GROUND-WATER AVAILABILITY FROM THE HAWI AQUIFER 

IN THE KOHALA AREA, HAWAII

By Mark R. Underwood William Meyer, and William R. Souza

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 95-4113

Prepared in cooperation with the

DEPARTMENT OF WATER SUPPLY 

COUNTY OF HAWAII

Honolulu, Hawaii 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government

For sale by the U.S. Geological Survey 

Earth Science Information Center 

Open-File Reports Section 

Box 25286, MS 517 

Denver Federal Center 

Denver, CO 80225

For additional information write to:

District Chief

U.S. Geological Survey

677 Ala Moana Blvd., Suite 415

Honolulu, HI 96813
CONTENTS

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

Introduction ............................................................ 2

Scope............................................................ 2

Previous investigations.............................................. 6

Description of study area .................................................. 8

Geology............................................................... 10

Ground-water occurrence ................................................. 13

Ground-water development ......................................... 17

Ground-water quality .............................................. 19

Vertical extent of freshwater......................................... 23

Hydraulic conductivity of the basal aquifer ................................... 26

Numerical simulation of the basal aquifer .................................... 32

Description of the model. ........................................... 32

Calibration of the steady-state model.................................. 36

Response of the basal aquifer to ground-water pumpage................... 44

Summary and conclusions ................................................ 55

References cited ........................................................ 56

in
FIGURES

1. Map showing Hawaiian islands and aquifer systems in the Kohala area,

island of Hawaii............................................... 3

2. Generalized section of Kohala Mountain showing basal aquifer and

direction of ground-water movement. .............................. 4

3-7. Maps showing:

3. Mean annual rainfall, Kohala area, island of Hawaii. ..................... 5

4. Streams, ditches, and tunnels in the Kohala area, island of Hawaii .......... 7

5. Surficial geology of the Kohala area, island of Hawaii................... 11

6. Ground-water recharge, Kohala area, island of Hawaii. .................. 14

7. Observed ground-water levels in the Hawi basal aquifer and test-well

sites, Kohala area, island of Hawaii. .............................. 15

8. Graph showing water levels measured in well 7347-02 and shaft 7652-01,

Kohala area, island of Hawaii ................................... 16

9. Map showing locations of existing and historic wells, existing tunnels, and

test holes in the Kohala area, island of Hawaii ...................... 18

10. Graph showing chloride concentrations and depth for wells D and I,

Kohala area, island of Hawaii ................................... 24

11-12. Maps showing:

11. Aquifer-test values for horizontal hydraulic conductivity, Kohala area,

island of Hawaii.............................................. 27

12. Model grid and boundaries used in the numerical simulation,

Kohala area, island of Hawaii ................................... 33

13-14. Graphs showing:

13. Model-calculated water levels at test wells E and H, and values of

hydraulic connection, K'/m', between the basal aquifer and the ocean, 

Kohala area, island of Hawaii ................................... 37

14. Model-calculated water levels and observed water levels for various

distributions of hydraulic conductivity (K) and the hydraulic connection 

between aquifer and ocean (K'/m'), Kohala area, island of Hawaii....... 39

IV
FIGURES-Continued

15-24. Maps showing:

15. Distribution of horizontal hydraulic conductivity and hydraulic 

connection between the aquifer and ocean as modeled, 

Kohala area, island of Hawaii ................................... 40

16. Model-calculated and observed ground-water levels, Kohala area,

island of Hawaii.............................................. 42

17. Model-calculated depth of the freshwater-saltwater interface,

Kohala area, island of Hawaii ................................... 43

18. Pumping rates used for the two simulated pumping scenarios,

Kohala area, island of Hawaii ................................... 45

19. Model-calculated drawdown for pumping scenario 1, Kohala area,

island of Hawaii.............................................. 46

20. Model-calculated drawdown for pumping scenario 2, Kohala area,

island of Hawaii.............................................. 47

21. Depth of model-calculated fresh water-salt water interface for pumping

scenario 1, Kohala area, island of Hawaii.......................... 49

22. Depth of model-calculated freshwater-saltwater interface for pumping

scenario 2, Kohala area, island of Hawaii.......................... 50

23. Model-calculated ground-water levels for pumping scenario 1,

Kohala area, island of Hawaii ................................... 53

24. Model-calculated ground-water levels for pumping scenario 2,

Kohala area, island of Hawaii ................................... 54
TABLES

1. Concentrations of common ions and other water-quality characteristics of

water samples from the Hawi basal aquifer, Kohala area, island of Hawaii .......... 20

