The Response of the Iao Aquifer to 

Ground-Water Development, Rainfall, and 

Land-Use Practices Between 1940 and 1998,

Island of Maui, Hawaii

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 00-4223
The Response of the Iao Aquifer to Ground-Water

Development, Rainfall, and Land-Use Practices

Between 1940 and 1998, Island of Maui, Hawaii

By William Meyer and Todd K. Presley

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 00-4223

Honolulu, Hawaii

2001
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: Copies of this report can be purchased

from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

677 Ala Moana Blvd., Suite 415 Box 25286

Honolulu, HI 96813 Denver, CO 80225-0286

U. S. DEPARTMENT OF THE INTERIO

R

MARCH 3, 1849
Contents iii

CONTENTS

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

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

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Well-Numbering System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Previous Geohydrologic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Description of the Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Physical Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Climate, Rainfall, and Fog Drip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Irrigation Ditches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Ground-Water Occurrence and Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Dike-Impounded System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Freshwater-Lens System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Perched System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Effect of Land-Use Change on Ground-Water Recharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Ground-Water Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Aquifer Response to Pumpage, Rainfall, and Land-Use Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Water-Level Changes Since the Introduction of Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Water-Level Changes Relative to Rainfall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Movement of the Freshwater/Saltwater Transition Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Changes in the Chloride Concentration of Pumped Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Sustainable Yield. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

FIGURES

1–3. Maps showing:

1. The Hawaiian islands, island of Maui, West Maui Mountain, and the Iao aquifer study area, Hawaii. . . . 2

2. Springs, dike-impounded water-development tunnels, and pumped wells in the Iao aquifer and

surrounding area, Maui, Hawaii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Selected wells in the Iao aquifer, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4. Graph showing annual mean pumpage in the Iao aquifer, and annual rainfall for Waiehu Camp rain

gage (station 484), Maui, Hawaii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5–7. Maps showing:

5. Mean annual rainfall in the Iao aquifer area, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Geology of West Maui Volcano, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

7. Geology of the Iao aquifer area, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8–10. Diagrams showing:

8. Geologic cross sections of the Iao aquifer area, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
iv The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

9. Geologic logs of wells and test holes in the Iao aquifer area, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . 17

10. Geologic cross section of the Iao aquifer area showing ground-water occurrence and movement,

Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

11. Map showing areal distribution of ground-water occurrence in the Iao aquifer area based on

predevelopment conditions, Maui, Hawaii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

12. Graphs showing water level above sea level as a function of altitude of bottom of hole during drilling for

test holes 15C, 15D, and 15E (wells 5330-06, 5330-07, and 5330-08), Maui, Hawaii . . . . . . . . . . . . . . . . . 22

13. Map showing agricultural land use for three different periods, Iao aquifer area, Maui, Hawaii . . . . . . . . . . . . 24

14–19. Graphs showing:

14. Annual mean pumpage for shaft 33 and Mokuhau well field, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . 27

15. Annual mean pumpage for Waiehu Heights well field, Waihee well field, and Kepaniwai well,

Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

16. Water levels for test holes T-102 and T-112, monthly mean pumpage for shaft 33, and monthly rainfall

for Waiehu Camp rain gage for 1940–70, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

17. Water levels and monthly pumpage for shaft 33, and monthly rainfall for the Waiehu Camp rain gage

for the years 1973–75 and 1996–98, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

18. Water levels and annual mean pumpage for the Mokuhau well field, and annual rainfall for Waiehu Camp

rain gage, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

19. Water levels for test holes in the northern part of the Iao aquifer, and pumpage for Iao aquifer and for

Waiehu Heights and Waihee well fields, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

20–22. Maps showing:

20. Mean water levels in the flank flows of Wailuku Basalt, April 1977, for the northern part of the Iao

aquifer, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

21. Water-level declines in the flank flows of Wailuku Basalt between April 1977 and April 1997 for the

northern part of the Iao aquifer, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

22. Water-level rise in the flank flows of Wailuku Basalt between the seasonal low of 1996 and the

seasonal low of 1998 for the Iao aquifer, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

23–24. Graphs showing:

23. Water levels for Waikapu wells 1 and 2, and North Waihee well 1, Maui, Hawaii. . . . . . . . . . . . . . . . . . . 41

24. Water levels and monthly mean pumpage for the Kepaniwai wells, and rainfall for Waiehu Camp

rain gage for 1980–98, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

25. Map showing mean water levels in the flank flows of Wailuku Basalt in 1998 for the Iao aquifer,

Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

26–34. Graphs showing:

26. Rainfall departure of backward-looking 12-month moving mean from long-term mean for

Waiehu Camp rain gage, water levels for test hole E, and monthly mean total pumpage for

the Iao aquifer, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

27. Water levels and rainfall before the start of ground-water withdrawal in the Iao aquifer, Maui, Hawaii . . 47

28. Chloride concentration with time for selected altitudes at the Waiehu deep monitor well

(5430-04, 05), Maui, Hawaii, 1985–98 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

29. Chloride concentration with depth for selected depths and dates in the Waiehu deep monitor well

(5430-04, 05), Maui, Hawaii, 1985–98 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

30. Chloride concentration, monthly mean pumpage, and annual mean pumpage for shaft 33, Maui, Hawaii 51

31. Chloride concentration, monthly mean pumpage, and annual mean pumpage for the Mokuhau well

field, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Contents v

32. Chloride concentration, monthly mean pumpage, and annual mean pumpage for the Waiehu Heights

well field, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

33. Chloride concentration, monthly mean pumpage, and annual mean pumpage for the Waihee well field,

Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

34. Chloride concentration, monthly mean pumpage, and annual mean pumpage for the Kepaniwai well,

Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

35. Diagram showing altitude of the top of transition zone for estimated thicknesses of the upper part

of the transition zone of 100 and 175 feet, relative to the bottoms of pumping wells in the Iao aquifer

area, Maui, Hawaii, using assumed stabilized 1996 or 1998 water levels . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

TABLES

1. Wells described in this report, Iao aquifer area, Maui, Hawaii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2. Springs in the dike complex of the Iao aquifer area, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3. Ground-water development tunnels in the Iao aquifer area, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4. Annual ground-water withdrawal from the Iao aquifer, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5. Mean water levels for April 1977, April 1997, and the year 1998; and seasonal low water levels for 1996

and 1998 for wells in the Iao aquifer area, Maui, Hawaii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6. Altitude of bottom of wells and estimated altitude of the top of the transition zone at pumped wells in the

freshwater lens part of the Iao aquifer, Maui, Hawaii, for assumed stabilized 1996 and 1998 water levels, and

for estimated thicknesses of the upper part of the transition zone of 100 ft (column a) and 175 ft (column b). . . 57

Abbreviations used in water-quality descriptions:

mg/L, milligrams per liter

μS/cm, microsiemen per centimeter at 25° Celsius

Conversion Factors

Multiply By To obtain

foot (ft) 0.3048 meter

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

mile (mi) 1.609 kilometer

square mile (mi2

) 2.590 square kilometer

inch (in.) 25.4 millimeter

inch per year (in/yr) 2.54 centimeter per year
Abstract 1

Abstract

Ground water pumped from the Iao aquifer has

been used for agricultural purposes since 1948, and

domestic purposes since 1955. In 1990, the Hawaii

State Commission on Water Resource Management established a value of 20 million gallons per

day for the sustainable yield of the aquifer. Waterlevel data from observation wells throughout the

aquifer and information on the depth to and thickness of the transition zone between freshwater and

saltwater at the Waiehu deep monitor well indicate

that pumping rates near the sustainable yield value

of 20 million gallons per day could result in saltwater intrusion in some pumped wells.

Since the introduction of pumpage in 1948 and

the reduction of recharge in 1980, water levels have

declined, chloride concentrations of the pumped

water have increased, and the transition zone

between freshwater and saltwater has risen. Water

levels declined by about 18 feet between 1940 and

1998 in the area near Iao Stream, and by as much

as 6 feet between 1977 and 1997 in the vicinity of

the major well fields near Waiehu Stream. Chloride

concentrations of pumped water have risen at all

the well fields, but are presently below the U.S.

Environmental Protection Agency recommended

standard of 250 milligrams per liter. The chloride

concentration of water pumped from Mokuhau 2,

however, was 460 milligrams per liter in late 1996

when pumping was halted at this well. The midpoint of the transition zone, as measured at the

Waiehu deep monitor well, rose by about 108 feet

between 1985 and 1998.

INTRODUCTION

The Iao aquifer, on the eastern side of West

Maui Mountain (fig. 1), is the principal source of

domestic water supply for the island of Maui. Groundwater withdrawal from the aquifer provides water to the

Wailuku area (fig. 1) and the populated central isthmus

of the island. In 1998, about 76 percent of the ground

water supplied by the County of Maui Department of

Water Supply (DWS) to the island was from the Iao

aquifer (unpub. data from Maui Department of Water

Supply).

Plantation agriculture has been the dominant

land use overlying the coastal part of the Iao aquifer

since the formation of the Wailuku Sugar Company

(Shade, 1997), which was first organized in 1862 (Wilcox, 1996). Large-scale furrow irrigation of sugarcane

began after the completion of the Waihee Ditch in 1907.

The ditch derives its water from the Waihee River and

from higher elevation springs and tunnels along the river. Irrigation increased ground-water recharge above

the natural rate, although changes in irrigation practices

and land use over time have resulted in changes in the

rate of recharge.

Significant ground-water withdrawal from the

aquifer for irrigation began in 1948 to supplement the

water diverted by the ditch systems. Withdrawal for

domestic use began in 1955. Since 1985, water has been

pumped only for domestic use, and is withdrawn from

five locations (shaft 33, Kepaniwai well, Mokuhau well

field, Waiehu Heights well field, and Waihee well field)

(figs. 2 and 3, table 1). The annual average rate of

ground-water withdrawal from the aquifer has ranged

widely, with maximum pumpage in 1995 and 1996 (fig.

4, see table 4).

The Response of the Iao Aquifer to Ground-Water

Development, Rainfall, and Land-Use Practices Between

1940 and 1998, Island of Maui, Hawaii

By William Meyer and Todd K. Presley
2 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

West Mau i Mountain Crest

Wailuku

Waikapu

Maalaea

Olowalu

Kahului

Bay

Lahaina

Kaanapali

Honokowai

Kahana

Napili

Waihee

Waiehu

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Maalaea Bay

PACIFIC

OCEAN

Puu Kukui

Honokowai Str

Kahoma

Str

Kanaha

Str

Kauaula

Str

Launiupoko

Str

Olowalu Str

Ukumehame Str

Poohahoahoa Str

Nakalaloa Str

Waikapu Str

Waiehu Str

Iao Str

Iao Str

Waihee

River

South Waiehu Str

North Waiehu Str

Honokowai

Tunnel

Honokohau Ditch

Mahinahina Gl

Kalepa

Gulch

Everett Ditch

Mania n i a Ditch Waihee Ditch

Spreckels Ditch

156°40'

21°

20°50'

156°30'

0

0

1 2 3 MILES

1 2 3 KILOMETERS

22°

20°

160° 158° 156°

PACIFIC OCEAN

Kauai

Niihau

Oahu

Maui

Molokai

Lanai

Kahoolawe

Hawaii

HAWAII

East Maui

Volcano

West Maui

Volcano

Area

of map

Pacific Ocean

Study

area

Kahului

Bay

ISLAND

OF MAUI

EXPLANATION

STUDY-AREA (IAO AQUIFER)

 BOUNDARY

DITCH OR TUNNEL

STREAM

INTERMITTENT STREAM

Figure 1. The Hawaiian islands, island of Maui, West Maui Mountain, and the Iao aquifer study area, Hawaii.
Introduction 3

West Mau i Mountain

Crest

P A C I F I C

O C E A N

Iao-Waikapu Ditch

Waihee

Waihee

River

Spreckels

Ditch

Ditch

Maniania Ditch

North Waiehu

Stream

South Waiehu Stream

Waiehu Stream

Iao

Stream

Kama Ditch

Everett Ditch South Side Waikapu Ditch

Waikapu Stream

North

Waihee

Waihee Waiehu

Heights

Mokuhau

Kepaniwai

Shaft 33

Shaft 33

4

16

15

14

13 12

11 10

5

7

11 12

12

9

10

8

6

1

2

3

484

484

16

Waihee

Waiehu

Wailuku

Waikapu

0

0

1 2 MILES

1 2 KILOMETERS

EXPLANATION

INFERRED BOUNDARY BETWEEN DIKE-IMPOUNDED 

 GROUND WATER AND FRESHWATER-LENS SYSTEM

 (modified from Yamanaga and Huxel, 1970)

STUDY-AREA (IAO AQUIFER) BOUNDARY 

DITCH OR TUNNEL

RAIN GAGE AND STATE RAIN GAGE NUMBER

PUMPED WELL OR WELL FIELD AND NAME

SPRING AND NUMBER (Stearns and Macdonald, 1942)

WATER-DEVELOPMENT TUNNEL AND NUMBER 

 (Stearns and Macdonald, 1942)

Kahului

Bay

FRESHWATER-LENS

SYSTEM

DIKE IMPOUNDED

GROUND WATER

DIKE-IMPOUNDED FRESHWATER-LENS

SYSTEM

156°34'

21°56'

21°54'

21°52'

156°32' 156°30'

MAUI

Area

of map

Pacific Ocean Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Figure 2. Springs, dike-impounded water-development tunnels, and pumped wells in the Iao aquifer and

surrounding area, Maui, Hawaii. (Spring and tunnel locations from Stearns and Macdonald, 1942; rain gage

locations from State of Hawaii, 1973.)
4 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

P A C I F I C

O C E A N

Waiehu Stream

Iao

Stream

0

0

1 MILE

1 KILOMETER DIKEIMPOUNDED

GROUND

WATER

FRESHWATER-LENS

SYSTEM

INFERRED BOUNDARY 

 BETWEEN DIKE-IMPOUNDED 

 GROUND WATER AND 

 FRESHWATER-LENS SYSTEM

 (modified from Yamanaga and 

 Huxel, 1970)

STUDY-AREA (IAO AQUIFER) 

 BOUNDARY

PUMPED WELL OR 

 WELL FIELD AND NAME

TEST HOLE OR WATER-LEVEL 

 OBSERVATION WELL AND

 NAME

EXPLANATION

20°56'

20°54'

20°52'

156°32' 156°30'

MAUI

Area

of map

Pacific Ocean

A1

T-103

D and

Waiehu

deep

monitor

well

B

E

15D 15E 15C

T-112

Waikapu 2

Waikapu 1

T-102

T-113

T-102

North

Waihee

Waihee

Waiehu

Heights

Mokuhau Kepaniwai

Shaft 33

Waihee River

Waikapu Stream

Mokuhau

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Figure 3. Selected wells in the Iao aquifer, Maui, Hawaii.
Introduction 5

a

 Portal to shaft has an altitude of 401 ft.

Table 1. Wells described in this report, Iao aquifer area, Maui, Hawaii

[--, none]

State well

number Well name Alternatewell name Use Altitude of topof well, in feet

Altitude of

bottom of

well,

in feet

Year

drilled

5130-01 Waikapu 1 -- Water-level observation 551 -206 1961

5130-02 Waikapu 2 -- Water-level observation 518 -502 1974

5330-03 Test hole T-112 Field 63 Water-level observation 457 -20 1945

5330-04 Test hole T-113 Wailuku mill TH Test hole 180 -524 1945

5330-05 Shaft 33 Wailuku shaft Production, water-level

observation

a

32 -280 1946

5330-06 Mokuhau test hole 15C EX1 Test hole 310 -121 1950

5330-07 Mokuhau test hole 15DEX2 Test hole, water-level

observation

484 -101 1951

5330-08 Mokuhau test hole 15E EX3 Test hole 364 -102 1953

5330-09 Mokuhau pump 2 15A Production, water-level

observation

353 -247 1953

5330-10 Mokuhau pump 1 15B Production 353 -247 1953

5330-11 Mokuhau pump 3 15F Production 354 -251 1967

5330-12 Test hole C -- Test hole 398 -212 1975

5331-01 Test hole T-102 -- Water-level observation 454 -21 1940

5332-04 Kepaniwai observation well Kepaniwai test

hole

Water-level observation 713 459 1973

5332-05 Kepaniwai well -- Production 713 413 1974

5430-01 Waiehu Heights 1 -- Production 337 -338 1975

5430-02 Waiehu Heights 2 -- Production 337 -206 1975

5430-03 Test hole E -- Water-level observation 415 -165 1976

5430-04 Test hole D-- Water-level observation 382 -108 1975

5430-05 Waiehu deep monitor well -- Water-level observation,

transition-zone monitoring

380 -1,020 1982

5431-01 Test hole B -- Water-level observation 494 -61 1974

5431-02 Waihee 1 -- Production 498 -182 1976

5431-03 Waihee 2 -- Production 493 -150 1976

5431-04 Waihee 3 -- Production 493 -157 1981

5530-02 Test hole T-103 -- Test hole 80 -97 1933

5631-01 Test hole A1 -- Water-level observation 248 -52 1974

5631-02 North Waihee 1 -- Production, water-level

observation

281 -106 1981

5631-03 North Waihee 2 -- Production 281 -106 1981

Since the introduction of pumping, groundwater levels have declined significantly and the chloride

concentration of pumped water has risen above predevelopment levels at all of the well fields. In addition, the

transition zone between freshwater and saltwater,

which has been monitored at the Waiehu deep monitor

well (fig. 3) since 1985, has been moving upward during the period of record.

