U.S. Department of the Interior COPY

Site Selection for a Deep Monitor Well, 

Kualapuu, Molokai, Hawaii

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99-4291

i; ' 1 ,

-Water table . ' " N - 

Prepared in cooperation with the 

DEPARTMENT OF HAWAIIAN HOME LANDS

science 

USGS

for a changing world

Site Selection for a Deep Monitor Well, 

Kualapuu, Molokai, Hawaii

EyDelwynS. Oki

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99-4291

Prepared in cooperation with the 

DEPARTMENT OF HAWAIIAN HOME LANDS

Honolulu, Hawaii 

2000
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

For additional information write to:

District Chief

U.S. Geological Survey

677 Ala Moana Blvd., Suite 415

Honolulu, HI 96813

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286
CONTENTS

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

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

Purpose and Scope ............................................................................ 4

Well-Numbering System........................................................................ 4

Description of Study Site ....................................................................... 9

Physical Setting......................................................................... 9

Land Use.............................................................................. 9

Climate ............................................................................... 9

Geology.......................................................................................... 9

West Molokai Volcano ......................................................................... 14

East Molokai Volcano.......................................................................... 16

Coastal Deposits.............................................................................. 16

Kualapuu Area ............................................................................... 16

Hydraulic Conductivity of the Rocks ................................................................... 16

Lava Flows .................................................................................. 17

Dikes....................................................................................... 17

Weathering .................................................................................. 17

Coastal Deposits.............................................................................. 17

Regional Ground-Water Flow System................................................................... 17

Ground-Water Recharge ........................................................................ 18

Ground-Water Discharge to Streams .............................................................. 18

Ground-Water Levels .......................................................................... 20

Freshwater Lens .............................................................................. 20

Dike-Impounded Ground Water.................................................................. 23

Ground-Water Flow System in the Kualapuu Area......................................................... 23

Ground-Water Levels .......................................................................... 23

Depth to Saline Water.......................................................................... 24

Chloride Concentration......................................................................... 24

Vertical Profiles of Chloride Concentration in Well 0801-01............................................ 24

Ground-Water Withdrawals........................................................................... 28

Numerical Simulation to Determine Areas of Possible Future Ground-Water Development......................... 31

Model Description ............................................................................ 31

Model Mesh ........................................................................... 31

Boundary Conditions .................................................................... 31

Hydraulic-Conductivity Zones............................................................. 32

Recharge.............................................................................. 32

Model Results................................................................................ 32

Withdrawal at the March 1999 Allocation Rate................................................ 36

Withdrawal of 0.5 Mgal/d from a Single Site.................................................. 36

Withdrawal of 1.0 Mgal/d from a Single Site.................................................. 36

Model Limitations ............................................................................ 40

Contents iii
Additional Criteria for Deep Monitor Well Site Selection ................................................... 44

Ground-Water Level........................................................................... 44

Ground-Surface Altitude ....................................................................... 44

Land Classification, Ownership, and Accessibility ................................................... 44

Geology .................................................................................... 44

Locations of Existing Production Wells............................................................ 45

Historical Ground-Water Quality Information....................................................... 45

Site Selection for a Deep Monitor Well ................................................................. 45

Summary and Conclusions ........................................................................... 47

References Cited................................................................................... 48

FIGURES

1-2. Maps showing:

1. Geographic features, island of Molokai, Hawaii, and location of Kualapuu study area .............. 2

2. Hawaii Commission on Water Resource Management aquifer-system boundaries, Molokai, Hawaii ... 3 

3. Schematic cross section of the regional ground-water flow system in the Kualapuu area, Molokai, Hawaii.. 5 

4-7. Maps showing:

4. Locations of selected wells, Molokai, Hawaii .............................................. 8

5. State land-use districts for 1995, Molokai, Hawaii .......................................... 10

6. Land ownership in the Kualapuu area, Molokai, Hawaii ...................................... 11

7. Mean annual rainfall, Molokai, Hawaii ................................................... 12

8. Chart showing annual rainfall at Kualapuu and Kaunakakai rain gages, Molokai, Hawaii ............... 13

9-11. Maps showing:

9. Generalized surficial geology of Molokai, Hawaii ........................................... 15

10. Surface-water gaging stations, Molokai, Hawaii ............................................ 19

11. Arithmetic mean of measured water levels for the period 1938-97, water-table altitude from 

resistivity measurements, and depth to saline water body determined from a time-domain 

electromagnetic survey, Molokai, Hawaii .............................................:.. 21

12-15. Charts showing:

12. Chloride concentration of water samples from different depths in Department of Hawaiian Home 

Lands well 1 (0801-01), and diagram of well-casing depth and type for wells 0801-01 to -03, 

Molokai, Hawaii ................................................................... 25

13. Estimated pumping rates at Department of Hawaiian Home Lands well 2 (0801-02) and Department

of Water Supply well 0801-03 during May 1995, Molokai, Hawaii............................ 26

14. Chloride concentration of water samples from Department of Hawaiian Home Lands well 2 (0801-02)

during May 1995, Molokai, Hawaii .................................................... 27

15. Monthly or annual mean pumpage from selected wells, Molokai, Hawaii ........................ 29

16-25. Maps showing:

16. Ground-water discharge zones used in the ground-water flow model, Molokai, Hawaii ............. 33

17. Horizontal hydraulic-conductivity zones used in the ground-water flow model, Molokai, Hawaii...... 34

18. Average annual recharge used in the ground-water flow model and nodes with simulated

1999-allocated ground-water withdrawal, Molokai, Hawaii ................................. 35

iv Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
19. Model-calculated steady-state water levels at well 0901-01 (model node 3094) caused by

withdrawing 0.5 million gallons per day, above the March 1999 total allocated withdrawal rate,

from the Kualapuu area (outside of the modeled dike complex), Molokai, Hawaii ................ 37

20. Model-calculated steady-state water levels at wells 0801-01 to -03 (model node 3142) caused by 

withdrawing 0.5 million gallons per day, above the March 1999 total allocated withdrawal rate, 

from the Kualapuu area (outside of the modeled dike complex), Molokai, Hawaii ................ 38

21. Model-calculated zone where withdrawing 0.5 million gallons per day from a single site does not

cause the steady-state water level at the withdrawal site to drop below 10 feet above sea level, and 

model-calculated zone where withdrawing 0.5 million gallons per day from a single site does not 

cause the water level at well 0901-01 (model node 3094) to drop below 7 feet above sea level, 

Kualapuu area (outside of the modeled dike complex), Molokai, Hawaii ....................... 39

22. Model-calculated steady-state water levels at well 0901-01 (model node 3094) caused by

withdrawing 1.0 million gallons per day, above the March 1999 total allocated withdrawal rate,

from the Kualapuu area (outside of the modeled dike complex), Molokai, Hawaii ................ 41

23. Model-calculated steady-state water levels at wells 0801-01 to -03 (model node 3142) caused by 

withdrawing 1.0 million gallons per day, above the March 1999 total allocated withdrawal rate, 

from the Kualapuu area (outside of the modeled dike complex), Molokai, Hawaii ................ 42

24. Model-calculated zone where withdrawing 1.0 million gallons per day from a single site does not 

cause the steady-state water level at the withdrawal site to drop below 10 feet above sea level, 

and model-calculated zone where withdrawing 1.0 million gallons per day from a single site does 

not cause the water level at well 0901-01 (model node 3094) to drop below 7 feet above sea level, 

Kualapuu area (outside of the modeled dike complex), Molokai, Hawaii ....................... 43

25. Possible area for drilling a deep monitor well in the Kualapuu area, Molokai, Hawaii............... 46

TABLES

1. State of Hawaii Commission on Water Resource Management March 1999 water-use permits,

Molokai, Hawaii ........................................................................ 6

2. Estimated base flow for northeastern Molokai streams, Hawaii ....................................... 20

3. Summary of measured, nonpumping water levels at selected wells, Molokai, Hawaii ..................... 22

Contents v
Conversion Factors

Multiply

foot (ft)

foot per day (ft/d)

inch (in.)

inch per year (in/yr)

mile (mi)

square mile (mi )

gallons per minute (gal/min)

million gallons per day (Mgal/d)

By

0.3048

0.3048

2.54

2.54

1.609

2.590

3.785

0.04381

To obtain

meter

meter per day

centimeter

centimeter per year

kilometer

square kilometer

liters per minute

cubic meters per second

Abbreviations

mg/L Milligrams per liter

vi Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
Site Selection for a Deep Monitor Well, Kualapuu, 

Molokai, Hawaii

ByDelwynS. Oki

Abstract

Management of the ground-water resources 

near Kualapuu on the island of Molokai, Hawaii, is 

hindered by the uncertainty in the vertical salinity 

structure in the aquifer. In the State of Hawaii, vertical profiles of ground-water salinity are commonly obtained from deep monitor wells, and these 

profiles are used to estimate the thicknesses of the 

freshwater part of the ground-water flow system 

and the freshwater-saltwater transition zone. Information from a deep monitor well would improve 

the understanding of the ground-water flow system 

and the ability to effectively manage the groundwater resources near Kualapuu; however, as of 

mid-1999 no deep monitor wells had been drilled 

on the island of Molokai.

Selection of an appropriate site for drilling a 

deep monitor well in the Kualapuu area depends 

partly on where future ground-water development 

may occur. Simulations using an areally twodimensional, steady-state, sharp-interface groundwater flow model previously developed for the 

island of Molokai, Hawaii, indicate that the southeastern part of the Kualapuu area is a possible area 

of future ground-water development because (1) 

withdrawals from this area have a small effect on 

water levels at existing wells in the Kualapuu area 

(relative to effects from withdrawals in other parts 

of the Kualapuu area that are outside of the dike 

complex), and (2) model-calculated water levels in 

this part of the Kualapuu area are high relative to 

water levels in other parts of the Kualapuu area that 

are outside of the dike complex.

Additional site-selection criteria include 

(1) ground-water level, (2) ground-surface altitude, 

(3) land classification, ownership, and accessibility* (4) geology, (5) locations of existing production wells, and (6) historical ground-water quality 

information. A deep monitor well in the Kualapuu 

area will likely be most useful for management 

purposes if it is located (1) in the vicinity of future 

ground-water development, (2) in an area where 

water levels are between about 8 and 12 feet above 

sea level, (3) at a ground-surface altitude that is 

between about 1,000 and 1,100 feet, (4) on government-owned land, (5) outside of the dike complex 

and as far from known volcanic vents as possible, 

(6) at least about 1,000 feet from, but within the 

same hydrogeologic setting as, existing or proposed production wells, and (7) east of well 0902- 

01. A viable area for drilling a deep monitor well is 

about a half mile southeast of existing wells 0801- 

01 to -03 and a half mile north of a known volcanic 

vent, Puu Luahine.

INTRODUCTION

Management of the ground-water resources of the 

island of Molokai, Hawaii is hindered by the uncertainty in the vertical salinity structure in the aquifer near 

the town of Kualapuu (fig. 1), where projected demand 

for water is high. The Kualapuu study area corresponds 

to the Kualapuu aquifer system (fig. 2) as defined by the 

State of Hawaii Commission on Water Resource Management (CWRM) (State of Hawaii, 1990). The Kualapuu study area extends about 7 mi in an east-west direction and about 4 mi in a north-south direction, and has

f\

an area of about 18 mi . In the Kualapuu area, the 

ground-water flow system consists of a body of

Introduction 1
if 

*

I31

0)oI_ 0) o

c co_ off

I

160° 158° 156°

157°15'

Ilio Point

WEST MOLOKAI VOLCANO

Cape 

Halawa

Base modified from U.S. Geological Survey digital data, 

1:24,000, 1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 

157°00'45". Relief from U.S. Geological Survey 

digital elevation models, 1:250,000

EXPLANATION 

MOLOKAI IRRIGATION SYSTEM PIPELINE

_._._._. MOLOKAI IRRIGATION SYSTEM TUNNEL ______ BOUNDARY OF KUALAPUU STUDY AREA

Figure 1. Geographic features, island of Molokai, Hawaii, and location of Kualapuu study area.
157°15'

Ilio Point

024 MILES 

024 KILOMETERS

KALAUPAPA^ Jlahiu Point

156°45'

Halawa

'( PELEKUNU J.

f' WAILAU "VBase modified from U.S. Geological Survey digital data 

1:24,000, 1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 157°00'45"

Kamalo

EXPLANATION

[" | KUALAPUU STUDY AREA AND 

AQUIFER SYSTEM

KAWELA AQUIFER-SYSTEM BOUNDARY AND 

NAME OF AQUIFER SYSTEM

3 aa5'

Figure 2. Hawaii Commission on Water Resource Management aquifer-system boundaries, Molokai, Hawaii (modified from State of Hawaii, 1990).
freshwater floating on denser saltwater. A brackishwater transition zone of unknown thickness exists 

between the freshwater and underlying saltwater (fig. 

3).

In response to projected water demand on the 

island of Molokai, CWRM designated the entire island 

as a Ground Water Management Area in 1992. This 

action authorized the State to manage ground-water 

withdrawals on Molokai through a permitting process to 

protect the water resources of the island. Ground-water 

withdrawals on Molokai are currently limited by 

sustainable-yield estimates for 16 areas denoted as aquifer systems (fig. 2), primarily delineated on the basis of 

geologic conditions and topographic divides (Mink and 

Lau, 1992). As of March 1999, CWRM had issued 

water-use permits authorizing a total of 6.6432 million 

gallons per day (Mgal/d) of ground-water withdrawals 

from wells and tunnels on Molokai (table 1, fig. 4). In 

addition, the State of Hawaii Department of Hawaiian 

Home Lands (DHHL) has a reservation for 2.905 

Mgal/d of ground water from the Kualapuu area, and 

Kukui (Molokai), Inc. is currently attempting to 

increase its authorized water use for well 0901-01 (table 

1, fig. 4) in the Kualapuu area from 0.871 Mgal/d to 

1.259 Mgal/d.

A numerical ground-water flow model of the entire 

island of Molokai was previously developed to estimate 

the regional effects of ground-water withdrawals on 

water levels and coastal discharge (Oki, 1997). Current 

knowledge of the effects of ground-water withdrawals 

on water quality, however, is limited partly because of 

the current lack of knowledge of the salinity structure of 

the ground-water flow system. The bottoms of production wells are usually located above the top of the 

brackish-water transition zone to reduce the risk of 

pumping high-salinity water. A deep monitor well 

drilled through the freshwater and into the brackishwater transition zone would improve the understanding 

of the salinity structure in the aquifer, thickness of the 

freshwater, and estimates of freshwater availability in 

the vicinity of the well.

The U.S. Geological Survey (USGS) undertook 

this investigation to locate a hydrologically suitable 

area for a deep monitor well in the Kualapuu area on the 

island of Molokai. Factors considered in selecting a 

suitable area include (1) possible sites of future groundwater development, (2) ground-water level, (3) groundsurface altitude, (4) land classification, ownership, and 

accessibility, (5) geology, (6) locations of existing production wells, and (7) historical ground-water quality 

information. The existing numerical ground-water flow 

model (Oki, 1997) for Molokai was used to estimate 

water-level declines caused by possible ground-water 

development near Kualapuu.

Purpose and Scope

The purpose of this report is to describe (1) results 

of model simulations that assess the hydrologic effects 

of withdrawals at rates in excess of the March 1999 permitted rates, and (2) the area where a deep monitor well 

could be located to provide information needed to effectively manage the ground-water resources in the Kualapuu area.

No new data were collected as part of this study. 

The existing USGS numerical ground-water flow 

model (Oki, 1997) of Molokai was used to estimate the 

effects of different withdrawal scenarios on regional 

ground-water levels.