2. Concentrations of trace metals and organic compounds in water samples from

the Hawi basal aquifer, Kohala area, island of Hawaii. .......................... 21

3. Summary of characteristics of wells used for exploration and aquifer testing

of the Hawi basal aquifer, Kohala area, island of Hawaii. ........................ 28

4. Summary of aquifer-test data and results, Kohala area, island of Hawaii ................ 31

5. Steady-state ground-water budget (existing pumpage) for the numerical

model, Kohala area, island of Hawaii. ....................................... 41

6. Water levels and interface locations at simulated pumping sites, Kohala area,

island of Hawaii ........................................................ 51

7. Steady-state ground-water budget (existing plus simulated pumpage) for the

numerical model, Kohala area, island of Hawaii ............................... 51

Conversion Factors

Multiply By To obtain

acre 

foot (ft)

foot per mile (ft/mi)

foot per day (ft/d)

foot per day per foot (ft/d)/ft

cubic foot per second (ft3/s)

gallon per minute (gal/min)

million gallons per day (Mgal/d)

mile (mi)

ij

square mile (mi )

inch (in.)

inch per year (in/yr)

4,047

0.3048

0.1894

0.3048

1

0.02832

0.06308

0.04381

1.609

2.590

25.4

25.4

square meter

meter

meter per kilometer

meter per day

meter per day per meter

cubic meter per second

liter per second

cubic meter per second

kilometer

square kilometer

millimeter

millimeter per year

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

using the equation:

°F = (1.8 x°C) +32

Abbreviations used in water quality descriptions:

|LLg/L, micrograms per liter ppm, parts per million

o

mg/L, milligrams per liter gm/cm , grams per cubic centimeter 

|LiS/cm, microsiemens per centimeter at 25° Celsius

VI
GROUND-WATER AVAILABILITY FROM THE HAWI AQUIFER 

IN THE KOHALA AREA, HAWAII

by

Mark R. Underwood, William Meyer, and William R. Souza

ABSTRACT

A ground-water study consisting of test-well drilling, aquifer tests, and numerical 

simulation was done to investigate ground-water availability in the basal part of the Hawi aquifer 

between the western drainage divide of Pololu Valley and Upolu Point in Kohala, Hawaii. The test 

well drilling provided information on geology, water levels, water quality, vertical extent of the 

freshwater, and the thickness of the freshwater-saltwater transition zone in that aquifer. A total of 

12 test wells were drilled at eight locations. Aquifer tests were done at five locations to estimate 

the hydraulic conductivity of the aquifer. Using information on the distribution of recharge, 

vertical extent of freshwater, hydraulic conductivity, and geometry of the basal aquifer, a 

numerical model was used to simulate the movement of water into, through, and out of the basal 

aquifer, and the effect of additional pumping on the water levels in the aquifer.

Results of the modeling indicate that ground-water withdrawal of 20 million gallons per 

day above the existing withdrawal of 0.6 million gallons per day from the basal aquifer is 

hydrologically feasible, but that spacing, depth, and pumping rates of individual wells are 

important. If pumping is concentrated, the likelihood of saltwater intrusion is increased. The 

additional withdrawal of 20 million gallons per day would result in a reduction of ground-water 

discharge to the ocean by an amount equal to pumpage. Although model-calculated declines in 

water-level outside the area of pumping are small, pumping could cause some reduction of 

streamflow near the mouth of Pololu Stream.
INTRODUCTION

The Kohala area of the island of Hawaii is the northwest peninsula of the island (fig. 1). 