In 1990, the Hawaii State Commission on

Water Resource Management (CWRM) established a

value of 20.0 Mgal/d for the sustainable yield of the Iao

aquifer (Commission on Water Resource Management,

1990). This pumpage value was surpassed by 0.50 and

0.35 Mgal/d in 1995 and 1996, respectively.

The combination of high pumpage, decline in

water levels, and the rise in the transition zone has

caused concern by CWRM regarding the long-term

availability of ground water from existing well fields. In

1996, CWRM required that annual mean pumpage be

reduced below 20 Mgal/d. In response to this requirement, the DWS reduced pumpage by about 1 Mgal/d in

1997 and 1998.
6 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

0

24

2

4

6

8

10

12

14

16

18

20

22

ANNUAL MEAN PUMPAGE, IN MILLION GALLONS PER DAY

YEAR

AGRICULTURE (SHAFT 33 PRIOR TO 1985)

DOMESTIC

TOTAL PUMPAGE

194519501955196019651970197519801985199019952000

0

80

20

40

60

ANNUAL RAINFALL,

 IN INCHES

Waiehu Camp rain gage (station 484)

Mean annual rainfall

Figure 4. Annual mean pumpage in the Iao aquifer, and annual rainfall for Waiehu Camp rain gage,

Maui, Hawaii. (Pumpage data from Maui Department of Water Supply and unpublished data from

Wailuku Sugar Company in U.S. Geological Survey well files, Honolulu; unpublished rainfall data from

Commission on Water Resource Management and the University of Hawaii Meteorology Department.)
Introduction 7

Purpose and Scope

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

hydrogeologic framework of the Iao aquifer, (2) discuss

the response of the freshwater lens in the Iao aquifer to

changes in pumpage, rainfall, and land-use practices in

terms of water-level and chloride changes, and (3) discuss the sustainability of withdrawals from the aquifer

on the basis of historical data.

This report uses water-level, chloride-concentration, rainfall, and pumpage data collected by the

USGS, the National Oceanic and Atmospheric Administration (NOAA), and the Maui Department of Water

Supply (DWS), and unpublished data available in

USGS well files. Ground-water data are available from

14 pumping wells and at least 14 observation wells in

the Iao aquifer area. Nearly 30 years of pumpage, rainfall, and water-level data have been collected since the

last study of the Iao aquifer (Yamanaga and Huxel,

1970).

Well-Numbering System

Wells mentioned in this report are referred to

by the local names used by the DWS and noted in USGS

well folders (on file at U.S. Geological Survey, Honolulu). Wells also 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;

cc is the minutes of longitude of the

southeastern corner of the one-minute grid;

dd is the sequential well number within the

one-minute grid.

An island code of “6” is used for all wells on

Maui, however, this number is omitted in this report.

The location of all the wells mentioned in this report are

shown in figure 3, and the corresponding local well

names, numbers, and well information are listed in

table 1.

Previous Geohydrologic Studies

The first comprehensive description of the

geology and hydrology of the island of Maui was written by Stearns and Macdonald (1942). This description

chronicled the initial development of the ground-water

and surface-water resources on Maui, and includes

early hydrologic data.

Yamanaga and Huxel (1970) described the general geohydrologic setting of the Wailuku area, including the Iao aquifer area, and included descriptions of

rainfall, runoff characteristics of streams, ground-water

occurrence, and the location and construction history of

wells. Yamanaga and Huxel (1970) also provided a limited account of the effect of ground-water development

on the aquifer in terms of changes in water levels, and

chloride concentration of pumped water.

The State of Hawaii Water Resources Protection Plan (Commission on Water Resource Management, 1990) contains descriptions of the aquifer areas of

the State of Hawaii and estimates of sustainable yields

for the aquifer areas.

Description of the Study Area

Physical Setting

The island of Maui is formed by the East Maui

Volcano and the West Maui Volcano (fig. 1). The Iao

aquifer lies on the flank of the West Maui Volcano and

covers about 24.7 mi2

. The boundaries of the Iao aquifer, as defined by the Commission on Water Resource

Management (1990) and Mink and Lau (1990), are: the

ridge south of Waihee River and north of Kalepa Gulch

extending from the coast to the summit of West Maui

Mountain; the crest of the West Maui Mountain; the

ridge north of Waikapu Stream extending from the crest

to the isthmus; and the southern divide of Iao Stream to

Kahului Bay (figs. 1 and 2). The area is characterized by

a steep and mountainous region to the west, an area of

sloping alluvial and colluvial plains extending east from

the mountains, and an area of lithified sand dunes and

coastal plains near the ocean.

The mountainous areas have steep ridges and

deep valleys, and range in altitude from about 300 ft at

the base to 5,788 ft at the summit (Puu Kukui). The

mountainous areas are typically rain forests and are
8 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

designated as Hawaii State conservation lands. The

alluvial and colluvial plains range in altitude from about

100 ft near the coast to more than 600 ft near Waikapu

Stream. In the past, sugarcane was grown on the alluvial

plains, but presently the land is used for macadamia

nuts, pineapple, papaya, or left fallow. The urban areas

of Waihee, Wailuku, and Waikapu also are on the alluvial plains. Lithified sand dunes are found on the flat

coastal plains and extend from the outlet of Iao Stream

to Waikapu. The sand dunes are generally less than 100

ft high.

Two major streams (Iao and Waiehu) drain the

Iao aquifer area. Waikapu Stream is situated immediately south of the aquifer. Waihee River flows immediately north of the aquifer and forms the northern

boundary of the aquifer near the shoreline (fig. 2). The

streams and the river emanate radially from the center

summit region of the volcano, forming deep valleys that

expose thousands of feet of lava flows.

Climate, Rainfall, and Fog Drip

The climate of Hawaii is characterized by mild

and fairly uniform temperatures, seasonal variation in

rainfall, and great geographic variation in rainfall (Blumenstock and Price, 1967). The cooler rainy season

lasts from October through April, and the hotter dry season is between May and September. Average temperature in Wailuku, near the coast, is 75 degrees, and

tradewinds blow 50 to 80 percent of the time in the winter and 80 to 95 percent of the time in the summer (Blumenstock and Price, 1967).

The Iao aquifer area has large range in rainfall

between Puu Kukui, and the shoreline (fig. 5) (Giambelluca and others, 1986). The mean annual rainfall at

Puu Kukui (greater than 355 in.) is the second highest

recorded in the State. Mean annual rainfall declines rapidly toward the ocean and is 30 in. or less at the shoreline. Precipitation in the area is actually a combination

of rainfall over all elevations and fog drip at higher elevations where the montane forest canopy intercepts

cloud water. Fog drip has not been measured on West

Maui Mountain, but studies of other high mountains in

Hawaii have shown that fog drip can significantly augment rainfall at elevations above 2,000 ft (Juvik and

Ekern, 1978; Juvik and Nullet, 1995).

Irrigation Ditches

Two major ditches and nine smaller ditches

were built in the early 1900’s in the Iao aquifer area

(Wilcox, 1996); however, only six of the ditches are still

in use (figs. 1 and 2). The ditches divert springs, tunnel

water, and streams. The Spreckels Ditch and the

Waihee Ditch, the two major ditches in the area, divert

and capture an average of about 40 Mgal/d from Waihee

River and from tunnels driven into the valley walls

above the stream, and about 6 Mgal/d from diversions

on the two forks of Waiehu Stream (Yamanaga and

Huxel, 1970). Two smaller ditches, the Maniania Ditch

and the Iao-Waikapu Ditch, carry about 18 Mgal/d from

diversions in the Iao Stream and from tunnels driven in

Iao Stream valley. A third small ditch, the Kama Ditch,

diverts water from Iao Stream at a lower elevation than

the shared diversion of the Maniania and Iao-Waikapu

Ditches. In Waikapu Stream valley, the South Side

Waikapu Ditch carries about 3 Mgal/d from a diversion

in the Waikapu Stream and from tunnels. A second

ditch in Waikapu Valley, the Everett Ditch, is no longer

in use because of blockage from rock slides.

GEOLOGY

West Maui Volcano is a deeply eroded volcanic

dome (fig. 6). The volcano is composed of three volcanic units: the shield-stage Wailuku Basalt; the postshield-stage Honolua Volcanics, and the rejuvenatedstage Lahaina Volcanics (Stearns and Macdonald,

1942; and Langenheim and Clague, 1987).

The structure of the West Maui Volcano consists of a central caldera, two main rift zones (trending

northwest and southeast), and the flanks (Stearns and

Macdonald, 1942). The assemblage of rock within the

caldera, known as the caldera complex, is composed of

dikes, vent breccias, talus, crater-filling flows, pyroclastic material, and faulted rock.

Emanating from the caldera are thousands of

dikes composed of Wailuku Basalt and Honolua Volcanics. The number of dikes generally decreases with distance from the caldera, and increases with depth. Most

of the dikes intruded along the main rift zones (fig. 6).

Other dikes intruded outside of the general trends of the

rift zones, creating a radial pattern of dike emplacement

emanating from the caldera (Macdonald and others,
Geology 9

P A C I F I C

O C E A N

Iao-W aikapu Ditch

Waihee

Ditch

Maniania Ditch

North Waiehu Stream

South

Waiehu Stream

Waiehu Stream

Iao

Stream

Waihee

Waiehu

Wailuku

Waikapu

355

355

315

275

235

195

155

120

100

80

60

40

30

20

0

0

1 2 MILES

1 2 KILOMETERS

EXPLANATION

MEAN ANNUAL RAINFALL--Interval, in inches, is variable

INFERRED BOUNDARY BETWEEN DIKE-IMPOUNDED GROUND WATER AND 

 FRESHWATER-LENS SYSTEM (modified from Yamanaga and Huxel, 1970)

STUDY-AREA (IAO AQUIFER) BOUNDARY

DITCH

Puu Kukui

Kahului

Bay

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

156°34'

21°56'

21°54'

21°52'

156°32' 156°30'

Figure 5. Mean annual rainfall in the Iao aquifer area, Maui, Hawaii (modified from Giambelluca and others, 1986).
10 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

156°40'

21°

20°50'

156°30'

0

0

1 2 3 MILES

1 2 3 KILOMETERS

Wailuku

Puu

Kukui

Maalaea

Olowalu

Lahaina

Kaanapali Waihee

Waiehu

Kahului

Bay

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Figure 6. Geology of West Maui Volcano, Hawaii (modified from Stearns and Macdonald, 1942, and Langenheim and

Clague, 1987).
Geology 11

EXPLANATION

SEDIMENTARY DEPOSITS

Unconsolidated deposits, chiefly younger alluvium 

careous sand dunes

solidated earthy deposits, chiefly older alluvium

VOLCANIC ROCKS

Lahaina Volcanics

olua Volcanics

a Volcanics

Wailuku Basalt

CONES, VENTS, DOMES, CRATERS, BOSSES, AND DIKES

Cone or vent of Lahaina Volcanics

Cones or domes of Honolua Volcanics

es or vents of Wailuku Basalt

Pit craters

Bosses of Wailuku Basalt

Dikes of either Wailuku Basalt or Honolua Volcanics

Dikes, where too numerous to show individually

Axis of main rift zones (from Macdonald and others, 1983)

 Caldera boundary, dotted where concealed, dashed where inferred
12 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

1983). The aggregate of dikes and the rocks they intrude

is known as a dike complex (Stearns and Vaksvik,

1935). The dike complex is exposed in the upper parts

of the valleys of West Maui, such as Ukumehame,

Waikapu, and Iao Stream valleys, and Waihee River

valley.

The flanks of the volcano are predominantly

composed of Wailuku Basalt, which is exposed over

most of the volcano’s surface (Stearns and Macdonald,

1942). Thousands of flows of the tholeiitic Wailuku

Basalt emanated from vents in the caldera and along the

rift zones. The thin-bedded flows of Wailuku Basalt

consist of both aa and pahoehoe (Stearns and Macdonald, 1942). Thin, localized ash deposits and pyroclastic layers lie within the upper part of the sequence of

flows.

In a relatively small part of the study area, north

of Waiehu Stream valley and south of Waihee River,

the Wailuku Basalt is overlain by lava flows of the

Honolua Volcanics (fig. 7 and fig. 8, section A-A′). The

Honolua Volcanics forms only a thin veneer of dense,

massive flows of lighter colored rock. The Honolua

Volcanics is not considered important to the hydrology

of the Iao aquifer mainly because of its geographic distribution.

Sedimentary deposits overlie the volcanic

rocks on the southeastern and southwestern sides of

West Maui Volcano, and the entire coast of the Iao aquifer area (fig. 7). Stearns and Macdonald (1942) divided

the sedimentary deposits into consolidated earthy

deposits, calcareous sand dunes, and unconsolidated

deposits. The consolidated earthy deposits are primarily

older alluvium which forms the bulk of the alluvial

plains stretching from the mountains to the coast (fig.

8). The consolidated earthy deposits are composed primarily of poorly sorted conglomerates. Within these

deposits, a thin layer of Lahaina Volcanics was found

while drilling one of the test holes in the area (test hole

E), but this lava flow is not exposed on the surface. The

older alluvium has since been eroded by Waikapu, Iao,

and Waiehu Streams, and Waihee River. Calcareous

sand dunes are found seaward of the exposed consolidated earthy deposits. Unconsolidated deposits are

found in streambeds and in the coastal areas, and are

primarily composed of younger, poorly sorted alluvium.

In the Iao aquifer area, sedimentary deposits

generally increase in thickness and width in a north-tosouth direction, creating an increasingly wider coastal

plain (fig. 8, sections A-A′, B-B′, and C-C′). To the

north, the coastal plain does not extend past Waihee

River valley.

Within the valleys in the Iao aquifer area, zones

of weathered basalt probably extend downward beneath

the stream floor alluvium. On Oahu, wells drilled in the

valley floors indicate that zones of weathered basalt,

tens to hundreds of feet thick, lie beneath stream valleys

below the contact between the basalt and the alluvium

(R.M. Towill Corporation, 1978; and Hunt, 1996).

Lavas from East Maui Volcano are not exposed

in the Iao aquifer, but have been found in wells drilled

closer to the coast and East Maui Volcano. Data from

drilling logs (unpub. data from well files at U.S. Geological Survey, Honolulu) were used to locate the upper

contact of the lavas of East Maui Volcano and the sedimentary deposits (fig. 8, section C-C′). These lavas

likely interfinger with sedimentary deposits derived

from West Maui Volcano.

Geologic setting of wells and test holes.--The

geologic logs (unpub. data from well files at U.S.

Geological Survey, Honolulu) for most of the wells in

the Iao aquifer area are summarized in figure 9. Because

the location of the contact between the sedimentary

deposits and the underlying basalt was of primary

importance for the construction of pumping wells, the

general stratigraphy of most of the test holes and pumping wells was logged as the holes were constructed. The

first occurrence of ground water during drilling was

also documented for some of the wells.

The geologic logs of the wells drilled in the Iao

aquifer area show that the thickness of the sedimentary

deposits, and the depth of the contact between the sedimentary deposits and the volcanic rocks, is variable

throughout. The greater depths and thicknesses are

assumed to be relative to where stream valleys are buried. For wells north of Iao Stream and south of Waihee

River (fig. 3), the thicknesses of the sedimentary deposits range from 31 ft at test hole B to 491 ft at test hole E,

and the altitude of the contact between the sedimentary

deposits and the basalt ranges from 463 ft at test hole B

to -76 ft at test hole E, which is about 0.5 mi away. Test

hole E and the wells and test holes 15Dand 15E of the

Mokuhau well field, are the only wells in the Iao aquifer

that penetrate the Wailuku Basalt near or below sea

level.
Geology 13

0

0

1 2 MILES

1 2 KILOMETERS

P A C I F I C

O C E A N

Wailuku

Waikapu

Waihee

Puu

Kukui

Waihee

Waiehu

Waikapu

EXPLANATION

SEDIMENTARY DEPOSITS

 Unconsolidated deposits, 

 chiefly younger alluvium 

 Calcareous sand dunes

 Consolidated earthy deposits, 

 chiefly older alluvium

VOLCANIC ROCKS

 Honolua Volcanics

 Wailuku Basalt

LINE OF SECTION

CONES, VENTS, DOMES, CRATERS, BOSSES, AND DIKES

 Cones or domes of Honolua Volcanics

 Pit craters

 Bosses of Wailuku Basalt

 Dikes of either Wailuku Basalt or Honolua Volcanics

 Dikes, where too numerous to show individually

 Axis of rift zone (from Macdonald and others, 1983)

 Caldera boundary, dotted where concealed, dashed where inferred

INFERRED BOUNDARY BETWEEN DIKE-IMPOUNDED 

 GROUND WATER AND FRESHWATER-LENS SYSTEM 

 (modified from Yamanaga and Huxel, 1970)

STUDY-AREA BOUNDARY

Waiehu

Heights

well 1

Test

hole

A1

A

B'

B

A'

C C'

C C'

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

156°34'

21°56'

21°54'

21°52'

156°32' 156°30'

Figure 7. Geology of the Iao aquifer area, Maui, Hawaii (modified from Stearns and Macdonald, 1942, and

Langenheim and Clague, 1987).
14 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii A A' FEET 2,000 1,000

0

-1,000-2,000

Test hole A1

Ocean

00

1/2 1 MILE

0.5 1 KILOMETER

2x VERTICAL EXAGGERATION

EXPL

SEDIM

Uncochief

ca

s

ef

VOLCA Hono

East 

ilu

DIKES

BEND IN SECTION

sea level

Figure 8. Geologic cross sections of the Iao aquifer area, Maui, Hawaii (geology modified from Stearns and Macdonand Langenheim and Clague, 1987).
Geology 15

B 

FEET

4,0003,0002,0001,000

0

-1,000-2,000-3,000

Waiehu Heights 1

00

1/2 

0.5 1 

2x VERTICAL EXAG

BEND IN SECTION

BEND IN SECTION

BEND IN SECTION

sea level

Figure 8. Geologic cross sections of the Iao aquifer area, Maui, Hawaii (geology modified from Stearns and Macdonand Langenheim and Clague, 1987)--Continued.
16 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

C

FEET

4,0003,0002,0001,000

0

-1,000-2,000-3,000

00

1/2 

0.5 1 KILOM

2x VERTICAL EXAGGERAT

BEND IN SECTION

BEND IN SECTION

sea level

Figure 8. Geologic cross sections of the Iao aquifer area, Maui, Hawaii (geology modified from Stearns and Macdonaand Langenheim and Clague, 1987)--Continued.
Geology 17

-1,200

600

FEET

EAST MAUI LAVA FLOWSWAILUKU BASALTHONOLUA VOLCANICSLAHAINA VOLCANICSSEDIMENTARY DEPOSITS

SEA LEVEL

-1,000

-800-600-400-200

200

400

?

North Waihee 1 (5631-02)

Test hole A1 (5631-01)

T-103 (5530-02)

Waiehu deep monitor

well (5430-05)

Test hole D (5430-04)

Waihee 3 (5431-04)

Test hole E (5430-03)

Mokuhau pump 2 (5330-09)

T-102 (5331-01)

T-112 (5330-03)

Shaft 33 (5330-05)

Waikapu 2 (5631-03)

Test hole B (5431-01)

Waiehu Heights 1 (5430-01)

T-113 (5330-04)

Waikapu 1 (5631-02)

Test hole 15D (5330-07)

Test hole 15C (5330-06)

Test hole 15E (5330-08)

Figure 9. Geologic logs of wells and test holes in the Iao aquifer area, Maui, Hawaii. (Unpublished data from U.S. Geological Survey well

files, Honolulu.)
18 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

South of Iao Stream, wells penetrate basalt

above sea level. These wells have thicknesses of the

sedimentary deposits of 150 ft at test hole T-102 to 332

ft at shaft 33, and penetrate the basalt at altitudes ranging from 68 ft at shaft 33 to 383 ft at Waikapu 1.

Three test holes, T-103, T-113, and 15C, never

penetrate into the Wailuku Basalt. Test hole T-103,

located about 0.5 mi from the coast at an altitude of 80

ft, penetrated 177 ft of sediment to an altitude of -97 ft.

Test hole T-113, located about 1.3 mi from the coast at

an altitude of 181 ft, penetrated sediment to -16 ft, lava

flows between the altitudes of about -16 and -191 ft, and

more sedimentary deposits beneath the flows from -191

to -524 ft (unpub. data from well files at U.S. Geological

Survey, Honolulu). It is assumed that the lava flows

penetrated by this well are from East Maui Volcano, but

this cannot be verified. Test hole 15C, located about 2.1

mi from the coast at an altitude of 310 ft, penetrated 431

ft of sediment to an altitude of -121 ft. Drilling was

aborted at this site prior to reaching the basalt. However, two other test holes (15Dand 15E) were drilled a few

hundred feet farther inland, and reached basalt near sea

level (fig. 9).

HYDROLOGY

Ground-Water Occurrence and Movement

The fresh ground-water system in the Iao aquifer contains: (1) dike-impounded water, (2) a freshwater

lens floating on saltwater, and (3) perched water (fig.

10). The dike-impounded water body is found in the

mountainous interior part of the aquifer. A freshwaterlens system is found within the dike-free volcanic rocks

and also in the coastal sedimentary deposits. Perched

water in the sedimentary deposits overlying volcanic

rocks is vertically separated from the freshwater lens by

a zone of unsaturated rock. Within the sedimentary

deposits, however, perched water may be continuous

with and upgradient from the part of the freshwater-lens

system in the sedimentary deposits. The general movement of fresh ground water in the Iao aquifer area is

from the dike-impounded water body into the freshwater-lens system and then to the ocean.

Dike-Impounded System

A dike-impounded system is found in the dike

complex of the West Maui Volcano where low-permeability dikes have intruded other rocks. The inferred

eastern boundary of the dike-impounded water body

(figs. 10 and 11), as drawn by Yamanaga and Huxel

(1970), is located at the seaward extent of dikes mapped

by Stearns and Macdonald (1942). Near-vertical dikes

tend to compartmentalize areas of permeable volcanic

rocks. The dike-impounded flow system includes a

freshwater body, and where it exists, underlying brackish water and saltwater. In the Iao aquifer area, dikes

impound freshwater to heights ranging from hundreds

of feet to over 2,000 ft. Water levels, as indicated by

tunnels and springs, range in altitude from 787 to 2,050

ft (see tables 2 and 3). Information is unavailable to

determine whether, or at what depth, saltwater exists

beneath the freshwater body in the dike complex of

West Maui Volcano.

Water enters the dike-impounded system mainly by direct infiltration of some part of rainfall and fog

drip and from seepage from streams where they overlie

the water table. Water discharges from the system as

springs, seepage to streams where they intersect the

water table, withdrawal from wells and tunnels, and

ground-water flow to the downgradient freshwater-lens

system. Where erosion has exposed dike compartments

in stream valleys and where the ground-water level is

higher than the stream, ground water can discharge

directly to the streams. Discharge from the dikeimpounded water body maintains perennial flow in

Waihee River and Iao, Waikapu, and North and South

Waiehu Streams.

Freshwater-Lens System

The freshwater-lens system includes a lensshaped freshwater body, an intermediate transition zone

of brackish water, and underlying saltwater. The freshwater lens floats on denser saltwater. The brackishwater transition zone is formed by mixing of seawardflowing freshwater with landward-flowing saltwater.

Meinzer (1930) defined water below the lowest

water table as basal ground water to distinguish it from

perched water. According to this broad definition,

ground water in freshwater-lens and dike-impounded

systems both can be considered basal ground water.

Descriptions of ground water in Hawaii have generally

limited the use of the term “basal” to freshwater-lens

systems with a water table near sea level in high-permeability rocks, although Meinzer's definition of basal

ground water was not so restrictive. In the Iao aquifer,
Hydrology 19

0 

4x VER

4,000

FEET

EAST 

3,5003,0002,5002,0001,5001,000

500

SEA

LEVEL

500

1,0001,500

DIKE COMPLEX

DIKE-IMPOUNDED GROUND WFRESHWATER LENS Unconfined ground water in W Confined ground water in Wa Unconfined ground water in sPERCHED GROUND WATER IN 

 DEPOSITS CONTINUOUS W

 LENSWAILUKU BASALTSEDIMENTARY DEPOSITSGENERALIZED DIRECTION OF  FLOWWATER TABLEWATER TABLE OF PERCHED G BODY

Oc

saltwater

freshwater

transition z ansition zone

transition zone

EXPLANATION

Dike-impounded

ground water

Unconfined fresh ground water in 

dike-free Wailuku Basalt

Confined fresh ground water in 

the Wailuku Basalt

Inferred boundary between dike-impounded 

ground water and freshwater lens

Boundary between unconfined and confined

ground water in the Wailuku Basalt

Approximate seaward extent of freshwater in

Wailuku Basalt for predevelopment conditions

3

2C

2B

2A

1

1

2A

2B

2C

3

Figure 10. Geologic cross section of the Iao aquifer area showing ground-water occurrence and movement, Maui, Hawaii
20 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

P A C I F I C

O C E A N

?

?

?

?

?

?

?

?

?

?

Waiehu Stream

Iao

Stream

0

0

1 MILE

1 KILOMETER

DIKEIMPOUNDED

GROUND

WATER

UNCONFINED

GROUND WATER IN

DIKE-FREE

WAILUKU BASALT

CONFINED FRESH GROUND 

 WATER IN THE WAILUKU 

 BASALT

APPROXIMATE SEAWARD 

 EXTENT OF FRESHWATER IN 

 WAILUKU BASALT FOR 

 PREDEVELOPMENT CONDITIONS

BOUNDARY BETWEEN 

 UNCONFINED AND CONFINED

 WATER IN THE WAILUKU

 BASALT. UNCONFINED 

 GROUND WATER IN 

 SEDIMENTARY DEPOSITS 

 SEAWARD OF BOUNDARY

INFERRED BOUNDARY 

 BETWEEN DIKE-IMPOUNDED 

 GROUND WATER AND 

 FRESHWATER-LENS SYSTEM

 (modified from Yamanaga and 

 Huxel, 1970)

STUDY-AREA BOUNDARY

PUMPED WELL OR 

 WELL FIELD AND NAME

TEST HOLE OR WATER-LEVEL 

 OBSERVATION WELL AND

 NAME

EXPLANATION

Kahului

Bay

20°56'

20°54'

20°52'

156°32' 156°30'

MAUI

Area

of map

Pacific Ocean

A1

T-103 D and

Waiehu deep monitor well

B

E

15D

T-112

Waikapu 2

Waikapu 1

T-102

T-102

North

Waihee

Waihee

Waiehu

Heights

Mokuhau

Shaft 33

Waihee River

Waikapu Stream

Mokuhau

Kepaniwai

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Figure 11. Areal distribution of ground-water occurrence in the Iao aquifer area based on predevelopment conditions, Maui,

Hawaii.
Hydrology 21

the freshwater-lens system is vertically extensive and

forms a continuous ground-water system within both

the dike-free volcanic rocks and the coastal sedimentary

deposits and, according to Meinzer's definition, may be

considered basal ground water.

The permeability of the dike-free volcanic

rocks, which extend to depths far below sea level, generally is high. Although the permeability of the sedimentary deposits that overlie the dike-free volcanic

rocks may be high in places, the overall permeability of

the sedimentary deposits is low relative to that of the

dike-free volcanic rocks. Thus, the sedimentary deposits form a confining unit above the dike-free volcanic

rocks within the coastal plain.

Where sedimentary deposits do not overlie the

dike-free volcanic rocks, and where sedimentary deposits overlie the dike-free volcanic rocks but are above the

water table in the volcanic rocks, ground water in the

volcanic rocks is unconfined (fig. 10). Seaward of

where the base of the sedimentary deposits intersects

the water table in the dike-free volcanic rocks, ground

water in the volcanic rocks is confined by the lowpermeability sedimentary deposits. Water levels in

wells completed in the dike-free volcanic rocks generally are less than a few tens of feet above sea level. Prior

to the end of 1998, water-level altitudes measured in

wells in the Iao aquifer area have ranged from a high of

37 ft at test hole T-102 in the early 1940's to a low of

9.21 ft at shaft 33 in 1996. In the dike-free volcanic

rocks, the freshwater lens is truncated by the base of the

sedimentary deposits before reaching the coast (figs. 10

and 11).

Within the sedimentary deposits, ground water

in the freshwater-lens system is unconfined from where

the sediments overlie the confined ground water in the

volcanic rocks to the coast (2C in fig. 10), and may be

saturated to altitudes as high as a few hundred feet.

Ground water in the sedimentary deposits was documented in driller's logs from test holes 15C, 15D, and

15E (5330-06 to -08), test hole E (5430-03), and T-103

(5530-02). In test holes 15E and T-103, water levels

ranged from 332 ft to 65 ft, respectively, where the

holes were entirely in the sedimentary deposits and the

bottoms of the holes were below sea level (unpub. well

logs on file at the U.S. Geological Survey, Honolulu).

 Water-level measurements were made in the

small diameter test holes 15C, 15D, and 15E as they

were being drilled. In wells 15D and 15E, water levels

dropped below 30 ft (fig. 12) once the holes penetrated

into the volcanic rocks, and the confined ground water

was reached. Since construction of the wells, waterlevel altitudes in the volcanic rocks in the area of these

test holes have ranged from 25.5 ft in 1954 to 9.25 ft in

1998.

Sedimentary deposits are generally heterogeneous, and ground-water occurrence in the sedimentary deposits may be more complex than indicated. The

presence of water in the test holes, and the measurements of the water levels as the wells were being drilled

(fig. 12), do not rule out the possibility that perched

water may exist above the freshwater-lens system in the

sedimentary deposits. Data from the small-diameter test

holes may not accurately reflect water-level changes in

the aquifer with depth.

Water enters the freshwater-lens system by

direct infiltration of precipitation and irrigation water,

seepage from irrigation ditches and streams where the

water table is below the streambed, and inflow from

upgradient ground-water bodies. Ground-water flow

from the dike-impounded water body recharges the

downgradient freshwater lens in the dike-free volcanic

rocks. Downward moving water from perched-water

bodies also recharges the freshwater-lens system. Discharge of freshwater from and inflow of saltwater to the

dike-free volcanic rocks is impeded by the overlying,

low-permeability sedimentary deposits. Discharge from

the dike-free volcanic rocks to the sedimentary deposits

may occur by flow parallel to the coast to areas where

the sedimentary deposits are either more permeable or

less extensive. Discharge from the freshwater lens in the

sedimentary deposits is by seepage to streams where the

ground-water level is higher than the stream, by subaerial and submarine coastal springs, and by diffuse

seepage near the coast.

Perched System

Perched water may occur where saturated, lowpermeability sedimentary deposits or low-permeability

volcanic rocks overlie unsaturated rocks. Perched water

in the sedimentary deposits was found in Waikapu 1

(5130-01), T-102 (5331-01), and shaft 33 (5330-05) at

altitudes of 386, 246, and 135 ft, respectively. Although

perched water bodies in the sedimentary deposits are

separated from the freshwater lens in the volcanic rocks

by a zone of unsaturated volcanic rocks, they may be
22 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

0

400

100

200

300

0

400

100

200

300

400 300 200 100 0 -100 -200

ALTITUDE OF BOTTOM OF HOLE, IN FEET

0

400

100

200

300

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

Mokuhau 15C (5330-06)

Test hole did not penetrate into

Wailuku Basalt

Wailuku Basalt

Wailuku Basalt

sedimentary deposits

sedimentary deposits

sea level sea level sea level

Mokuhau 15D (5330-07)

Mokuhau 15E (5330-08)

Figure 12. Water level above sea level as a function of altitude of bottom of hole during drilling

for test holes 15C, 15D, and 15E (wells 5330-06, 5330-07, and 5330-08), Maui, Hawaii.

(Unpublished data in U.S. Geological Survey well files, Honolulu.)
Ground-Water Development 23

adjacent to and hydrologically continuous with the

freshwater lens in the sedimentary deposits (fig. 10).

Recharge to the perched water body is from

direct infiltration of precipitation and irrigation water.

Discharge from the perched system is downward to a

lower water table in the volcanic rocks, downgradient to

the freshwater lens in the sedimentary deposits, or to

streams that intersect perched-water tables.

Effect of Land-Use Change on Ground-Water

Recharge

Shade (1997) estimated that under natural (or

predevelopment) conditions, recharge within the dike

complex and to the ground-water system seaward of the

dike complex was about 24 Mgal/d and 10 Mgal/d,

respectively, for a total of 34 Mgal/d. The water-budget

accounting method used by Shade (1997) probably

maximizes values for ground-water recharge, and as a

result, actual recharge rates may be less by some

unknown amount.

The introduction of sugarcane agriculture in

1856 increased rates of ground-water recharge in areas

under cultivation (fig. 13). For these areas, Shade

(1997) estimated changes in the rate of recharge for

three time periods: 1926–79, when sugarcane was the

only crop and the furrow irrigation method was used;

1980–85, when sugarcane acreage was reduced and

replaced by macadamia nut tree acreage, and when the

furrow irrigation method was largely replaced by drip

irrigation; and 1986–95, when sugarcane acreage was

reduced further to only about 5 percent of the original

sugarcane acreage and some of the remaining acreage

was used for pineapple. These time periods represent

relatively constant but different periods of land use (fig.

13) and irrigation practices. Land use did not change for

the dike-impounded ground-water area during these

times. Total recharge rates estimated by Shade (1997)

for the area underlain by the freshwater lens are 27, 16,

and 12 Mgal/d for 1926–79, 1980–85, and 1986–95,

respectively. Given a natural recharge rate to the dikeimpounded ground-water area of 24 Mgal/d, the total

recharge to the Iao aquifer ranged from a natural rate of

34 Mgal/d, to 51 Mgal/d for 1926–79, 40 Mgal/d for

1980–85, and 36 Mgal/d for 1986–95.

GROUND-WATER DEVELOPMENT

Development of ground water from the Iao

aquifer began with the construction of tunnels to divert

dike-impounded water and with the improvement of

springs (fig. 2, table 2). Twelve tunnels in the Iao aquifer area were excavated between 1900 and 1926 (table

3), but most of the history and records of their construction has been lost (Stearns and Macdonald, 1942). Eight

of the tunnels penetrate into the dike complex and tap

dike-impounded water. The other four tunnels penetrate

alluvium beneath Iao and Waiehu Streams and collect

water from beneath the streams in the valley-floor alluvium (Stearns and Macdonald, 1942).

The total amount of ground water developed by

the tunnels in the Iao aquifer area and the adjacent

Waikapu Stream and Waihee River valleys that penetrate into the dike compartments was about 9.0 Mgal/d

as of 1970 (Yamanaga and Huxel, 1970). Stearns and

Macdonald (1942) and Yamanaga and Huxel (1970)

suggested that most of the tunnel water (about 7.5

Mgal/d) would have discharged naturally into the

streams had it not been intercepted by the tunnels. The

remaining water (about 1.5 Mgal/d) would have

recharged the freshwater-lens system beyond the dike

complex had it not been captured by the tunnels. Tunnels 1 and 2, in Waihee River valley, discharge dikeimpounded water at an altitude of about 1,650 ft into

Waihee River. A diversion on Waihee River farther

downstream feeds ditches used for irrigation in the

study area. Other tunnels, such as tunnel 7 in Iao Stream

valley and tunnel 11 in Waikapu Stream valley, tap

dike-impounded water at altitudes of 787 ft and 1,800 ft,

respectively. The descriptions of the other four tunnels

(4, 5, 6, and 12) indicate that they were not as successful

as tunnels 1, 2, 7, and 11 (Stearns and Macdonald,

1942). Tunnels 3, 8, 9, and 10 were dug in the alluvium

and captured about 0.75 Mgal/d (Stearns and Macdonald, 1942).

At the present time (1998), ground water is

pumped from five major wells or well fields (shaft 33,

Mokuhau, Waiehu Heights, Waihee, and Kepaniwai).

Most ground water is pumped from the freshwater lens

in the dike-free volcanic rocks, except for the Kepaniwai well, which pumps ground water from the dikeimpounded system.

Large-scale withdrawal of ground water from

the freshwater lens of the Iao aquifer began in 1948 with
24 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

Wailuku

Waihee

Waiehu

0

0

1 2 MILES

1 2 KILOMETERS

EXPLANATION

AGRICULTURAL LAND USE

 Sugarcane

 Macadamia nut

 Pineapple

20°52'

20°54'

20°56'

156°30'

156°32'

156°34'

(1980–85)

Wailuku

Waihee

Waiehu

20°52'

20°54'

20°56'

156°30'

156°32'

156°34'

Wailuku

Waihee

Waiehu

20°52'

20°54'

20°56'

156°30'

156°32'

156°34'

(1986–95)

(1926–79)

MAUI

Pacific Ocean

Study

area

Figure 13. Agricultural land use for three different periods, Iao aquifer area, Maui, Hawaii (modified from Shade, 1997).
Ground-Water Development 25

Table 2. Springs in the dike complex of the Iao aquifer area, Maui, Hawaii

[Data from Stearns and Macdonald, 1942]

Spring

number Name Location(stream valley)

Altitude

(feet)

Average daily

discharge

(gallons)

10 Lower North Waiehu 1,500 150,000

11 Upper North Waiehu 2,050 1,500,000

12 Lower South Waiehu 1,000 200,000

13 Middle South Waiehu 1,325 500,000

14 Upper South Waiehu 1,500 1,000,000

15 Black Gorge Iao 1,200 600,000

16 Needle Iao 1,100 250,000

Table 3. Ground-water development tunnels in the Iao aquifer area, Maui, Hawaii

[Mgal/d, million gallons per day; --, no data]

Tunnel

number

Location

(stream or

river valley)

Altitude

(feet)

Yield from

Stearns and

Macdonald,

1942

(Mgal/d)

Yield from

Yamanaga and

Huxel, 1970

(Mgal/d)

Length

(feet) Geology

1 Waihee 1,625 4.6 4.6 2,200 Older alluvium and dike complex

2 Waihee 1,650 1.0 1.0 2,500 Older alluvium, dike complex in the Wailuku

Basalt, and pit crater breccia

3 Waiehu 300 0.25 0.25 500 Alluvium

4 Iao 1,425 0.075 -- 2,500 Dike complex, Wailuku Basalt

5 Iao 1,475 0.075 -- caved Caldera complex, Wailuku Basalt

6 Iao 1,305 0.6 -- 1,413 Dike complex, Wailuku Basalt

7 Iao 787 2.05 2.30 2,630 Dike complex, Wailuku Basalt

8 Iao 700 0.1 -- caved Alluvium and Wailuku Basalt

9 Iao 440 0.15 0.15 1,000 Alluvium

10 Iao 240 0.25 0.25 2,000 Alluvium

11 Waikapu 1,800 1.0 1.0 2,943 Dikes in Wailuku Basalt

12 Waikapu 1,770 0.0007 -- 1,500 Vent breccia of the caldera complex
26 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

the completion of shaft 33 (figs. 2 and 3). A 647-ft tunnel inclined at 30 degrees was excavated, terminating in

a large vault at an altitude of 30 ft. Three wells were

drilled about 10 ft apart and about 310 ft deep in the

vault. The bottoms of the wells are about -280 ft in altitude.

Shaft 33 was used for agricultural supply during 1948–84, with an annual mean withdrawal ranging

from 0.00 to 11.65 Mgal/d (fig. 14). The monthly mean

pumpage depended largely on rainfall and varied significantly, ranging from 0.00 to 20.90 Mgal/d (probably

the maximum pump capacity). Shaft 33 was not used

between 1985 and early 1991. In mid-1991, the Maui

DWS started using the shaft for domestic supply. The

DWS uses only one of the pumps in the shaft, which

was pumped continuously at a rate of about 5.16 Mgal/d

between July 1991 and June 1994, about 5.31 Mgal/d

between January 1995 and December 1996, and at a

reduced rate of about 4.72 Mgal/d from August 1997

through the end of 1998. Pumping was interrupted during the last 6 months of 1994 and the first 7 months of

1997.

The Mokuhau well field was the second major

ground-water withdrawal site added to the Iao aquifer

with the construction of two wells in 1953. A third well

was added to the well field in 1967. Water from the

Mokuhau well field was the sole source of domestic

water from the Iao aquifer until 1977. The well field

was placed at an altitude of about 353 ft, and the three

wells were drilled to a bottom altitude of about -250 ft

each. Annual mean pumpage from the well field

increased steadily from about 1.10 Mgal/d in 1955 to

more than 8 Mgal/d in 1977 and 1978 (fig. 14). These 2

years represent the peak annual pumpage from the well

field. Since 1978, the mean annual pumpage has been

about 5.53 Mgal/d, ranging from 3.05 to 7.66 Mgal/d. In

1997, the annual mean pumpage from Mokuhau well

field was about 6.29 Mgal/d.

Waiehu Heights and Waihee well fields were

added in 1977 and 1979, respectively, in order to meet

increasing domestic demand. Pumpage from the Waiehu Heights well field has been lower relative to the

Mokuhau well field. The mean annual pumpage at

Waiehu Heights well field was 0.43 Mgal/d from 1977

to 1990, with annual mean pumpage ranging between

0.18 and 0.92 Mgal/d (fig. 15). From 1991 to the end of

1997, annual mean pumpage from the Waiehu Heights

well field ranged between 1.08 and 1.96 Mgal/d. Pumpage in 1998 dropped to 0.23 Mgal/d. Pumpage from

Waihee well field, which initially consisted of two wells

(Waihee 1 and 2), was about 1.37 Mgal/d in 1979; annual mean pumpage increased to 6.15 Mgal/d in 1980.

Annual mean pumpage ranged from about 4.87 to 6.63

Mgal/d between 1981 and 1986 (fig. 15), and increased

to about 8.53 Mgal/d in 1987, after a third well (Waihee

3) was constructed at this well field in late 1981. Annual

mean pumpage has been between 7.24 Mgal/d and 9.11

Mgal/d since 1987.

The next well added to the DWS distribution

system in the Iao aquifer was the Kepaniwai well in

1991. Before being added to the distribution system, the

well was used locally during 1977–86. The well is just

north of Iao Stream at an altitude of about 713 ft and

taps dike-impounded water. Annual mean pumpage

reached a maximum in 1991 at 1.03 Mgal/d (fig. 15).

After 1991, the mean annual pumpage has been about

0.55 Mgal/d.

In 1997, two wells immediately north of

Waihee River were added to the distribution system.

The North Waihee well field (fig. 3) is considered outside of the Iao aquifer area for management purposes,

thus the pumpage from this well field is not added to the

total pumpage for the aquifer. The well field is at an altitude of about 285 ft. The bottoms of both wells are at

altitudes of about -106 ft. The two wells have submersible pumps installed that can deliver about 1.6 Mgal/d

each. The maximum monthly pumpage from this well

field has been about 2.38 Mgal/d.

The annual mean rate of ground-water withdrawal from the Iao aquifer has ranged widely, but generally has increased with time (fig. 4 and table 4). The

record of total pumpage can be divided into four periods

where pumpage trends are distinguishable. The trends

are a result of differences in rainfall, irrigation, and

pumpage for domestic needs.

Between 1948 and 1970, the annual mean total

pumpage ranged from 1.49 to 10.01 Mgal/d and the

average was about 5.57 Mgal/d. From 1971 through

1977, annual mean total pumpage ranged from a low of

13.06 Mgal/d in 1973 to a high of 16.96 Mgal/d in 1975,

and the average was 14.69 Mgal/d. Years with high

pumpage within these two periods were years with low

rainfall and high water demand for irrigation. Annual

mean total pumpage between 1978 and 1985 (fig. 4)

was less than during the 1970’s primarily because of

changes in agricultural practices. The maximum annual

mean total pumpage for this time was 12.57 Mgal/d, the
Ground-Water Development 27

0

14

2

4

6

8

10

12

ANNUAL MEAN PUMPAGE

IN MILLION GALLONS PER DAY

19451950195519601965197019751980198519901995

2000

0

10

2

4

6

8

ANNUAL MEAN PUMPAGE

IN MILLION GALLONS PER DAY

YEAR

Mokuhau well field (5330-08, -09, -10, -11)

Shaft 33 (5330-05)

Figure 14. Annual mean pumpage for shaft 33 and Mokuhau well field, Maui, Hawaii. (Data from

Maui Department of Water Supply and unpublished data from Wailuku Sugar Company in U.S.

Geological Survey well files, Honolulu.)
28 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

Waiehu Heights well field (5430-01 and -02)

Waihee well field (5431-02, -03, -04)

Kepaniwai well (5332-05)

0

5

2

4

ANNUAL MEAN PUMPAGE

IN MILLION GALLONS PER DAY

0

10

2

4

6

8

ANNUAL MEAN PUMPAGE

IN MILLION GALLONS PER DAY

1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

0

5

2

4

ANNUAL MEAN PUMPAGE

IN MILLION GALLONS PER DAY

YEAR

Figure 15. Annual mean pumpage for Waiehu Heights well field, Waihee well field, and Kepaniwai

well, Maui, Hawaii. (Data from Maui Department of Water Supply.)
Ground-Water Development 29

Table 4. Annual ground-water withdrawal from the Iao aquifer, Maui, Hawaii

[Values in million gallons per day, or percentage where noted; --, not applicable; data from Maui Department of Water Supply and unpublished data from

Wailuku Sugar Co. in U.S. Geological Survey well files, Honolulu]

Well field Domestic

(percentage

Year of total)

Waiehu

Heights Waihee Kepaniwai Mokuhau Shaft 33 Total Domestic

1948 -- -- -- -- 2.00 2.00 0.00 0.0

1949 -- -- -- -- 1.99 1.99 0.00 0.0

1950 -- -- -- -- 1.77 1.77 0.00 0.0

1951 -- -- -- -- 3.70 3.70 0.00 0.0

1952 -- -- -- -- 3.05 3.05 0.00 0.0

1953 -- -- -- -- 9.77 9.77 0.00 0.0

1954 -- -- -- -- 6.11 6.11 0.00 0.0

1955 -- -- -- 1.10 1.31 2.41 1.10 45.6

1956 -- -- -- 0.66 0.83 1.49 0.66 44.3

1957 -- -- -- 1.19 6.12 7.31 1.19 16.3

1958 -- -- -- 1.22 0.67 1.89 1.22 64.6

1959 -- -- -- 1.43 4.15 5.58 1.43 25.6

1960 -- -- -- 1.59 5.65 7.24 1.59 22.0

1961 -- -- -- 2.25 5.64 7.89 2.25 28.5

1962 -- -- -- 2.04 7.97 10.01 2.04 20.4

1963 -- -- -- 2.06 0.85 2.91 2.06 70.8

1964 -- -- -- 2.79 6.00 8.79 2.79 31.7

1965 -- -- -- 2.67 4.68 7.35 2.67 36.3

1966 -- -- -- 3.12 4.69 7.81 3.12 39.9

1967 -- -- -- 2.91 3.08 5.99 2.91 48.6

1968 -- -- -- 2.88 6.28 9.16 2.88 31.4

1969 -- -- -- 3.61 1.18 4.79 3.61 75.4

1970 -- -- -- 3.98 5.08 9.06 3.98 43.9

1971 -- -- -- 4.34 11.65 15.99 4.34 27.1

1972 -- -- -- 4.66 9.45 14.11 4.66 33.0

1973 -- -- -- 5.16 8.11 13.27 5.16 38.9

1974 -- -- -- 5.44 9.11 14.55 5.44 37.4

1975 -- -- -- 6.40 10.56 16.96 6.40 37.7

1976 -- -- -- 6.69 6.37 13.06 6.69 51.2

1977 0.38 -- 0.04 8.10 6.39 14.91 8.52 57.1

1978 0.46 -- 0.01 8.29 3.14 11.90 8.76 73.6

1979 0.48 1.37 0.02 6.51 3.29 11.67 8.38 71.8

1980 0.48 6.15 0.02 3.05 0.00 9.70 9.70 100.0

1981 0.59 4.87 0.03 5.80 1.18 12.47 11.29 90.5

1982 0.49 5.20 0.007 4.60 0.00 10.30 10.30 100.0

1983 0.34 5.39 0.14 5.82 0.00 11.69 11.69 100.0

1984 0.29 5.41 0.11 6.39 0.37 12.57 12.20 97.1

1985 0.18 5.12 0.03 6.52 0.00 11.85 11.85 100.0

1986 0.27 6.63 0.003 6.42 0.00 13.32 13.32 100.0

1987 0.34 8.53 0.003 5.11 0.00 13.98 13.98 100.0

1988 0.35 8.06 0.00 6.71 0.00 15.12 15.12 100.0

1989 0.51 7.34 0.00 7.49 0.00 15.34 15.34 100.0

1990 0.92 8.66 0.07 7.66 0.00 17.31 17.31 100.0

1991 1.96 8.22 1.03 5.72 1.90 18.83 18.83 100.0

1992 1.08 7.96 0.82 3.17 5.10 18.13 18.13 100.0

1993 1.51 7.24 0.49 3.60 5.56 18.40 18.40 100.0

1994 1.20 8.15 0.45 6.49 2.91 19.20 19.20 100.0

1995 1.71 7.92 0.49 4.92 5.46 20.50 20.50 100.0

1996 1.56 8.22 0.28 5.13 5.16 20.35 20.35 100.0

1997 1.23 8.94 0.80 6.29 1.84 19.10 19.10 100.0

1998 0.23 9.11 0.51 3.21 4.84 17.90 17.90 100.0
30 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

minimum was about 9.70 Mgal/d, and the average was

about 11.52 Mgal/d.

Since 1984, all of the water pumped from the

Iao aquifer has been for domestic use. Between 1985

and 1995, annual mean total pumpage increased by

about 0.86 Mgal/d each year as domestic needs

increased. In 1995, annual mean total pumpage peaked

at a value of 20.50 Mgal/d.

In 1996, the annual mean total pumpage was

20.35 Mgal/d, nearly equal to the peak pumpage of the

previous year. Annual mean total pumpage was reduced

in the Iao aquifer area by about 1 Mgal/d per year in

1997 and 1998, primarily as a result of the additional

pumpage from the North Waihee well field. Annual

mean total pumpage decreased to 19.10 Mgal/d in 1997,

and further decreased to 17.90 Mgal/d in 1998.

AQUIFER RESPONSE TO PUMPAGE,

RAINFALL, AND LAND-USE CHANGES

Water-Level Changes Since the Introduction of

Pumping

Pumping from the Iao aquifer has caused an

overall decline in water levels that has resulted in a corresponding rise of the freshwater/saltwater transition

zone. Pumping from each well field generates a cone of

depression that overlaps with the cones of depression

from one or more of the other well fields.

Water levels in the Iao aquifer have been monitored for various periods of time since 1940, beginning

with measurements from test hole T-102 (fig. 3). However, the temporal-spatial distribution of observation

wells has been insufficient to permit documentation of

the change in water levels throughout the aquifer since

the start of pumping. Also, sufficient data do not exist

to fully document the temporal changes in water levels

since the inception of pumping at any particular location.

Available data indicate that water levels not

only respond to pumping, but also correspond closely to

the departure of the mean of the preceding 12 months of

rainfall from the mean rainfall of the entire record.

Yamanaga and Huxel (1970) were the first to recognize

that water levels in the Iao aquifer respond to changes

in rainfall, and the findings of the present study reinforce this conclusion. In addition, water levels fluctuate

seasonally, with annual highs occurring during the winter and lows occurring during the late summer or early

fall.

Ground-water levels near shaft 33.--Initial

efforts to monitor water levels in the Iao aquifer were at

wells T-102 and T-112 (fig. 3). Water-level measurements were made at the two wells for various periods

between 1940 and 1970, beginning at well T-102 in

1940, 8 years before pumping started at shaft 33. T-102

is about 1,800 ft upgradient of shaft 33 and about 2,300

ft from the Mokuhau well field (figs. 2 and 3). Test hole

T-112, which was destroyed in 1996, was about 260 ft

from the location of the wells in shaft 33. Water levels

from T-112 are assumed for this report to be representative of water levels at shaft 33.

Water levels were measured monthly at T-102

during 1940–55 and continuously during 1968–70 (fig.

16). Water levels were measured monthly in T-112 during 1948–55 and continuously during 1968–70. Some

breaks in the record exist for both wells. Pre-development water-level altitudes measured during 1940–47 at

T-102 ranged from a low of about 26.9 ft to a high of

36.6 ft and averaged 31.4 ft. Water-level altitudes measured at T-112 ranged from 26.1 to 31.3 ft between

1946 and 1947 and averaged about 28.5 ft.

The hydrographs for T-102 and T-112 indicate

that water levels near shaft 33 declined more than 5 ft

within the first year of pumping at the shaft (1948), but

thereafter, no definitive change was observed in the two

wells until 1953. Water levels at the two wells fell an

additional 5 to 10 ft during 1953, a year for which

pumpage was high compared with preceding and following years and rainfall was low (fig. 16). Water levels

recovered to within a few feet of pre-1953 levels

between 1954 and 1955 when increased rainfall resulted

in pumpage below that of 1953. Water levels at T-102

and T-112 during 1968–70 fell within the general range

observed at these wells in 1954–55.

Water-level measurements were made in shaft

33 during 1973–75, and ranged from 23.5 to 19.3 ft.

Water-level measurements resumed in the shaft in 1996

and have continued to the present time (1998) in one of

the unused pumping wells located about 10 ft from the

pumped well. The annual mean pumpage from the shaft

between 1991 and 1996 ranged from 1.90 Mgal/d to

5.56 Mgal/d and averaged 4.35 Mgal/d. Water levels

during 1996 dropped to a low of 9.2 ft above sea level

after 22 months of continuous pumping (fig. 17). The

shaft was not pumped during the first 7 months of 1997,

and water levels rose to 16.4 ft. Pumping resumed in

August 1997, and the water level in the shaft has since
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 31

0

40

20

0

40

20

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

0

30

10

20

MONTHLY MEAN PUMPAGE,

IN MILLION GALLONS PER DAY

1940 1942 1944 1946 1948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970

0

24

4

8

12

16

20

MONTHLY RAINFALL,

 IN INCHES

Test hole T-102 (5331-01)

Test hole T-112 (5330-03)

Shaft 33 (5330-05)

Waiehu Camp rain gage (station 484)

YEAR

Figure 16. Water levels for test holes T-102 and T-112, monthly mean pumpage for shaft 33 and monthly rainfall for

Waiehu Camp rain gage, for 1940–70, Maui, Hawaii. (Hydrographs from unpublished data in U.S. Geological Survey

well files, Honolulu; unpublished pumpage data from Wailuku Sugar Company in U.S. Geological Survey well files,

Honolulu; unpublished rainfall data from Commission on Water Resource Management and the University of Hawaii

Meteorology Department.)
32 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

0

24

8

16

MONTHLY PUMPAGE IN 

MILLION GALLONS PER DAY

J F M A M J J A S O N D J F M A M J J A S O N 

1996 1997 

Shaft 33 (5330-05) monthly pumpageShaft 33 (5330-05) miscellaneous water-level measurements Shaft 33 (5330-05) continuous record water-lev

Waiehu Camp rain gage (station 484)

Shaft 33 (5330-05) monthly pumpage

Waiehu Camp rain gage (station 484)

8

2812162024

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

1973 1974 1975

0

2010

MONTHLY RAINFALL,

IN INCHES

Figure 17. Water levels and monthly pumpage for shaft 33, and monthly rainfall for Waiehu Camp rain gage, for 1973–75 and (Pumpage data from Maui Department of Water Supply; unpublished rainfall data from Commission on Water Resource Mansity of Hawaii Meteorology Department.)
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 33

declined by about 5 ft. The altitudes of the water levels

remained between 10 and 11 ft for the last 9 months of

1998.

Water-level changes at Mokuhau well field.--

The initial water-level measurements in the vicinity of

Mokuhau well field were measured in test hole 15D

(fig. 3) in 1951. This well is located about 250 ft northwest of the well field. Water levels for test hole 15Dare

available for May 15, 1951 and for March 17, August 4,

September 14, and October 5, 1952, as well as April 16,

1968 (fig. 18). Water levels in 1952 ranged from an altitude of 24.5 ft on March 17 to 23.2 ft on September 14

(fig. 18).

Water levels for the Mokuhau well field

(Mokuhau pump 1, also known as well 15B; and Mokuhau pump 2, also known as well 15A, in figure 18) were

recorded between 1954 and 1979. Pumpage from

Mokuhau well field increased from about 1.10 Mgal/d

in 1955 when pumping started, to about 2.91 Mgal/d in

1967 (fig. 18), but the effect of this pumping is not discernible from the hydrograph. Water levels may have

dropped 2 or 3 ft over the first year of pumping, but this

is not verifiable with the available data.

From 1951 to 1970, water levels generally

ranged from seasonal lows around 16 to 17 ft to seasonal highs around 21 to 23 ft. Water levels at the well field

began a steady decline between 1969 and 1975 with the

seasonal low falling from over 20 ft in 1969 to about 8

ft in 1975–76. Water levels remained between 8 and

10.5 ft through 1976, 1977, and most of 1978. This

decline occurred during a period when rainfall, for the

most part, was below normal and pumping was significantly above previous rates (fig. 18 and table 4). Water

levels rose to about 16 ft in early 1979 in apparent

response to reduced pumpage and higher rainfall. However, after late 1979, water levels were not recorded

again until 1998. During 1998, the water level in Mokuhau pump 2 ranged from a seasonal low of 9.1 ft in September to a high of 12.0 ft in December.

Ground-water levels in the northern part of

the Iao aquifer.--Ground-water levels between Iao

Stream and the Waihee River were first measured in

January 1977 when five test holes were drilled (A1, B,

C, D, and E) (fig. 3). Test hole C caved before it could

be properly cased, but the other four test holes were successfully completed and have since been used as waterlevel observation wells. All the holes were drilled to

obtain stratigraphic and hydrologic information as part

of an exploratory program for a new well field in the

vicinity of Waiehu Stream. Additionally, the Waiehu

deep monitor well was drilled in 1982 to characterize

and monitor the movement of the transition zone of the

aquifer at this location. The deep monitor well is about

15.5 ft away from test hole Dand is also in the vicinity

of Waihee and Waiehu Heights well fields (fig. 3). The

Waiehu deep monitor well was drilled to an altitude of

-1,020 ft below sea level, and is uncased and open to the

aquifer from an altitude of -20 ft to -1,020 ft.

Except for a period of about a year (1981–82),

water-level measurements have continued in test hole E

to the present (1998) (fig. 19). Measurements in the other test holes were interrupted between May 1977 to

1979, 1982, or 1983 depending on the location (fig. 19).

The hydrograph for test hole D and the Waiehu deep

monitor well is a composite of the measurements at both

wells through time. Thirteen measurements, made in

both wells within a few minutes of each other between

1983 and 1989, indicate that the altitude of water levels

in the two wells are identical, allowing the data from the

two wells to be combined into one hydrograph. In 1986,

a water-level recorder was installed on the Waiehu deep

monitor well.

The configuration of water-level contours in

the flank flows of the Wailuku Basalt north of Iao

Stream in April 1977 is consistent with the concept of a

general movement of water from the dike complex to

the ocean between Waiehu Stream and Waihee River,

and water levels decreasing in an seaward direction (fig.

20). Water-level contours are based on the average

water levels in the test holes for the month (table 5). On

the basis of the contours, the average water level in the

vicinity of Waihee well field (which had yet to be constructed) was about 15 ft at this time, and the average

water level at Waiehu Heights well field was about 15.6

ft. The average water level at Mokuhau well field was

10.3 ft.

Contours bend inland near Waiehu Heights and

Waihee well fields, and there is an inferred bending of

contours inland at Mokuhau well field. The upstream

bending of the contours in the Mokuhau area is expected given ground-water withdrawal at the well field. The

cause of the bending of contours near Waiehu Heights

and Waihee well fields is not clear. Pumping from

Waiehu Heights well field began in 1977, but the exact

date pumping started cannot be determined from
34 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii Mokuhau pump 2 (53

Mokuhau pumps 1 (5Mokuhau test hole 15

5

30

10152025

WATER LEVEL, IN FEET

ABOVE MEAN SEA LEVEL

0

10

5

ANNUAL MEAN PUMPAGE,

IN MILLION GALLONS PER DAY

1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1

YEAR

0

80204060

ANNUAL RAINFALL, 

IN INCHES

Waiehu Camp rain gage (station 484)

Mean annual rainfall

Mokuhau well field (5330-09, -10, -11)

Figure 18. Water levels and annual mean pumpage for the Mokuhau well field, and annual rainfall for Waiehu Camp rain gagelevel data prior to 1998 from unpublished data in U.S. Geological Survey well files, Honolulu; 1998 water-level data from U.S. base; unpublished rainfall data from Commission on Water Resource Management; pumpage data from Maui Department o
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 35

4

28

8

12

16

20

24

PUMPAGE,

IN MILLION GALLONS PER DAY

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

Monthly total Iao aquifer

Annual total Iao aquifer

Monthly for Waiehu Heights and Waihee well fields

Annual for Waiehu Heights and Waihee well fields

6

22

10

14

18

Test hole A1 (5631-01)

6

22

10

14

18

6

22

10

14

18

YEAR

8

24

12

16

20

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

Test hole B (5431-01)

Test hole D and Waiehu deep monitor well (5430-04, -05)

Test hole E (5430-03)

Figure 19. Water levels for test holes in the northern part of the Iao aquifer, and pumpage for Iao aquifer and

for Waiehu Heights and Waihee well fields, Maui, Hawaii. (Water levels from unpublished data in U.S. Geological Survey well files and data base; pumpage data from Maui Department of Water Supply; monthly

pumpage not available for Waiehu Heights and Waihee well fields for years 1977 and 1978, thus total

monthly pumpage not shown for these 2 years.)
36 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

P A C I F I C

O C E A N

15

16

?

?

?

?

?

?

?

?

?

?

17

Waiehu Stream

Iao

Stream

Waihee

Waiehu

Wailuku

Waikapu

0

0

1 MILE

1 KILOMETER

DIKEIMPOUNDED

GROUND

WATER

UNCONFINED

GROUND WATER IN

DIKE-FREE

WAILUKU BASALT

 CONFINED FRESH GROUND 

 WATER IN THE WAILUKU 

 BASALT

LINE OF EQUAL WATER-LEVEL

 ALTITUDE--Contour 

 interval is one foot. Dashed where 

 approximately located. 

 Datum is mean sea level

APPROXIMATE SEAWARD 

 EXTENT OF FRESHWATER IN 

 WAILUKU BASALT FOR 

 PREDEVELOPMENT CONDITIONS

BOUNDARY BETWEEN 

 UNCONFINED AND CONFINED

 WATER IN THE WAILUKU

 BASALT. UNCONFINED 

 GROUND WATER IN 

 SEDIMENTARY DEPOSITS 

 SEAWARD OF BOUNDARY

INFERRED BOUNDARY 

 BETWEEN DIKE-IMPOUNDED 

 GROUND WATER AND 

 FRESHWATER-LENS SYSTEM

 (modified from Yamanaga and 

 Huxel, 1970)

STUDY-AREA (IAO AQUIFER) 

 BOUNDARY

PUMPED WELL OR WELL FIELD

WATER-LEVEL OBSERVATION 

 WELL

EXPLANATION

Kahului

Bay

20°56'

20°54'

20°52'

156°32' 156°30'

Waihee River

Waikapu Stream

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Figure 20. Mean water levels in the flank flows of Wailuku Basalt, April 1977, for the northern part of the Iao aquifer,

Maui, Hawaii (see table 3).
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 37

Table 5. Mean water levels for April 1977, April 1997, and the year 1998; and seasonal low water levels for 1996 and 1998 for wells in Hawaii[All values in feet; --, no data; datum is mean sea level; negative values are declines]

Well name

Mean

water level

of April 1977

(figure 20)

Mean

water level

of April 1997

Difference

between mean

water level

of April 1977 and

mean water level

of April 1997

(figure 21)

Seasonal waterlevel low of 1996

(October)

(figure 22)

Seasonal waterlevel low of 1998

(August)

(figure 22)

Differenc

season

level low 

(Octob

season

1998 (

(figu

Test hole A1 15.9 13.6 -2.3 12.4 12.5 0Test hole B 15.3 9.4 -5.9 7.8 9.4 1Waiehu deep monitor well

and test hole D

15.5 10.2 -5.3 9.0 10.0 1

Test hole E 16.4 11.4 -5.0 9.7 11.6 2Waikapu 1 -- 15.9 -- 12.0 12.0 -- Waikapu 2 -- 13.0 -- 11.9 12.1 -0North Waihee well field -- -- -- -- -- -- Waihee well field -- -- -- -- -- -- Waiehu Heights well field 15.6 -- -- -- -- -- Mokuhau well field 10.3 -- -- -- 9.3 -- Shaft 33 -- -- -- 9.2 10.5 1
38 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

existing data. Pumping from Waihee well field began in

1979.

The hydrograph from test hole E (fig. 19) indicates that, despite the introduction of pumping from

Waiehu Heights and Waihee well fields in 1977 and

1979, respectively (table 4), and despite a general

increase in total pumpage from the aquifer between

1980 and 1996 (fig. 19), water levels actually began to

rise in mid-1978 and in late 1982 to early 1983. Water

levels were as much as 5 ft higher in the test holes than

those measured in 1977. This peak was followed by a

general decline in water levels in all the test holes (fig.

19) that lasted until late 1985 to early 1986 when water

levels were as much as 3 ft below those recorded in

1977. Beginning in early 1986, water levels generally

increased through late 1989 to early 1990, rising to several feet or more above those recorded for April 1977.

The possible reason for water-level increases during

1977–83 and during 1985–90 is discussed in the following section on water-level changes relative to rainfall.

From mid-1990 or mid-1991 (depending on the

test well) to the present (1998), water levels dropped

below 1977 recorded values. A declining water-level

trend continued until 1996–97, after which water levels

started to increase. The water-level increase corresponds to the decrease in the annual rate of groundwater withdrawal that began in 1997 (table 4, fig. 4).

The contoured areal decline in water levels from April

1977 to April 1997 for the northern half of the aquifer is

shown in figure 21. Maximum water-level decline is

greater than 6 ft and centered around Waihee and Waiehu Heights well fields. The decline decreases in an outward direction from the well fields.

The lowest water levels measured during 1990–

97, when water levels generally were declining,

occurred in August 1996. These water levels were also

the seasonal-low water levels for 1996. Following the

reduction in pumping in 1997 and 1998, water levels

increased such that the seasonal-low water levels in

1997 were greater than the seasonal-low water levels in

1996, and the seasonal-low water levels in 1998 were

greater than the seasonal-low water levels of 1997. The

contoured areal rise in the seasonal low between 1996

and 1998 is shown in figure 22. The greatest rise (2.0 ft)

in water levels is centered around Waiehu Heights well

field, where annual average pumpage decreased from

1.56 Mgal/d in 1996 to 0.23 Mgal/d in 1998. The water

levels also rose by about 1.3 ft at shaft 33, where the

monthly pumping rate was reduced from about 5.16

Mgal/d to 4.84 Mgal/d during 1996–98. No water-level

data are available during 1996 for the Mokuhau well

field, but pumpage was reduced at the well field from an

annual average of 5.13 Mgal/d to 3.21 Mgal/d during

1996–98.

Other water-level measurements.--Water levels have been consistently measured near Waikapu

Stream at Waikapu wells 1 and 2 since 1989 and 1983,

respectively (fig. 23). Water levels were measured in

Waikapu 1 for a few months in 1967 and in Waikapu 2

for a few months during 1974–75. Water levels were

measured at North Waihee 1 between 1988 and 1996

(fig. 23). Water-level measurements continued at this

well following the start of pumping in 1997. Since

1991, water levels have been measured in the Kepaniwai observation well near the Kepaniwai well (fig. 24).

The water-level trends for the two Waikapu

wells follow the trends of the other wells in the Iao aquifer, with a general decline in water levels from 1990–

96. The water-level record for Waikapu 1 shows greater

seasonal fluctuation than other wells in the Iao aquifer

and a greater overall decline between 1990 and 1996

than Waikapu 2. Seasonal water-level trends at

Waikapu 2 more closely parallel those at observation

wells farther north in the aquifer. Water levels increased

in 1997–98 in both of the Waikapu wells, which may

have been a response to the decrease in pumpage over

these 2 years.

The water-level trend at North Waihee 1 during

1988–96 is similar to the trends of the other monitor

wells in the Iao aquifer (fig. 19), although the water levels were 4 to 5 ft lower than the closest monitor well

south of the Waihee River (test hole A1). Although

water-level measurements continued since the start of

pumping at this well in 1997, the measurements are of

limited value for monitoring aquifer trends and are not

discussed in this report.

Water levels have been measured at Kepaniwai

observation well (fig. 3) since 1991. The well head is at

an elevation of about 713 ft above sea level, and penetrates into the dike-impounded water body. The observation well is about 10 ft from the Kepaniwai well. The

initial water level for the pumped well was about 675 ft

above sea level in 1973. Water levels in the observation

well have ranged from 630.3 to 673.6 ft above sea level
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 39

P A C I F I C

O C E A N

?

?

?

?

?

?

?

?

?

?

2

Waiehu Stream

Iao

Stream

Waihee

Waiehu

Wailuku

Waikapu

0

0

1 MILE

1 KILOMETER

DIKEIMPOUNDED

GROUND

WATER

UNCONFINED

GROUND WATER IN

DIKE-FREE

WAILUKU BASALT

CONFINED FRESH GROUND 

 WATER IN THE WAILUKU 

 BASALT

LINE OF EQUAL WATER-LEVEL

 DECLINE--Contour interval 

 is one foot. Dashed where 

 appoximately located. Datum is 

 mean sea level.

APPROXIMATE SEAWARD 

 EXTENT OF FRESHWATER IN 

 WAILUKU BASALT FOR 

 PREDEVELOPMENT CONDITIONS

BOUNDARY BETWEEN 

 UNCONFINED AND CONFINED

 WATER IN THE WAILUKU

 BASALT. UNCONFINED 

 GROUND WATER IN 

 SEDIMENTARY DEPOSITS 

 SEAWARD OF BOUNDARY

INFERRED BOUNDARY 

 BETWEEN DIKE-IMPOUNDED 

 GROUND WATER AND 

 FRESHWATER-LENS SYSTEM

 (modified from Yamanaga and 

 Huxel, 1970)

STUDY-AREA (IAO AQUIFER)

 BOUNDARY

PUMPED WELL OR WELL FIELD

WATER-LEVEL OBSERVATION 

 WELL

EXPLANATION

Kahului

Bay

20°56'

20°54'

20°52'

156°32' 156°30'

2

3

4

5

6

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Waihee River

Waikapu Stream

Figure 21. Water-level declines in the flank flows of Wailuku Basalt between April 1977 and April 1997 for the northern

part of the Iao aquifer, Maui, Hawaii (see table 3).
40 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

P A C I F I C

O C E A N

?

?

?

?

?

?

?

?

?

?

1

Waiehu Stream

Iao

Stream

Waihee

Waiehu

Wailuku

Waikapu

0

0

1 MILE

1 KILOMETER

DIKEIMPOUNDED

GROUND

WATER

UNCONFINED

GROUND WATER IN

DIKE-FREE

WAILUKU BASALT

CONFINED FRESH GROUND 

 WATER IN THE WAILUKU 

 BASALT

LINE OF EQUAL WATER-LEVEL 

 RISE--Contour interval is 

 0.5 foot. Dashed where 

 approximately located. Datum is 

 mean sea level

APPROXIMATE SEAWARD 

 EXTENT OF FRESHWATER IN 

 WAILUKU BASALT FOR 

 PREDEVELOPMENT CONDITIONS

BOUNDARY BETWEEN 

 UNCONFINED AND CONFINED

 WATER IN THE WAILUKU

 BASALT. UNCONFINED BASAL 

 GROUND WATER IN 

 SEDIMENTARY DEPOSITS 

 SEAWARD OF BOUNDARY

INFERRED BOUNDARY 

 BETWEEN DIKE-IMPOUNDED 

 GROUND WATER AND 

 FRESHWATER-LENS SYSTEM

 (modified from Yamanaga 

 and Huxel, 1970)

STUDY-AREA (IAO AQUIFER)

 BOUNDARY

PUMPED WELL OR WELL FIELD

WATER-LEVEL OBSERVATION 

 WELL

EXPLANATION

Kahului

Bay

20°56'

20°54'

20°52'

156°32' 156°30'

2

1.5

1

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Waihee River

Waikapu Stream

Figure 22. Water-level rise in the flank flows of Wailuku Basalt between the seasonal low of 1996 and the seasonal

low of 1998 for the Iao aquifer, Maui, Hawaii (see table 5).
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 41

8

20

10

12

14

16

18

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

Waikapu 1

Waikapu 2

Waikapu 1 and 2 (5130-01 and -02)

4

16

6

8

10

12

14

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

YEAR

North Waihee 1 (5631-02)

Figure 23. Water levels for Waikapu 1 and 2, and North Waihee 1, Maui, Hawaii (from U.S.

Geological Survey data base and unpublished data in U.S. Geological Survey well files, Honolulu).
42 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

0

20

5

10

15

MONTHLY RAINFALL, IN INCHES

620

680

630

640

650

660

670

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

YEAR

Kepaniwai observation well (5332-04)

0

1.6

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

MONTHLY MEAN PUMPAGE, 

IN MILLION GALLONS PER DAY

Waiehu Camp rain gage (station 484)

Kepaniwai well (5332-05)

Figure 24. Water levels and monthly mean pumpage for the Kepaniwai wells, and rainfall for Waiehu Camp rain

gage, for 1980–98, Maui, Hawaii. (Pumpage data from Maui Department of Water Supply; unpublished rainfall

data from Commission on Water Resource Management and the University of Hawaii Meteorology Department.)
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 43

over the period of record with no discernible declining

or rising trend (fig. 24). Water-level fluctuations in the

Kepaniwai observation well correspond to pumping

fluctuations in the pumped well.

Water levels in the Iao aquifer, 1998.--The

water-level measurements at Mokuhau well field in

1998, combined with the measurements at Waikapu 1

and 2, shaft 33, the Waiehu deep monitor well, and test

holes A1, B, and E, make it possible to construct a map

of mean water-level contours between Waikapu Stream

and Waihee River for 1998 (fig. 25 and table 5). The

contours indicate a general movement of water toward

the ocean, as would be expected. They also bend upgradient near Mokuhau well field, and near Waihee and

Waiehu Heights well fields, similar to the water-level

contours for 1977 (fig. 20).

The average water level near the major well

fields was about 10 ft in 1998. Water levels were about

18 ft below the predevelopment water level at shaft 33,

as measured in T-112 in 1940, and about 14 ft below the

average water level measured at Mokuhau well field in

1952. The average 1998 water level at Waihee and

Waiehu Heights well fields is more than 5 ft below

average water levels at these locations during April

1977 (fig. 20).

Water-Level Changes Relative to Rainfall

From 1948 to 1980, large fluctuations in total

pumpage were primarily due to increased agricultural

irrigation pumping during dry periods. Low rainfall

during 1952–54, 1962, 1964, and 1975–78, corresponds

to periods of increased pumpage from shaft 33, which

was used only for agriculture during this time. Water

levels in the Iao aquifer declined during these periods

indicating that these declines may be a combined result

of increased pumping and low rainfall (figs. 4, 16, and

18).

The best data available for examining the relation among water levels, rainfall, and pumpage are the

hydrographs for test holes A, B, and E, and the combined hydrograph for test hole D and the Waiehu deep

monitor well (fig. 19). Assuming no other influence,

water levels in the general area encompassed by the test

holes would be expected to decline after the start of

pumping from Waiehu Heights and Waihee well fields,

at least on an annual basis. However, several times during the 1980’s, water levels rose above 1977 water levels (the year that pumping started at Waiehu Heights)

and above 1979 water levels (the year that pumping

started at Waihee well field). The increase in water levels indicates that another factor such as rainfall, or a

combination of factors, influence ground-water levels,

and that these factors are sufficient, at least temporarily,

to offset declines induced by the combined pumping at

the two well fields.

One factor, other than rainfall, that would probably cause a temporary increase in water levels above

1977 values in the area of the Waiehu Heights and

Waihee well fields, is that during 1975–80, annual rates

of pumping from Mokuhau well field and shaft 33 were

reduced from about 17 Mgal/d to below 10 Mgal/d.

Most of the reduction was at shaft 33. Ground-water

pumpage from the aquifer did not reach or exceed that

of 1975 until 1990 (fig. 4). However, offsetting the

reduction of pumpage was the loss of recharge to the

aquifer in the late 1970’s and early 1980’s as a result of

changes in irrigation practices (furrow to drip), changes

in crops grown, and a reduction of agricultural acreage.

The reduction of recharge from 51 Mgal/d during 1926–

79 to 40 Mgal/d during 1980–85, together with the

introduction of pumping at the Waiehu Heights and

Waihee well fields, would collectively have caused

water levels to decline below 1977 values if rainfall and

pumpage from Mokuhau well field and shaft 33 had

remained constant.

Yamanaga and Huxel (1970) concluded that

water levels in the aquifer seemed to respond to annual

or seasonal changes in rainfall rather than monthly fluctuations, and this apparent relation was investigated further for this study. Daily and monthly rainfall data were

used to compare trends in rainfall and water levels.

Plots of the percent departure of 6-, 12-, 18-, and 24-

month average from the long-term mean rainfall for station 484 (fig. 2) were also compared to the hydrographs

of the test holes. The plot using the departure of the 12-

month average best matched the long-term trend of the

hydrographs (fig. 26). Prior to 1991, the match is apparent. The match of the trends indicates that the time

required for the infiltrating rainfall to affect the altitude

of the water tables is similar to the 6-month time lag

resulting from the averaging process of the rainfall

record. Although not shown, this relation is similar for

the other wells.

Long-term trends in water levels during 1977–

91 in the area of Waiehu Heights and Waihee well fields
44 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

P A C I F I C

O C E A N

?

?

?

?

?

?

?

?

?

13

Waiehu Stream

Iao

Stream

Waihee

Waiehu

Wailuku

Waikapu

0

0

1 MILE

1 KILOMETER

DIKEIMPOUNDED

GROUND

WATER

UNCONFINED

GROUND

WATER IN

DIKE-FREE

WAILUKU

BASALT

CONFINED FRESH GROUND 

 WATER IN THE WAILUKU 

 BASALT

LINE OF EQUAL WATER-LEVEL

 ALTITUDE--Contour interval 

 is one foot. Dashed where 

 approximately located. Datum is 

 mean sea level

APPROXIMATE SEAWARD 

 EXTENT OF FRESHWATER IN 

 WAILUKU BASALT FOR 

 PREDEVELOPMENT CONDITIONS

BOUNDARY BETWEEN 

 UNCONFINED AND CONFINED

 WATER IN THE WAILUKU

 BASALT. UNCONFINED 

 GROUND WATER IN 

 SEDIMENTARY DEPOSITS 

 SEAWARD OF BOUNDARY

INFERRED BOUNDARY 

 BETWEEN DIKE-IMPOUNDED 

 GROUND WATER AND 

 FRESHWATER-LENS SYSTEM

 (modified from Yamanaga and 

 Huxel, 1970)

STUDY-AREA (IAO AQUIFER)

 BOUNDARY

PUMPED WELL OR WELL FIELD

WATER-LEVEL OBSERVATION 

 WELL

Kahului

Bay

EXPLANATION

20°56'

20°54'

20°52'

156°32' 156°30'

10

11

12

13

10

10

11

12

13

10

Waihee River

Waikapu Stream

Base modified from U.S. Geological Survey

digital data, 1:24,000, 1983, Albers equal area 

projection, standard parallels 20°35' and 21°00', 

central meridian 156°20'

Figure 25. Mean water levels in the flank flows of Wailuku Basalt in 1998 for the Iao aquifer, Maui, Hawaii.
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 45

-100

150

-50

0

50

100

RAINFALL DEPARTURE OF

12-MONTH MOVING MEANFROM LONG-TERM MEAN,

IN PERCENT

8

28

12

16

20

24

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

Test hole E (5430-03)

Waiehu Camp rain gage (station 484)

0

28

4

8

12

16

20

24

MONTHLY MEAN PUMPAGE

 IN MILLION GALLONS PER DAY

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

YEAR

Iao aquifer

Figure 26. Rainfall departure of backward-looking 12-month moving mean from long-term mean for Waiehu

Camp rain gage, water levels for test hole E, and monthly mean total pumpage for the Iao aquifer, Maui,

Hawaii. (Water-level data prior to 1982 from unpublished data in U.S. Geological Survey well files, Honolulu;

water-level data including and after 1982 from U.S. Geological Survey data base; unpublished rainfall data

from Commission on Water Resource Management and the University of Hawaii Meteorology Department;

pumpage data from Maui Department of Water Supply; monthly pumpage is not available for Waiehu Heights

and Waihee well fields for 1977 and 1978, thus total monthly pumpage is not shown for these 2 years.)
46 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

correspond to the departure of rainfall for the preceding

12 months from mean rainfall, and not pumpage (fig.

26). Water levels failed to mimic the higher rainfall in

1993 and other changes in rainfall thereafter, indicating

that pumpage from 1993 onward had reached a magnitude (18.40 Mgal/d) such that the effect of pumpage on

water levels exceeded the effect of rainfall variation on

water levels.

Seasonal Variation.--In addition to the longterm trend, water levels in test holes A1, B, D, and E

also fluctuated seasonally as much as 2 ft (fig. 19) during 1991–98. The highest water levels were in either

January or February of the year, and the lowest water

levels were in August, September, or October. Seasonal

changes in water levels are probably induced by seasonal changes in rainfall and pumpage. The highest rainfall

months in the Wailuku area are in December through

March, and the lowest rainfall months are in June

through September (Blumenstock and Price, 1967).

Pumpage is generally highest in August through October, and lowest during the winter months.

An indication that seasonal change in water levels is most influenced by pumpage is shown by the comparison of hydrographs prior to pumping (1941–48) for

test holes T-102 and T-112 (fig. 27), to the hydrograph

of test hole E during the more recent period of pumping

(1991–98) (fig. 26). The hydrographs for test holes T102 and T-112 show little seasonal variation and no

apparent correlation with seasonal fluctuations of rainfall, whereas the hydrographs of test holes A1, B, D, and

E (fig. 19) show seasonal fluctuation of water levels corresponding to the seasonal fluctuation in pumpage.

Movement of the Freshwater/Saltwater

Transition Zone

The decline in water levels from increased

pumpage and reduced recharge has resulted in an

upward movement of the saltwater that underlies the

freshwater body. Ultimately, the amount of fresh

ground water that can be developed is limited by the

upward and landward movement of saltwater.

For the purposes of this report, freshwater is

defined as having a chloride concentration of less than

250 mg/L, on the basis of the USEPA secondary maximum contaminant levels for chloride concentration of

domestic water (U.S. Environmental Protection Agency, 1996). The chloride concentration of seawater

around the Hawaiian islands is about 19,600 mg/L

(Wentworth, 1939). The change from freshwater to saltwater at depth in an aquifer is not abrupt; rather, freshwater and saltwater mix and the change is gradual

through the transition zone. The transition zone

includes water with chloride concentrations between

250 mg/L and 19,600 mg/L.

On the basis of the density difference between

freshwater and saltwater, freshwater is sometimes

assumed to extend below sea level to a depth equal to 40

times the water level above sea level, assuming that

hydrostatic conditions prevail. This relation is referred

to as the Ghyben-Herzberg relation (see, for example,

Bear, 1979, p. 560). In practice, the depth defined by the

Ghyben-Herzberg relation approximates the point in the

transition zone where the salinity of the ground water is

equivalent to a mixture of 50 percent freshwater and 50

percent saltwater, and therefore is commonly referred to

as the midpoint (Lau, 1962). This midpoint, however,

may not represent the half-way point of the total thickness of the transition zone. As used in this report, the

term midpoint represents the altitude in a well at which

the chloride concentration is 9,800 mg/L.

Because the amount of water that can be developed from a freshwater lens for potable use is constrained by the salinity of the water, the altitude of the

top of the transition zone and the thickness of the transition zone are of great practical interest. Additionally,

since the approximate altitude of the midpoint of the

transition zone can be estimated from static water levels

in wells, the thickness of the upper part of the transition

zone between the freshwater limit and the midpoint is

also of interest.

Waiehu deep monitor well.--The deep monitor

well, drilled to an altitude of about -1,020 ft, penetrates

most of the transition zone. Since 1985, water samples

for chloride-concentration analysis have been collected

quarterly from discrete depths (fig. 28). Chlorideconcentration data from the Waiehu deep monitor well

would be expected to most reflect changes in the aquifer

as a result of pumping from the Waihee and Waiehu

Heights well fields because the deep monitor well is

located closest to these well fields. Because the well is

uncased, it is possible that the value of chloride concentration of the water collected at any depth in the well is

influenced by borehole flow. Thus, chloride concentration for a given depth may not reflect the actual chloride

concentration of water in the aquifer at that depth.
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 47

20352530

WATER LEVEL, IN FEET

ABOVE MEAN SEA LEVEL

25403035

WATER LEVEL, IN FEET

ABOVE MEAN SEA LEVEL

Test hole T-112 (5330-03)

Test hole T-102 (5331-01)

1940 1941 1942 1943 1944 1945 1946 

0

2010

RAINFALL, IN INCHES Waiehu Camp rain gage (station 484)

YEAR

-100

100

0

RAINFALL DEPARTURE OF

12-MONTH MOVING MEAN

FROM LONG-TERM MEAN,

IN PERCENT

Waiehu Camp rain gage (station 484)

Figure 27. Water levels and rainfall before the start of ground-water withdrawal in the Iao aquifer, Maui, Hawaii. (Unpublish

U.S. Geological Survey well files, Honolulu; unpublished rainfall data from Commission on Water Resource Management aHawaii Meteorology Department.)
48 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

0

30

10

20

0

200

100

0

300

100

200

0

1,400

200

400

600

800

1,000

1,200

0

20,000

5,000

10,000

15,000

0

20,000

5,000

10,000

15,000

85 90 95 00

YEAR

-400 feet

-200 feet

-600 feet

-675 feet

-750 feet

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

-800 feet

Nevertheless, discrete sampling with depth over time

provides a general pattern of transition-zone thickness

and movement.

Chloride concentrations have gradually

decreased since 1985 for altitudes down to -600 ft.

Chloride concentrations have risen and continue to

increase with time at -675 and -750 ft. At -800 ft, chloride concentration initially rose, but has essentially stabilized at 17,000 mg/L.

Chloride concentrations at selected altitudes for

1985, 1990, 1995, and 1998 are shown in figure 29. By

linearly interpolating between the chloride concentrations of the sampling points, the midpoint of the transition zone in August 1985 was about -823 ft. By

September 1990 and August 1995 the midpoint rose to

about -777 ft and -727 ft, respectively. By July 1998, the

midpoint of the transition zone rose to about -715 ft.

The total change in the position of the midpoint from

1985 to 1998 was about 108 ft, corresponding to an

average rate of 8.4 ft/yr.

The top of the transition zone, on the basis of

the interpolation of the sampling points in the well, was

at about -648 ft in 1985 and about -613 ft in 1998. Thus,

the top of the transition zone moved upward about 35 ft

between 1985 and 1998 compared with 108 ft for the

midpoint over the same time period. The thickness of

the upper part of the transition zone, which is the water

between the midpoint and top of the transition zone,

thinned from about 173 ft in 1985 to about 102 ft in

1998. This apparent thinning of the upper part of the

transition zone may be actual, or it may be an artifact

resulting from the interpolation between data points to

determine the depth where water with a chloride concentration of 250 mg/L occurs. Significant borehole

flow, if present, could also influence the conclusion

concerning the thinning of the upper part of the transition zone. Using the chloride-concentration data, it is

probably more appropriate to conclude that the thickness of the upper part of the transition zone is somewhere between 100 to 175 ft, rather than to assume an

actual thickness for the transition zone at a given time

or that it has thinned over time.

Assuming a 40-to-1 Ghyben-Herzberg relation

between the altitude of the midpoint below sea level and

the altitude of the water table, the midpoint altitudes

estimated for 1985, 1990, 1995, and 1998 correspond to

water levels of 20.6, 19.4, 18.2, and 17.9 ft, respectively. The actual water level in the well was lower for each

Figure 28. Chloride concentration with time

for selected altitudes at the Waiehu deep

monitor well (5430-05), Maui, Hawaii, 1985–

98.
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 49 -1,100

0

-1,000

-900-800-700-600-500-400-300-200-100

DEPTH, IN FEET BELOW SEA LEVEL

0 5,000 10,000 15,000 

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

August 1985September 19August 1995July 1998

Figure 29. Chloride concentration with depth for selected depths and dates at the Waiehu deep monitor well (5430-05), Ma
50 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

of these years, however. The water level was between

13 and 14 ft in 1985, 13 to 18 ft in 1990, between 10 to

13 ft in 1995, and between 10 and 11 ft in 1998. The discrepancy between the heads calculated from the

Ghyben-Herzberg relation for the position of the midpoint of the transition zone in 1985, 1990, 1995, and

1998 and the actual water levels for these times is not

surprising. Movement of the transition zone in response

to a change in the position of the water table lags considerably behind the change in the water table (Essaid,

1986). This delay may be on the order of years to tens

of years. The present position of the water table at the

well is 10 to 11 ft above sea level, which corresponds to

an altitude of the midpoint of about -400 to -440 ft. This

indicates that the upward movement of the transition

zone may continue if water levels remain near their

present altitude.

The Ghyben-Herzberg relation assumes, however, that the head in the well does not change vertically

through the thickness of the freshwater lens. (Hydraulic

head at a given point is commonly measured by water

levels in wells that are open to the aquifer only at that

point.) If a vertical head gradient exists at the location

of the Waiehu deep monitor well, such that there is

greater head with depth in the well, then the depth to the

midpoint in equilibrium conditions would actually be

deeper than that calculated using the Ghyben-Herzberg

relation (Izuka and Gingerich, 1998). If so, the upward

movement of the transition zone could possibly halt at a

depth greater than what would be expected if the water

levels in the aquifer are maintained at present levels.

Conversely, if the well is located within the area of the

aquifer with less head with depth in the well, the upward

movement of the midpoint would rise above the level

predicted by the Ghyben-Herzberg relation. However,

as discussed previously, water-level measurements

made in test hole D(open to the aquifer between -68 to

-108 ft) and the Waiehu deep monitor well (open to the

aquifer between -20 and -1,020 ft) indicate that water

levels in the two wells are the same. Thus, in the vicinity

of the deep monitor well, the Ghyben-Herzberg

assumption of no vertical head loss is met.

Changes in the Chloride Concentration of

Pumped Water

Values of chloride concentration of the pumped

water from the well fields are available from 1972 to the

present. Comparison of these values with chlorideconcentration values obtained at the well fields prior to

pumping indicates that chloride concentrations of the

water from pumped wells have increased above predevelopment levels.

Causes for changes in chloride concentration of

the pumped water over time can be a function of pumping rate of the particular well, pumping at nearby wells,

depth of the well, and overall aquifer trends. Therefore,

the evaluation of aquifer conditions based on chlorideconcentration trends from the pumping wells has limitations. If pumpage is held constant, however, changes

in chloride concentration of water from a production

well can be valuable for interpreting chloride-concentration trends at that location.

Despite the increase in chloride concentration

at all of the wells, the chloride concentration of the

pumped water since pumping began has been below the

USEPA maximum contaminant level of 250 mg/L for

all pumped wells in the aquifer, except for the water

from Mokuhau pump 2. Chloride concentration at this

well has been as high as 450 mg/L. Chloride concentrations in pumps 1 and 3 at the Mokuhau well field have

occasionally exceeded 200 mg/L.

Predevelopment chloride concentrations in

shaft 33 were between 18 and 22 mg/L (unpub. data in

well files at U.S. Geological Survey, Honolulu). During

1991–98, shaft 33 produced water with chloride concentrations generally ranging between 30 and 65 mg/L

(fig. 30). In 1998, the chloride concentration was about

40 mg/L.

Predevelopment chloride concentrations at the

Mokuhau well field in 1953 were about 15 mg/L. During an aquifer test in 1964, chloride concentration of the

pumped water was about 30 to 35 mg/L (unpub. data in

well files at U.S. Geological Survey, Honolulu).

Between 1972 and 1977, the chloride concentration of

the pumped water ranged from about 20 to 80 mg/L in

Mokuhau pump 1, from 20 to 90 mg/L in Mokuhau

pump 2, and from 20 to 30 mg/L in Mokuhau pump 3

(fig. 31). During 1977–98, chloride concentration of the

pumped water at Mokuhau well field fluctuated greatly

and generally rose; the pumped water from Mokuhau

pump 1 and pump 3 ranged from 20 to 240 mg/L. In

Mokuhau pump 2, the chloride concentration of the

pumped water ranged from 100 to 360 mg/L during

1977–89 with most of the measurements falling below

220 mg/L, and during 1989–96, the chloride concentration ranged between 100 to 460 mg/L. Mokuhau pump
Aquifer Response to Pumpage, Rainfall, and Land-Use Changes 51

0

2

4

6

8

10

12

14

PUMPAGE, IN MILLION GALLONS PER DAY

1990 1992 1994 1996 1998 2000

0

100

20

40

60

80

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

YEAR

Shaft 33 (5330-05)

monthly mean

annual mean

Shaft 33 (5330-05)

Figure 30. Chloride concentration, monthly mean pumpage, and annual mean pumpage for shaft 33, Maui,

Hawaii. (Data from Maui Department of Water Supply.)
52 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

0

16

4

8

12

PUMPAGE, IN MILLION

GALLONS PER DAY

1970 1975 1980 1985 1990 1995 2000

0

500

100

200

300

400

0

500

100

200

300

400

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

500

0

100

200

300

400

YEAR

Mokuhau pump 1 (5330-10)

Mokuhau pump 2 (5330-09)

Mokuhau pump 3 (5330-11)

monthly mean 

annual mean 

Mokuhau well field (5330-09, -10, -11)

Figure 31. Chloride concentration, monthly mean pumpage, and annual mean pumpage for the Mokuhau well

field, Maui, Hawaii. (Data from Maui Department of Water Supply.)
Sustainable Yield 53

2 has produced the saltiest water from the aquifer and

23 percent of the samples collected between 1972 and

1996 have exceeded the USEPA standard. Mokuhau

pump 2 has not been used since late 1996.

Predevelopment chloride concentration of

water collected during initial tests of the production

wells at the Waiehu Heights well field in 1975 ranged

between 20 and 52 mg/L (unpub. data in well files at

U.S. Geological Survey, Honolulu). Since then, the well

field has shown a pattern of increasing chloride concentration of the pumped water (fig. 32), similar to that at

the Mokuhau well field. Waiehu Heights 1, which is the

deepest production well in the aquifer at 338 ft below

sea level, had a general increase in chloride concentrations of the pumped water from the start of pumping in

1977 to 1989 with values that ranged between about 15

and 100 mg/L. During 1989–98, chloride concentration

of the pumped water increased, ranging from about 80

to 160 mg/L. In the Waiehu Heights 2, chloride concentration ranged from about 15 to 60 mg/L during 1977–

86, and ranged from about 25 to 100 mg/L during 1986–

98.

Predevelopment chloride concentration of

water at Waihee well field was about 15 mg /L as measured in Waihee 1 and 2 in late 1976 (unpub. data in

well files at U.S. Geological Survey, Honolulu). Water

from Waihee 1 and 2 remained at nearly the same chloride concentration during 1980–87, with values that

ranged between 10 and 20 mg/L (fig. 33). During 1987–

98, chloride concentrations ranged between about 10

and 60 mg/L. Chloride concentration in the water

pumped from Waihee 3 was initially recorded in 1985.

The initial value, 16.5 mg/L, is similar to that recorded

in the other two wells at this time. Chloride concentration of water pumped from this well remained similar to

those measured in Waihee 1 and 2 during 1985–95.

During 1995–98, concentrations ranged from 15 to 30

mg/L.

Predevelopment chloride concentration of

water at Kepaniwai well, which was drilled in 1974,

was 24 mg/L (unpub. data in well files at U.S. Geological Survey, Honolulu). During 1977–86, chloride concentrations of the pumped water ranged from about 16

to 24 mg/L (fig. 34). Since the connection of the well to

the municipal distribution system in 1991, chloride concentrations have ranged from about 10 mg/L to more

than 50 mg/L. Chloride concentration of the pumped

water has remained at about 20 mg/L since late 1995.

SUSTAINABLE YIELD

The sustainable yield of the Iao aquifer was set

at 20.0 Mgal/d by CWRM (Commission on Water

Resource Management, 1990). As defined by the State

of Hawaii, sustainable yield refers to the amount of

water that can be withdrawn from an aquifer indefinitely, without affecting either the quality of pumped water

or the rate of pumping (Commission on Water Resource

Management, 1990). Hydrologically, in a setting such

as the Iao aquifer, the upward movement of the transition zone caused by pumping is the process that is generally expected to limit ground-water development.

Although the value for sustainable yield of the Iao aquifer is presently set at 20 Mgal/d, the response of the

aquifer to pumping (in terms of water-level declines and

the rise in the freshwater/saltwater interface) has raised

questions concerning this value, at least with regard to

existing rates and distribution of pumping.

The ability to maintain a pumping rate of about

20 Mgal/d at existing well fields was examined by comparing the estimated top of the transition zone for average 1996 water levels to the depths of the production

wells, assuming that water levels had stabilized. Water

levels were still declining in 1996 with pumpage at

20.35 Mgal/d, however. Thus, if the calculations indicate intrusion of the transition zone into the wells for

assumed stable water levels, sustained pumpage at

20.35 Mgal/d from existing infrastructure would not be

feasible.

The first step in calculating the top of the transition zone involved use of the Ghyben-Herzberg relation to calculate the altitude of the midpoint of the

transition zone. The altitude of the top of the transition

zone was then estimated by adding the thickness of the

upper part of the transition zone as indicated by field

data at the Waiehu deep monitor well to the altitude of

the midpoint. The altitude of the top of the transition

zone was then compared to the altitude of the bottom of

the wells in the major well fields to estimate the difference between the two (table 6). Two values (100 and

175 ft) for the thickness of the upper part of the transition zone were used in these calculations owing to the

uncertainty in the actual value for the thickness.

The results of the above calculations indicate

that the transition zone would ultimately intrude into

Waiehu Heights 1 for a thickness of the upper part of the

transition zone of 100 ft (fig. 35). The top of the
54 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

0

4

1

2

3

PUMPAGE, IN MILLION

GALLONS PER DAY

1970 1975 1980 1985 1990 1995 2000

0

200

50

100

150

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

0

200

50

100

150

YEAR

Waiehu Heights 1 (5430-01)

Waiehu Heights 2 (5430-02)

monthly mean

annual mean

Waiehu Heights well field (5430-01 and -02)

Figure 32. Chloride concentration, monthly mean pumpage, and annual mean pumpage for the Waiehu

Heights well field, Maui, Hawaii. (Data from Maui Department of Water Supply.)
Sustainable Yield 55

0

12

2

4

6

8

10

PUMPAGE, IN MILLION

GALLONS PER DAY

1970 1975 1980 1985 1990 1995 2000

0

140

50

100

0

140

50

100

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

0

140

50

100

YEAR

Waihee 1 (5431-02)

Waihee 2 (5431-03)

Waihee 3 (5431-04)

monthly mean

annual mean

Waihee well field (5431-02, -03, -04)

Figure 33. Chloride concentration, monthly mean pumpage, and annual mean pumpage for the Waihee

well field, Maui, Hawaii. (Data from Maui Department of Water Supply.)
56 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

0

2

1

PUMPAGE, IN MILLION GALLONS PER DAY

1970 1975 1980 1985 1990 1995 2000

0

80

10

20

30

40

50

60

70

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

YEAR

Kepaniwai well (5332-05)

monthly mean

annual mean

Kepaniwai well (5332-05)

Figure 34. Chloride concentration, monthly mean pumpage, and annual mean pumpage for the Kepaniwai

well, Maui, Hawaii. (Data from Maui Department of Water Supply.)
Sustainable Yield 57

transition zone would be below other wells by values

ranging from 28 to 106 ft for this thickness of the transition zone. For a thickness of the upper part of the transition zone equal to 175 ft, the transition zone intrudes

into shaft 33, the two wells at Waiehu Heights, and just

reaches Waihee 1. The top of the transition zone is still

below Waihee 2 and 3 by 31 and 24 ft, respectively.

The 1996 water levels are not available for the

Mokuhau well field so that the above calculations were

made for this well field using average 1998 water levels. Available data indicate that 1998 water levels at

Mokuhau well field were higher than those in 1996 and

thus the use of 1998 water levels is somewhat more conservative than the use of 1996 water levels at the other

well fields.

Calculations for the Mokuhau well field indicate that the top of the transition zone would be 49 to 53

ft below the bottom of the Mokuhau wells for a thickness of the upper part of the transition zone equal to 100

ft. The top of the transition zone would intrude into the

wells, however, for a thickness of the upper part of the

transition zone equal to 175 ft.

Given the above results and the continuing

decline in water levels in 1996 for a pumping rate of

20.35 Mgal/d, it is unlikely that 20 Mgal/d can be

obtained from existing infrastructure on a long-term

basis. Pumping in the Iao aquifer area is concentrated in

the middle one-third of the area which thus intensifies

water-level declines. In freshwater lenses subject to

saltwater intrusion, development of freshwater is best

accomplished by minimizing drawdowns at pumping

centers (McWhorter and Sunada, 1977, p. 162). Therefore, total yield from the aquifer could be enhanced,

above that possible with the existing distribution of

pumpage, if wells were distributed more evenly

between Waikapu Stream and Waihee River, thereby

reducing water-level decline at any given location in the

aquifer for the same amount of pumpage.

Away from a well or well field, the decline in

water levels induced by pumping is reduced, and as a

1 Estimated from average 1996 water levels in test holes B and E, and Waiehu deep monitor well

Table 6. Altitude of bottom of wells and estimated altitude of the top of the transition zone at pumped wells in the freshwater lens

part of the Iao aquifer, Maui, Hawaii, for assumed stabilized 1996 and 1998 water levels, and for estimated thicknesses of the

upper part of the transition zone of 100 ft (column a) and 175 ft (column b)

[All values in feet; --, no measurement; datum is mean sea level]

Well field Well

Altitude of

bottom of

well

 Average

water

level in

1996

Estimated

altitude of top

of transition

zone using

1996 water levels

Estimated

difference

between bottom

of well and

top of transition

zone for 1996

Average

water

level in

1998

Estimated

altitude of top

of transition

zone using

1998 water levels

Estimated

difference

between bottom

of well and

top of transition

zone for 1998

a b a b ab ab

Shaft 33 unused pumping well-280 10.2 -308 -233 28 -47 -- -- -- -- --

pumping well -280 -- -- -- -- -- -- -- -- -- --

Mokuhau pump 1 -247 -- -- -- -- -- -- -- -- 53 -22

pump 2 -247 -- -- -- -- -- 10.0 -300 -225 53 -22

pump 3 -251 -- -- -- -- -- -- -- -- 49 -26

Waiehu

Heights

 1 -338 19.4 -276 -201 -62 -137 -- -- -- -- --

 2 -206 -- -- -- 70 -5 -- -- -- -- --

Waihee 1 -182 18.9 -256 -181 74 -1 -- -- -- -- --

 2 -150 -- -- -- 106 31 -- -- -- -- --

 3 -157 -- -- -- 99 24 -- -- -- -- --
58 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii 600 -400 -300 -200 -100 100 SEA LEVEL FEET 200 300 400 500

Waihee 3 (5431-04)

Mokuhau pump 1 

(5330-10)

Waiehu Heights 

2 (5430-02)

Waiehu Heights 

Mokuhau pump 2

(5330-09)

Mokuhau pump 3

(5330-11)

1 (5430-01)

Waihee 1 (5431-02)

Waihee 2 (5431-03)

Waihee well field

Waiehu Heights well field

Mokuhau well field

solid casing

screenopen hole

100 ft thick

175 ft thick

175 ft thick

175 ft thick 

100 ft thick

100 ft thick 

EXPLANATION

Figure 35. Altitude of the top of the transition zone for estimated thicknesses of the upper part of the transition zonative to the bottoms of pumping wells in the Iao aquifer area, Maui, Hawaii, using assumed stabilized 1996 or 1998 
Summary and Conclusions 59

result, the area of greatest upward movement of the

transition zone is at the well or well field. Depending on

spacing between wells, pumpage from one well may

cause water-level declines in the other wells. This

results in a greater upward movement of the transition

zone in excess of that caused by pumping at that particular well. Within the Iao aquifer, for example, pumping

at Waihee well field is affecting water levels at Waiehu

Heights and vice versa. Pumping from both well fields

may also be causing water levels at the Mokuhau well

field and shaft 33 to decline. However, sufficient data

are not available to permit a complete analysis of the

influence one well field in the Iao aquifer has on others.

Not only is the areal distribution of wells

important with regard to the amount of water that can be

developed, the depth of a given well in comparison to

the altitude of the transition zone is also of major importance. The closer the well bottom is to the transition

zone before pumping is started, the less water-level

decline in the well is required for saltwater to intrude

the well. As a result, yield from a well can potentially

be increased simply by raising the altitude of the bottom

of the well relative to the altitude of the top of the transition zone.

SUMMARY AND CONCLUSIONS

The Iao aquifer is located on the eastern side of

the West Maui Mountain on the island of Maui. The

mountain is primarily composed of volcanic rocks from

West Maui Volcano, which are overlain by sedimentary

deposits in stream valleys and near the coast. The volcano is mostly composed of Wailuku Basalt, and the Iao

aquifer is primarily within this rock unit. The aquifer is

actually part of a larger regional ground-water system

because the Wailuku Basalt and sediments extend

beyond the geographic boundaries of the Iao aquifer.

Ground water in the Iao aquifer occurs as: (1) dikeimpounded water in the dike complex, (2) a freshwaterlens system in the dike-free volcanic rocks and sedimentary deposits, and (3) as water perched in the sedimentary deposits.

Recharge to the aquifer is from the infiltration

of rainfall and fog drip, from seepage through streambeds that overlie the water table, and from irrigation.

The predevelopment rate of ground-water recharge to

the aquifer was about 34 Mgal/d. With the introduction

of sugarcane agriculture in 1856, recharge increased to

as high as 51 Mgal/d owing to irrigation in some of the

areas of the aquifer. The irrigation water has primarily

been supplied by diverted streamflow and development

tunnels in Waihee River valley and Waiehu, Waikapu,

and Iao Stream valleys. Irrigation practices have

changed through time, and this has resulted in a

reduction of recharge to about 40 Mgal/d during 1980–

85, and to about 36 Mgal/d during 1986–95.

Ground water from the Iao aquifer has been

used for agricultural and domestic supply. Development of ground water from the Iao aquifer began with

the construction of tunnels that penetrate into dike compartments to divert the dike-impounded water and with

the improvement of springs. The total amount of ground

water developed by the tunnels was about 9.0 Mgal/d in

1970. Large-scale withdrawal of ground water from the

dike-free part of the Iao aquifer began in 1948 with the

completion of shaft 33. Water from shaft 33 was used

for agriculture during 1948–84, with annual average

withdrawal ranging from zero to 11.65 Mgal/d. The

Mokuhau well field was the second major area of

ground-water withdrawal from Iao aquifer. Two wells

were constructed in 1953 and a third well was added to

the well field in 1967. The Waihee and Waiehu Heights

well fields were added in 1977 and 1979, respectively,

to supply the increasing domestic demand. Shaft 33 was

refurbished to supply domestic water in 1991. The

annual average rate of ground-water withdrawal from

the aquifer has varied widely, but generally has

increased with time, reaching more than 20 Mgal/d in

1995 and 1996. Pumpage has diminished by about 1

Mgal/d each in 1997 and 1998.

Ground-water data indicate that withdrawal

from the freshwater lens from four well fields, and the

reduction of recharge to the aquifer, have resulted in a

decline of ground-water levels and a corresponding rise

and upward movement of the transition zone. Waterlevel data, collected since 1940, have responded to seasonal variations in pumpage, the overall long-term

increase in pumpage, and to changes in recharge resulting from variations in rainfall and changes in land use.

Water levels generally decreased between 1990 and

1998. Water samples for chloride-concentration analysis from the Waiehu deep monitor well have been collected quarterly from discrete depths since 1985. The

chloride-concentration data from the well indicate that

the midpoint of the transition zone rose about 108 ft

from 1985 to 1998, corresponding to an average rate of

8.4 feet per year.
60 The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii

In 1990, the Hawaii State Commission on

Water Resource Management established a value of

20.0 Mgal/d for the sustainable yield of the Iao aquifer

area. Water-level data and changes in the vertical extent

of freshwater measured at the Waiehu deep monitor

well, however, indicate that pumping rates near the sustainable yield of 20.0 Mgal/d could result in saltwater

intrusion in some of the existing wells. The withdrawal

from the Iao aquifer is concentrated in the middle onethird of the area. Also, some of the wells are deep relative to the altitude of the top of the transition zone. In

aquifers susceptible to saltwater intrusion, the maximum development of ground water generally can be

obtained by distributing pumpage to minimize waterlevel decline at any location for a given pumping rate,

and by using shallower wells.

REFERENCES CITED

Bear, Jacob, 1979, Hydraulics of groundwater: New York,

McGraw-Hill, 569 p.

Blumenstock, D.I., and Price, Saul, 1967, Climates of the

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Commission on Water Resource Management, 1990, Water

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Yuen and Associates, Inc., for Department of Land and

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Cox, D.C., 1951, Lowland ground-water development prospects in and north of Iao Valley, Wailuku, Maui: Honolulu, Hawaii, Hawaii Sugar Planter’s Association, 12 p.

Essaid, H.I., 1986, A comparison of the complex fresh water–

salt water flow and the Ghyben-Herzberg SHARP interface approaches to modeling of transient behavior in

coastal aquifer systems: Journal of Hydrology, v. 86, p.

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Giambelluca, T.W., Nullet, M.A., and Schroeder, T.A., 1986,

Rainfall atlas of Hawaii: State of Hawaii, Department of

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Land Development, Report R76, 267 p.

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1412-B, 54 p.

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Juvik, J.O., and Ekern, P.C., 1978, A climatology of mountain fog on Mauna Loa, Hawai’i island: University of

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volcanic rocks of the Hawaiian islands, chap. 1 of

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hydraulic model studies: Honolulu, Hawaii, City and

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Macdonald, G.A., Abbott, A.T., and Peterson, F.L., 1983,

Volcanoes in the sea, the geology of Hawaii (2d ed.):

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hydrology and hydraulics: Fort Collins, Colo., Water

Resources Publications, 163 p.

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in Geology and water resources of the Kau district,

Hawaii: U.S. Geological Survey Water-Supply Paper

616, p. 1–28.

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Circular C61, 43 p.
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