Well-Numbering System

Wells mentioned in this report are numbered 

according to the State of Hawaii numbering system. 

Well numbers contain seven digits and are based on a 

latitude-longitude one-minute grid system. Well numbers are of the form:

a-bbcc-dd,

where:

a is the island code;

bb is the minutes of latitude of the southeastern

corner of the one-minute grid cell containing 

the well;

cc is the minutes of longitude of the southeastern 

corner of the one-minute grid cell containing 

the well; and

dd is the sequential well number within the oneminute grid cell containing the well.

An island code of "4" is used for all wells on Molokai and is omitted in this report.

4 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
Line of Section

EXPLANATION

[~ ' | VOLCANIC ROCKS

|::-X:| SEDIMENTARY DEPOSITS

 WATER TABLE

-«l GENERAL DIRECTION OF GROUND-WATER FLOW

Kualapuu 

study area

Hb^_

' /FRESJHWAT^FT

 ^

BRACKISH WATER'

SALTWATER

2345 

DISTANCE FROM COASTLINE, IN MILES

Vertical scale greatly exaggerated

Figure 3. Schematic cross section of the regional ground-water flow system in the Kualapuu 

area, Molokai, Hawaii.

Introduction 5
Table 1. State of Hawaii Commission on Water Resource Management March 1999 water-use permits, Molokai, Hawaii

[Mgal/d, million gallons per day]

9 

OT 

2.

S o'

Well no. Well name Applicant name Permit no.

Date 

approved

Permitted 

(Mgal/d)

Kahanui aquifer system

1058-01 Waihanau#239

Kamiloloa aquifer system

I(D

TJ 

O

nitor Wel

0501-04

0501-06 

0501-07

0601-01 

0759-01

Kupa shaft

Home Pumehana 

Kaunakakai Park

Oloolo Kaunakakai 

Waiola no. 1

^ Kawela aquifer system

0)

0)

TO

, Molokai

z

1

0352-10

0352-11 

0354-01

0354-02 

0354-03 

0354-04

0354-07

0456-04 

0456-06

0456-08

0456-09

0456-16

0456-17

0457-01

0457-04

Kamalo-Curtis

Shige's Farm 

Meyer Incorporated, #1

Meyer Incorporated, #2 

Well #3 

Meyer Incorporated, #4

Bostwick well no. 1

Breadfruit well 

DW3

DW2

DW1

Kawela-Iaea #3

Johnson

Kawela shaft

Ag#l

Kualapuu aquifer system

0801-01 

0801-02 

0801-03 

0901-01 

1059-01

DHHL1 

DHHL2

Kualapuu Mauka 

Well #17 

Waikalae tunnel

Manawainui aquifer system

0602-03 Kalaiakamanu Hou

0603-01 Umipaa

0603-07 Naiwa dug

0604-03 Naiwa MECO open pit

National Park Service 201 8/4/93

Hawaiian Research, Limited 273

HaleMahaolu 312

Maui County Dept. of Parks and Recreation 195

Hawaiian Research, Limited 274

Waiola Molokai, Incorporated 429

Curtis, David

Inouye, Shigenobu

T.T. Meyer, Incorporated

T.T. Meyer, Incorporated

Kanukuawa Ranch

T.T. Meyer, Incorporated

Bostwick, Charles

Kawela Plantation Homeowners Assoc.

Kawela Plantation Homeowners Assoc.

Kawela Plantation Homeowners Assoc.

Kawela Plantation Homeowners Assoc.

laea, John Wm., Sr.

Granger, R.M.

Maui County Dept. of Water Supply

Kawela Plantation Homeowners Assoc.

Hawaii State Dept. of Hawaiian Home Lands 267 9/15/93

Hawaii State Dept. of Hawaiian Home Lands 267 9/15/93

Maui County Dept. of Water Supply 359 10/20/95

Kukui (Molokai), Incorporated 341 3/14/95

Maui County Dept. of Water Supply 269 9/15/93

Kalaiakamanu Hou Church 200 8/4/93

Hawaiian Research, Limited 216 11/17/93

Maui Electric Company 293 11/17/93

Maui Electric Company 294 11/17/93

0.094

0.056

0.005

0.075

0.075

0.656

177

184

308

299

338

300

442

207

208

208

208

266

268

223

209

3/14/95

8/4/93

4/14/94

4/14/94

4/14/94

4/14/94

9/24/96

3/14/95

3/14/95

3/14/95

3/14/95

3/14/95

3/14/95

3/14/95

3/14/95

0.012

0.004

0.029

0.040

0.017

0.005

0.045

0.285

included with wel

included with wel

included with wel

0.017

0.016

0.330

included with wel

0.367

included with wel

0.516 

0.871 2 

0.036

0.005

0.046

0.0001

0.0001
Table 1. State of Hawaii Commission on Water Resource Management March 1999 water-use permits, Molokai, Hawaii-Continued

[Mgal/d, million gallons per day]

Well no. Well name Applicant name Permit no.

Date 

approved

Permitted 

(Mgal/d

Manawainui aquifer system Continued

0605-01

0605-02

0605-03

0705-05

0706-02

Orca shaft #1

Orca #2

Orca #3

Naiwa

South Hoolehua

Molokai Ranch, Limited

Molokai Ranch, Limited

Molokai Ranch, Limited

Cargill, Incorporated

Palaau Prawn and Shrimp

276

276

211

339

278

9/15/93

9/15/93

9/15/93

4/14/94

11/17/93

0.600

included with we

0.040

0.012

0.864

Ualapue aquifer system

0350-05

0350-07

0352-09

0352-12

0448-01

0448-03

0449-01

0549-01

Wescoatt

Manawai #1

Kamalo

Urauchi #1

Mapulehu shaft #1

Mapulehu shaft

Ualapue shaft

Mapulehu tunnel

Wescoatt, Wren

Hawaiian Research, Limited

Mahealani Ranch

Urauchi, John N.

HSPA

Kerner, Greg

Maui County Dept. of Water Supply

Petro, Pearl Friel

354

462

206

291

214

461

221

310

3/14/95

6/17/98

8/4/93

11/17/93

11/16/93

4/16/97

9/15/93

11/17/93

0.004

0.015

0.010

0.001

0.003

0.007

0.185

0.010

Waialua aquifer system

0546-02

0844-01

Puelelu well

Puu O Hoku no. 1

Kainalu Ranch

Puu O Hoku Ranch

222

490

6/2/94

11/6/98

0.202

0.235

Waikolu aquifer system

0855-01

0855-02

0855-03

Waikolu #22

Waikolu #23

Waikolu #24

Hawaii State Dept. of Agriculture

Hawaii State Dept. of Agriculture

Hawaii State Dept. of Agriculture

220

220

220

1/12/94

1/12/94

1/12/94

0.853

included with we

included with we

1 Finite element node number and element number can be determined from the row and column number of the cell: 

Node number (upper left or northwest comer of the cell) = row + 48 x (column -1) 

Node number (lower left or southwest comer of the cell) = row + 48 x (column -1) + 1 

Node number (upper right or northeast corner of the cell) = row + 48 x (column) 

Node number (lower right or southeast corner of the cell) = row + 48 x (column) + 1 

Element number = row + 47 x (column - 1)

Applicant has requested a permit for 1.259 Mgal/d

o

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deea JO)juo|/\| '||e/w 'nndeienx '|B>|O|0|/\| 

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o 

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Description of Study Site 

Physical Setting

The island of Molokai, which has an area of 261 

mi2, is the fifth largest of the Hawaiian islands and is 

located between longitude ISV^O'W and 156°4QW 

and between latitude 21°00'N and 21°15'N (fig. 1). It is 

composed mainly of two shield volcanoes (Stearns and 

Macdonald, 1947): the older West Molokai Volcano, 

which rises to an altitude of 1,430 ft, and the younger 

East Molokai Volcano, which rises to an altitude of 

4,961 ft. The town of Kualapuu lies on the Hoolehua 

Plain in the central saddle area of the island.

Land Use

Land use on Molokai is classified by the Hawaii 

State Land Use Commission into conservation, urban, 

rural, and agricultural zones (fig. 5). The conservation 

districts cover 77.8 mi2 (Oliver, 1995) mainly in the 

wet, northeastern part of the island. Urban and rural districts cover 6.8 mi2 (Oliver, 1995) mainly near the 

towns of Maunaloa, Kualapuu, Kalae, and Kalaupapa 

and along the southern, eastern, and western coasts. The 

remainder of the island is classified as agricultural land 

used for, among other things, field crops, nurseries, and 

livestock grazing. In the Kualapuu study area, land use 

is classified as 72 percent (12.8 mi2) agriculture, 26 percent (4.7 mi2) conservation, and 2 percent (0.4 mi2) 

urban.

Molokai Ranch, Limited controls about a third of 

the land on Molokai. Their land is used mainly for grazing and recreation. In central and eastern Molokai, the 

land is controlled mainly by DHHL, other State of 

Hawaii agencies, and private land owners. DHHL controls 39.7 mi2, or about 15 percent of the land on Molokai. Within the Kualapuu area, land is owned primarily 

by Molokai Ranch, Limited and the State of Hawaii 

(including DHHL) (fig. 6).

Climate

The climate of Molokai is characterized by mild 

temperatures, cool and persistent tradewinds, a rainy 

winter season from October through April, and a dry 

summer season from May through September (B lumenstock and Price, 1967). Climate is controlled primarily 

by topography and the location of the north Pacific anticyclone and other migratory systems relative to the 

island. During the dry season the stability of the north

Pacific anticyclone produces persistent northeasterly 

winds known locally as tradewinds. Summer 

tradewinds blow 80 to 95 percent of the time. During 

the rainy season migratory high-pressure systems often 

move past the Hawaiian islands resulting in less persistent tradewinds. Winter tradewinds blow 50 to 80 percent of the time. Southerly winds associated with lowpressure systems can bring heavy rains to the island. 

The dry coastal areas receive much of their rainfall as a 

result of these low-pressure systems.

Rainfall

Rainfall on Molokai is characterized by maxima at 

high altitudes and steep spatial gradients (fig. 7). Highest mean annual rainfall occurs in northeastern Molokai. The maximum mean annual rainfall is near the 

summit of the East Molokai Volcano and exceeds 150 

in. Over West Molokai Volcano, the maximum mean 

annual rainfall is about 25 in. Along the coastal areas of 

southern and western Molokai, mean annual rainfall is 

less than 16 in. Annual rainfall at Kualapuu (rain gage 

534, fig. 7) varied from about 13 to 59 in. during 1900- 

93 (fig. 8). Farther south at Kaunakakai (rain gage 536, 

fig. 7) annual rainfall is less and has varied from about 

3 to 35 in. during 1933-94 (fig. 8). For comparison, 

mean annual rainfall over the open ocean is estimated to 

be between 22 in. and 28 in. (Elliot and Reed, 1984; 

Dorman and Bourke, 1979).

Evaporation

Published pan-evaporation records for Molokai are 

available for only three sites; two are located in the 

Hoolehua Plain and one is near the town of Maunaloa in 

west Molokai. Mean annual pan evaporation at the three 

sites ranges from 81 in. at Maunaloa to 118 in. in the 

Hoolehua Plain (Ekern and Chang, 1985). The high 

pan-evaporation rate on the dry, windy uplands of central Molokai is attributed to the extreme positive advection of heat from the dry surrounding areas (Ekern and 

Chang, 1985). Over the open ocean, the computed evaporation rate is about 65 in/yr (Seckel, 1962).

GEOLOGY

The evolution of Hawaiian volcanoes generally 

progresses through four stages: preshield, shield, postshield, and rejuvenated. However, not all Hawaiian volcanoes have a postshield stage or a rejuvenated stage. 

The preshield stage is the earliest, submarine phase of

Geology 9
co 

S£ao33"n>om. m o

c co. oa> i

157°15'

Ilio Point

024 MILES

I I I I I I Tl II 

024 KILOMETERS

Kahiu Point

Base modified from U.S. Geological Survey digital data, 1:24,000,1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 157°00'45"

156°45'

EXPLANATION

p7| AGRICULTUREH CONSERVATION ^| RURAL B] URBAN

imalo

Halawa ..- .' ' ' ! .'. kualap'uu^,

Figure 5. State land-use districts for 1995, Molokai, Hawaii (modified from State of Hawaii, 1999).
157°04' 157°02' 157°00' 156°58'

21°08

21°06' 

Kalaupapa 

Peninsula

Contour interval approximately 328 feet (100 meters) 

Datum is mean sea leve

PACIFIC OCEAN

?v-. KUALAPUUEXPLANATION

LAND OWNED BY

Department of Hawaiian Home Lands 

(DHHL)

State of Hawaii, other than DHHL

Maui County or Federal Government

Molokai Ranch, Limited

Private party other than Molokai Ranch, Limited

. AQUIFER-SYSTEM BOUNDARY 

AND NAME

WELL AND NUMBER

Base modified from Geographic Decision Systems 

International digital data, 1999, Albers equal area 

projection, standard parallels 21°04'50" and 21°12'10 

central meridian

Figure 6. Land ownership in the Kualapuu area, Molokai, Hawaii (modified from digital data, Geographic Decision 

Systems International, 1999).

Geology 11
5 

3"

0)

o

i

TJ 

O

Q)

 o 

c

o.

o

3T

n>

D)

i

157°15'

Ilio Point

024 MILES

I I I I I

024 KILOMETERS

Kahiu Point

Base modified from U.S. Geological Survey digital data, 

1:24,000,1983, Alters equal area projection, standard 

parallels 21°04'50" and 2ri2'10", central meridian 

157°00'45"

EXPLANATION

 16 LINE OF EQUAL MEAN ANNUAL RAINFALL-Interval, 

(400) ^ inches, is variable. Number in parentheses is value in 

millimeters

 MOLOKAI IRRIGATION SYSTEM PIPELINE 

<$F® RAIN GAGE AND STATE KEY NUMBER

Figure 7. Mean annual rainfall, Molokai, Hawaii (modified from Giambelluca and others, 1986).
C/3

LLJ

X

o

< 

cc

70

60

50

40

30

20

10

KUALAPUU (rain gage 534)

Annual rainfall not available for 1983,1987,1989-90,1993-95.

 I

Mean 

annual 

rainfalh

o

CM

O>

O

CO 

O5

o in 

o>

o

en 

en

70

60

50

40

iii

x 

o

rf 30

i i i iTpp ' T^ 1 ' 'II ' ' ' T ' ' ' ' II' ' ' ' ' ' ' ' I I ' ' ' ' ' ' ' ' II " ' ' ' ' ' ' I I

KAUNAKAKAI (rain gage 536)

Annual rainfall not available for 1900-1932,1981,1993, 1995

Figure 8. Annual rainfall at Kualapuu and Kaunakaki rain gages, Molokai, Hawaii (Data from National 

Climatic Data Center, 1984-94, and Commission on Water Resource Management, 1996, unpub.).

Geology 13
activity. Lava from the preshield stage consists predominantly of alkalic basalt (basalt that is low in silica and 

high in sodium and potassium). Lava from the principal 

stage of volcano building, called the shield stage, consists of fluid tholeiitic basalt (silica-saturated basalt) 

that characteristically forms thin flows. This basalt 

forms during submarine, as well as subaerial, eruptions. 

A large central caldera can form during the preshield or 

shield stages and might later be partly or completely 

filled during subsequent eruptions. Thousands of lava 

flows originate from the central caldera and from two or 

three rift zones that radiate out from the central part of 

the volcano. Intrusive dikes formed by rising magma 

exist within the rift zones and caldera area. The shield 

stage is the most voluminous phase of eruptive activity 

during which more than 95 percent of the volcano is 

formed. The postshield stage is marked by a change in 

lava chemistry and character. Postshield-stage lava 

includes alkalic basalt, and more viscous hawaiite, 

ankaramite, mugearite, and trachyte. Lava from the 

postshield stage may erupt from locations outside of the 

rift zones formed during the shield stage and forms a 

veneer atop the shield-stage basalt. After a period of 

quiescence, lava might issue from isolated vents on the 

volcano during the rejuvenated stage.

Volcanic rocks in Hawaii can be divided into three 

main groups on the basis of modes of emplacement: 

lava flows, dikes, and pyroclastic deposits. In general, 

lava flows that erupt from rift zones are less than 10 ft 

thick and are either pahoehoe, which is characterized by 

smooth or ropy surfaces, or aa, which contains a massive central core sandwiched between rubbly clinker 

layers. Aa flows are typically more abundant at greater 

distances from eruptive sources (Lockwood and Lipman, 1987).

Dikes are thin, near-vertical sheets of massive rock 

that intrude existing rocks, such as lava flows. Dikes are 

commonly exposed by erosion within the rift zones of 

older volcanoes (see for example Takasaki and Mink, 

1985), including West and East Molokai Volcanoes.

In the central part of a rift zone, the number of 

dikes can be as many as 1,000 per mile of horizontal distance and the dikes compose 10 percent or more of the 

total rock volume (Takasaki and Mink, 1985). The 

number of dikes decreases toward the outer edges of a 

rift zone. At the outer part of the rift zone, within the 

marginal dike zone, dikes usually constitute less than 5 

percent of the total rock volume (Takasaki and Mink,

1985). Wentworth and Macdonald (1953) estimate that 

200 dikes are needed to build 1,000 ft of a shield volcano.

Takasaki and Mink (1985, p. 5) define a dike complex as the "aggregates of dikes and the rocks they 

intrude..." By this definition, the marginal dike zone 

should be considered part of the dike complex. However, in their earlier description of a dike complex, 

Steams and Vaksvik (1935, p. 97) recognized that dikes 

do occur outside of the dike complex. Thus, for the purposes of this report, the dike complex is considered as 

the central part of the rift zone, where dikes compose 10 

percent or more of the total rock volume, and the marginal dike zone is adjacent to the dike complex.

Pyroclastic rocks form by explosive volcanic 

activity and are deposited by transport processes related 

to this activity. Pyroclastic rocks, such as ash, cinder, 

and spatter, can be deposited during all of the-subaerial 

stages of eruption and probably form less than 1 percent 

of the mass of a Hawaiian volcano (Wentworth and 

Macdonald, 1953).

The overall geology of Molokai has been 

described by numerous investigators (see for example 

Lindgren, 1903; Stearns and Macdonald, 1947; Macdonald and others, 1983; Stearns, 1985). Langenheim 

and Clague (1987) described the stratigraphic framework of volcanic rocks on Molokai.

The island of Molokai is formed primarily by the 

shield- and postshield-stage volcanic rocks of West 

Molokai Volcano and East Molokai Volcano, and secondarily by rejuvenated-stage.volcanic rocks at Kalaupapa Peninsula (Langenheim and Clague, 1987). 

Intrusive volcanic rocks in the form of dikes associated 

with rift zones and volcanic vents exist on both West 

and East Molokai. Coastal deposits consisting of sediments and limestone reefs are found along the southern 

coast.

West Molokai Volcano

The primary rift zones of the West Molokai Volcano trend roughly northwest and southwest (fig. 9) in 

the direction of broad ridges that extend from near the 

summit of the volcano. A few southeast-trending dikes 

exposed near the southern coast may be evidence of a 

third rift zone associated with West Molokai Volcano. 

There is no surface evidence of a summit caldera on

14 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
4 MILES

157°15'

Ilio Point

Laau 

Point

Base modified from U.S. Geological Survey digital data, 

1:24,000, 1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 157°00'45"

EXPLANATION 

SEDIMENTARY DEPOSITS (Holocene and Pleistocene)EAST MOLOKAI VOLCANO 

Kalaupapa Volcanics (Pleistocene)East Molokai Volcanics (Pleistocene and Pliocene) 

Upper memberLower member

WEST MOLOKAI VOLCANO 

West Molokai Volcanics (Pleistocene and Pliocene)

Kamalo

VOLCANIC VENT DIKE 

CONTACT

CALDERA BOUNDARY AXIS OF RIFT ZONE-Approximately located 

_._._._ BOUNDARY OF KUALAPUU STUDY AREA

- - ESTIMATED SEA-LEVEL LOCATION OF WESTMOLOKAI CONFINING UNIT

Cape 

Halawa

O 

n> oo IQ

Figure 9. Generalized surficial geology of Molokai, Hawaii (from Steams and Macdonald, 1947; and Langenheim and Clague, 1987).
West Molokai Volcano (Langenheim and Clague, 

1987). The exposed rocks of West Molokai are shieldstage tholeiitic basalt and postshield-stage hawaiite and 

alkalic basalt. Collectively, the volcanic rocks of West 

Molokai Volcano are known as the West Molokai Volcanics (Langenheim and Clague, 1987).

East Molokai Volcano

The primary rift zones of the East Molokai Volcano trend northwest and east from a central caldera 

complex (fig. 9). Macdonald (1956) suggests that there 

also may be a southern rift zone emanating from the 

caldera. Furthermore, Malahoff and Woollard (1966) 

interpreted results from a magnetic survey and indicated 

that there may be a southwest rift zone emanating from 

the caldera complex. The northwest and east rift zones 

are marked by numerous vertical to nearly vertical 

intrusive dikes (Stearns and Macdonald, 1947). The 

caldera complex of East Molokai Volcano is exposed in 

Pelekunu and Wailau Stream valleys (fig. 1), and is 

composed of stocks, plugs, crater fills, ponded lavas, 

and talus and fault breccias cut by dike swarms (Stearns 

and Macdonald, 1947).

Stearns and Macdonald (1947) mapped numerous 

vent features, including cinder and spatter cones, along 

the western and southern flanks of the East Molokai 

Volcano (fig. 9). Many of these features do not appear 

to lie along the trends of the two primary rift zones of 

the volcano, which may indicate that (1) a marginal dike 

zone exists or (2) more than two primary rift zones 

exist.

The exposed rocks of East Molokai Volcano are 

named the East Molokai Volcanics and Kalaupapa Volcanics (Langenheim and Clague, 1987). The East Molokai Volcanics are divided into two informal members. 

The lower member consists of shield-stage tholeiitic, 

olivine-tholeiitic, and picritic-tholeiitic basalts, and 

postshield-stage alkalic basalt. The upper member consists of postshield-stage mugearite, with lesser amounts 

of hawaiite and trachyte. The upper member of the East 

Molokai Volcanics forms a relatively thin veneer, about 

50 to 500 ft thick, over the lower member (Stearns and 

Macdonald, 1947). The Kalaupapa Volcanics include 

the rejuvenated-stage alkalic basalt and basanite that 

form Kalaupapa Peninsula (Langenheim and Clague, 

1987).

Estimated ages of rocks from West and East Molokai Volcanoes (Naughton and others, 1980; McDougall, 

1964; Langenheim and Clague, 1987) indicate that the 

volcanoes may have formed almost contemporaneously. Stearns and Macdonald (1947) noted, however, 

that an erosional unconformity, which dips about 10° to 

the east, is exposed at an altitude of 250 ft in the east 

bank of Waiahewahewa Gulch (fig. 1). At this site, East 

and West Molokai Volcanics are separated by 3 ft of 

soil and 6 ft of spheroidally weathered basalt, with the 

West Molokai Volcanics at the bottom of the sequence. 

The sequence indicates that West Molokai Volcanics 

are older than East Molokai Volcanics at the site of the 

exposed unconformity.

Coastal Deposits

Along southern Molokai, a coral reef extends from 

the coast to about 1 mi offshore, and limestone has also 

been described in a geologic log from a well near the 

southern coast of the island (Lindgren, 1903). In addition, along the southern shore of East Molokai Volcano 

and the Hoolehua Plain, an apron of alluvium has 

formed by deposition of eroded soil. Off the northern 

coast of Molokai, only a thin veneer of recent sediments 

exists (Mathewson, 1970).

Kualapuu Area

The surface rocks in the Kualapuu study area consist of the shield- and postshield-stage volcanic rocks of 

East Molokai Volcano (fig. 9). Numerous volcanic 

vents associated with East Molokai Volcano exist in the 

Kualapuu area. Available geological, hydrological, and 

geophysical information indicate that the northeastern 

part of the Kualapuu area is probably within the dike 

complex of the northwest-trending rift zone of East 

Molokai Volcano, and the southwestern part of the 

Kualapuu area may be within a marginal dike zone.

HYDRAULIC CONDUCTIVITY OF THE 

ROCKS

Hydraulic conductivity is a quantitative measure of 

the capacity of a rock to transmit water. The hydraulic 

conductivity of a rock can be qualitatively described by 

permeability. Permeability describes the ease with

16 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
which fluid can move through a porous rock (see for 

example Domenico and Schwartz, 1990). The permeability of volcanic rocks is variable and depends on the 

mode of emplacement of the rocks.

Lava Flows

In a layered sequence of subaerial, shield-stage 

lava flows of a volcano, where dike intrusions are not 

present, the overall permeability is high (Stearns and 

Macdonald, 1947). The main features of lava flows contributing to the high permeability are (1) clinker zones 

associated with aa flows, (2) voids along the contacts 

between flows, (3) cooling joints normal to flow surfaces, and (4) lava tubes associated with pahoehoe 

flows. Oki (1997) estimated the horizontal hydraulic 

conductivity of the dike-free, shield-stage lava flows of 

Molokai to be 1,000 feet per day (ft/d).

Dikes

Although most dikes are less than 10 ft thick, dikes 

are hydrologically important because they have low 

permeability and can extend vertically and laterally for 

thousands of feet. Within a dike complex, dikes intersect at various angles and compartmentalize the more 

permeable intruded rock so that ground water can be 

impounded to high altitudes. Because dikes lower overall rock porosity and permeability, the average hydraulic conductivity of a dike complex decreases as the 

number of dike intrusions increases. Although the 

geometry and the local-scale hydrologic effects of the 

dikes that fed the scattered vents of East Molokai Volcano near Kualapuu are not known, these dikes intrude 

the aquifer and probably lower the overall permeability 

of the aquifer.

From a ground-water flow model, Oki (1997) estimated the overall hydraulic conductivity of the dike 

complexes of West and East Molokai to be 2 and 0.02 

ft/d, respectively. The horizontal hydraulic conductivity 

of the marginal dike zone near Kualapuu was estimated 

to be 100 ft/d (Oki, 1997).

Weathering

Weathering tends to reduce the permeability of the 

volcanic rocks. The zone of weathered West Molokai

Volcanics and soil beneath the contact of the West and 

East Molokai Volcanics likely impedes ground-water 

flow between East and West Molokai. This weathered 

zone is referred to as the West Molokai confining unit 

in this report. No data are available to determine 

whether this unit is truly an effective barrier to groundwater flow. However, on the basis of information from 

Oahu on weathered volcanic rocks and a similar geohydrologic barrier (Oki, 1998), the hydraulic conductivity 

of the West Molokai confining unit is probably on the 

order of 1 ft/d.

Coastal Deposits

Coastal deposits and underlying weathered volcanic rocks impede the seaward discharge of freshwater 

on southern Molokai. The permeability of the interbedded coastal deposits may vary widely, from low-permeability compacted alluvium to cavernous limestone 

deposits. In southern Molokai, Oki (1997) estimated the 

overall vertical hydraulic conductivity of the coastal 

deposits to range from 0.5 to 5 ft/d.

REGIONAL GROUND-WATER FLOW 

SYSTEM

Precipitation is the source of all freshwater on 

Molokai. The precipitation either (1) runs off, (2) evaporates or is transpired by vegetation, or (3) recharges the 

ground-water system. Water that recharges the groundwater system flows from zones of higher to lower 

hydraulic head, as measured by water levels. Water levels are highest in the mountainous interior parts of the 

island, particularly in the northeastern part of the island, 

and lowest near the coast and, thus, ground water flows 

from the mountainous interior areas to coastal discharge 

areas. Ground water originating from eastern and western Molokai also flows toward the central Hoolehua 

Plain, from where it flows to either the northern or 

southern coast.

Ground water that is not withdrawn from wells and 

tunnels discharges naturally from the aquifer at onshore 

springs and seeps in deeply incised valleys and at subaerial and submarine coastal springs and seeps. In 

northeastern Molokai, springs form where stream erosion has cut through dike compartments below the level 

of the water table. Ground-water discharge at these 

springs contributes to the base flow of streams. Along

Regional Ground-Water Flow System 17
the southern coast, native Hawaiians built fishponds in 

shallow coastal waters by constructing rock-wall enclosures extending from the shoreline. References to fishpond construction on Molokai date back to the sixteenth 

century, and the most recently constructed fishpond on 

Molokai was built around 1829 (Farber, 1997). Because 

freshwater discharge to the ponds is necessary for 

growth of plants on which fish feed, the ponds may be 

evidence of the existence of springs (Stearns and Macdonald, 1947).

Ground water on Molokai is unconfmed in the 

inland areas. Along the southern coast, ground water 

may be confined by sedimentary deposits that impede 

the seaward discharge of fresh ground water. A measured seismic-velocity discontinuity at an altitude of 

about -6,000 ft measured in southwestern Oahu may 

coincide with a reduction in permeability of the volcanic rocks (Furumoto and others, 1970). Kauahikaua 

(1993) also suggests that a reduction in porosity on the 

island of Hawaii may occur near an altitude of-6,000 ft. 

The base of the aquifer on Molokai is unknown, but 

may extend down to an altitude near -6,000 ft. However, freshwater probably occurs in only the upper part 

of the aquifer, and perhaps in only a small fraction of the 

total thickness of the aquifer.

Fresh ground water on Molokai is found in two 

main forms: (1) as a lens-shaped body of freshwater, 

called a freshwater lens, floating on denser, underlying 

saltwater within permeable dike-free lava flows, and (2) 

as dike-impounded water with higher water levels (ten 

to hundreds of feet above sea level) where overall permeability is reduced because of the presence of dikes. 

Steams and Macdonald (1947) also suggest that 

perched water exists on Molokai.

Ground-Water Recharge

Ground water is recharged by direct infiltration of 

rainfall over much of Molokai. Over West Molokai 

Volcano and the Hoolehua Plain, however, groundwater recharge rates are low because of the low rainfall 

and high evaporation rates. The area of greatest 

recharge lies near the topographic peak of East Molokai 

Volcano, where rainfall is greatest.

Ground-water recharge on Molokai was estimated 

to be 144 Mgal/d with an annual water budget (State of

Hawaii, 1990). More recently, Shade (1997) estimated 

that ground-water recharge was 188.6 Mgal/d for natural vegetation conditions. This represents an average of 

about 15 in/yr over the island. However, recharge varies 

greatly areally from a minimum of near zero in/yr in 

western Molokai to a maximum of about 100 in/yr in 

northeastern Molokai.

Ground-Water Discharge to Streams

Streams on Molokai have steep gradients in the 

mountainous, high-rainfall regions and flatter gradients 

near the coast. No perennial streams exist in western 

Molokai or the central Hoolehua Plain. In contrast, 

streams in the windward, northeastern valleys of Molokai are perennial throughout most of their lengths. Most 

streams that drain to the southern coast of East Molokai 

Volcano are perennial only in the upper reaches where 

rainfall is persistent or where water is drained from 

marsh areas or springs. These streams are generally 

perennial only where they flow over lavas of the upper 

member of the East Molokai Volcanics. Where streams 

flow over the more permeable lavas of the lower member, surface water is more readily lost to infiltration 

(Stearns and Macdonald, 1947, p. 47).

Daily streamflow records are available at nine 

stream-gaging stations on streams in the windward, 

northeastern valleys of Molokai (fig. 10). Streamflow 

consists of direct runoff of rainfall and base flow. Base 

flow represents ground-water discharge. Oki (1997) 

used a computerized base-flow separation method 

(Wahl and Wahl, 1995) to estimate the base-flow component of streamflow at the gaging stations. The sum of 

the estimated average annual base flow in the gaged 

streams of northeastern Molokai is 38.8 Mgal/d (table 

2). Base flow at gaging station 16403900 was not 

included in the total base-flow estimate because flow 

measured at this gaging station is included in the baseflow estimate at downstream station 16404000. At station 16408000, Waikolu Stream, only data prior to 1961 

were used because of diversions after November 1960. 

The total base flow on Molokai is unknown because not 

all streams are gaged at their mouths. Base flow estimates were used to estimate the leakance (see "Boundary Conditions" subsection of the "Model Description" 

section) for streams in northeastern Molokai.

18 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
2 4 MILES

I I I I

I I I I I 

024 KILOMETERS

157°15'

Ilio Point

Kahiu Point

Base modified from U.S. Geological Survey digital data, 

1:24,000,1983, Albers equal area projection, standard 

parallels 21°04'50' and 21°12'10°, central meridian 

157°00'45-

3D

(D

O

I

3 

Q.

I

i 

to

400000

402000

A

SWAMP (from Stearns and Macdonald, 1947) 

MOLOKAI IRRIGATION SYSTEM TUNNEL 

MOLOKAI IRRIGATION SYSTEM PIPELINE

GAGING STATION AND ABBREVIATED 

NUMBER-Complete number is preceded 

by 16

Continuous-record, active station 

Continuous-record, inactive station 

SPRING (from Stearns and Macdonald, 1947)

amalo

Figure 10. Surface-water gaging stations, Molokai, Hawaii.
Table 2. Estimated base flow for northeastern Molokai streams, Hawaii

[BFI, base flow index program (Wahl and Wahl, 1995); Mgal/d, million gallons per day]

Station Station name

Complete years of record used Average streamflow1 Average base flow, 

in BFI program (Mgal/d) (Mgal/d)

16400000 Halawa Stream near Halawa

16401000 Papalaua Stream near Pukoo

16402000 Pulena Stream near Wailau

16403000 Waiakeakua Stream near Wailau

16403900 Kawainui Stream near Pelekunu

16404000 Pelekunu Stream near Pelekunu

16404200 Pilipililau Stream near Pelekunu

16405000 Lanipuni Stream near Pelekunu

16408000 Waikolu Stream below pipeline 

	crossing near Kalaupapa

1918, 1921-31,1938-95 

1921-28

1920-27,1938-56 

1920-28,1938-56 

1969-79, 1995

1920-28,1938-46,1949-56, 

1972-81

1969-95

1920-28, 1938-56

1921-29, 1938-47,1949-60

19.5

13.8

22.2

7.5

5.1

10.6

1.0 

9.1

12.3

5.2 

4.1 

8.9 

3.8 

2.2 

4.6

0.7 

4.3 

7.2

Data available at URL http://wwwdhihnl.wr.usgs.gov.

Ground-Water Levels

Measured water levels are available primarily at 

wells along the southern coast and in the central plain 

area (figs. 4 and 11, table 3). In the vicinity of Kualapuu, water levels are about 8 to 12 ft above sea level. 

Along the south shore, water levels are 1 to 3 ft above 

sea level between Umipaa and Kawela, and 4 to 5 ft 

above sea level between Kamalo and Mapulehu.

Within the northwest rift zone of the East Molokai 

Volcano near Waikolu Stream valley, water levels at 

wells 0855-01 to -03 were about 900 ft above sea level 

in 1961. At the northern margin of the dike complex, 

near Kalaupapa Peninsula, the water level at well 1058- 

01 was reported to be 9 ft above sea level. The 9-ft water 

level at well 1058-01 probably represents an upper limit 

for the water-table altitude in the dike-free Kalaupapa 

Volcanics. Results from an electrical resistivity survey 

indicated that the lens in the Kalaupapa Volcanics was 

thin (Takasaki, 1986).

In the West Molokai Volcanics, the water level at 

well 1011-01 was reported by Stearns and Macdonald 

(1947, p. 61) to be 5.6 ft above sea level in 1946.

The distribution of water levels on Molokai, averaged from measurements taken over a period of about 

60 years, is shown in figure 11. A detailed contour map 

of water levels for the entire island cannot be drawn 

using existing well data. MacCarthy (1941) used electrical-resistivity measurements to determine the depth 

to saltwater, then applied the Ghyben-Herzberg principle (see following discussion in the section "Freshwater

Lens") to estimate the altitude of the water table in 

western Molokai (fig. 11). MacCarthy (1941) estimated 

that the water-table altitude in western Molokai ranges 

from about 1 to 14 ft above sea level. The water-level 

estimates made from the resistivity measurements are 

only approximate because use of the Ghyben-Herzberg 

principle to predict water levels from estimated depths 

to saltwater (1) ignores the freshwater-saltwater transition zone separating freshwater from saltwater and (2) 

does not account for dynamic conditions in the system 

where vertical flow is present. It is also likely that 

unquantified errors are associated with the resistivity 

measurements and the geophysical models used to represent the actual subsurface conditions.

Freshwater Lens

Water levels measured in wells drilled into the permeable dike-free lava flows of the island are less than 

15 ft above sea level. Within the dike-free lava flows, a 

lens of freshwater floats on denser, underlying saltwater. The source of freshwater in the lens is groundwater recharge from (1) the upgradient dike complex 

where water levels are high, (2) infiltration of rainfall, 

and (3) irrigation water. Fresh ground water flows from 

inland recharge areas to coastal discharge areas. A saltwater-circulation system exists beneath the lens (fig. 3) 

(Cooper and others, 1964; Souza and Voss, 1987). Saltwater flows landward in the deeper parts of the aquifer, 

rises, and then mixes with seaward-flowing freshwater. 

This mixing creates a freshwater-saltwater transition 

zone.

20 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
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Regional Ground-Water Flow System 21
Table 3. Summary of measured, nonpumping water levels at selected wells, Molokai, Hawaii

[Data from: Stearns and Macdonald, 1947; Anthony, 1995; Takasaki, 1986; USGS Hawaii District well files; --, none or not applicable]

Water level, in feet

Well no.

0352-01

0352-05

0448-02

0449-01

0456-01

0456-02

0456-03

0456-04

0456-98b

0456-99C

0457-01

0457-02

0458-01

0458-02

0458-03

0459-01

0459-02

0459-03

0459-04

0459-05

0500-01

0500-03

0501-03

0501-04

0501-05

0559-01

0600-01

0601-01

0601-02

0603-01

0700-01

0705-04

0801-01

0801-02

0801-03

0807-01

0855-01

0855-02

0855-03

0901-01

0902-01

0905-01

0915-01

1011-01

1014-01

1058-01

1114-01

1114-02

Period

08/47-06/61

11/27/61

09/70-07/97

06/47-07/97

05/48

no date

no date

no date

no dates

no date

06/47-08/97

no date

no date

no date

no date

no date

no date

no date

no date

no date

no date

no date

06/38-08/45

no date

06/47-01/54

no date

08/39-09/39

05/54-07/97

no date

12/67-09/77

07/76-01/97

11/22/72

03/48-06/95

no date

07/87-09/97

11/03/75

06/29/61

04/03/61

07/10/61

05/52-06/76

06/53-06/63

06/38-05/46

no date

01/18/46

no date

no date

no date

no date

Arithmetic mean8

4.63

4.97

4.28

4.20

1.03

1.77

1.96

1.8

1.83

2.5

2.00

1.7

1.14

1.39

1.22

1.17

1.17

1.19

1.13

1.10

1

1.25

1.27

2.34

2.45

1.63

8.5

2.66

2.50

2.46

6.22

5.21

9.12

7.8

9.88

3

989.15

866.52

947.04

12.24

11.67

5.69

6.7

5.65

5.7

9

0.6

7.3

Maximum

4.10

3.67

2.09

1.36

1.83

~

1.47

~

-

-

~

~

0.75

2.03

--

8.9

1.60

2.10

5.72

10.74

~

11.7

~

~

8.65

7.53

4.47

--

~

-

~

Minimum

5.40

5.45

6.05

0.56

_

1.83

2.78

«

--

--

2.00

3.27

--

7.7

3.30

3.52

6.70

8.4

8.06

~

~

..

17.90

14.46

6.52

-

Standard 

deviation 

(feet)

0.26

0.31

0.91

0.27

..

0.52

0.20

«

~

--

~

0.35

~

0.19

--

0.67

0.23

0.24

0.26

0.74

2.57

-

-

1.24

1.69

0.56

-

-

-

~

~

-

No. of daily 

measurements

80

1

191

283

10

1

1

1

3

1

201

1

1

1

1

1

1

1

1

1

1

1

87

1

52

1

3

290

1

69

81

1

12

1

2

1

1

1

1

274

118

92

1

1

1

-

1

1

a The arithmetic mean is an average obtained by dividing the sum of the water-level measurements by the number of water-level measurements.

For some wells, only a single water-level measurement was available. 

b No state number assigned to this well, old well 13 (Stearns and Macdonald, 1947). 

c No state number assigned to this well, old well 14 (Stearns and Macdonald, 1947).

22 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
For hydrostatic conditions, the thickness of a freshwater lens can be estimated by the Ghyben-Herzberg 

principle. If the specific gravities of freshwater and saltwater are assumed to be 1.000 and 1.025, respectively, 

and if the freshwater and saltwater are assumed to be 

separated by a sharp interface, then the GhybenHerzberg principle predicts that every foot of freshwater above sea level must be balanced by 40 ft of freshwater below sea level. For dynamic conditions, the 

Ghyben-Herzberg principle tends to underestimate 

freshwater-lens thickness near the discharge zone and 

overestimate lens thickness near the recharge zone.

Dike-Impounded Ground Water

Within the dike complex of the East Molokai Volcano, fresh ground water is impounded to high levels in 

the volcanic rocks between low-permeability dikes. In 

the valleys of northeast Molokai, the presence of 

springs indicates that ground water in the dike complex 

is probably impounded to altitudes greater than 2,000 ft 

above sea level (Stearns and Macdonald, 1947, p. 75). 

Because of low recharge rates in west Molokai, water 

levels in the dike complex of West Molokai Volcano are 

probably less than about 15 ft above sea level (MacCarthy, 1941).

The abundance of dikes in a rift zone increases 

with depth, which reduces the overall permeability of 

the dike complex with depth. No data exist that indicate 

the depth to which rocks are saturated with freshwater 

in the dike complex. In the marginal dike zone where 

dike intrusions are few, freshwater floats on saltwater.

GROUND-WATER FLOW SYSTEM IN THE 

KUALAPUU AREA

The northeastern part of the Kualapuu area is 

within the dike complex of the northwest rift zone of 

East Molokai Volcano. No wells have been drilled in 

the northeastern part of the Kualapuu area, but ground 

water there is probably impounded to high levels (tens 

to hundreds of feet above sea level) in the volcanic 

rocks between low-permeability dikes. Within the 

Kualapuu area, ground water generally flows to the 

southwest, where water levels are about 8 to 12 ft above 

sea level. The southwest part of the Kualapuu area lies 

within a marginal dike zone of East Molokai Volcano. 

In the southwest part of the Kualapuu area, a freshwater

body of unknown thickness overlies a brackish-water 

transition zone of unknown thickness, which in turn 

overlies saltwater. Ground water in the Kualapuu area 

that is not withdrawn from wells flows either to the west 

or south, where ground-water levels are lower, or 

directly to the ocean in the north.

Ground-Water Levels

As of 1999, only five wells (0801-01 to -03,0901- 

01, and 0902-01) (fig. 4) had been drilled in the Kualapuu area, and four of them are within about 2,000 ft of 

each other. The earliest reported water level in the 

Kualapuu area is from well 0902-01, and measured 10.5 

ft above sea level in 1946 (unpub. data in U.S. Geological Survey, Honolulu, well files). Hydrographs for 

wells 0902-01 and 0901-01 show relatively large variations in water levels from one measurement to the next 

(Oki, 1997). Water levels in well 0902-01 ranged from 

7.53 to 14.46 ft above sea level during the period 1953- 

63, and water levels in well 0901-01 ranged from 8.65 

to 17.90 ft above sea level during the period 1952-76. 

The large variations could be caused by pumping or 

changes in recharge, but are more likely associated with 

the inaccuracy of the pressure measurements made with 

the air-line device used to monitor water levels in these 

wells.

The water level in DHHL well 1 (0801-01) measured 10.48 to 10.69 ft above sea level on March 31, 

1948, when the well was 1,011 ft deep, and 10.74 ft 

above sea level on May 11, 1949, when the well was 

1,055 ft deep (unpub. data in U.S. Geological Survey, 

Honolulu, well files). The final completed depth of well 

0801-01 was 1,095 ft in 1949. During 1995 after the 

pump in well 0801-01 was removed, water levels in this 

well were measured by the USGS and ranged from 8.4 

to 9.2 ft above sea level during May and June 1995. The 

water levels measured in the late 1940's are higher than 

those measured during 1995, which may be the result of 

increased withdrawals or decreased recharge in the 

Kualapuu area since the late 1940's.

The only reported water-level measurement from 

DHHL well 2 (0801-02), which was drilled in 1979, 

was 7.8 ft above sea level in July 1979 (unpub. data in 

Commission on Water Resource Management well 

files). Maui County Department of Water Supply 

(DWS) well 0801-03 was drilled in 1987, and the initial 

water-level measurement at this well was 11.70 ft above

Ground-Water Flow System in the Kualapuu Area 23
sea level on July 27, 1987 (unpub. data in Commission 

on Water Resource Management well files). The USGS 

measured a water level of 6.91 ft above sea level on 

April 5, 1995 in well 0801-03 with the pump in operation, and Tom Nance Water Resource Engineering 

(State of Hawaii, 1998) measured a water level of 8.06 

ft above sea level on September 18,1997 after the pump 

had been turned off for an unspecified period of time.

Depth to Saline Water

A time-domain electromagnetic survey was. made 

in the vicinity of the Kualapuu area by Blackhawk Geosciences, Inc. (State of Hawaii, 1997) to estimate the 

depth to saline (unspecified salinity) water. Results of 

the survey indicate that the altitude of the saline-water 

zone in the aquifer rises to the southwest (fig. 11). The 

estimated altitude of the saline-water zone near wells 

0801-01 to -03 is about 1,000 ft below sea level. Near 

the western part of the Kualapuu area, the altitude of the 

saline water is about 400 ft below sea level. The results 

of the time-domain electromagnetic survey are consistent with the expected decrease in water levels in a 

southwesterly direction.

In the Kualapuu area, the estimated depths to 

saline water from the time-domain electromagnetic survey cannot be used to accurately predict water levels 

using the Ghyben-Herzberg principle. For example, the 

estimated depth to saline water of 1,000 ft below sea 

level near wells 0801-01 to -03 would correspond to a 

water level of 25 ft if the Ghyben-Herzberg principle 

were assumed to be applicable. However, measured 

water levels near wells 0801-01 to -03 are less than 12 

ft above sea level.

Chloride Concentration

Chloride concentration is commonly used as an 

indicator of salinity, which may increase as a result of 

saltwater intrusion into the ground-water system. The 

chloride concentration of water from wells in the Kualapuu area has generally been below 200 mg/L except at 

well 0902-01. During the period from 1950 through 

1961, the chloride concentration of water withdrawn 

from well 0902-01 ranged from 252 to 430 mg/L. For 

comparison, the chloride concentration of rainfall is 

typically less than 10 to 20 mg/L (Swain, 1973), and the

chloride concentration of seawater is about 19,500 

mg/L (Wentworth, 1939).

During the 1990's, chloride concentrations of 

pumped water from wells 0801-01, 0801-02, 0801-03, 

and 0901-01 in the Kualapuu area have been less than 

200 mg/L. DWS began pumping well 0801-03 in mid1991. Chloride concentrations of water pumped from 

DHHL wells 0801-01 and 0801-02 rose by about 20 

mg/L and 10 mg/L, respectively, following the start of 

pumping from the DWS well (0801-03).

Vertical Profiles of Chloride Concentration in 

Well 0801 -01

During 1994-95, the pump in DHHL well 1 (0801- 

01) was removed, making it possible to measure chloride concentration with depth on four separate dates. 

The well is at a ground-surface altitude of about 1,006 

ft in the Kualapuu area, and has a solid casing from the 

top of the well to an altitude of about -3 ft, a perforated 

casing between -3 ft and -71 ft, and is open from -71 ft 

to the bottom of the hole at -89 ft (fig. 12). The well is 

about 130 ft from DHHL well 2 (0801-02) and 560 ft 

from the DWS well (0801-03). DHHL well 2 has perforated casing between an altitude of -27 ft and the bottom 

of the hole at -87 ft. The DWS well has perforated casing between altitudes of 13 ft and -37 ft, and is open 

from -37 ft to the bottom of the hole at -96 ft (fig. 12).

In the days prior to May 15,1995, both DHHL well 

2 and the DWS well were pumped intermittently (fig. 

13). The pump capacities for DHHL well 2 and the 

DWS well are about 700 and 750 gallons per minute 

(gal/min), respectively. During May 15-22, 1995, 

DHHL well 2 was pumped continuously at a rate of 

about 700 gal/min, and the DWS well was not pumped. 

Pumped water samples were collected from DHHL well 

2 during the period May 15-22, 1995 (fig. 14). 

Chloride-concentration profiles in DHHL well 1 were 

obtained on two separate dates prior to May 15, 1995 

(November 3,1994 and May 12,1995), and on two separate dates during the period when DHHL well 2 was 

being pumped continuously (May 18,1995 arid May 22, 

1995) (Fontaine and others, 1997).

Profile of November 3, 1994.--The chlorideconcentration profile in DHHL well 1 from November 

3, 1994 indicates a chloride concentration of about 35 

mg/L within the part of the borehole with solid casing 

(fig. 12). Beneath the solid casing, chloride

24 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
DHHLwell 1 (0801-01)

10

-10

LU

-20

LU

O -30

LU

-40

LU

§ -50

-60

-70

-80

-90

-100

\ I I I i l l l l

 0-- Sampled November 3, 1994

 ® Sampled May 12,1995

- -Sampled May 18, 1995

- - Sampled May 22, 1995

0 20 40 60 80 100 120 140 160 180 200 

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

to ground surface j

10-

o-

-10-

-20-

O -30

LU

1

LU

-40-

-50-

-60-

-70-

-80-

-90-

-100 J

0)V5S

EXPLANATION

SOLID CASING

PERFORATED CASING

OPEN HOLE

wo

Figure 12. Chloride concentration of water samples from different depths in Department of Hawaiian Home Lands

well 1 (0801-01), and diagram of well-casing depth and type for wells 0801-01 to -03, Molokai, Hawaii. There were no reported

withdrawals from well 0801-01 from October 1993 to December 1995. During the period October 1993 to May 15,1995,

Department of Hawaiian Home Lands well 2 (0801-02) and Maui County Department of Water Supply well 0801-03

were pumped on an intermittent basis. During the period May 15-22,1995, well 0801-02 was pumped at a continuous

rate of about 700 gallons per minute and well 0801-03 was not pumped.

Ground-Water Flow System in the Kualapuu Area 25
. Department of Hawaiian Home Lands 

well 2 (0801-02)

1,000

900

800

700

600

500

400

300

200

100

0

LLJ

Z)

DC 

LU 

0. 

05

cc

(3 

Z 

Q.

2 =) 

o.

Department of Water Supply 

well 0801-03

J I L

1234567 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 

May 1995

23 24 25 26 27 28 29 30 31

Figure 13. Estimated pumping rates at Department of Hawaiian Home Lands well 2 (0801-02) and Department of 

Water Supply well 0801-03 during May 1995, Molokai, Hawaii. Vertical dashed lines represent times of water 

sampling for chloride concentration profiles in Department of Hawaiian Home Lands well 1 (0801-01).

26 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
UJ

=i 88

CC

UJ

w 86

1 84

I 82

z_

z" 80

0

H

£ 78

hZ

O 76

z

O

0 74

UJ

O

cc 7«

0 72

I 

° yn

-

-

-

_

-

---- --.

" ~», _^

 - «. __

 -J

-

I W ^

/ I

I

I

I

1 _ 1

" """" ~91 

,

^^_ _^^

,,,,,,,,,

-

-

^_ _ ^ ~~ "~^ ^ _

""" " -~ ^ ^

-

-

-

-

i i i i i i i i i

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

May 1995

Figure 14. Chloride concentration of water samples from Department of Hawaiian Home Lands well 2 (0801-02) 

during May 1995, Molokai, Hawaii. Vertical dashed lines represent times of water sampling for chloride 

concentration profiles in Department of Hawaiian Home Lands well 1 (0801-01).

concentrations ranged from 161 to 183 mg/L. Reported 

pumpage from DHHL well 2 during October 1994 averaged 0.58 Mgal/d (400 gal/min), and reported pumpage 

from the DWS well during October 1994 averaged 0.68 

Mgal/d (470 gal/min).

Profile of May 12, 1995. The chloride-concentration profile in DHHL well 1 from May 12, 1995 indicates a chloride concentration of about 111 to 120 mg/L 

within the part of the borehole with solid casing (fig. 

12). Beneath the solid casing, chloride concentrations 

ranged from 171 to 175 mg/L. Thus, beneath the solid 

casing, chloride concentrations in the borehole were 

approximately uniform with depth. At the time the 

water samples were being collected on May 12, 1995, 

both DHHL well 2 and the DWS well were being 

pumped (fig. 13).

Profile of May 18,1995. The chloride-concentration profile in DHHL well 1 from May 18,1995 indicates a chloride concentration of about 113 mg/L from 

the top of the sampled water column within the solid 

casing to an altitude of -36 ft (fig. 12). Between altitudes of -61 and -74 ft, the chloride concentration was 

168 mg/L.

Profile of May 22, 1995. -The chloride-concentration profile in DHHL well 1 from May 22, 1995 indicates about a 10 mg/L decrease in chloride concentration, relative to the profile obtained on May 18,1995, 

from the top of the sampled water column to an altitude 

of -36 ft (fig. 12). The chloride concentration near the 

bottom of the borehole, between altitudes of -61 to -74 

ft, remained at about 168 mg/L.

From November 3,1994 to May 12, 1995, the 

chloride concentration in the part of DHHL well 1 with 

solid casing increased from about 35 mg/L to 111 to 120 

mg/L. The two profiles obtained before May 15, 1995 

indicate that the chloride concentration within the part 

of the DHHL well 1 borehole with solid casing is lower 

than in the screened part of the borehole. Furthermore, 

the chloride concentrations beneath the solid casing 

were almost constant with depth. These data indicate 

that upward flow within the borehole may cause mixing 

within the screened part of the borehole. Pumping at the 

DWS well and DHHL well 2 could cause a reduction in 

head near the top of the aquifer, which could create a 

vertical head gradient. This vertical gradient could 

cause saltier water from near the bottom of the DHHL 

well 1 borehole to flow upward in the borehole, mix

Ground-Water Flow System in the Kualapuu Area 27
within the borehole, and flow out of the borehole into a 

permeable zone near the top of the aquifer.

The profile obtained on May 12,1995 indicates a 

chloride concentration of 111 to 120 mg/L within the 

part of DHHL well 1 with solid casing, and a chloride 

concentration of 171 to 175 mg/L beneath the solid casing. The chloride-concentration profile obtained on 

May 18,1995 indicates an approximately uniform chloride concentration of about 113 mg/L within the part of 

the borehole with solid casing and down to an altitude 

of -36 ft. Thus, over a period of 3 days (May 15-18, 

1995), continuous pumping at DHHL well 2 and no 

pumping from the DWS well apparently caused a 

greater amount of fresher water near the top of the aquifer to enter the screened part of the borehole of DHHL 

well 1. There is no evidence, however, that the chloride 

concentration of pumped water from DHHL well 2 

decreased between May 15 and May 18,1995. (fig. 14).

On May 22,1995, the chloride concentrations in 

DHHL well 1 above an altitude of -36 ft decreased by 

about 10 mg/L relative to the chloride concentrations 

from May 18,1995. Thus, continuous pumping from 

May 18 to May 22,1995 at DHHL well 2 (and no pumping from the DWS well) caused a further freshening of 

the water in the DHHL well 1 borehole above an altitude of -36 ft. At the bottom of the DHHL well 1 borehole, however, the chloride concentration was relatively 

constant (168 mg/L) from May 18 to May 22, 1995.

The chloride concentration of water in the DHHL 

well 1 borehole is apparently affected by nearby pumping from DHHL well 2 and the DWS well. The magnitude and duration of pumping, the locations of pumped 

wells, and the structure of the aquifer are some of the 

factors that can control the vertical distribution of chloride concentration in a nonpumped, open monitor well.

Salinity profiles from open boreholes are used in 

Hawaii to estimate the location of the transition zone. 

However, the estimated location of the transition may 

be unreliable if significant vertical flow causes mixing 

of water within the borehole. A flow meter can be used 

to estimate the rate of vertical flow at different depths in 

an open borehole. Alternatively, the mixing effects 

caused by vertical flow can be eliminated by using a 

series of piezometers, each open at a different depth in 

the aquifer, rather than a single open borehole.

GROUND-WATER WITHDRAWALS

Most of the ground water withdrawn on Molokai is 

from the Kualapuu area, the south shore of eastern 

Molokai, and the dike complex in northeastern Molokai. The reported annual mean pumpage for Molokai 

during 1996 was 4.336 Mgal/d (computed from data 

supplied by Neal Fujii, CWRM, written commun., 

1997).

Five production wells (0801-01 to -03, 0901-01, 

and 0902-01) have been drilled in the Kualapuu area for 

either irrigation or domestic use. Wells 0902-01 and 

0901-01 (fig. 4), drilled in 1946 and 1950, respectively, 

were originally used to irrigate pineapple fields in the 

Hoolehua Plain area. Well 0902-01 was abandoned in 

1964 when surface water from the Molokai Irrigation 

System (MIS) tunnel (fig. 4) became available. Since 

1976, water from well 0901-01 has been used for 

domestic and irrigation purposes in western Molokai. 

Prior to the completion of the MIS tunnel, combined 

withdrawals from wells 0901-01 and 0902-01 varied 

seasonally from near zero to about 1 Mgal/d (fig. 15). 

DHHL wells 0801-01 and 0801-02 (fig. 4) were completed in 1949 and 1979, respectively, and DWS well 

0801-03 was drilled in 1987. Monthly mean withdrawal 

rates from each of wells 0801-01 to -03 have remained 

below 1 Mgal/d (fig. 15). During 1996, annual mean 

combined withdrawal from the four active wells in the 

Kualapuu area was 2.029 Mgal/d.

Along the south shore of eastern Molokai, groundwater withdrawals are mainly from two Maui-type 

wells (consisting of a shaft excavated to or below the 

water table, and one or more infiltration tunnels extending outward from the shaft); one near Kawela (0457-01) 

completed in 1921, and the other near Ualapue (0449- 

01) completed in 1936 (fig. 4). During 1996, annual 

mean withdrawals from wells 0457-01 and 0449-01 

were 0.398 and 0.204 Mgal/d, respectively. Total unreported withdrawals from several other drilled wells and 

numerous shallow dug wells along the southern coast of 

Molokai are probably small.

Three production wells (0855-01 to -03) (fig. 4) 

drilled in 1961 withdraw water from the dike complex 

in northeastern Molokai. Water from these wells enters 

the Molokai Irrigation System. Monthly mean combined withdrawal from these three wells averages about

28 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
1.0

0.8

0.6

0.4

0.2

1.0 

0.8 

0.6 

0.4 

0.2

IT IT TT TT I I ' ' ' I I TT TT TT

WELL 0449-01

Data prior to 1980 represent annual mean pumpage

I I

o in in in

O) O)

I I I I I I ' ' ' I I I I TT

WELL 0457-01

Q

CC 

LJJ 

Q.

C/D

Data prior to 1980 represent annual mean pumpage

in 

CO

O)

=3 1.0 I I I I \ I I I T r i i i i TI I I ' ' ' I I ' ' ' I I

WELL 0801-01

LJJ

S

Q.

^

Z)

o_

0.8 

0.6 

0.4 

0.2 

0

1.0 

0.8 

0.6 

0.4 

0.2 

0

I I

Data prior to 1961 represent 

annual mean pumpage

I I

I I I I I I

WELL 0801-02

I I I I I I I I I I I I m

s

in in 

en

o

(O

o>

in hO)

Figure 15. Monthly or annual mean pumpage from selected wells, Molokai, Hawaii.

Ground-Water Withdrawals 29
CJ o

PUMPAGE, IN MILLION GALLONS PER DAY

3:

CD 

(A 

2. 

<D

2 5*3

(D

TJ

O 

| 

<D

SL o> oc

2! 1935

 n 

<5

Ol 1940 

_

j- 1945

0>

3 

C 

P. 3

CD 

0) 3

T3C3

CO 

CDO3

CO

cp_

CD O

CD 

Q.

CD

I 

&) &

1950

1990

1995
1 Mgal/d (fig. 15). During 1996, annual mean combined 

withdrawal was 1.438 Mgal/d.

Because parts of the MIS tunnel are below the 

water table, ground water discharges directly into the 

tunnel by gravity. During periods in which wells 0855- 

01 to -03 were not in use, direct discharge of ground 

water to the tunnel was estimated from the difference 

between the discharges at stations 16405300 and 

16405100, and averaged 1.822 Mgal/d.

NUMERICAL SIMULATION TO DETERMINE 

AREAS OF POSSIBLE FUTURE GROUNDWATER DEVELOPMENT

One of the factors to be considered in selecting a 

site for a deep monitor well in the Kualapuu area is 

where future ground-water development may occur. A 

deep monitor well located near an area of possible 

future ground-water development would provide useful 

information related to the availability of freshwater in 

the area. Salinity profiles obtained from a deep monitor 

well located near an area where production wells are 

likely to be drilled could establish (1) whether the production wells can be successfully developed, or (2) suitable construction details for the production wells. 

Furthermore, if the production wells are eventually 

drilled, a deep monitor well located in the vicinity of the 

production wells could provide relevant, long-term 

water-quality information.

A numerical ground-water flow model was previously developed to simulate steady-state regional 

ground-water flow on Molokai (Oki, 1997). The numerical model was used in this study to estimate water-level 

declines caused by possible future ground-water development in the Kualapuu area of Molokai.

Model Description

The regional model used the two-dimensional 

(areal), finite-element code AQUIFEM-SALT (Voss, 

1984). This code was designed to simulate flow of confined or unconfined fresh ground water in systems that 

may have a freshwater body floating on denser underlying saltwater. AQUIFEM-SALT treats freshwater and 

saltwater as immiscible fluids separated by a sharp 

interface. The depth of the interface is determined by 

the Ghyben-Herzberg relation. In reality, a diffuse transition zone exists between the core of freshwater and the 

underlying saltwater. In this study, it was assumed that 

the position of the surface of 50-percent seawater salinity is approximated by the sharp-interface position. 

AQUIFEM-SALT simulates the vertically averaged 

freshwater head in the aquifer and assumes that flow is 

entirely horizontal and all wells fully penetrate the 

fresh water body.

The model mesh, boundary conditions, hydraulic 

characteristics, and recharge used in this study are the 

same as those used in the regional model (Oki, 1997). 

For a complete description of these features and other 

information regarding the construction details of the 

model, refer to Oki (1997). A brief summary of the 

modebis provided below.

Model Mesh

The finite-element mesh used in this study consists 

of 6,432 nodes and 6,251 square elements, 1,640 ft on a 

side, arranged in a rectangular array with 47 rows and 

133 columns. The mesh covers the entire island of 

Molokai and extends at least a mile offshore to include 

the entire zone where fresh ground water discharges to 

the ocean.

Boundary Conditions

AQUIFEM-SALT supports three types of boundary conditions: (1) specified head, (2) specified flow 

(which includes no flow), and (3) head-dependent discharge. Specified-head boundary conditions were not 

used for this study. The perimeter of the mesh is a noflow boundary. The aquifer bottom was treated as a noflow boundary at an altitude of 6,000 ft below sea level.

All elements representing offshore areas and subaerial springs near the coast and in northeastern Molokai valleys were modeled using a head-dependent 

discharge boundary condition. All elements not simulated as a head-dependent discharge boundary were 

unconfined, water-table elements. Head-dependent discharge elements associated with streams in northeastern 

Molokai valleys were simulated as confined elements if 

the model-calculated head was above the base of the 

stream, and as water-table elements otherwise. The base 

of the stream within an element was estimated from the 

average stream-channel altitude (U.S. Geological Survey, 1952) within that element.

Flow out of the model at head-dependent discharge 

elements was assumed to be linearly related to the

Numerical Simulation to Determine Areas of Possible Future Ground-Water Development 31
difference between the head in the aquifer and the head 

overlying the confining unit at the discharge site 

according to the equation:

Q=(K'/B')A(h-hQ) (1)

where:

<2 is the rate of discharge within a model element

[L3/T], 

K' is the vertical hydraulic conductivity of the

confining unit overlying the aquifer [L/T], 

Bf is the thickness of the confining unit overlying the

aquifer [L],

A is the area of the model element [L2], 

h is the head in the aquifer [L], and 

/i0 is the head above the confining unit [L].

Six discharge zones were defined (fig. 16): (1) 

southeastern Molokai coast (K'= 0.5 ft/d), (2) southwestern Molokai coast (K'= 5 ft/d), (3) northern Molokai coast exclusive of Kalaupapa Peninsula (K'IB'= 0.1 

per day), (4) Kalaupapa Peninsula (K'/B'= 0.1 per day),

(5) northeastern Molokai streams in the dike complex 

within the caldera area (K'IB'= 6.1 x 10"5 per day), and

(6) northeastern Molokai streams in the dike complex 

outside the caldera area (K'IB'= 1.2 x 10"3 per day). The 

confining-unit vertical hydraulic conductivity (K'} 

divided by the confining-unit thickness (B') forms a 

lumped parameter known as the leakance. No attempt 

was made to estimate separate values for confining-unit 

thickness and vertical hydraulic conductivity for the 

northern Molokai coast (including Kalaupapa Peninsula) and the northeastern Molokai streams.

The head, /IQ, overlying the confining unit above 

onshore coastal-discharge elements is unknown but 

probably near mean sea level, and therefore was 

assumed to be zero. For offshore elements, /IQ was 

assigned a value corresponding to the freshwater equivalent head of the saltwater column overlying the ocean 

floor within the element. For these offshore elements, 

the freshwater equivalent head, measured relative to a 

mean sea level datum, was computed from the equation:

hQ= -Z/40 (2)

where Z is the altitude of the ocean floor.

For elements representing the springs in northeastem Molokai valleys, /IQ was assigned a value corresponding to the average altitude (U.S. Geological 

Survey, 1952) of the stream channel within the model 

element.

Hydraulic-Conductivity Zones

The island of Molokai was divided into seven 

zones (fig. 17) with separate horizontal hydraulicconductivity values: (1) 1,000 ft/d for the dike-free 

West Molokai Volcanics, (2) 1,000 ft/d for the dike-free 

East Molokai Volcanics, (3) 500 ft/d for the Kalaupapa 

Volcanics, (4) 1 ft/d for the West Molokai Volcanics 

confining unit, (5) 2 ft/d for the West Molokai Volcanics dike complex, (6) 0.02 ft/d for the East Molokai 

Volcanics dike complex, and (7) 100 ft/d for the East 

Molokai Volcanics marginal dike zone. The West 

Molokai confining-unit zone represents the zone 

formed by weathered volcanic rocks and soil between 

West and East Molokai Volcanics. Although the confining unit dips at an angle of only 10° it extends throughout the freshwater thickness and impedes ground-water 

flow between the West and East Molokai Volcanics. In 

this study, the West Molokai confining unit was represented in the two-dimensional model as a zone of 

reduced hydraulic conductivity. The West and East 

Molokai dike-complex zones were modeled as zones of 

lower overall hydraulic conductivity relative to the 

dike-free volcanic-aquifer zones. A model zone was 

created for the assumed East Molokai marginal dike 

zone, where numerous volcanic-vent features exist. The 

hydraulic conductivity of the East Molokai marginal 

dike zone was presumed to be between the values for 

the dike-free volcanic rocks and the East Molokai dike 

complex.

Recharge

Ground-water recharge to the model was distributed over the modeled area according to the mean 

annual recharge rates estimated by Shade (1997). The 

areal distribution of recharge used in the model is 

shown in figure 18. Total island-wide recharge estimated by Shade (1997) was 188.6 Mgal/d. Total 

recharge used in the model was 187.3 Mgal/d because 

discretization at the coastline resulted in land area that 

receives 1.3 Mgal/d recharge being assigned to offshore 

elements. This loss in recharge is less than 1 percent of 

the total value, and is well within the range of uncertainty associated with the recharge estimates.

Model Results

The numerical model was used to estimate groundwater levels that result from pumping at rates equal to

32 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
157°20' 157°10' 157° 156°50' 156°40'

4 MILES

Z

o3

3

O 

<D (D(D>(A O

<D O3 Q. i

o1

o a

21°

II II 

024 KILOMETERS

10 20 30 40 50

COLUMN 

60 70 80 100

EXPLANATION 

HEAD-DEPENDENT DISCHARGE ELEMENT AND LEAKANCE OR VERTICAL HYDRAULIC-CONDUCTIVITY VALUE

Southern Molokai Coast

lliiij] Southeastern part (vertical hydraulic conductivity = 0.5 feet per day) | I Southwestern part (vertical hydraulic conductivity = 5 feet per day) .Iff:] Northern Molokai Coast (leakance = 0.1 per day) [jjljij Kalaupapa Peninsula (leakance = 0.1 per day)

_ Northeastern Molokai streams

gggg Within caldera area (leakance = 6.1 x 10"5 per day)H Outside of caldera area (leakance = 1.2 x 10"3 per day)I | WATER-TABLE, NON-DISCHARGE ELEMENT

Figure 16. Ground-water discharge zones used in the ground-water flow model, Molokai, Hawaii.

uOJ
CO

»n> oo'6*^01<D

TJ

ISJ. fi) ao_ o

0)

157°20' 157°10'

21 "101

21°

10

157°

COLUMN 

60 70

156°50' 156°40'

024 MILES 024 KILOMETERS

110

NOTE: In the model grid, hydraulic-conductivity values are assigned to nodes. If a node is associated with elements in different zones in the figure, the hydraulic conductivity assigned to that node is the lowest among the values for these zones.

EXPLANATION

HYDRAULIC-CONDUCTIVITY ZONE AND HYDRAULIC-CONDUCTIVITY VALUE ASSIGNED IN THE GROUND-WATER FLOW MODEL

West Molokai Volcanics

Dike free (1,000 feet per day) Dike complex (2 feet per day) 

Confining unit (1 foot per day)

East Molokai Volcanics 

| | Dike free (1,000 feet per day)

Dike complex (0.02 feet per day) 

I_| Marginal dike zone (100 feet per day) 

ISa Kalaupapa Volcanics (500 feet per day)

_._._.. BOUNDARY OF KUALAPUU STUDY AREA

Figure 17. Horizontal hydraulic-conductivity zones used in the ground-water flow model, Molokai, Hawaii.
157°20' 157°10' 157° 156°50' 156MO'

21°10'

ZC3 io0)

3

0)o01i>ro0)

0

of Possible Future Ground-Water Develo

i i

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 ' ' - '-- , , ;;'';;''< Atflni^^Vt^f 1 :!:^: ' '''\Yi''\m\^f\\' ' ' ' '..,,-,,,.,,,,,,,,,,, ,,,,., , ,. -':|f:!fly|||^iiiitt|||i|iifl n ' ~=~ ~~ ^7^7 7 ~ 7 ^"T" ~ ~~~~~ ~ ~ ^.^-~ ~ ~ ~ ~ ~ ~ ~ ------ - ^R^^H^I^^^^^^I^^^^^^B^^^^V! ~ ~ ~ ~

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EXPLANATION

\VERAGE ANNUAL RECHARGE, IN INCHES 

0 Greater than 0 and less than or equal to 10 Greater than 10 and less than or equal to 25 Greater than 25 and less than or equal to 50 

Greater than 50

^JODE WITH SIMULATED GROUND- WATER WITHDRAWAL

2 4 MILES

1

4 KILOMETERS 

120 130

= =;;==,, __________ :ii!:i^:;:::::: ":~ ^iilillii - i t

'*^ *~^ ^ 2

^^- ------ LZ-, ___________ Figure 18. Average annual recharge used in the ground-water flow model and nodes with simulated 1999-allocated ground-water withdrawal, Molokai, Hawaii.
and in excess of the March 1999 allocated rates from 

existing and hypothetical wells in the Kualapuu area. 

Withdrawal rates of 0.5 and 1.0 Mgal/d in excess of the 

March 1999 total allocation rate were simulated at 

selected individual sites in the Kualapuu area, outside of 

the modeled dike complex. The simulations are consistent with the original assumptions upon which the 

numerical model was based, and the additional simulated withdrawals did not stress the modeled system 

beyond the original design limits of the model.

Withdrawal at the March 1999 Allocation Rate

The ground-water flow model was used to estimate 

the distribution of water levels in the Kualapuu area, 

outside of the modeled dike complex, for withdrawal 

rates corresponding to the March 1999 allocated rates. 

As of March 1999, CWRM had issued water-use permits authorizing a total of 6.6432 Mgal/d of groundwater withdrawals from wells and tunnels on Molokai 

(table 1), and these withdrawals were represented in the 

model. For the Kualapuu area, allocated withdrawals 

totaling 1.79 Mgal/d were represented in the model. The 

State of Hawaii Department of Hawaiian Home Lands 

reservation for 2.905 Mgal/d of ground water from the 

Kualapuu area was not represented in the model.

For withdrawal rates corresponding to the March 

1999 allocated rates, model results indicate that the 

steady-state water level is about 8.64 ft above sea level 

near existing wells 0801-01 to 03 and about 7.59 ft 

above sea level near well 0901-01. Because the 

thicknesses of the freshwater body and the freshwatersaltwater transition zone are not known, it is uncertain 

whether a water level of 7.59 ft above sea level is sufficient to protect all existing wells. A deep monitor well 

in the Kualapuu area would help to address this issue.

Withdrawal of 0.5 Mgal/d from a Single Site

The ground-water flow model was used to simulate steady-state water levels that result from pumping 

from a single site in the Kualapuu area at a rate of 0.5 

Mgal/d above the March 1999 total allocation rate. The 

0.5 Mgal/d withdrawal was simulated at a total of 56 

different sites in the Kualapuu area, outside of the modeled dike complex. Withdrawals in the dike complex 

were not simulated because there are no current plans to 

withdraw ground water from higher altitudes in the 

Kualapuu area, where costs associated with pumping 

and transmission would be high. For each of the 56 different scenarios (corresponding to the 56 different withdrawal sites), the hydrologic effects of the additional 

0.5 Mgal/d withdrawal were quantified by considering 

model-calculated water levels at (1) well 0901-01, (2) 

wells 0801-01 to -03, and (3) the site of the additional 

0.5 Mgal/d withdrawal.

The model-calculated water level at existing well 

0901-01 is lowest for cases in which the additional 0.5 

Mgal/d withdrawal is placed at or near well 0901-01, 

and highest for cases in which the additional 0.5 Mgal/d 

withdrawal is placed near the southwestern and southeastern boundaries of the Kualapuu area (fig. 19). The 

model-calculated water level at well 0901-01 is 5.96 ft 

above sea level for the case of pumping an additional 

0.5 Mgal/d from well 0901-01. Pumping the additional 

0.5 Mgal/d from near the southern extent of the Kualapuu area results in a model-calculated water level at 

well 0901-01 of 7.50 ft above^ea level.

Similarly, the model-calculated water level at 

wells 0801-01 to -03 is lowest for cases in which the 

additional 0.5 Mgal/d withdrawal is placed at or near 

existing wells 0801-01 to -03, and greatest for cases in 

which the additional 0.5 Mgal/d withdrawal is placed 

near the southwestern and southeastern boundaries of 

the Kualapuu area (fig. 20). The model-calculated water 

level at wells 0801-01 to -03 is 7.17 ft above sea level 

for the case of pumping an additional 0.5 Mgal/d from 

wells 0801-01 to -03. Pumping the additional 0.5 

Mgal/d from near the southwestern boundary of the 

Kualapuu area results in a model-calculated water level 

at wells 0801-01 to -03 of 8.55 ft above sea level.

The model-calculated water level at the site of the 

additional 0.5 Mgal/d withdrawal is less than 5 ft above 

sea level for sites in the southwestern part of the Kualapuu area, and greater than 10 ft above sea level for sites 

in the central and southeastern parts of the Kualapuu 

area (fig. 21).

Withdrawal of 1.0 Mgal/d from a Single Site

The ground-water flow model was also used to 

simulate steady-state water levels that result from 

pumping from a single site in the Kualapuu area at a rate 

of 1.0 Mgal/d above the March 1999 total allocation 

rate. The 1.0 Mgal/d withdrawal was simulated at a total 

of 54 different sites in the Kualapuu area, outside of the 

modeled dike complex. For each of the 54 different scenarios (corresponding to the 54 different withdrawal 

sites), the hydrologic effects of the additional 1.0 

Mgal/d withdrawal were quantified by considering

36 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
157°02' 157°00' 156°58' 

T

Kahiu Point

Kalaupapa 

Peninsula

well 0901-01 

(node 3094)

MAUNAWAINUI

KUALAPUU

EXPLANATION

MODEL-CALCULATED WATER LEVEL AT WELL 0901-01 CAUSED 

BY WITHDRAWING 0.5 MGAL/D FROM A SINGLE SITE IN THE 

INDICATED ZONE, IN FEET ABOVE MEAN SEA LEVEL

Less than 7.0

Greater than or equal to 7.0 and less than 7.2 

Greater than or equal to 7.2 and less than 7.4 

Greater than or equal to 7.4 

AQUIFER-SYSTEM BOUNDARY AND NAME

MODEL NODE WITH 0.5 MGAL/D WITHDRAWAL AND

CORRESPONDING MODEL-CALCULATED WATER LEVEL AT 

WELL 0901-01 CAUSED BY THE ADDITIONAL 0.5 MGAL/D 

WITHDRAWAL, IN FEET ABOVE MEAN SEA LEVEL

KAMILOLOA

KAWELA

Kawela

Base modified from U. S. Geological Survey digital data, 

1:24,000, 1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 

157W45"

Figure 19. Model-calculated steady-state water levels at well 0901-01 (model node 3094) caused by withdrawing 

0.5 million gallons per day, above the March 1999 total allocated withdrawal rate, from the Kualapuu area (outside 

of modeled dike complex), Molokai, Hawaii. The model-calculated water levels at well 0901-01 are shown adjacent 

to the selected sites where the additional 0.5 million gallons per day withdrawal was simulated. In all simulations, 

the 0.5 million gallons per day additional withdrawal was assumed to be from a single site.

Numerical Simulation to Determine Areas of Possible Future Ground-Water Development 37
156°58'

21°08'

21°06'

wells 0801-01 to-03 

(node 3142)

MAUNAWAINUI

KUALAPUU

EXPLANATION

MODEL-CALCULATED WATER LEVEL AT WELLS 0801-01 TO -03 

CAUSED BY WITHDRAWING 0.5 MGAL/D FROM A SINGLE SITE IN 

THE INDICATED ZONE, IN FEET ABOVE MEAN SEA LEVEL

Less than 8.0

Greater than or equal to 8.0 and less than 8.2 

Greater than or equal to 8.2 and less than 8.4 

Greater than or equal to 8.4 

AQUIFER-SYSTEM BOUNDARY AND NAME

MODEL NODE WITH 0.5 MGAL/D WITHDRAWAL AND

CORRESPONDING MODEL-CALCULATED WATER LEVEL AT 

WELLS 0801-01 TO -03 CAUSED BY THE ADDITIONAL 0.5 MGAL/D 

WITHDRAWAL, IN FEET ABOVE MEAN SEA LEVEL

KAMILOLOA

KAWELA

Kawela

Base modified from U. S. Geological Survey digital data, 

1:24,000, 1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 

157°00'45"

Figure 20. Model-calculated steady-state water levels at wells 0801-01 to -03 (model node 3142) caused by 

withdrawing 0.5 million gallons per day, above the March 1999 total allocated withdrawal rate, from the Kualapuu area 

(outside of modeled dike complex), Molokai, Hawaii. The model-calculated water levels at wells 0801-01 to -03 are 

shown adjacent to the selected sites where the additional 0.5 million gallons per day withdrawal was simulated. In 

all simulations, the 0.5 million gallons per day additional withdrawal was assumed to be from a single site.

38 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
21°04'

157°04' T~ 157°02' r~ 157°00f 156°58'

Kahiu Point

Kalaupapa 

Peninsula

well 0901-01 

(node 3094)

MAUNAWAINUI

EXPLANATION

iHi ZONE WHERE WITHDRAWING 0.5 MGAL/D FROM A SINGLE SITE

CAUSES MODEL-CALCULATED WATER LEVELS AT WELL 0901-01 

(MODEL NODE 3094) TO DROP BELOW 7 FEET ABOVE SEA LEVEL

H ZONE WHERE WITHDRAWING 0.5 MGAL/D FROM A SINGLE SITE

DOES NOT CAUSE MODEL-CALCULATED WATER LEVELS AT WELL 

0901-01 (MODEL NODE 3094) TO DROP BELOW 7 FEET ABOVE SEA 

LEVEL

CZD ZONE WHERE WITHDRAWING 0.5 MGAL/D FROM A SINGLE SITE

DOES NOT CAUSE MODEL-CALCULTATED WATER LEVELS AT THAT 

WITHDRAWAL SITE TO DROP BELOW 10 FEET ABOVE SEA LEVEL

KUALAPUU

- - - AQUIFER-SYSTEM BOUNDARY AND NAME

o6 41 MODEL NODE WITH 0.5 MGAL/D WITHDRAWAL AND

CORRESPONDING MODEL-CALCULATED WATER LEVEL AT 

THAT NODE CAUSED BY WITHDRAWING THE 0.5 MGAL/D ABOVE 

THE 1999 ALLOCATED WITHDRAWAL RATE

KAMILOLOA

KAWELA

I I

Base modified from U. S. Geological Survey digital data, 

1:24,000, 1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 

157°00'45"

Figure 21. Model-calculated zone where withdrawing 0.5 million gallons per day from a single site does not cause the 

steady-state water level at the withdrawal site to drop below 10 feet above sea level, and model calculated zone where 

withdrawing 0.5 million gallons per day from a single site does not cause the water level at well 0901-01 (model node 

3094) to drop below 7 feet above sea level, Kualapuu area (outside of modeled dike complex), Molokai, Hawaii.

Numerical Simulation to Determine Areas of Possible Future Ground-Water Development 39
model-calculated water levels at (1) well 0901-01, (2) 

wells 0801-01 to -03, and (3) the site of the additional 

1.0 Mgal/d withdrawal.

The model-calculated water level at existing well 

0901-01 is lowest for cases in which the additional 1.0 

Mgal/d withdrawal is placed at or near well 0901-01, 

and greatest for cases in which the additional 1.0 

Mgal/d withdrawal is placed near the southwestern and 

southeastern boundaries of the Kualapuu area (fig. 22). 

The model-calculated water level at well 0901-01 is 

3.83 ft above sea level for the case of pumping an additional 1.0 Mgal/d from well 0901-01. Pumping the additional 1.0 Mgal/d from near the southern extent of the 

Kualapuu area results in a model-calculated water level 

at well 0901-01 of 7.41 ft above sea level.

Similarly, the model-calculated water level at 

wells 0801-01 to -03 is lowest for cases in which the 

additional 1.0 Mgal/d withdrawal is placed at or near 

existing wells 0801-01 to -03, and greatest for cases in 

which the additional 1.0 Mgal/d withdrawal is placed 

near the southwestern and southeastern boundaries of 

the Kualapuu area (fig. 23). The model-calculated water 

level at wells 0801-01 to -03 is 5.38 ft above sea level 

for the case of pumping an additional 1.0 Mgal/d from 

wells 0801-01 to -03. Pumping the additional 1.0 

Mgal/d from near the southwestern boundary of the 

Kualapuu area results in a model-calculated water level 

at wells 0801-01 to -03 of 8.46 ft above sea level.

The model-calculated water level at the site of the 

additional 1.0 Mgal/d withdrawal is less than 5 ft above 

sea level for sites in the western and southwestern parts 

of the Kualapuu area, and greater than 10 ft above sea 

level for sites in the central and southeastern parts of the 

Kualapuu area (fig. 24).

Model results indicate that the southeastern part of 

the Kualapuu area is a possible area of future groundwater development because (1) withdrawals from this 

area have a small effect on water levels at existing wells 

in the Kualapuu area (relative to effects from withdrawals in other parts of the Kualapuu area that are outside 

of the dike complex), and (2) model-calculated water 

levels in this part of the Kualapuu area are high relative 

to water levels in other parts of the Kualapuu area that 

are outside of the dike complex. Although it may be 

possible to successfully withdraw additional ground 

water from other parts of the Kualapuu area, model 

results indicate that the effects on existing wells will be

greater, water levels at the site of the additional pumping will be lower, or both.

Model Limitations

The ground-water flow model of Molokai used in 

this study has several limitations. There are an insufficient number of monitor wells to define the spatial distribution of water levels in western Molokai, the inland 

parts of southeastern Molokai, and the dike complex of 

northeastern Molokai. Furthermore, the distribution of 

model-calculated water levels in the Kualapuu area is 

unverified in places.

Because of the lack of sufficient water-level data, 

the model developed for this study is not unique. That 

is, it is possible that different distributions of hydraulic 

conductivity and leakance can be used in a model to 

produce equally acceptable model-calculated-water levels. A model zone was created to represent the marginal 

dike zone of East Molokai Volcano, and two coastal 

discharge zones were created for southern Molokai. 

Although this zonation is plausible, it is probable that 

other zonation geometries could produce similar 

results. The model developed for this study can be 

refined and a better representation of the flow system 

can be obtained as more data become available to constrain the model.

Because the ground-water flow model contains 

only a single layer, vertical hydraulic gradients cannot 

be simulated and model-calculated drawdown caused 

by additional withdrawals underestimates actual drawdown near partially penetrating wells. In addition, the 

model should not be viewed as a quantitatively precise 

predictive tool because of the uncertainty in the hydraulic-conductivity distribution of the model. The model is, 

nevertheless, the best available tool for analyzing the 

possible regional hydrologic effects of additional withdrawals on Molokai for steady-state conditions. The 

transient hydrologic effects of additional withdrawals 

were not modeled in this study.

The AQUIFEM-SALT code, which assumes a 

sharp interface between freshwater and saltwater, was 

used to simulate the regional flow system on Molokai. 

No deep monitor wells exist on Molokai which can provide information on the thickness of the transition zone 

between freshwater and underlying saltwater. A deep 

monitor well would provide information that is necessary to evaluate (1) the extent to which the sharp40 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
156°58'

Kalaupapa 

Peninsula

PACIFIC OCEAN

complex 

o o

646 - "alae

well 0901-01 

(node 3094)

MAUNAWAINUI

EXPLANATION

MODEL-CALCULATED WATER LEVEL AT WELL 0901-01 CAUSED 

BY WITHDRAWING 1.0 MGAL/D FROM A SINGLE SITE IN THE 

INDICATED ZONE, IN FEET ABOVE MEAN SEA LEVEL

Less than 6.0

Greater than or equal to 6.0 and less than 6.5 

Greater than or equal to 6.5 and less than 7.0 

Greater than or equal to 7.0

AQUIFER-SYSTEM BOUNDARY AND NAME

MODEL NODE WITH 1.0 MGAL/D WITHDRAWAL AND

CORRESPONDING MODEL-CALCULATED WATER LEVEL AT 

WELL 0901-01 CAUSED BY THE ADDITIONAL 1.0 MGAL/D 

WITHDRAWAL. IN FEET ABOVE MEAN SEA LEVEL

Base modified from U. S. Geological Survey digital data,

1:24,000, 1983, Albers equal area projection, standard :

parallels 21°04'50" and 21°12'10", central meridian 157°00'45"

Figure 22. Model-calculated steady-state water levels at well 0901-01 (model node 3094) caused by withdrawing 

1.0 million gallons per day, above the March 1999 total allocated withdrawal rate, from the Kualapuu area (outside 

of modeled dike complex), Molokai, Hawaii. The model-calculated water levels at well 0901-01 are shown adjacent 

to the selected sites where the additional 1.0 million gallons per day withdrawal was simulated. In all simulations, 

the 1.0 million gallons per day additional withdrawal was assumed to be from a single site.

Numerical Simulation to Determine Areas of Possible Future Ground-Water Development 41
157°04' 157°02'

21°08'

I Kahiu Point

Kalaupapa 

Peninsula

wells 0801-01 to-03 

(node 3142)

MAUNAWAINUI

n

KUALAPUU

EXPLANATION

MODEL-CALCULATED WATER LEVEL AT WELLS 0801-01 TO -03 

CAUSED BY WITHDRAWING 1.0 MGAL/D FROM A SINGLE SITE IN 

THE INDICATED ZONE, IN FEET ABOVE MEAN SEA LEVEL

Less than 7.0

Greater than or equal to 7.0 and less than 7.6 

Greater than or equal to 7.6 and less than 8.2 

Greater than or equal to 8.2 

AQUIFER SYSTEM BOUNDARY AND NAME

MODEL NODE WITH 1.0 MGAL/D WITHDRAWAL AND

CORRESPONDING MODEL-CALCULATED WATER LEVEL AT 

WELLS 0801-01 TO -03 CAUSED BY THE ADDITIONAL 1.0 MGAL/D 

WITHDRAWAL, IN FEET ABOVE MEAN SEA LEVEL

KAMILOLOA

KAWELA

Kawela

j_

Base modified from U. S. Geological Survey digital data, 

1:24,000, 1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 

157°00'45"

Figure 23. Model-calculated steady-state water levels at wells 0801-01 to -03 (model node 3142) caused by 

withdrawing 1.0 million gallons per day, above the March 1999 total allocated withdrawal rate, from the Kualapuu area 

(outside of modeled dike complex), Molokai, Hawaii. The model-calculated water levels at wells 0801-01 to -03 are 

shown adjacent to the selected sites where the additional 1.0 million gallons per day withdrawal was simulated. In 

all simulations, the 1.0 million gallons per day additional withdrawal was assumed to be from a single site.

42 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
157°04'

21°04'

well 0901-01 

(node 3094)

MAUNAWAINUI

KUALAPUU

EXPLANATION

ZONE WHERE WITHDRAWING 1.0 MGAL/D FROM A SINGLE SITE 

CAUSES MODEL-CALCULATED WATER LEVELS AT WELL 0901-01 

(MODEL NODE 3094) TO DROP BELOW 7 FEET ABOVE SEA LEVEL

ZONE WHERE WITHDRAWING 1.0 MGAL/D FROM A SINGLE SITE

DOES NOT CAUSE MODEL-CALCULATED WATER LEVELS AT WELL 

0901-01 (MODEL NODE 3094) TO DROP BELOW 7 FEET ABOVE SEA 

LEVEL

ZONE WHERE WITHDRAWING 1.0 MGAL/D FROM A SINGLE SITE

DOES NOT CAUSE MODEL-CALCULTATED WATER LEVELS AT THAT 

WITHDRAWAL SITE TO DROP BELOW 10 FEET ABOVE SEA LEVEL

AQUIFER-SYSTEM BOUNDARY AND NAME

MODEL NODE WITH 1.0 MGAL/D WITHDRAWAL AND

CORRESPONDING MODEL-CALCULATED WATER LEVEL AT 

THAT NODE CAUSED BY WITHDRAWING THE 1.0 MGAL/D ABOVE 

THE 1999 ALLOCATED WITHDRAWAL RATE

KAMILOLOA

KAWELA

Kawela

J_

Base modified from U. S. Geological Survey digital data, 

1:24,000, 1983, Albers equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 

157°00'45"

Figure 24. Model-calculated zone where withdrawing 1.0 million gallons per day from a single site does not cause the 

steady-state water level at the withdrawal site to drop below 10 feet above sea level, and model calculated zone where 

withdrawing 1.0 million gallons per day from a single site does not cause the water level at well 0901-01 (model node 

3094) to drop below 7 feet above sea level, Kualapuu area (outside of modeled dike complex), Molokai, Hawaii.

Numerical Simulation to Determine Areas of Possible Future Ground-Water Development 43
interface assumption deviates from reality and (2) how 

accurately the surface of 50-percent seawater salinity in 

the transition zone can be represented by a sharp interface.

ADDITIONAL CRITERIA FOR DEEP 

MONITOR WELL SITE SELECTION

In addition to possible sites of future ground-water 

development, factors to be considered in selecting a site 

for a deep monitor well in the Kualapuu area include (1) 

ground-water level, (2) ground-surface altitude, (3) land 

classification, ownership, and accessibility, (4) geology, (5) locations of existing production wells, and (6) 

historical ground-water quality information. Each of 

these additional factors is discussed below.

Ground-Water Level

Measured water levels in existing production wells 

in the Kualapuu area are between about 8 and 12 ft 

above sea level. Thus, a deep monitor well drilled in an 

area with water levels of 8 to 12 ft above sea level may 

provide hydrologic information that is relevant to the 

existing production wells. The depth to saline water has 

been estimated from a geophysical survey (State of 

Hawaii, 1997) and is a function of the water-level altitude.

Ground-Surface Altitude

Drilling at ground-surface altitudes significantly 

greater than 1,000 ft will result in increased costs and 

the well may be located in the dike complex or conservation district (fig. 5), which presents additional problems (see following sections). Existing water-level 

information indicates that wells drilled at groundsurface altitudes significantly less than 1,000 ft will 

likely have lower water levels than existing production 

wells in the Kualapuu area. Ground-surface altitudes 

are about 1,000 ft at the sites of existing production 

wells in the Kualapuu area. Because model-calculated 

water-level contours (Oki, 1997) generally are aligned 

with the topographic contours, a deep monitor well 

drilled at an altitude of about 1,000 to 1,100 ft in the 

Kualapuu area may provide hydrologic information that 

is relevant to the existing production wells.

Land Classification, Ownership, and 

Accessibility

A deep monitor well drilled in a conservation district would be problematic because (1) the well would 

be located at an altitude significantly greater than 1,000 

ft, (2) the well would likely be located in the dike complex, and (3) it may be difficult to secure the necessary 

legal permits to drill on conservation land.

Land ownership is an additional consideration. 

Access to a deep monitor well located on privately 

owned property may be lost if the property is sold or the 

landowner no longer desires the well on the property. 

Locating a well on government-owned land, particularly DHHL or DWS land, would probably minimize 

the possibility of losing access to the well in the future 

because (1) land ownership is less likely to change, and 

(2) both DHHL and DWS have expressed an interest in 

drilling a deep monitor well in the Kualapuu area.

From a logistical standpoint, accessibility to the 

proposed drilling site is a factor to consider. Locating a 

deep monitor well near an existing road would reduce 

the costs associated with drilling the well and future 

monitoring of water levels and the vertical distribution 

of salinity in the well.

Geology

Existing production wells in the Kualapuu area 

were drilled in the East Molokai Volcanics and are 

located outside of the dike complex. Within the dike 

complex near the eastern part of the Kualapuu area, 

water levels probably are at least several tens of feet 

above sea level, which is considerably higher than the 

water levels of 8 to 12 ft measured at existing wells. 

Thus, a deep monitor well drilled within the dike complex may not provide useful information that is relevant 

to the existing wells. Furthermore, there are no plans to 

drill additional pumped wells in the dike complex 

within the Kualapuu area that would make a deep monitor well in the dike complex relevant.

Because hydrologic conditions in the immediate 

vicinity of the volcanic vents may differ from conditions near the existing production wells, a deep monitor 

well drilled near a volcanic vent may not provide relevant information. Existing production wells in the 

Kualapuu area are located about a mile from known volcanic vents.

44 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
Locations of Existing Production Wells

Limited information on the vertical distribution of 

salinity in DHHL well 1 (0801-01) indicates that pumping from nearby wells can cause borehole flow, and 

thereby affect the salinity profile, in a deep monitor 

well. A salinity profile obtained from an ideal deep 

monitor well would be an exact representation of the 

vertical distribution of salinity in the aquifer. Because 

of borehole flow, however, the salinity profile obtained 

from a deep monitor well may not correspond to the 

salinity distribution in the aquifer. Although borehole 

flow in a deep monitor well can exist even in the 

absence of nearby pumping, information from DHHL 

well 1 (0801-01) indicates that pumped wells located 

within about 1,000 ft from the monitor well can further 

enhance borehole flow. Pumped wells located farther 

than 1,000 ft from a deep monitor well can also cause 

borehole flow in the monitor well depending on pumping rate and the permeability of the aquifer.

To reduce the possible effects of borehole flow, a 

deep monitor well in the Kualapuu area would ideally 

be located as far as possible from existing or proposed 

production wells. On the other hand, a deep monitor 

well that is located in the vicinity of existing or proposed wells, and that is not affected by borehole flow 

induced by pumping from other wells, would provide 

the most relevant information.

Historical Ground-Water Quality Information

Chloride concentrations of pumped water from 

well 0902-01 indicate that chloride concentrations near 

the western part of the Kualapuu area may exceed 250 

mg/L. Thus, it is unlikely that future drinking-water 

wells will be drilled in the Kualapuu area west of well 

0902-01. Locating a deep monitor well in the area west 

of well 0902-01 would reduce the possible effects of 

borehole flow caused by pumping from existing or proposed wells, but the monitor well may not provide useful information for the management of existing or future 

ground-water development.

SITE SELECTION FOR A DEEP MONITOR 

WELL

As discussed previously, a deep monitor well in 

the Kualapuu area will be most useful if it is located (1)

in the vicinity of future ground-water development, (2) 

in an area where water levels are about 8 to 12 ft above 

sea level, (3) at a ground-surface altitude of 1,000 to 

1,100 ft, (4) on government-owned land that is outside 

of the conservation district, (5) outside of the dike complex and as far from known volcanic vents as possible, 

(6) at least about 1,000 ft from existing or proposed production wells, and (7) east of well 0902-01. The intersection of the areas defined by (1) model-calculated 

water levels between 8 and 12 ft above sea level (with 

withdrawal rates equal to the March 1999 allocated 

rates), (2) ground-surface altitudes between 1,000 and 

1,100 ft, (3) government-owned property that is outside 

of the dike complex, and (4) the area east of well 0902- 

01 forms two zones; a smaller intersection zone north of 

well 0902-01 and a larger intersection zone mostly 

southeast of the existing production wells in the Kualapuu area (fig. 25). The intersection zone to the north of 

well 0902-01 is near a volcanic vent, Eleuweuwe, and is 

thus ruled out as a possible site for a deep monitor well 

because of geological considerations.

The intersection zone that is southeast of the existing Kualapuu production wells is outside of the modeled dike complex and near the area where future 

ground-water development is possible. Within this 

southeastern intersection zone, a site must be chosen 

that is as far from known volcanic vents as possible, and 

at least about 1,000 ft from existing production wells. A 

viable area for drilling a deep monitor well is located 

about a half mile southeast of existing wells 0801-01 to 

-03 and a half mile north of the central part of a known 

volcanic vent, Puu Luahine (fig. 25). This area represents the central part of the intersection zone that is 

southeast of existing Kualapuu production wells. In this 

central area, existing geophysical information indicates 

that the saline water is about 1,000 ft below sea level, 

which is the same depth indicated at the existing production wells (fig. 25). Although there are unimproved 

roads leading to this central area, existing topography 

may make access difficult for large vehicles. Thus, 

alternative sites may need to be considered that are outside of the central area but that are within the intersection zone that is southeast of existing Kualapuu 

production wells.

Site Selection for a Deep Monitor Well 45
157°04'

21°08'

21°06'

21°04'

157°02' 

I

2 MILES

1570 00' 

I Kahiu Point

Kalaupapa 

Peninsula

MAUNAWAINUI

EXPLANATION

[JJJJJJ] POSSIBLE AREA FOR DRILLING A 

DEEP MONITOR WELL

[Tx] ZONE WITHIN THE KUALAPUU AREA THAT 

IS EAST OF WELL 0902-01, BETWEEN 

ALTITUDES OF ABOUT 1,000 AND 1,100 FEET, 

ON GOVERNMENT-OWNED LAND, AND 

WITHIN THE AREA WHERE MODELCALCULATED WATER LEVELS ARE BETWEEN 

8 AND 12 FEET ABOVE MEAN SEA LEVEL 

FOR WITHDRAWALS AT THE MARCH 1999 

ALLOCATED RATE

^ VOLCANIC VENT 

^ALAPUU AQUIFER-SYSTEM BOUNDARY AND NAME

- -200 - LINE OF EQUAL DEPTH TO SALINE WATER 

BODY (STATE OF HAWAII, 1997) - 

Interval 200 feet. Datum is mean sea level.

0902-01

WELL AND NUMBER

Kamiloloa KAMILOLOA / KAWELA

Kawela

Base modified from U. S. Geological Survey digital data, 

1:24,000, 1983, Alters equal area projection, standard 

parallels 21°04'50" and 21°12'10", central meridian 157°00'45"

Figure 25. Possible area for drilling a deep monitor well in the Kualapuu area, Molokai, Hawaii. The possible 

area for drilling represents the central part of the zone within the Kualapuu area that is southeast of well 0901 -01, 

between altitudes of about 1,000 and 1,100 feet, on government-owned property, and within the area where 

model-calculated water levels are between 8 and 12 feet above mean sea level for withdrawals at the 

March 1999 allocated rate.

46 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii
SUMMARY AND CONCLUSIONS

Projected demand for ground water is high in the 

Kualapuu area of Molokai, which lies in the central part 

of the island. The northeastern part of the Kualapuu area 

is within the dike complex of the northwest rift zone of 

East Molokai Volcano. No wells have been drilled in 

the northeastern part of the Kualapuu area, but ground 

water there is probably impounded to high levels (tens 

to hundreds of feet above sea level) in the volcanic 

rocks between low-permeability dikes. Within the 

Kualapuu area, ground water generally flows from the 

northeast to the southwest. Measured water levels from 

wells that are outside of the dike complex are about 8 to 

12 feet above sea level. In the Kualapuu area and outside of the dike complex, a freshwater body of unknown 

thickness overlies a brackish-water transition zone of 

unknown thickness, which in turn overlies saltwater.

Results of a time-domain electromagnetic survey 

indicate that the altitude of the saline-water zone in the 

Kualapuu area rises in a southwesterly direction. The 

results of the time-domain electromagnetic survey are 

consistent with the expected increase in water levels in 

a northeasterly direction.

As of mid-1999 no deep monitor wells existed on 

Molokai. There is an acute need for such wells on Molokai, and in the Kualapuu area in particular, to evaluate 

the thickness of the freshwater body and brackish-water 

transition zone. Management of the ground-water 

resources in the Kualapuu area is hindered because of 

the uncertainty in the vertical salinity structure in the 

aquifer.

The main factors to be considered in selecting a 

site for a deep monitor well in the Kualapuu area 

include (1) possible sites of future ground-water development, (2) ground-water level, (3) ground-surface altitude, (4) land classification, ownership, and accessibility, (5) geology, (6) locations of existing production 

wells, and (7) historical ground-water quality information.

A deep monitor well near an area of possible future 

ground-water development would provide useful information related to the availability of freshwater in the 

area. A numerical ground-water flow model (Oki, 1997) 

that was developed to simulate steady-state regional 

ground-water flow on Molokai was used in this study to 

determine areas of possible future ground-water development in the Kualapuu area of Molokai.

The numerical model was used to estimate groundwater levels that result from pumping at rates equal to 

and in excess of the March 1999 allocated rates from 

existing and hypothetical wells in the Kualapuu area. 

Withdrawal rates of 0.5 and 1.0 Mgal/d in excess of the 

March 1999 total allocation rate were simulated at 

selected individual sites in the Kualapuu area, outside of 

the modeled dike complex.

Model results indicate that the southeastern part of 

the Kualapuu area is a possible area of future groundwater development because (1) withdrawals from this 

area have a small effect on water levels at existing wells 

in the Kualapuu area (relative to effects from withdrawals in other parts of the Kualapuu area that are outside 

of the dike complex), and (2) model-calculated water 

levels in this part of the Kualapuu area are high relative 

to water levels in other parts of the Kualapuu area that 

are outside of the dike complex. Although it may be 

possible to successfully withdraw additional ground 

water from other parts of the Kualapuu area, model 

results indicate that the effects on existing wells will be 

greater, water levels at the site of the additional pumping will be lower, or both.

Measured water levels in existing production wells 

in the Kualapuu area are between about 8 and 12 ft 

above sea level. Thus, a deep monitor well drilled in an 

area with water levels of about 8 to 12 ft above sea level 

may provide hydrologic information that is relevant to 

the existing production wells.

Drilling at ground-surface altitudes significantly 

greater than 1,000 ft will result in increased costs, 

whereas drilling at ground-surface altitudes significantly less than 1,000 ft will likely be in an area with 

water levels that are not representative of those measured at exiting production wells. Ground-surface altitudes are about 1,000 ft at the sites of existing 

production wells in the Kualapuu area. Because modelcalculated water-level contours (Oki, 1997) generally 

are aligned with the topographic contours, a deep monitor well drilled at an altitude of about 1,000 to 1,100 ft 

in the Kualapuu area may provide hydrologic information that is relevant to the existing production wells.

Access to a deep monitor well located on privately 

owned property may be lost if the property is sold or the 

landowner no longer desires the well on the property. 

Locating a well on government-owned land, particularly DHHL or DWS land, would probably minimize 

the possibility of losing access to the well in the future

Summary and Conclusions 47
because (1) land ownership is less likely to change, and 

(2) both DHHL and DWS have expressed an interest in 

drilling a deep monitor well in the Kualapuu area.

Existing production wells in the Kualapuu area 

were drilled in the East Molokai Volcanics and are 

located outside of the dike complex. A deep monitor 

well will be most useful if it is drilled in a similar hydrogeologic setting as the existing production wells. 

Because hydrologic conditions in the immediate vicinity of volcanic vents may differ from conditions near the 

existing production wells, a deep monitor well will 

probably be most relevant if it is drilled as far from 

known volcanic vents as possible.

Limited information on the vertical distribution of 

salinity in DHHL well 1 (0801-01) indicates that pumping from nearby wells can cause borehole flow, and 

thereby affect the salinity profile, in a deep monitor 

well. A salinity profile obtained from an ideal deep 

monitor well would be an exact representation of the 

vertical distribution of salinity in the aquifer. Because 

of borehole flow, however, the salinity profile obtained 

from a deep monitor well may not correspond to the 

salinity distribution in the aquifer. Although borehole 

flow in a deep monitor well can exist even in the 

absence of nearby pumping, information from DHHL 

well 1 indicates that pumped wells located within about 

1,000 ft of the monitor well can further enhance borehole flow. Pumped wells located farther than 1,000 ft 

from a deep monitor well can also cause borehole flow 

in the monitor well depending on pumping rate and the 

permeability of the aquifer. To reduce the possible 

effects of borehole flow, a deep monitor well in the 

Kualapuu area would ideally be located as far as possible from existing or proposed production wells. On the 

other hand, a deep monitor well that is in the vicinity of 

existing or proposed wells, and that is not affected by 

borehole flow induced by pumping from other wells, 

would provide the most relevant information.

Chloride concentrations of pumped water from 

well 0902-01 indicate that ground-water quality near 

the western part of the Kualapuu area may exceed 250 

mg/L. Thus, it is uncertain whether future drinkingwater wells will be drilled in the Kualapuu area west of 

well 0902-01. Locating a deep monitor well in the area 

west of well 0902-01 would reduce the possible effects 

of borehole flow caused by pumping from existing or 

proposed wells, but because it would be far from existing or future wells, it may not provide useful information for management of the water resources.

A viable area for drilling a deep monitor well is 

about a half mile southeast of existing wells 0801-01 to 

-03 and a half mile north of a known volcanic vent, Puu 

Luahine. In this area, existing geophysical information 

indicates that the saline water is at about the same depth 

as at wells 0801-01 to -03. Although there are unimproved roads leading to this area, existing topography 

may make access difficult for large vehicles. Thus, 

nearby sites may need to be considered that are within 

the zone that is southeast of existing Kualapuu production wells, between model-calculated water levels of 8 

and 12 ft above sea level, between altitudes of 1,000 and 

1,100 ft, on government-owned property, and east of 

well 0902-01.

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50 Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii

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