The area is dominated by the asymmetrical, elongated Kohala Mountain. Kohala Mountain is an 

extinct volcanic dome reaching an altitude of 5,605 ft and is the oldest of the five volcanoes 

forming the island. The Hawaii Water Resources Protection Plan (George A.L. Yuen and 

Associates, Inc., 1992) delineated three aquifer systems in the Kohala area: Hawi, Waimanu, and 

Mahukona (fig. 1). Ground water in these systems is found as basal ground water along the flanks 

of the volcanic dome, and as high-level water in the rift zones (Stearns and Macdonald, 1946) (fig. 

2). The basal ground water extends from the outer edges of the rift zones to a discharge area 

seaward of the shoreline, and is a roughly lens-shaped body of freshwater floating on seawater. The 

maximum altitude of the water table of the basal water is unknown.

The windward (northern) side of Kohala Mountain is relatively cool and wet, receiving 

more than 160 in/yr of rainfall near the summit (fig. 3). The leeward (southern) side of the 

mountain is in the rain shadow of Kohala Mountain. Average rainfall is less than 10 in/yr along the 

coast near Kawaihae. Tourism on the leeward side of the Kohala area is expected to grow 

considerably because of the dry, warm climate. One of the primary limitations for growth and 

development in this area is the availability of water. The Hawaii County Department of Water 

Supply (DWS) estimates that water demand in the year 2005 will increase by about 20 Mgal/d 

more than the present use of 0.6 Mgal/d (Thompson, 1988). Ground water is the preferred source 

to supply the projected demand, because of its quality, reliability, and lack of required treatment as 

compared with surface water.

To address an increased demand for water, the DWS and the U.S. Geological Survey 

(USGS) began a cooperative study to investigate ground-water availability from the basal part of 

the Hawi aquifer. This area was selected because of its high rainfall and because of the known 

existence of an extensive basal aquifer. This report describes the results of the study and includes 

a summary of exploratory well drilling and aquifer tests, and numerical simulation of the changes 

in the aquifer resulting from increased pumpage of 20 Mgal/d.

Scope

The primary study area for the field investigations was the basal part of the Hawi aquifer 

between the western drainage divide of Pololu Stream and Upolu Point (fig. 1). This basal aquifer 

extends to the southeast beyond the Pololu drainage divide and is contiguous with the basal part of 

the Waimanu aquifer. The Waimanu aquifer was not considered for ground-water development 

because of its rugged terrain; however, the Pololu drainage basin, which is within the Waimanu 

aquifer (fig. 1), was included in the numerical ground-water flow model constructed for this study. 

Because the basal aquifers are part of a larger ground-water system within the Kohala area, it was 

necessary to quantify rates of ground-water movement into the basal aquifers from areas adjacent
155° 50' 155° 40'

Upolu Point

4 MILES

20° 10'

20° 00'

160° 158° 156°

EXPLANATION 

HAWI AQUIFER SYSTEM 

WAIMANU AQUIFER SYSTEM 

MAHUKONA AQUIFER SYSTEM 

DRAINAGE DIVIDE 20°

Kauai HAWAII

Niihau Oahu

Lanai 

Kahoolawe

Molokai

Maui

Area of 

map

Figure 1 . Hawaiian islands and aquifer systems in the Kohala area, island of Hawaii 

(aquifer systems are from George A. L. Yuen and Associates, Inc., 1992).
SOUTHWEST NORTHEAST

FEET 

6,000 

5,000 - 

4,000 

3,000 

2,000 - 

1,000 - 

Sea level 

-1,000 

-2,000 -J

-3,000

RIFT ZONE

Kohala 

Mountain

01] leye |_____Basal_water_table^ _ 

__w Freshwater

Ocean

Saltwater

2 MILES

Vertical scale greatly exaggerated

i I

2 KILOMETERS

Figure 2. Generalized section of Kohala Mountain showing basal aquifer and 

direction of ground-water movement.
155° 50' 155° 40'

20° 10' 

20° 00' 

80

EXPLANATION

LINE OF EQUAL ANNUAL RAINFALL 

Interval, in inches, is variable

DRAINAGE DIVIDE

Figure 3. Mean annual rainfall, Kohala area, island of Hawaii (from Giambelluca 

and others, 1986).
to the basal aquifers. This information was provided in a study of ground-water recharge for the 

entire Kohala area by Shade (in press).

A program of test-well drilling was instituted in the Hawi basal aquifer to obtain 

information on geology, water levels, water quality, and vertical extent of freshwater and the 

transition zone. A total of 12 test wells were drilled at eight locations at altitudes ranging from 

about 108 to 630 ft above sea level (see fig. 7). Aquifer tests were done at five locations to estimate 

the horizontal hydraulic conductivity of the aquifer. Using information on the distribution of 

recharge, vertical extent of freshwater, horizontal hydraulic conductivity, and geometry of the 

basal aquifer, a numerical model was constructed to simulate the movement of water into, through, 

and out of the basal aquifer.The model was then used to estimate the hydrologic feasibility of 

developing 20 Mgal/d of fresh ground water in addition to the existing use of 0.6 Mgal/d.

Previous Investigations

Early studies in the Kohala area focused on streamflow in the area southeast of Pololu 

Stream. During a 10-month period in 1890, Lydgate measured streamflow in Waipio Valley (Davis 

and Yamanaga, 1963). During a 5-month period in 1889 and 1890, Brunner (Davis and Yamanaga, 

1963) measured flow in the 17 streams that are tributary to Honokane Nui Stream (fig. 4). In 1901 

and 1902, Tuttle (Martin and Pierce, 1913) investigated water resources in the Honokane Nui and 

Waipio Valleys as well as regions southeast of Waipio Valley by collecting information on 

streamflow, springs, and wells. Systematic streamflow measurement in the Kohala area was begun 

in 1907. Since 1912, stream and irrigation discharge ditch data have been published annually by 

the USGS. Many of the stream and ditch measurements were discontinued when sugarcane 

production was halted in the early 1970's.

The geology and ground-water resources of the island of Hawaii were described by Stearns 

and Macdonald (1946) who also provided a conceptual model of the hydrologic system of the 

Kohala area. Hydrologic information provided by Stearns and Macdonald (1946) was updated by 

Davis and Yamanaga (1963). A thorough inventory of basic water resources and historical records 

was compiled by the State of Hawaii (1970).

A project by the Kohala Sugar Company in 1964 drilled five successful observation holes 

into the Hawi basal aquifer between Upolu Point and Pololu Stream (Bowles and others, 1974). A 

water-level map was drawn from the data obtained from these holes and from existing data from 

wells in the area. Bowles and others (1974) evaluated surface-water and ground-water resources in 

the Kohala area with a focus on the flow in the Kohala ditch (fig. 4). Using a simplified water 

budget and Darcy's law they estimated that 40 to 45 Mgal/d flowed through the Hawi aquifer; of 

that quantity, 30 to 35 Mgal/d (75 percent of flow through the system) was available for 

development.
155° 50' 155° 40'

20° 10'

20° 00'

EXPLANATION 

DRAINAGE DIVIDE 

ROAD 

STREAM 

DITCH

Figure 4. Streams, ditches, and tunnels in the Kohala area, island of Hawaii.
DESCRIPTION OF STUDY AREA

The Hawi basal aquifer is bounded along the southeast by the Pololu drainage divide and 

along the north and northwest by the Pacific Ocean. The southern and southwestern boundary is 

the rift zone along the crest of Kohala Mountain that extends from the summit toward the coast 

near Upolu Point (fig. 1). A slightly extended study area for modeling purposes included the 

continuation of the basal aquifer into the drainage area of Pololu Stream (fig. 4).

Altitude of the land surface on the Kohala Mountain increases gradually from sea level at 

the shoreline to 5,605 ft above sea level at the mountain summit (fig. 1). In general, the surface 

relief of the study area is moderately dissected. It is covered dominantly by pastures and orchards, 

with forests covering upland areas. Sugarcane was produced over much of this area from the early 

1900's until the early 1970's. Two major irrigation ditches, Kohala and Kehena (fig. 4), were 

constructed in the early 1900's to transport water from the area east of Pololu Stream into the study 

area to support agriculture.

The Kohala ditch, constructed in 1901-02 by the Kohala Sugar Company, presently diverts 

water from the east branch of Honokane Nui Stream at an altitude of 2,000 ft. The ditch carries the 

water for 18 mi northwest, mostly as tunnel, toward Hawi where it delivers most of its water to the 

intake of a hydroelectric plant near Hawi (fig. 4). The ditch also captures minor flows from springs 

and streams west of Pololu Stream.

During 1928-60, median monthly flow rates in the Kohala ditch out of Honokane Nui 

Stream ranged from 21.9 to 31.9 Mgal/d (Bowles and others, 1974). Present flow rates are about 

10 to 15 Mgal/d. Much of the decline in flow in recent years can be attributed to the loss of water 

previously derived from streams east of Honokane Nui Stream. At the hydroelectric plant at Hawi, 

the water released by the plant is injected into the basal aquifer through a holding pond into a row 

of five shallow injection wells located about 1.7 mi inland at about 520 ft altitude. Depths of these 

injection wells range from 160 to 240 ft altitude and lateral spacing between them is about 35 ft. 

The average rate of injection is about 8 Mgal/d.

The Kehena ditch diverts water from an intake at Honokane Nui Stream above the canyon 

rim at an altitude of about 4,200 ft (fig. 3) and carries the water westward for about 8 mi. Discharge 

measurements were made in the ditch from 1917 through 1919 and from 1928 through 1966. 

Average discharge in the ditch for these periods was 7.38 Mgal/d. In recent years the ditch has 

fallen into disrepair and does not flow continuously.

The extended study area, included for purposes of ground-water modeling, contains the 

Pololu Stream drainage area. As discussed by Stearns and Macdonald (1946, p. 228), the source of 

water in Pololu Stream is from springs in its canyon walls below the dike zone and Stearns and 

Macdonald have described these springs as perched. Pololu Stream is sufficiently entrenched near 

the ocean to presumably intersect the basal aquifer. If this occurs, the area near the mouth of the 

stream would be expected to gain water from the Waimanu basal aquifer.
The geohydrologic setting of that part of the Waimanu aquifer beyond Pololu Valley is 

highly complex and to some extent poorly understood. The work of Stearns and Macdonald (1946) 

remains the definitive report for the area, but even so, the extent of the high-level and basal aquifers 

and the relation between surface water and ground water is unclear. Stearns mapped numerous 

dikes and dike-associated springs and described an area of high-level water about 6 mi wide 

underlying the higher part of Kohala Mountain (Stearns and Macdonald, 1946, p. 228). This width 

would extend the high-level aquifer considerably into the Waimanu aquifer. Stearns' description, 

however, does not allow the extent of the high-level aquifer to be closely delineated. Ground water 

discharging from behind dikes causes streams to gain along their entire course to the ocean, 

indicating that most of the area may consist of high-level water. However, Stearns and Macdonald 

mapped much of the area as basal aquifer. Three streams, Pololu, Waimanu, and Waipio may 

intersect the basal aquifer near the ocean, but the other streams are hundreds to thousands of feet 

above sea level along their entire courses and discharge to the ocean from high cliffs.
GEOLOGY

Kohala Mountain was formed by volcanic rocks derived from the now-extinct Kohala 

Volcano. The mountain is composed mainly of basaltic and andesitic (Pololu Basalt and Hawi 

Volcanics, respectively) lavas that erupted from two main rift zones that trend N 35° W and S 65° 

E from Kohala summit. These rift zones are referred to as the northwest and southeast rift zones, 

respectively. The estimated location and orientation of the central part of the rift zones are shown 

in figure 5. The estimated outer margin of the rift zone in the Waimanu aquifer, on the basis of 

mapped dikes (Stearns and Macdonald, 1946, plate 1), is also shown (figs. 1 and 5). The southern 

flanks of Kohala Mountain merge with Mauna Kea Volcano and form a saddle that slopes toward 

the eastern and western coasts of the island of Hawaii.

Pololu Basalt.~The Pololu Basalt contains a thick sequence of basalt lava flows composed 

of hundreds of individual pahoehoe and aa lava flows that range from a few to 50 ft in thickness 

and dip from 3 to 10 degrees away from the rift area (Stearns and Macdonald, 1946, p. 174). The 

basal aquifer is contained within lava flows of the Pololu Basalt. Lavas of the Pololu Basalt make 

up most of Kohala Mountain above sea level, and, because of subsidence of the island, extend an 

unknown distance below sea level.

Most of the eruptive areas of the Pololu Basalt have been buried by later lava flows of the 

Hawi Volcanics. Typically the upper 50 to 200 ft of the Pololu Basalt is decomposed from 

weathering processes (Stearns and Macdonald, 1946, p. 174). Greater weathering occurred in the 

wet areas and where the rocks were not covered by the later flows. Intercalated soils greater than 

a few inches thick are rare below the upper part of the Pololu Basalt, indicating that periods of time 

between flows were short. Thin vitric ash beds can be found throughout the basalts but rarely on 

the windward slopes of Kohala Mountain (Stearns and Macdonald, 1946).

In general, the Pololu Basalt resembles the Koolau Basalt on Oahu, which forms aquifers 

with high transmissivities. The clinker areas on either side of aa flows have high horizontal 

hydraulic conductivities as do the interflow faces between pahoehoe flows, fractures, and cooling 

joints in the rock. Because these lavas are layered and nearly horizontal, horizontal hydraulic 

conductivity is likely to be greater than the vertical hydraulic conductivity. Aquifer tests and 

numerical studies in the Pearl Harbor aquifer of Oahu indicate that the greatest-to-least (horizontalto-vertical) hydraulic conductivity ratios, or anisotropy ratios, are from 10:1 to 1,000:1. In an 

analysis of the Pearl Harbor aquifer, Souza and Voss (1989) estimated a ratio of horizontal to 

vertical hydraulic conductivity of 200:1.

Intrusive dikes cut the Pololu Basalt flows in the deep canyons on the northeast slope. Dikes 

trend N 50° to 80° W and dip about 75° northeast (Stearns and Macdonald, 1946, p. 175). Widths 

of Pololu dikes range from a few inches to 10 ft and average about 2 ft. Dike concentrations become 

greater with depth and closeness to the rift areas. Their dense composition, late-stage intrusion, and

10
155° 50' 155° 40'

Upolu Point

4 MILES

20° 10'

20° 00'

EXPLANATION 

DRAINAGE DIVIDE 

ALLUVIUM - 

MAUNA KEA [

Undifferentiated 

lava flows

KOHALA 

Hawi Volcanics 

Cinder cones 

Lava flows

Pololu Basalt 

Cinder cones 

Lava flows

Pacific 

Ocean

Figure 5. Surficial geology of the Kohala area, island of Hawaii (modified from 

Langenheim and Clague, 1987; and Stearns and Macdonald, 1946).

11
near-vertical emplacement give dikes relatively low horizontal hydraulic conductivity that 

impedes ground-water flow. On the basis of a description of the area between Pololu Stream and 

Waipio Stream (fig. 4) by Stearns and Macdonald (1946), the northern margin of the southeast rift 

zone, and the associated high-level water, may extend more than 4 mi from the summit area 

towards the ocean (fig. 5). As discussed, Stearns and Macdonald (1946) indicate a total width of 

about 6 mi for high-level water in the area of the Waimanu aquifer.

Hawi Volcanics. The Hawi Volcanics form a cap over most of the rift zone and cover 

about a third of the northeast slope and about half of the southwest slope below the rift zone (fig. 

5). Lava flows of the Hawi Volcanics were composed almost entirely of highly viscous aa that 

flowed onto the existing Pololu Basalt surface. An unconformity, that in places is marked by a red 

soil, exists between the Pololu Basalt and the overlying Hawi Volcanics. The red soil prevails on 

the southeastern slopes and indicates that most of the original topographic slopes were not eroded 

before being covered by lava flows of the Hawi Volcanics.

Only a few lava flows of the Hawi Volcanics extend to the coast. Thickness of individual 

lava flows range from 10 to 150 ft and average about 40 ft. In up-slope areas, flows overlap to reach 

a composite thickness as great as 500 ft near the summit. Many of the Hawi flows followed existing 

topography and filled the shallow canyons. Thin, ashy soils and gravel can be found between 

individual Hawi flows indicating longer periods of time between flows than in the Pololu Basalt. 

Soils that form on Hawi rocks are gray and can exceed 3 ft in thickness. These soils support 

agriculture when adequate moisture is present.

Alluvium.-- Alluvial deposits are found in the floors of the larger canyons on the windward 

side of Kohala Mountain (Stearns and Macdonald, 1946). These deposits consist of unconsolidated 

deposits of poorly sorted silts, sands, and boulders; landslide deposits characterized by blocks of 

volcanic rock in an earthy mix; and consolidated alluvium consisting of poorly sorted boulder 

conglomerates that form terraces in the lower stretches of the larger canyons and gulches. Stearns 

and Macdonald (1946, p. 172) indicate a thickness for the consolidated alluvium on the order of 

500 ft and thickness of the unconsolidated deposits of about 25 ft. Both deposits are thought to be 

poorly permeable.

12
GROUND-WATER OCCURRENCE

The general movement of water in the Kohala area is from the mountainous areas toward 

the ocean. A schematic representation of this movement is shown in figure 2. In the mountainous 

areas, the movement of water is impeded by the presence of dikes within the rift zones. This 

impedance results in relatively high water levels in the rift zone that can be more than 2,000 ft 

above sea level (Stearns and Macdonald, 1946). Orientation of dikes is subparallel to the main 

orientation of the rift zone so that intersecting dikes are common and ground water is 

compartmentalized between the dikes. In Hawaii, this water is referred to as high-level water.

The water budget for the study area was estimated by Shade (in press). The major source 

of water to the basal part of the Hawi aquifer is from direct infiltration of precipitation. Recharge 

derived from this source is 53.1 Mgal/d on a mean annual basis. The movement of water into the 

basal aquifer from the adjacent rift zone constitutes another source of water that equals 6.9 Mgal/d 

on a mean annual basis. Mean annual recharge from infiltration of precipitation falling on the basal 

aquifer in the Pololu Stream drainage area is 7.2 Mgal/d and mean annual recharge to this aquifer 

from the rift zone above it is 1.2 Mgal/d. The actual rate of recharge at any point is dependent on 

the rate of precipitation minus water lost to evapotranspiration, direct runoff, and soil-moisture 

storage. Each of these factors varies from location to location. The areal distribution of groundwater recharge for the entire area is shown in figure 6.

A small amount of ground-water recharge is derived from water that infiltrates from the 

Kohala ditch. Discharge measurements made in the ditch during this study indicate a seepage loss 

ranging from 0.16 to 0.33 Mgal/d per mile with a total loss of about 2.0 Mgal/d in the study area. 

An additional source of recharge to the aquifer is derived from the injection of an estimated 8 

Mgal/d at the Hawi hydroelectric plant.

Ground water moves out of the rift zone into the basal aquifer where the direction of 

ground-water motion is mainly horizontal. The altitudes of water levels in the basal aquifer 

measured on March 22, 1990 in the test wells drilled for this study ranged from a high of 11.3 ft 

above sea level to a low of 2.5 ft (fig. 7). Water levels higher than those shown in figure 7 would 

be expected inland toward the rift zone. As indicated by the water levels, there is a general 

northerly movement of water in the basal aquifer. Existing hydrologic conditions in the basal 

aquifer in terms of recharge to and discharge from the aquifer have not changed significantly for 

many years, indicating that water levels shown in figure 7 would represent equilibrium or near 

equilibrium conditions, although seasonal variations in water levels would be expected. Extensive 

agriculture and irrigation of crops ceased in the mid-1970's and water previously used for irrigation 

has been injected at the Hawi hydroelectric plant since about 1979. The only regularly pumped well 

in the basal aquifer is the DWS well. Production at this well (7449-02; see fig. 9) averages about 

0.6 Mgal/d. Water levels were regularly measured in shaft 7652-01 from 1972 to 1995 and 

periodically measured in well 7347-02 (see fig. 9) from 1986 to February 1991 (fig. 8). This data 

supports the concept that equilibrium conditions exist in the aquifer.

13
155° 50'

20° 10' 

20° 00' 

GROUND-WATER RECHARGE, 

IN INCHES PER YEAR

Greater than 40 

30 to 40

20 to 30

10 to 20 

I to 10 

Less than 1

DRAINAGE DIVIDE

Figure 6. Ground-water recharge, Kohala area, island of Hawaii (Shade, in press).

14
155° 50' 155° 45'

20° 15

20° 10'

A 

(8.0)

EXPLANATION

LINE OF EQUAL GROUND-WATER LEVEL 

Interval 2 feet. Datum is mean sea level

TEST SITE WITH 2 WELLS 

TRANSITION-ZONE WELL SITE 

SITE IDENTIFIER

OBSERVED GROUND-WATER LEVEL, 

IN FEET ABOVE MEAN SEA LEVEL

Area of map

Pacific 

Ocean

Figure 7. Observed ground-water levels in the Hawi basal aquifer and test-well sites, 

Kohala area, island of Hawaii.

15
Ground water discharges from the basal aquifer into the ocean and, near the shoreline, there 

is an upward component of ground-water flow. The upper 50 to 250 ft of the basaltic basal aquifer 

is largely decomposed by weathering, but it is not known if the weathered layer extends under the 

ocean. Waves along the coast may have eroded all or part of this layer. If a weathered layer exists, 

it would help impede the movement of ground water from the basal aquifer into the ocean because 

the hydraulic conductivity of weathered basalt is generally much lower than that of fresh basalt. 

Sedimentary deposits similar to those found along the shoreline of other islands in Hawaii are not 

present along the Kohala shoreline because of the relatively young age of the basalts in the area.

Springflow in the study area occurs primarily between 600 and 1,800 ft altitude and has 

been described by Stearns and Macdonald (1942) as perched water that, for the most part, issues 

from a clinker zone at the bottom of the Hawi Volcanics and above the red soil at the top of the 

Pololu Basalt.

uu 

>

uu

uu 

en

uu 

O

CD

LJU 

UU

UU 

>

UU_l

cc 

uu

I

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

I I I I I I i i r

Well 7347-02

Shaft 7652-01

J__I

oo oo oo oo oo oo oo oo oo oo

YEAR

Figure 8. Water levels measured in well 7347-02 and shaft 7652-01, Kohala area, 

island of Hawaii.

16
Ground- Water Development

The locations of wells, shafts, and test holes constructed in the Hawi basal aquifer prior to 

this study are shown in figure 9. Altogether there are four wells, four shafts, and five test holes. 

With the exception of the DWS well (well 7449-02) near Hawi, all of the other wells, shafts, and 

test holes were drilled by sugar plantations. The first well (7448-01) was drilled in 1898 by the 

Kohala Sugar Company (State of Hawaii, 1970). