data from these atolls indicate that 

patterns of ground-water flow are affected by the permeability contrast between the Holocene and Pleistocene deposits, and sea-level fluctuations that impose 

stresses from both the lagoon and the ocean-side of the

island. Increasing tidal efficiencies with depth indicate 

that the permeability of the Pleistocene deposits is much 

greater than that of the overlying Holocene deposits. On 

Majuro, tidal lags on the order of 15 to 30 minutes and 

tidal efficiencies near 90 percent recorded just above 

the Holocene/Pleistocene contact indicate that these tidal signals have undergone very little attenuation as they 

propagate through the Pleistocene deposits (Anthony 

and others, 1989). It is likely that the tidal signal in an 

atoll island is propagated vertically within the Holocene 

deposits, because the depth to the highly permeable 

Pleistocene deposits is relatively small compared to the 

width of the island.

Plots of tidal lag and efficiency for selected watertable wells as a function of distance from the nearest 

shore on Kahlap Island are shown in figure 7. The plots 

indicate that water-table tidal responses vary parabolically inland from the nearest shore, indicating that these 

tidal responses are not complicated by the permeability 

contrast between the Holocene and Pleistocene deposits 

as was observed on Bikini, Enewetak, Kwajalein, and 

Majuro Atolls.

12 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
1

5

* 

o_

<§

w -nIf

2. 

^

CD* fl> 

Q. -si Q. '

05 SL3 03

5 5'

CD. 3«" CT

- CD

1|

"EL*

5T * ! Q.Q. ffi.

"^ 2, S. =5 5 o'

§ 

®

M03 §

5 Da

o!

8L

oT

CO

§.5

Q. (/)'1" O 

CD

3

Bcf3CD

03

c5SIw3-oc5o"

a

§z

o 

mT|DO

0

^z mDOmCO

1

COT

0

DOmz

T|m

i

TIDAL LAG, IN HOURS TIDAL EFFICIENCY, IN PERCENT

IV) ^ CJ) 00 C O-J- IV) CO -&.CJ1O OOO OC o -" 

IV) o 

oo o8

008

t

o8

L

1 1 1 1

DO

0

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-

,

9

-

%

1 1 1 1

\~>

IV)

8o

O)800

o o

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8o

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9

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1 1 1 1

o 

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0
On Mwoakilloa, tidal lag and tidal efficiency data 

do not show a systematic decrease and increase with 

depth, respectively, further indicating that groundwater flow patterns are not affected by the assumed permeability contrast between Holocene and Pleistocene 

deposits (table 5 and fig. 8). Without subsurface geologic information and relying on the dual-aquifer system 

model, it can only be inferred that a low-permeability 

layer, perhaps lagoonal muds at depth within the 

Holocene deposits or an impermeable rind of a solutionaltered Pleistocene soil zone, may retard the vertical 

propagation of the tidal signal at Kahlap Island.

Calculated hydraulic conductivities for Kahlap 

range from less than 1 to greater than 100 ft/day (fig. 9). 

Recovery times for the slug (bail) tests ranged from too 

fast to measure to greater than 4 hours. The tests were 

analyzed using the method of Hvorslev (1951) and the 

shape factor presented by Freeze and Cherry (1979). 

This analysis assumes a homogeneous, isotropic, infinite medium in which both soil and water are incompressible. The calculated hydraulic-conductivity values 

for Kahlap are consistent with those obtained from other atoll islands: 0.3 to 200 ft/day (Underwood, 1990). A 

hydraulic conductivity of 0.1 ft/day was calculated for 

driven well B-50 that penetrates 50 ft below land surface at the B-site. Although B-50 is the only driven well 

that penetrates more than 25 ft below land surface, the 

data from this well supports the hypothesis of low-permeability lagoonal muds at depth within the Holocene 

deposits.

To fully understand the complexities of subsurface 

layering and the effect of layering on the occurrence and 

flow of ground water in the Holocene deposits, core 

samples of these deposits must be collected; however, 

without an efficient and cost-effective means of obtaining core samples on a remote island, this cannot be 

accomplished.

GROUND-WATER RESOURCES

The following sections describe recharge to, and 

the occurrence, quantity, and quality of fresh ground 

water beneath Kahlap Island. Estimates of water 

demand and sustainable yield of the ground-water 

resource, development alternatives, and the need for 

additional data are discussed.

Table 5. Tidal response data, Kahlap Island, Mwoakilloa Atoll

[hr:min, hours and minutes; ND, not determined]

Well no.: Letter is cluster-site designation, WT is water-table well, and number is depth of well below land surface.

Tidal efficiency: ratio of water-level fluctuation in a well to the tidal fluctuation in the ocean. 

Tidal lag: time difference between ocean and ground-water signals.

Well no.

Tidal 

efficiency

Tidal lag 

hrimin

A-WT 

A-15 

A-20 

A-24

B-14 

B-24 

B-50

C-12 

D-18.75

E-WT 

E-19

E-22.5

F-19 

F-24

G-11.5 

G-16

H-14 

H-19 

H-24

I-WT

1-15

1-20

J-12.5

0.27 

ND 

ND

0.27

0.34

0.16

0.27

0.15

0.21

0.29 

ND 

0.36

0.22

0.29

ND

0.39

0.58

ND

0.58

0.89

0.42

0.51

0.28

3:10 

ND 

ND

3:10

2:50 

2:30 

2:40

3:05 

3:55

2:15 

ND

2:25

3:00 

3:10

ND

3:00

1:00 

ND 

1:00

1:25 

1:40 

1:20

3:50

Recharge

A water balance based on regional climatic data 

was used to estimate the average annual recharge to the 

freshwater lens at Kahlap. The components of the water 

balance are rainfall, surface-water runoff, evapotranspiration, and ground-water recharge. Because there is no 

surface-water runoff on atoll islands, the rate of 

recharge to the freshwater lens at Kahlap is equal to 

rainfall minus evapotranspiration. For Kahlap, no mea14 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
10

LLJ 

O

cc

ID

Q 

Z 

<

O

LU 

CD

LU 

LU 

LL

Q. 

LU 

Q

20

30

40

50

20 40 60

TIDAL EFFICIENCY, IN PERCENT

80 100

Figure 8. Variation in tidal efficiency with depth for selected driven wells, Kahlap Island, Mwoakilloa Atoll.

Ground-Water Resources 15
10

LLJ 

O 

<

LL 

DC

20

LJJ

m

ILU 

LJJ 

LL

Q. 

LJJ 

Q

30

40

50

0.1

T I I I I I I I I I I I I I I I I I I I I

I I I I I I I I I I I

1 10 

HYDRAULIC CONDUCTIVITY, IN FEET PER DAY

100

Figure 9. Variation in hydraulic conductivity with depth for selected driven wells, Kahlap Island, 

Mwoakilloa Atoll.

16 Hydrology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
surements have been made for any component of the 

water balance, except rainfall.

The mean annual rainfall at Kahlap is 121 in. with 

a standard deviation of 27 in. This data is based on daily 

rainfall totals recorded intermittently by a local U.S. 

National Oceanic and Atmospheric Administration 

observer between 1962 and 1977, and continuously 

between July 1988 and May 1990 by a local USGS 

observer (table 6). The data consists of 134 monthly and 

5 annual totals. Figure 10 shows that on average rainfall 

is highest in August (12.86 in.) and lowest in February 

(6.66 in.). There is no significant dry season.

A regional rainfall map of the western Pacific 

(Taylor, 1973) shows the average annual rainfall at 

Kahlap to be 200 in. (fig. 11), which is significantly 

higher than the 121 in. of mean annual rainfall recorded 

at Kahlap. The position of the 160- and 200-in. rainfall 

line shown in figure 11 is probably the result of orographic effects reflected in the long-term rainfall 

records of the high volcanic islands of Kosrae and 

Pohnpei, which are located to the southeast and northwest of Kahlap, respectively. A comparison of monthly 

rainfall totals for Kahlap and Pohnpei shows little or no 

correlation; the lack of correlation indicates that Kahlap 

is not influenced by orographic effects, as on Pohnpei, 

and that the average annual rainfall at Kahlap is probably closer to 120 in. than to 200 in. The long-term average annual rainfall at Kahlap is therefore assumed to be 

120 in. because it provides a more conservative estimate 

of ground-water recharge in the following analysis.

Evapotranspiration, a term used to describe evaporation of rainfall and transpiration of soil moisture by 

plants, was not measured at Kahlap. It can be estimated, 

however, by evaluating previous studies in the western 

Pacific. In a study of the Chuuk (Truk) Islands, Takasaki (1989) developed a relation between mean annual 

rainfall and evapotranspiration on the basis of data from 

the islands of Guam, Johnston, and Yap and determined 

that the evapotranspiration loss was about 50 percent of 

the rainfall at Chuuk. The islands of Chuuk and Kahlap 

lie within an elongate belt of greater than 120 in. of 

annual rainfall that extends eastward beyond Majuro 

and westward to Palau (fig. 11). Mullet (1987) estimated 

ground-water recharge rates for Pacific atolls under 

average climatological conditions and rainfall as shown 

in Taylor (1973). The method assumes that climatological conditions over atolls are similar to those over the 

open ocean. Nullet (1987) concluded that ground-water

recharge for Kahlap is about 50 percent of the rainfall. 

The 50 percent recharge rate also compares favorably 

with estimates by Hunt and Peterson (1980) and Hamlin 

and Anthony (1987) for Kwajalein and Majuro Atolls, 

respectively. Evapotranspiration at Kahlap is, therefore, 

estimated to be 50 percent of rainfall.

To provide a conservative estimate of groundwater recharge it is assumed that the long-term average 

annual rainfall at Kahlap is 120 in. and evapotranspiration losses are 50 percent of the average annual rainfall. 

Given these assumptions, ground-water recharge at 

Kahlap is 60 in/yr. This rate of recharge over a freshwater lens catchment area of 28 acres underlain by at least 

5 ft of potable water results in an average daily recharge 

of 125,000 gal. This is an average value; actual values 

would vary with rainfall and evapotranspiration.

Occurrence

The size and shape of the lens of freshwater at 

Kahlap is governed principally by recharge, hydraulic 

characteristics of the aquifer, and the size and shape of 

the island. The configuration of the lens was estimated 

using chloride-concentration data from the dug and 

driven wells. For the purposes of resource evaluation, 

the term "freshwater nucleus" is applied to the potable 

part of the ground-water body. The World Health Organization's (WHO)(1971) international standard for 

drinking water recommends a maximum permissible 

level of 600 mg/L for chloride; this criterion was adopted in this study for definition of the freshwater nucleus.

Samples from dug wells were collected to define 

the lateral extent of the freshwater lens. The chlorideconcentration values for ground water at the water table 

at Kahlap Island indicate that the freshest part of the 

ground-water body is located beneath the taro patch and 

slightly lagoonward of the island center (fig. 12).

Samples were collected from driven wells to determine the vertical distribution of chloride concentration 

and to develop relative-salinity profiles that can be used 

to calculate the thickness of the freshwater lens. A chloride-concentration depth profile at the B-site shows the 

typical S-shaped curve (fig. 13). The nearly vertical left 

limb of the curve represents the nucleus of the freshwater lens. The chloride concentration increases rapidly 

with depth between points a and b. Below point b, the 

chloride concentration increases gradually with depth.

Ground-Water Resources 17
Table 6. Monthly and annual rainfall data, Kahlap Island, Mwoakilloa Atoll

[M, missing data; N, number of years of data; , not applicable; values are in inches]

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1988

1989

1990

Mean

N

32.18

8.22

2.69

8.72

6.58

15.41

5.89

4.68

0.20

M

8.39

M

M

M

M

M

M

9.39

7.45

9.15

12

11.27

10.36

9.45

4.90

0.40

7.79

5.95

4.07

3.44

M

10.75

3.66

M

M

M

M

M

11.41

3.18

6.66

13

13.11

11.72

10.29

8.28

9.28

11.40

M

10.32

3.24

M

9.57

4.42

M

M

M

M

M

6.24

9.75

8.96

12

14.10

11.11

11.58

16.32

5.44

18.32

14.24

5.44

5.75

M

15.99

M

M

M

M

M

M

15.65

13.04

12.25

12

11.50

15.71

21.21

7.78

19.06

9.09

7.27

6.05

11.23

M

M

12.15

M

5.70

M

10.16

M

15.45

18.16

12.18

14

M

12.24

18.14

7.79

8.31

13.21

11.82

5.56

10.87

M

M

M

M

M

M

9.37

M

10.40

--

10.77

10

M

10.33

11.14

15.46

9.88

8.52

M

8.39

7.85

M

M

10.84

M

M

M

13.84

10.43

15.86

-

11.14

11

M

23.81

12.18

18.19

10.38

10.49

M

8.04

M

M

M

M

M

M

M

7.31

13.12

12.25

-

12.86

9

5.20

11.50

13.06

6.97

8.63

7.74

M

5.47

M

10.82

18.87

M

M

M

M

18.25

16.37

7.13

-

10.83

12

10.80

9.23

8.04

11.06

12.67

11.62

M

6.64

M

12.94

1.55

M

M

M

M

M

16.17

12.29

--

10.27

11

11.30

9.28

8.45

12.54

11.25

M

M

6.90

M

5.54

M

M

M

M

M

M

9.08

8.65

--

9.22

9

7.40

7.52

M

10.09

9.60

4.90

3.48

7.77

M

5.16

M

M

M

M

M

M

12.55

22.60

-

9.11

10

M

141.03

M

128.10

111.48

M

M

79.33

M

M

M

M

M

M

M

M

M

147.32

--

121.44

5

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

MONTH

Figure 10. Mean monthly rainfall, based on intermittent monthly totals between 1962 and 1977, and 

continuous monthly totals from July 1988 through May 1990, Kahlap Island, Mwoakilloa Atoll.

18 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
140° E

20° N 

PACIFIC \ OCEAN

\ COMMONWEALTH 

\ OF THE NORTHERN 

MARIANA ISLANDS

# ISLANDS

FEDERATED STATES 

OF MICRONESIA

EXPLANATION

40 LINE OF EQUAL MEAN ANNUAL RAINFALL 

Interval, in inches, is variable

Figure 11. Mean annual rainfall in the western Pacific (modified from Taylor, 1973).
I

o_ oo

s

30 (DwO3(D W

159°47' 159°47'10" 159°47'20"

6°41'

6°40'50"

6°40'40"

50-

EXPLANATION

- LINE OF EQUAL CHLORIDE 

CONCENTRATION, IN 

MILLIGRAMS PER LITER

fleet flat

PACIFIC 

OCEAN

Figure 12. Chloride concentration of ground water at the water table, November 1989, Kahlap Island, Mwoakilloa Atoll.
10

LLJ 

O

cc

CO 20

Q

Z

3 30

LU

CO

LLJ 

LJL

? 40

D_

LLJ 

Q

50

60

^Electromagnetic-inteipretedjnterface^

B

2,000 4,000 6,000 8,000 10,000 12,000 

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

14,000

Figure 13. Variation in chloride concentration with depth at driven-well site B, Kahlap Island, Mwoakilloa Atoll. 

S-shaped curve shows gradual increase in chloride concentration in the freshwater nucleus to point A, rapid 

increase between points A and B, and gradual increase below point B.

The driven-well clusters installed during the study 

allow for the determination of chloride-concentration 

data at specific depths without disrupting the natural 

chloride-concentration distribution within the freshwater lens. In order to interpolate between specific depths, 

a consistent methodology was used. In this study, the 

point-chloride-concentration determinations are 

expressed as relative salinity in percent seawater. The 

relative salinity values are then plotted on probability 

paper against depth, permitting linear interpolation 

between points (Vacher, 1974). Relative salinity (RS) is 

defined as

RS= 100

(Cl-Clf) 

(Cl -CL)

v s f

0)

where Cl is the chloride concentration in the water sample and Cls and Clp represent chloride concentrations in 

saltwater and freshwater, respectively.

It is assumed that the limit for chloride concentration in potable water is 600 mg/L; which is equivalent 

to a relative salinity of 3.1 percent. The depth of the 600 

mg/L chloride-concentration contour was interpolated 

from relative-salinity graphs. These depths were then 

used to construct cross-sections of the freshwater nucleus on the basis of the position of the 600 mg/L chlorideconcentration contour. Figure 14 is a cross-section 

through the Kahlap freshwater lens showing lines of 

equal relative salinity. The 3.1 percent relative-salinity 

contour defines the base of the freshwater nucleus. The 

thickest part of the freshwater nucleus coincides with 

the area within and around the taro patch.

Ground-Water Resources 21
to to

I

3 o

CD«< 

D>a 

poc3a

I 5T a5T5T

a

A 1

Sea level

-10 - -20 - -30 - -40 - -50 3.7 EXPLANATION LINE OF EQUAL RELATIVE SALINITY IN PERCENT SEAWATER-Dashed where approximate Chloride concentration, in milligrams per liter, used to calculate / relative salinity L- 13,600

Altitude of land surface is estimated 

Datum is sea level

Vertical scale greatly exaggerated

J__I

500 FEET

\ \ I I

100 METERS

Figure 14. Hydrologic section through the Kahlap freshwater lens, November 1989, Kahlap Island, Mwoakilloa Atoll. Line of section is 

shown in figure 3.
Information on the thickness and shape of the 

freshwater lens was also derived from the geophysical 

survey. The purpose of the geophysical survey was to 

interpolate lens thickness data between driven-well 

clusters and areally map the thickness of freshwater. A 

three layered-earth solution that includes unsaturated, 

freshwater, and saltwater zones was used to obtain a 

quantitative interpretation of electromagnetic (terrainconductivity) data measured in the field.

The following assumptions are used when applying electromagnetic profiling methods and layered earth 

modeling to the mapping of freshwater lenses: (1) the 

earth consists of horizontal, infinite, homogeneous, and 

isotropic layers, (2) the conductivity of each layer is 

constant and changes abruptly at each boundary, (3) the 

deepest layer is assumed to have an infinite thickness, 

and (4) the water table is close to sea level (Anthony, 

1992).

The first layer of the three-layer solution represents the unsaturated zone. The thickness of the first 

layer is assumed to be the elevation of land surface 

above sea level; this is reasonable considering the 

height of the water table above sea level on atoll islands 

is less than 3 ft. The second layer is the freshwater-saturated zone. The third layer is the saltwater-saturated 

zone, assumed to be infinite in thickness. A conductivity of 1.0 jxmho/m is assumed for the unsaturated layer, 

because the contrast between the conductivity of the 

unsaturated layer and the freshwater layer is not large 

enough to be resolved (Kauahikaua, 1987). Conductivity values for the freshwater and saltwater layers, and 

thickness of the second layer, which represents the 

freshwater lens, were determined by inversion modeling of the electromagnetic (terrain-conductivity) data 

with the computer program EMIX 34.

The overall shape of the freshwater lens was adequately determined by use of electromagnetic profiling 

methods and layered earth modeling. The model 

applied includes the unsaturated zone, freshwater zone, 

and saltwater zone, but excludes the transition zone. 

Chloride-concentration data from driven wells indicate 

that the electromagnetic-interpreted interface is located 

in the upper part of the transition zone, where the chloride-concentration depth profile shows a rapid increase 

with depth (fig. 13 and 15). Considerable variability 

was found at the island margins. Figure 16 shows the 

depth to the electromagnetic-interpreted interface 

below the water table in plan view as well as freshwater-thickness data at driven-well clusters. Electromagnetic profiling methods produce poor results where the 

island is underlain by brackish water rather than freshwater, because the layered-earth model used assumes 

that a freshwater layer exists and that there is a large 

contrast in conductivity between the freshwater and 

saltwater layers.

Storage

Storage of potable ground water in the Kahlap 

freshwater lens is estimated to be 21.3 Mgal, on the 

basis of the volume of the freshwater nucleus adjusted 

to account for porosity. Porosity was estimated to be 20 

percent to provide a conservative estimate of the storage 

of ground water in unconsolidated carbonate sediments. 

The volume of the freshwater nucleus was approximated by combining chloride-concentration and geophysical data to estimate the depth to the base of the 

freshwater nucleus in plan view (fig. 17).

Storage of potable ground water in a freshwater 

lens varies with recharge from rainfall. Samples for 

analysis of chloride concentration were collected at four 

different times during a 17-month period to determine 

the response of the freshwater lens to variations in 

recharge from rainfall. Figure 18 shows plots of the 

variation in relative salinity with depth at driven-well 

clusters. The thickness of potable freshwater, as defined 

by 3.1 percent relative salinity, has varied less than 5 ft 

between sampling periods, indicating a relatively constant thickness of freshwater. Freshwater thickness is 

calculated by subtracting the depth to water, usually 

about 5 to 10 ft below land surface, from the depth corresponding to 3.1-percent relative salinity. The thickness of freshwater remained relatively constant because 

each of the sampling periods was preceded by monthly 

rainfall of greater than 10 in. (fig. 19).

Mean monthly rainfall at Mwoakilloa is consistently greater than 10 in. except for the months of 

November through March. February is the only month 

in which rainfall falls below 8 in. (fig. 10). Numerical 

computer simulations have shown that atoll island 

freshwater lenses respond quickly to increased rainfall, 

but are less responsive to decreased rainfall (Underwood, 1990). Therefore, several months of little or no 

rainfall would be required to show a reduction in the 

thickness of freshwater under natural conditions. 

Because this did not occur during the 17-month study

Ground-Water Resources 23
a

FEET

10 -i

Sea level -

-10 -

-20 -

-30 -

-40 -

-50

B'

Freshwater zone

\

^

Electromagneticinterpreted interface

Saltwater zone

no

- io4

-7,120

.Chloride concentration, 

in milligrams per liter

100 2ooFEET

i

25 50 METERS

-13,600

Altitude of land surface is estimated 

Datum is sea level

Vertical scale greatly exaggerated

u_

0

C'

FEET

10 -i

5 -

-5 - 

-10-

-15-

-20 -

o> o>

c c

0 0

Q Q

Water table ± L

\ / "^\ -248

\ / \ Freshwater zone 

V \ ~

\ L 780 

\ -48^

/* - 

Electromagnetic- ' \ 

interpreted interface ' "" " t^.0

Saltwater zone ,

(

_ ^\

Unsaturated zone ^

^Chloride concentration, 

X^ in milligrams per liter

/'"

50 ___^.-^ /

D 100 200 FEET

I I I I

I I I I 

3 25 50 METERS

Altitude of land surface is estimated 

Datum is sea level

Vertical scale greatly exaggerated

Figure 15. Hydrologic and geophysical sections through the Kahlap freshwater lens, August 1989, 

Kahlap Island, Mwoakilloa Atoll. Lines of section shown in figure 3.

24 Hydrology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
159°47' 159°47'10" 159°47'20"

O 

3

Q.

6°41'

6°40'50"

6°40'40"

PACffTC 

OC&tiV

22 LINE OF EQUAL DEPTH TO ELECTROMAGNETICINTERPRETED INTERFACE, IN FEET BELOW THE 

WATER TABLE-Interval 4 feet

ELECTROMAGNETIC SURVEY TRANSECT LINES 

WITH SOUNDING STATIONS

DRIVEN-WELL SITE AND THICKNESS, IN FEET 

BELOW THE WATER TABLE, OF POTABLE FRESHWATER

PACIFIC 

OCEAN

Figure 16. Estimated depth to electromagnetic-interpreted interface and thickness of potable freshwater, Kahlap Island, Mwoakilloa Atoll.
O 

(O

B> 

O.O Oc

0. I

3J 

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OCEAN

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Reef fiat

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I I I T I

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6°40'40"

10 

EXPLANATION

LINE OF EQUAL DEPTH TO BASE OF 

FRESHWATER NUCLEUS, IN FEET BELOW WATER TABLE-Interval 5 feet

PACIFIC 

OCEAN

Figure 17. Estimated depth to the base of the freshwater nucleus, Kahlap Island, Mwoakilloa Atoll.
LU

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DATE

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1 

1

 

0.01 0.1 1 2 10 20 30 40 50 60 70 80 90 95 98 99 99.9 99.99

RELATIVE SALINITY, IN PERCENT SEAWATER

Figure 18. Variation in relative salinity with depth for selected driven-well sites, Kahlap Island, 

Mwoakilloa Atoll.

Ground-Water Resources 27
u

5 

10

15 

20

25

30 

0.

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D1 0.1 12 5 10 20 30 40 50 60 70 80 90 95 98 99 99.9 99.99

RELATIVE SALINITY, IN PERCENT SEAWATER

Figure 18. Variation in relative salinity with depth for selected driven-well sites, Kahlap Island, 

Mwoakilloa Moll-Continued.

28 Hydrology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
25

20

C/3 

LU

X 

O

15

? 10

Sampling date for chloride 

concentration of ground water

O) 

CO 

C)

*

O

C) 

C)

J A S O N D 

1988

J FMAMJJASOND 

1989

J F M A 

1990

Figure 19. Monthly rainfall totals for July 1988 through April 1990, Kahlap Island, Mwoakilloa Atoll.

period, it was not possible to determine the response of 

the freshwater lens to climatic stress such as a drought.

Quality

Analyses for temperature, pH, total alkalinity, calcium, magnesium, strontium, sodium, potassium, chloride, sulfate, nitrate, and phosphate were made to 

determine the chemical characteristics of the water in 

the freshwater lens at Kahlap Island. Samples were collected from selected dug and driven wells (table 4). For 

this report, the basic criterion for determining potability 

is the chloride concentration of the water. The WHO 

(1971) reports maximum desirable and permissible levels for chloride of 200 and 600 mg/L, respectively. The 

lower figure represents the concentration where chloride begins to adversely affect the taste of water.

Another chemical factor to be considered is the 

hardness of water. Hardness of fresh ground water at 

Kahlap is attributed to calcium and magnesium, and to 

a lesser extent strontium. Hard water requires considerable amounts of soap to produce a foam or lather and 

can also contribute to incrustation that can develop during pumping, when water undergoes changes in temperature and pressure. All water from shallow wells 

surveyed had hardness values exceeding the WHO 

(1971) highest desirable level of 100 mg/L as calcium 

carbonate (CaCO3), but below the maximum permissible level of 500 mg/L as CaCO3 . The hardness of water 

from shallow wells ranged from 220 to 450 mg/L as 

CaCO3 . The WHO (1971) levels for hardness are recommended to avoid excessive scale formation in waterdelivery systems.

Ground-Water Resources 29
One of the most common contaminants in ground 

water is dissolved nitrogen in the form of nitrate. Excessive concentrations of nitrate in drinking water can 

cause methemoglobinemia in infants. The WHO (1971) 

level for nitrate in drinking water is 45 mg/L. Unlike 

most elements in ground water, nitrate is not derived 

from aquifer materials. Instead, nitrate enters the 

ground from nitrogen fixing plants, decomposing plant 

debris, animal and human waste, and fertilizers. Natural 

nitrate concentration in ground water ranges from 0.1 to 

10 mg/L (Davis and DeWiest, 1966). Because nitrate 

cannot be removed from water by boiling, the water 

must be treated by demineralization or distillation.

Nitrate concentrations in water from dug wells at 

Kahlap range from 0.02 to 24 mg/L with a mean of 7.0 

mg/L. While none of these waters exceed the WHO 

(1971) level for nitrate in drinking water, seven of them 

show concentrations elevated above the 10 mg/L commonly found in ground water (table 7). The wells from 

which these waters with elevated concentrations were 

obtained are located along the lagoon shore (fig. 3). The 

source of the elevated concentrations is probably from 

human waste owing to the large number of water-seal 

toilets in close proximity (inland) to these wells. Waterseal toilets discharge water and waste into an un-lined 

pit located directly beneath the toilet.

Some water-seal toilets on Kahlap have been constructed within 50 ft of an existing dug well. In one 

example, an existing dug well was abandoned to serve 

as a discharge pit for a water-seal toilet. Because ground 

water flows radially outward from the interior of the 

island to the sea, livestock and toilets located inland of 

shallow wells may contaminate well water. The potential for such contamination can be reduced by locating 

livestock and toilets on the seaward side of wells. 

Because the water table generally is shallow, the depth 

of the toilet discharge pit also plays a role in reducing 

potential contamination of ground water. Construction 

of water-seal toilets with at least 2 or 3 ft of unsaturated 

sediments beneath the discharge pit could promote 

some filtration of discharge water before it reaches the 

ground-water body.

Demand

The demand for water on Kahlap is expected to 

increase as a result of a desire to construct sanitary facilities such as showers, flush toilets, and laundry faciliTable 7. Nitrate concentrations in water from selected 

dug wells, Kahlap Island, Mwoakilloa Atoll

[NO3, nitrate; mg/L, milligrams per liter]

Well no.: DW is dug well; number is well designation.

Well no.

DW-1

DW-2

DW-4

DW-7

DW-10

DW-33

DW-34

N03 (mg/L)

24

17

16

12

18

10

14

ties. The population of Kahlap has remained rather 

constant over the past 55 years and in 1985 was about 

270 (Ashby, 1987). Assuming a demand of 50 gal/d per 

person and a population of 270 persons, 13,500 gal/d 

would be required to meet the demand for water.

Sustainable Yield

Sustainable yield from a lens of freshwater can be 

defined as the quantity of water that can be withdrawn 

from that lens on a long-term basis without producing 

an undesired result (Todd, 1959). The actual short-term 

yield is variable and depends on several factors, of 

which, periodic droughts may be the most important. 

The effect of overdraft on the freshwater lens would be 

degradation of water quality by saltwater intrusion. For 

example, when the water table is lowered by pumping 

water from a well penetrating only the upper freshwater 

part of the aquifer, a local rise in saltwater below the 

well will occur. This form of saltwater intrusion is 

known as upconing. An on-going monitoring program 

is needed to detect early signs of saltwater intrusion.

The withdrawal of ground water from the Kahlap 

lens will reduce natural discharge from the lens to the 

ocean and in turn reduce the thickness of freshwater. 

The reduction in the thickness of freshwater can be estimated by using Mather's (1975) method, which assumes 

that withdrawing ground water from an aquifer is equivalent to reducing vertical recharge. The estimate is 

based on the Dupuit assumptions where the thickness of 

freshwater below sea level (7) is proportional to the 

square root of the rate of uniform vertical recharge per 

unit area (/?). This relation,

T = ajR, (2)

30 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
Table 8. Relation between reduction in recharge and thickness of freshwater lens, Kahlap Island, Mwoakilloa Atoll

Assumes ground-water recharge is 60 inches per year and water is withdrawn from the 12.9 acres underlain by an average freshwater thickness 

of greater than 15 feet (fig. 17).

Percent of 

recharge reduced

0

20

25

40

60

80

Amount of 

recharge reduced 

(inches)

0

12

15

24

36

48

Equivalent quantity 

of water pumped 

(gallons per day)

0

11,500

14,400

23,000

34,600

46,000

Freshwater lens 

thickness 

(feet)

15.0

13.4

13.0

11.6

9.5

6.7

where a is the constant of proportionality, can be used 

to predict changes in the equilibrium position of the 

base of the freshwater lens with changes in uniform 

vertical recharge.

Annual recharge to the freshwater lens at Kahlap is 

estimated to be about 60 in. and produces a lens that 

extends to 15 ft below sea level. Application of the 

Mather (1975) method yields a proportionality constant 

of 1.94. If the effective recharge is reduced by 20 percent to 48 in. by pumping, then the new equilibrium 

freshwater thickness is 1.94 x A/48, or 13.4 ft below 

mean sea level. The effect of reducing vertical recharge 

on the thickness of freshwater is shown in table 8. This 

method assumes that water is withdrawn uniformly 

from the freshwater lens and that the thickness of freshwater will be reduced by an equal percentage at all locations.

It should be noted that the proportionality constant 

in equation (2) is dependent on the thickness of freshwater below sea level. Therefore, if ground-water withdrawals were to occur over an area underlain by 10 ft of 

freshwater, the proportionality constant would be 1.29. 

If the effective recharge is reduced by 20 percent or 48 

in. by pumping, then the new equilibrium freshwater 

thickness is 1.29 x A/48, or 8.9 ft below mean sea level.

The estimated changes in freshwater thickness predicted by the Mather (1975) method are only first-order 

approximations, because recharge to the aquifer is not 

steady but results from periodic rainfall. The estimate 

can be refined by taking into account the amount of 

storage that must remain within the lens to maintain 

pumping during periods of drought.

The estimated demand for water on Kahlap is 

about 13,500 gal/d. This demand can be met by reducing recharge by approximately 25 percent and the freshwater lens thickness by approximately 2.0 ft (table 8).

Experience on Kwajalein and Diego Garcia Atolls indicates that approximately 30 percent of mean annual 

recharge to the freshwater nucleus can be developed 

from a freshwater lens on a long-term basis without 

adversely affecting the resource (C.D. Hunt Jr., USGS, 

oral commun., 1990). Thirty percent of the mean annual 

recharge to the part of the lens underlain by at least 15 

ft of potable water (12.9 acres) at Kahlap is equivalent 

to a long-term average sustainable yield of 17,300 gal/d. 

It may be possible to increase ground-water pumpage 

beyond that quantity, provided that the transition zone 

is continually monitored with a series of driven-well 

clusters to allow early detection of saltwater intrusion.

Rainfall, and therefore recharge, is variable in 

time, and may require that well production be adjusted 

in response to changes in the condition of the lens. The 

estimate of long-term sustainable yield can be refined 

by monitoring the response of the freshwater lens to 

various pumping rates and rainfall. Pumping rates can 

be increased until water demand is met or saltwater 

intrusion is detected. Saltwater intrusion, if present, can 

be alleviated by reducing pumping rates until the chloride-concentration of the pumped water decreases to 

acceptable levels.

Development Alternatives

The ground-water resource of Kahlap Island has 

been developed in only a limited way, mostly through 

shallow dug wells from which water is dipped for washing purposes. These open wells are easily contaminated 

with waste materials and bacteria that can present a 

safety hazard. Two of the driven wells installed during 

this study were outfitted with a hand pump and a concrete pad. These wells are inexpensive, sanitary, and 

available to the community as a whole.

Ground-Water Resources 31
Use of ground water at Kahlap could be increased 

by installing vertical-tube or horizontal-infiltration 

wells. In the absence of an electrical power system, 

these wells could be outfitted with solar, windmill, or 

gasoline driven pumps. Schematic diagrams of the vertical and horizontal wells are shown in figures 20 and 

21, respectively. Both designs rely on the establishment 

of sea-level and mean-lower-low-water datum planes 

for proper location of the well screens.

Placement of production wells over the thickest 

part of the lens along the eastern side of the taro patch 

would aid in minimizing the possibility of saltwater 

intrusion. Regular spacing of wells would distribute the 

effects of pumpage evenly and prevent localized overdraft. Experience on Diego Garcia Atoll indicates that a 

shallow vertical well depth of about 10 ft and individual 

pumping rates of about 10 gal/min or less helps to prevent excessive drawdown and saltwater intrusion. Vertical-tube wells are less expensive and easier to 

construct than horizontal-infiltration wells, but horizontal wells allow pumping at higher rates from a single 

well without causing excessive drawdown and thus 

reduce the potential for upconing of saltwater. Horizontal-infiltration wells are similar in construction to the 

vertical-tube wells. Horizontal wells have a water-tight 

pump sump into which water can flow only from the 

horizontal-infiltration pipes (D.A. Davis, USGS, written commun., 1986). Fewer horizontal than vertical 

wells are needed to provide an equivalent total pumping 

rate.

Certain land-use activities may contaminate the 

ground-water resource. Disposal of human and animal 

waste represents the greatest potential for ground-water 

contamination at Kahlap. Farming activities also may 

contribute contaminants to the ground water in the form 

of fertilizers and pesticides. The separation of water 

development areas from sources of contamination will 

aid in the protection of the freshwater resource.

Need for additional data

An understanding of the relation between well production, recharge, and water quality is needed in the 

management of the ground-water resource at Kahlap. 

This relation can be developed from rainfall, pumpage, 

and water-quality data (chloride and nitrate) collected 

routinely from a network of shallow-water-table wells, 

deep driven wells, and production wells. The shallowwater-table wells and deep driven-wells need to be

located within and around the perimeter of the anticipated area of ground-water development to determine spatial and temporal changes in water quality, estimate 

changes in storage, and define responses to variations in 

recharge and discharge (pumpage).

SUMMARY

The demand for water on Kahlap Island, Mwoakilloa Atoll is expected to increase as a result of a desire to 

construct sanitary facilities such as showers, flush toilets, and laundry facilities. Water supplies on Kahlap 

are obtained from individual and community rainwatercatchment systems and from shallow dug wells yielding 

fresh to brackish ground water. During extended dry 

periods the demand for potable water commonly 

exceeds the supply. One way to alleviate the chronic 

water-supply shortage is to further develop groundwater resources.

The fresh ground-water lens at Kahlap is the most 

favorable additional source of freshwater in terms of 

storage and availability. Long-term average daily 

recharge to the freshwater lens at Kahlap is about 

125,000 gallons (50 percent of rainfall) on the basis of 

an average annual rainfall of 160 inches. It is estimated 

that about 21.3 million gallons of freshwater are stored 

in the Kahlap lens and about 17,300 gallons per day 

could be developed on a sustained basis. The estimated 

demand for water is about 13,500 gallons per day.

The hydrogeologic framework at Kahlap, to a large 

extent, controls the shape and size of the freshwater 

nucleus. A conceptual hydrogeologic model of Kahlap 

Island has been developed from near-surface observations combined with published descriptions of the 

hydrogeology of other atoll islands in the western Pacific. This model incorporates a dual-aquifer system consisting of surficial Holocene deposits overlying more 

permeable Pleistocene deposits. Layering and lateral 

gradation of back-reef and marginal-lagoon deposits 

affect the occurrence and flow of freshwater within the 

Holocene deposits. The hydraulics of the system are 

characterized by long-term, mainly horizontal flow that 

is driven by recharge, and short-term vertical fluctuations that are driven by semi-diurnal tides. Tidal fluctuations expand the transition zone, which ultimately 

reduces storage in the freshwater lens.

The extent of the potable part of the freshwater lens 

at Kahlap is delineated by the 600 milligrams per liter

32 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
Concrete pump base to be added

Membrane

Concrete curb

Sand backfill

6

o

C 

CO 

.C

0)

o

E

0

Solid-wall nonferrous pipe

Water table

Sea level

 . ' Sand backfill

0

C-X

0

[}

3

t less than 

o o

Q

y o

o o

o Q

o °

cb O

1 i

 Not less 

than 1 foot t

, ' ' i

1 i i

i ' |

1 , , 

 ,

;: : :

i

© &

© v ® © '- |

0

o

Perforated or slotted

(1/4 inch or more)

nonferrous pipe

Envelope of crushed

hard limestone 

(1/2-3/4 inch diameter) 

washed free of all fines

\

Concrete plug Not to scale

Figure 20. Schematic of vertical-tube well (modified from Hamlin and Anthony, 1987).

Summary 33
PLAN VIEW 

Not to scale

Circular or square pump sump

Infiltration pipes, 

each about 25 feet long

VERTICAL SECTION

Not to scale

Water table

CM

__l_

Mean-lower 

low-water datumInvert of infiltration pipeat 1 foot below 

mean-lower-low water 

sea-level datum plane

Membrane

-3 to 4 

feet Ground surface

Note: Depths below ground surface 

are approximate and may vary 

from place to place

Water-tight, nonferrous 

lining in pump sump

Plug at

end of

pipe

Envelope of crushed

hard limestone 

(1/2-3/4 inch diameter) 

washed free of all fines

Perforated or slotted

nonferrous infiltration pipe,

not less than 1 foot in

diameter

Figure 21. Schematic of horizontal-infiltration well (modified from Hamlin and Anthony, 1987).

34 Hydrology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
chloride-concentration contour. The depth of the 600 

milligrams per liter chloride-concentration contour was 

interpolated from relative salinity-depth profiles on the 

basis of chloride-concentration data from driven-well 

clusters. Profiles of relative salinity correlate fairly well 

with geophysical profiles of the lens. The volume of the 

freshwater nucleus was approximated by combining 

chloride-concentration and geophysical data to produce 

a plan-view map of the thickness of potable freshwater. 

Results of chemical analyses show that water from the 

Kahlap freshwater lens is within the WHO (1971) recommended maximum permissible drinking-water limits. Elevated nitrate concentrations in some locations, 

however, may indicate potentially harmful bacterial 

contamination.

The ground-water resource at Kahlap can be developed using a network of either vertical-tube wells or 

horizontal-infiltration wells. Network management 

would entail monitoring rainfall, pumpage, and water 

quality in shallow-water-table wells, deep driven wells, 

and production wells. Monitoring water quality will 

identify salinity increases that indicate saltwater intrusion. Well production can be adjusted in response to 

changes in the chloride concentration of the freshwater 

lens. Records of individual well and total well-field 

pumping rates will permit evaluation of sustainable 

yield under actual developmental conditions.

The ground-water resource on Kahlap Island, 

Mwoakilloa Atoll, can be used in conjunction with individual rainwater-catchment systems: rainwater can be 

used for drinking and cooking, and ground water can be 

used for sanitary uses. When rainwater-catchment systems fail during extended dry periods, ground water 

would be available to meet the total demand.

REFERENCES CITED

Anthony, S.S., 1992, Electromagnetic methods for mapping 

freshwater lenses on Micronesian atoll islands: Journal 

of Hydrology, v. 137, p. 99-111.

 , 1996a, Hydrogeology and ground-water resources of 

Pingelap Island, Pingelap Atoll, State of Pohnpei, Federated States of Micronesia: U.S. Geological Survey Water 

Resources Investigations Report 92-4005,40 p.

- , 1996b, Hydrogeology and ground-water resources of 

Ngatik Island, Sapwuahfik Atoll, State of Pohnpei, Federated States of Micronesia: U.S. Geological Survey Water Resources Investigations Report 93-4117,44 p.

Anthony, S.S., Peterson, F.L., Mackenzie, F.T., and Hamlin, 

S.N., 1989, Geohydrology of the Laura fresh-water lens, 

Majuro Atoll: A hydrogeochemical approach: Geological Society of America Bulletin, v. 101, p. 1066-1075.

Ashby, G., 1987, A guide to Pohnpei: An island argosy: Eugene, Oreg., Rainy Day Press, 317 p.

Ayers, J.F., and Vacher, H.L., 1986, Hydrogeology of an atoll 

island: A conceptual model from detailed study of a Micronesian example: Ground Water, v. 24, p. 185-198.

Buddemeier, R.W., and Holladay, Gale, 1977, Atoll hydrology, island groundwater characteristics and their relationship to diagenesis: Proceedings of the 3d Interconference 

Coral Reef Symposium, Miami, Fla., p. 167-173.

Cox, D.C., 1951, The hydrology of Arno Atoll, Marshall Islands: Scientific Investigations in Micronesia, Pacific 

Science Board [Hawaii], National Research Council, 

29 p.

Davis, S.N., and DeWiest, R.J.M., 1966, Hydrogeology: New 

York, Wiley, 463 p.

Emery, K.D., Tracey, J.I., Jr., and Ladd, H.S., 1954, Geology 

of Bikini and nearby atolls: U.S. Geological Survey Professional Paper 260-A, 265 p.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, N.J., Prentice-Hall, 604 p.

Griggs, J., 1989, Numerical simulation of groundwater development schemes for the Laura area of Majuro Atoll, 

Marshall Islands: Ph.D. dissertation, University of Hawaii at Manoa, Hawaii, 203 p.

Hamlin, S.N., and Anthony, S.S., 1987, Ground-water resources of the Laura area, Majuro Atoll, Marshall Islands: U.S. Geological Survey Water-Resources 

Investigations Report 87-4047, 69 p.

Herman, M.E., Buddemeier, R.W., and Wheatcraft, S.W., 

1986, A layered aquifer model of atoll island hydrology: 

A validation of a computer simulation: Journal of Hydrology, v. 84, p. 303-322.

Hogan, P., 1988, Modeling of freshwater-seawater interaction on Enjebi Island, Enewetak Atoll: M.S. thesis, San 

Jose State University, Calif., 141 p.

Hunt, C.D., Jr., and Peterson, F.L., 1980, Ground-water resources of Kwajalein Island, Marshall Islands: University of Hawaii, Water Resources Research Center, 

Technical Report 126,91 p.

Hvorslev, M.J., 1951, Time lag and soil permeability in 

groundwater observations: U.S. Army Corps of Engineers Waterways Experiment Station Bulletin 36, Vicksburg, Miss.

Interpex Limited, 1988, EMIX 34 user's manual: Golden, Colo., 89 p.

Kauahikaua, J., 1987, Description of a freshwater lens at 

Laura Island, Majuro Atoll, Republic of the Marshall IsReferences Cited 35
lands, using electromagnetic profiling: U.S. Geological 

Survey Open-File Report 87-582, 32 p.

Ladd, H.S., and Schlanger, S.O., 1960, Drilling operations on 

Enewetak Atoll: U.S. Geological Survey Professional 

Paper 260-Y, p. 863-903.

Ladd, H.S., Tracey, J.I., Jr., and Gross, M.G.. 1970, Deep 

drilling on Midway Atoll: U.S. Geological Survey Professional Paper 680-A, p. A1-A22.

Lalou, C., Labeyrie, J., and Delibrias, G., 1966, Datation des 

calcaires coralliens de 1' atoll de Mururoa (archipel des 

Tuamotu) de 1'e'poque actuelle jusqu'a - 5000, ans.: 

Comptes Rendus Acad. Sci. 263-D, p. 1946-1949.

Marshall, J.F., and Jacobson, G., 1985, Holocene growth of a 

mid-Pacific atoll: Tarawa, Kiribati: Coral Reefs, v. 4, 

p. 11-17.

Mather, J.D., 1975, Development of the groundwater resources of small limestone islands: Quarterly Journal of 

Engineering Geology, v. 8, p. 141-150.

McNeill, J.D., 1980a, Electromagnetic terrain conductivity 

measurement at low induction numbers: Geonics Ltd. 

Technical Note-6, Mississauga, Ontario, 15 p.

Nullet, D., 1987, Water balance of Pacific atolls: Water Resources Bulletin, v. 23, p. 1125-1132.

Schlanger, S.O., 1963, Subsurface geology of Enewetak 

Atoll: U.S. Geological Survey Professional Paper 260-B, 

p. 991-1066.

Selley, R.C., 1970, Ancient sedimentary environments - a 

brief survey: Ithaca, N.Y., Cornell University Press, 

237 p.

Stewart, M.T., 1982, Evaluation of electromagnetic methods 

for rapid mapping of salt-water interfaces in coastal aquifers: Ground Water, v. 20, p. 538-545.

 , 1988, Electromagnetic mapping of fresh-water lenses 

on small oceanic islands: Ground Water, v. 26, p. 187- 

191.

Takasaki, K.J., 1989, Ground-water resources of selected 

high volcanic islands of Truk with emphasis on small village supplies: U.S. Geological Survey Water-Resources 

Investigations Report 88-4163,60 p.

Taylor, R.C., 1973, An atlas of Pacific Islands rainfall: University of Hawaii, Hawaii Institute of Geophysics, Data 

Report 25, 5 p.

Todd, D.K., 1959, Ground-water hydrology: New York, 

Wiley, 336 p.

Tracey, J.I., and Ladd, H.S., 1974, Quarternary history of 

Enewetak and Bikini Atolls, Marshall Islands: Proceedings of the Second International Coral Reef Symposium, 

December 1974, Brisbane, Australia, p. 537-550.

Underwood, M.R., 1990. Atoll island hydrogeology: Conceptual and numerical models: Ph.D. dissertation, University of Hawaii at Manoa, Hawaii, 205 p.

Vacher, H.L., 1974, Groundwater hydrology of Bermuda: 

Hamilton, Bermuda, Government of Bermuda Public 

Works Department, 87 p.

van der Brug, Otto, 1986, The 1983 drought in the western 

Pacific: U.S. Geological Survey, Open-File Report 85- 

418, 89 p.

World Health Organization, 1971, International standards for 

drinking water: (2d ed.): Geneva, Switzerland, 72 p.

36 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
Table 2. Temperature, specific conductance, and chloride-concentration data from dug wells, Kahlap Island, 

Mwoakilloa Atoll

Well no.: DW is dug well and number is well designation.

[°C, degrees Celsius; jjS/cm, microsiemens per centimeter at 25°C; mg/L, milligrams per liter, --, no data]

Well no.

DW-1

DW-2

DW-3

DW-4

DW-5

DW-6

DW-7

DW-9

DW-10

DW-11

Date

12-11-86

12-12-86

07-13-88

08-22-89

11-07-89

12-11-86

07-13-88

08-22-89

11-09-89

12-11-86

07-13-88

08-22-89

11-08-89

12-11-86

07-13-88

08-22-89

11-09-89

12-11-86

07-13-88

08-22-89

11-09-89

12-11-86

12-12-86

07-13-88

10-01-88

08-22-89

12-11-86

07-13-88

10-01-88

08-22-89

11-08-89

12-11-86

07-13-88

08-22-89

11-08-89

12-11-86

07-13-88

10-01-88

08-22-89

11-08-89

12-11-86

07-13-88

10-01-88

Time

1000

1200

1515

0950

1038

1125

1320

0950

1410

1135

1225

1200

1550

1145

1310

0955

1420

1150

1300

1010

1430

1200

1410

1130

1010

1145

1210

1115

1000

1115

-

1225

1035

1045

1055

1355

1020

0845

1105

1040

1500

1520

0810

Temperature 

(°C)

27.5

27.5

28.0

27.1

27.5

28.0

27.3

27.5

28.0

27.0

27.5

28.0

27.1

27.5

28.0

27.2

27.0

27.5

28.0

26.6

-

27.0

28.0

27.7

-

27.7

27.0

28.0

27.8

27.5

28.0

27.1

27.9

27.5

28.0

27.2

Specific 

conductance 

(pS/cm)

1,400

1,410

921

937

-

1,060

940

635

700

500

940

653

520

1,100

1,030

834

915

1,090

1,050

735

750

880

910

658

416

318

900

2,070

2,540

1,400

1,230

810

1,970

1,110

925

770

2,560

2,700

1,370

1,280

2,800

752

1,650

Chloride 

(mg/L)

280

280

83

97

94.5

130

69.5

43

55

15

30.5

22.7

15.8

150

75

87

98.5

340

76

52.5

41.5

78

71

22.8

20.6

17

120

390

274

223

187

60

372

139

112

82

590

368

231

220

650

76

280

Owner

Daniel Obed

Arwo David

Ichiro John

Belep Johnson

Anrow Harry

Robis Lipai

Tom Neth

Mike Harris

Simion Hedson

Hemyl Luhda

Table 2 37
Table 2. Temperature, specific conductance, and chloride-concentration data from dug wells, Kahlap Island, 

Mwoakilloa Atoll-Continued

Well no.

DW-12

DW-13

DW-14

DW-15

DW-16

DW-18

DW-20

DW-22

DW-23

Date

08-22-89

11-09-89

12-11-86

07-13-88

10-01-88

08-22-89

11/08/89

12-11-86

07-13-88

08-22-89

11/08/89

12-11-86

07-13-88

10-01-88

08-22-89

11-08-89

12-11-86

07-13-88

10-01-88

08-22-89

11-08-89

12-12-86

07-06-88

10-01-88

12-12-86

07-13-88

10-01-88

08-22-89

11-08-89

12-12-86

07-13-88

10-01-88

08-22-89

11-08-89

12-12-86

07-13-88

10-01-88

08-22-89

07-13-88

08-22-89

11-08-89

Time

0945

1356

1505

1555

1235

0910

1740

1512

1600

0910

1745

1515

1605

1240

0905

1755

1630

1615

1240

0900

1800

1330

0940

1225

1430

1205

1105

1445

1335

1625

1150

1045

1500

1230

1630

1120

1010

1125

1055

1025

Temperature 

<°C)

-

27.2

27.5

28.0

27.0

-

26.6

27.0

28.0

27.2

27.0

28.0

27.6

26.9

27.0

28.0

27.9

-

26.7

28.0

27.5

27.9

27.5

28.0

27.4

26.5

28.0

28.0

27.2

29.2

27.0

28.0

26.7

-

28.0

27.5

Specific 

conductance 

(pS/cm)

448

1,680

1,000

865

607

497

490

820

1,240

619

780

475

1,960

2,020

1,100

1,500

1,190

1,220

948

913

940

4,750

4,300

2,600

590

1,300

724

454

725

485

771

402

454

390

500

658

550

192

1,490

914

790

Chloride 

(mg/L)

48.2

366

120

70

33.4

18.2

20

120

180

54.4

101.

27

388

416

153

301

180

139

88

94

111

1,500

1,100

578

45

225

67

25.8

73.4

8.5

17.2

13

6.8

14

17.2

19.8

11.3

354

116

80.2

Owner

Simion Edgar

John Henry

Nelson Albert

Wilson Ben

Jacob Jim

Asher Johnson

Andon Edwards

Sakaraies

Sepety

Johb Lumual

38 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
Table 2. Temperature, specific conductance, and chloride-concentration data from dug wells, Kahlap Island, 

Mwoakilloa Atoll-Continued

Well no.

DW-26

DW-29

DW-30

DW-32

DW-33

DW-34

DW-35

DW-37

Date

07-13-88

10-01-88

08-22-89

11-08-89

08-22-89

07-13-88

10-01-88

08-22-89

11-08-89

07-13-88

10-01-88

08-22-89

11-08-89

07-13-88

10-01-88

08-22-89

11-09-89

07-13-88

10-01-88

08-22-89

11-09-89

07-13-88

10-01-88

07-14-88

10-01-88

08-22-89

11-08-89

Time

1025

0840

1050

1050

1155

1105

0835

1020

1140

1240

1035

1150

0935

1250

0830

1015

1514

1255

0825

1015

1520

1315

0820

1345

1005

1115

1200

Temperature 

(°C)

28.0

27.0

27.1

-

28.0

26.7

26.7

28.0

26.9

26.5

28.0

27.2

-

27.6

28.0

27.1

27.6

28.0

26.6

28.0

26.9

28.0

Specific 

conductance 

(MS/cm)

1,200

583

486

455

592

1,490

912

757

740

1,030

491

301

625

1,420

1,110

1,100

1,320

921

1,030

626

785

959

676

875

651

584

765

Chloride 

(mg/L)

213

79.6

56.6

49.2

13.8

256

94

65.5

43

116

39.8

12.9

67.8

193

164

179

212

61.5

87.2

54.8

79.4

62

32

86

29.5

18.2

45.6

Owner

Banjamen Sebty

Welesen Julios

Smith Jack

Eshiam Ben

Lincoln Lebehn

Eshiam Ben

Dishon Hedson

Tom Neth

Table 2 39
Table 3. Temperature, specific conductance, and chloride-concentration data from driven wells, Kahlap Island, 

Mwoakilloa Atoll

[°C, degrees Celsius; j^S/cm, microsiemens per centimeter at 25°C; mg/L, milligrams per liter, --, no data; >, greater than] 

Well no.: Letter is cluster-site designation; WT is water table; number is depth of well below land surface.

Well no.

A-WT

A-15

A-18

A-20

A-21

A-24

B-WT

B-09

B-14

B-19

B-21.5

B-24

B-26

Date

12-12-86

07-01-88

10-01-88

08-22-89

11-08-89

07-01-88

10-01-88

08-22-89

11-08-89

07-01-88

10-01-88

08-22-89

11-08-89

07-01-88

07-01-88

10-01-88

08-22-89

11-08-89

12-12-86

07-13-88

10-01-88

08-22-89

11-08-89

07-01-88

07-01-88

10-01-88

08-22-89

11-08-89

07-01-88

10-01-88

08-22-89

11-08-89

07-02-88

07-01-88

07-02-88

10-01-88

08-22-89

11-08-89

07-02-88

Time

1605

0930

1135

1400

1620

0950

1135

1400

1630

1010

1135

1400

1635

1030

1055

1135

1400

1640

1515

1200

1050

1515

1245

1600

1635

1050

1515

1245

1655

1050

1515

1250

1415

1720

1425

1050

1515

1300

1445

Temperature

27.5

27.0

27.5

26.8

27.8

27.9

27.4

28.0

28.1

27.5

28.0

28.0

27.8

-

27.6

27.5

28.0

27.5

27.9

28.0

28.0

27.8

27.8

28.0

27.8

-

27.6

27.5

28.0

27.8

27.9

27.8

27.5

Specific 

conductance 

(|iS/cm)

685

864

785

690

565

2,440

1,430

690

1,330

7,050

7,350

4,750

5,400

18,100

28,700

>20,000

20,000

19,000

390

1,110

406

253

430

1,080

1,550

1,050

904

1,090

4,700

2,550

2,220

4,500

15,000

24,900

24,600

>20,000

21,900

21,500

30,400

Chloride 

(mg/L) Owner

65 Bunaszki Dannis

45.5

41.5

36

20.6

522

320

209

188

2,110

1,490

1,260

1,460

6,120

10,200

9,150

6,480

6,320

8.5 Rensile Lebehn

136

17.4

9.5

42.6

117

285

156

104

158

1,220

928

472

1,140

5,080

8,420

8,380

8,050

7,120

7,920

10,300

40 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei
Table 3. Temperature, specific conductance, and chloride-concentration data from driven wells, Kahlap Island, 

Mwoakilloa Atoll-Continued

Well no.

B-29

B-31

B-34

B-40

B-45

B-50

C-WT

C-9.25

D-18.75

E-WT

E-15

E-18

E-19

E-20

E-22.5

F-WT

Date

07-02-88

07-02-88

07-02-88

07-09-88

07-09-88

07-09-88 

10-01-88 

08-22-89 

11-08-89

07-05-88

07-05-88 

10-01-88 

08-22-89 

11-08-89

07-05-88 

10-01-88 

08-22-89 

11-08-89

07-04-88 

10-01-88 

08-22-89 

11-08-89

07-04-88 

10-01-88 

08-22-89 

11-08-89

07-04-88

10-01-88 

08-22-89 

11-08-89

07-04-88

07-04-88 

10-01-88 

08-22-89 

11-08-89

07-06-88 

10-01-88

Time

1500

1520

1540

0840

0930

1100 

1055 

1515 

1305

1215

1530 

1110 

1430 

1330

1315 

1700 

1450 

1325

1030 

1015 

1140 

0840

0940 

1015 

1140 

0920

1015

1020 

1140 

0855

1035

1055 

1025 

1140 

0910

0900 

1155

Temperature 

<°C)

26.8

26.0

26.5

28.0

28.0

27.9 

27.7

27.8

28.8 

27.6

27.9

28.8 

27.6

27.8

28.2 

27.7

27.6

28.0 

27.9

27.8

28.0

28.0 

27.5

28.0

28.0 

28.0

27.7

27.2 

27.9

Specific 

conductance 

((iS/cm)

32,600

35,300

35,900

36,700

38,000

37,800 

>20,000 

39,900 

38,000

1,120

1,150 

1,340 

952 

1,500

10,100 

10,000 

8,900 

9,150

1,410 

1,400 

1,540 

1,140

1,650 

1,600 

1,770 

1,500

7,520

6,200 

3,710 

4,090

17,400

28,200 

>20,000 

19,200 

20,000

750 

790

Chloride 

(mg/L)

11,000

12,000

12,400

12,800

13,400

13,200

13,600 

13,600

152

178 

74.2 

115 

281

3,010

3,030 

3,000

225 

214 

240 

190

260 

275 

322 

255

2,120

1,640 

920 

1,140

5,680

9,820 

7,550 

6,120 

7,100

39.5 

45.6

Owner

Aidel Obed

Roland Poll

Rensile Lebehn

Jacob Jim

Table3 41
Table 3. Temperature, specific conductance, and chloride-concentration data from driven wells, Kahlap Island, 

Mwoakilloa Atoll-Continued

Well no.

F-14

F-17

F-19

F-20

F-22

F-24

G-WT

G-11.5

G-13

G-16

H-WT

H-13

H-14

H-15.4

H-19

H-19.25

Date

07-06-88

10-01-88

08-22-89

11-08-89

07-06-88

07-13-88

10-01-88

08-22-89

11-08-89

07-06-88

07-06-88

07-13-88

10-01-88

08-22-89

11-08-89

07-06-88

10-01-88

08-22-89

07-06-88

10-01-88

08-22-89

11-08-89

07-06-88

07-06-88

10-01-88

08-22-89

11-08-89

07-13-88

10-01-88

07-07-88

10-01-88

08-22-89

11-08-89

07-07-88

10-01-88

08-22-89

11-08-89

07-07-88

Time

0950

1155

0940

1700

1010

1335

1155

0940

1705

1105

1130

1545

1155

0940

1705

0950

1220

0930

1640

1220

0930

1725

1600

1615

1220

0930

1720

1630

0855

1055

0900

1110

1000

1115

0905

1110

1010

1125

Temperature 

(°C)

27.5

27.9

26.7

28.0

28.0

28.0

27.5

28.0

28.0

28.0

27.9

-

27.6

27.5

27.9

-

28.0

27.6

-

27.3

28.0

28.0

28.2

27.5

28.0

27.8

28.0

27.8

-

27.6

28.0

27.8

27.8

28.0

Specific 

conductance 

(|.iS/cm)

931

755

710

1,100

2,350

7,520

4,320

2,210

2,400

9,400

21,600

27,300

>20,000

22,200

19,000

3,230

1,820

1,850

3,490

2,060

1,960

2,050

4,020

5,740

4,950

3,360

3,540

4,980

-

10,410

9,900

7,960

8,750

23,900

>20,000

31,500

35,000

37,200

Chloride 

(mg/L) Owner

51

36

65

116

520

1,670

965

484

480

2,890

7,200

9,350

10,100

7,050

6,320

815 Jacob Jim

354

334

810

396

248

394

990

1,510

930

780

720

1,320 Eleu Peter

63.6

3,120

2,700

2,240

2,620

7,920

11,200

11,000

12,300

13,100

42 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakiiioa Atoll, State of Pohnpei
Table 3. Temperature, specific conductance, and chloride-concentration data from driven wells, Kahlap Island, 

Mwoakilloa Atoll-Continued

Well no.

H-20.75

H-24

I-WT

1-10

1-12.5

1-15

1-17.5

1-19

1-20

J-WT

J-12.5

L-WT

L-11.5

M-WT

M-10.5

N-WT

N-8.0

Date

07-07-88

07-07-88

10-01-88

08-22-89

11-08-89

12-11-86

07-13-88

10-01-88

08-22-89

11-08-89

07-07-88

10-01-88

07-07-88

07-07-88

10-01-88

08-22-89

11-09-89

07-07-88

07-07-88

10-01-88

08-22-89

11-08-89

07-09-88

07-13-88

10-01-88

08-22-89

11-08-89

07-08-88

08-22-89

07-08-88

07-13-88

07-13-88

08-22-89

11-08-89

12-12-86

07-13-88

07-13-88

10-01-88

08-22-89

11-08-89

Time

1145

1200

0905

1110

1025

1220

1045

0935

1040

1110

1445

0935

1500

1530

0940

1040

1440

1545

1600

0940

1040

1115

1140

1640

1125

1000

1615

1415

1130

1405

1230

1240

1155

1540

1630

1140

1145

1040

1530

1215

Temperature 

(°C)

28.0

28.0

27.8

27.9

27.0

28.0

27.1

27.2

28.0

27.2

28.0

28.0

27.8

27.9

28.0

28.0

27.8

--

28.6

28.0

28.0

27.6

27.2

26.0

-

26.0

28.0

28.0

27.3

27.5

28.0

28.0

27.3

28.3

Specific 

conductance 

(jiS/cm)

43,900

47,200

>20,000

46,300

47,500

625

2,180

1,090

805

575

5,320

2,550

17,600

33,000

>20,000

16,900

18,500

43,000

46,000

>20,000

42,900

44,000

615

600

630

650

590

640

319

865

677

865

455

660

675

869

933

690

916

565

Chloride 

(mg/L)

15,600

17,400

16,800

16,600

17,400

38

458

149

88

83.5

1,420

-

5,620

11,400

6,380

5,420

6,120

15,400

16,500

16,300

15,200

15,800

39.6

36.9

20

44

14

22.2

11.8

21.6

21

22.5

19.8

19.2

31

89

137

58.5

107

31.2

Owner

Asher Johnson

Roland Poll

Robert Joel

Anrow Harry

Mike Harris

Table 3 43
Table 4. Water-quality data from selected dug and driven wells collected November 1989, Kahlap Island, Mwoakilloa 

Atoll

[Temp, temperature; ALK, total alkalinity; Ca, calcium; Mg, magnesium; Sr, strontium; Na, sodium; K, potassium; Cl, chloride; 804, sulfate; NC>3, 

nitrate; PO4, phosphate;0 C, degrees Celsius; meq/L, milliequivalents per liter, mg/L, milligrams per liter, <, less than]

Well no.: DW, dug well and number is well designation; letters A through N are driven-well cluster-site designations, and number is depth of well 

below land surface

Well no.

DW-1

DW-2

DW-3

DW-4

DW-5

DW-7

DW-9

DW-10

DW-11

DW-1 2

DW-13

DW-14

DW-15

DW-1 8

DW-20

DW-23

DW-26

DW-30

DW-32

DW-33

DW-34

DW-37

A-WT

A- 15

A-20

A-24

B-WT

B-14

B-19

B-24

B-50

C-9

D-18.7

E-WT

E-15

E-19

E-22.5

F-14

F-19

F-24

G-11.5

G-16

H-14

H-19

H-24

I-WT

1-15

1-20

J-12.5

M-10.5

N-8.0

Temp <°C)

27.1

27.3

27.0

27.1

27.2

27.7

27.9

27.9

27.2

26.6

27.2

26.9

26.7

26.5

29.2

27.5

27.1

26.7

26.5

27.6

27.6

28.0

26.8

27.4

27.5

27.6

27.9

27.8

27.6

27.8

27.7

27.9

27.8

27.6

27.8

27.5

27.7

26.7

27.5

27.6

27.3

27.5

27.6

27.8

27.9

27.2

27.9

28.6

27.2

27.3

28.3

PH

7.28

7.69

7.46

7.46

7.50

7.76

7.74

7.82

7.28

7.72

7.75

7.40

7.53

7.63

7.73

7.89

7.97

7.52

7.54

7.65

7.65

7.40

7.35

7.10

7.36

7.19

7.71

7.34

7.20

7.20

7.18

7.48

8.78

6.94

7.26

7.18

7.30

7.30

7.20

7.21

7.40

7.33

7.34

7.50

7.62

7.23

7.49

7.54

7.35

7.54

7.17

ALK 

(meq/L)

6.26

5.38

5.40

6.39

6.55

6.99

6.56

6.22

5.69

5.03

4.66

7.56

6.55

5.45

4.44

5.63

3.14

6.96

4.78

5.33

5.35

6.98

5.54

8.37

7.83

3.29

3.29

7.29

7.22

5.81

0.57

6.74

1.50

7.71

8.20

7.09

5.83

8.32

8.34

6.55

8.58

8.76

6.77

4.43

2.34

5.40

6.87

2.95

6.51

5.51

7.34

Ca 

(mg/L)

110

91.2

94.9

102

79.8

123

90.5

98.4

95.9

87.2

82.1

110

104

97

78

83.4

48.6

109

81.4

85.8

90.3

111

93.2

140

144

237

59.1

132

160

254

268

123

78.9

142

144

144

243

147

154

237

131

137

159

311

363

88.6

234

348

116

126

91.2

Mg 

(mg7L)

18.2

11.1

7.6

16.4

11.5

20.7

19.5

31.8

36.4

9.5

12.1

42.6

16.9

13.9

4.5

23.6

9.9

14.7

8.2

29

12.3

11.2

8.1

23.6

98.4

397

5.1

13.6

69.7

498

819

21.8

166

19.6

24.4

77.8

450

20.8

37

393

55

76.6

181

770

1,070

16.4

387

1,010

8

7.1

7.9

Sr 

(mg/L)

1.4

1.3

1.2

1.2

0.9

1.8

1.1

1.3

1.3

0.8

0.9

1.2

1.1

1.5

1.1

1.2

0.6

1.7

1.3

1.1

1.1

1.2

1.3

2.1

2.1

3.5

0.9

2.2

2.5

4.4

4.6

1.9

1.3

2.1

2.2

2.2

3.9

1.5

1.9

3.7

1.7

1.8

2.5

5.3

6.5

1.2

3.5

6.1

1.4

1.7

1.4

Na 

(mg/L)

73.7

45.0

10.7

80.0

57.9

123

107

206

212

19.3

62.5

181

86.6

86.1

6.4

132

36.4

35.9

41.0

154

65.1

31.4

16.9

112

842

3,450

27.4

88.3

622

4,300

7,680

187

1,830

134

188

640

3,890

95.8

267

3,430

279

476

1,470

6,780

9,500

63.6

3,320

8,900

8.4

10.0

23.4

K

(mg/L)

2.6

5.6

2.4

3.9

29.8

5.1

2.4

5.8

2.1

2.7

5.8

2.7

1.0

3.4

0.8

11.5

2.7

2.3

5.1

2.9

2.2

20.4

4.7

8.3

31.2

120

4.2

2.9

20.3

144

271

5.8

60.6

8.3

10.2

23.3

132

3.2

9.4

117

8.8

14.9

88

239

336

2.6

113

317

0.3

2.5

1.8

Cl 

(mg/L)

91.1

48.9

17.5

89.5

33.2

184

112

293

371

22.5

101

328

115

146

5.5

200

50.0

38.4

64.3

240

78.0

42.0

17.5

184

1,436

6,227

36.8

146

1,110

7,730

13,500

283

3,100

189

258

1,110

6,990

159

465

6,170

470

827

2,590

12,200

17,300

73.2

5,900

14,300

10.6

13

27.7

SO4 

(mg/L)

26.4

19.0

1.0

31.7

25.8

46.5

46.9

81.8

50.6

4.4

27.0

56.6

41.5

25.0

11.0

57.1

17.5

19.5

5.9

26.8

34.3

15.3

6.8

30.1

131

726

1.3

9.3

64.5

1,020

1,650

17.9

146

30.4

37.4

148

814

17.2

28.5

731

48

86

292

1,480

2,180

29.3

290

1,970

3.5

<1

3.8

N03 

(mg/L)

24.4

16.5

0.028

15.9

9.81

12.0

1.54

17.8

0.738

0.081

5.81

0.409

7.53

1.13

0.065

1.79

5.29

0.185

0.020

10.4

13.6

7.72

1.59

0.012

0.014

<0.001

0.020

0.003

0.001

0.004

<0.001

0.001

0.006

5.98

4.44

0.026

0.006

0.001

<0.001

0.003

<0.001

0.003

0.203

0.105

<0.001

10.8

1.85

2.63

0.539

0.003

0.113

PO4 

(mg/L)

0.436

0.387

2.39

0.420

0.177

1.31

1.29

0.299

0.344

0.387

0.382

0.127

0.347

0.623

0.628

0.261

0.881

0.165

0.865

0.191

0.239

0.418

0.404

0.061

0.087

0.045

1.41

0.703

1.20

0.230

0.055

0.299

0.06

0.464

0.248

0.455

0.644

0.165

0.143

0.066

0.140

0.197

0.159

0.106

0.020

0.385

0.340

0.051

0.102

0.045

0.083

44 Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilioa Atoll, State of Pohnpei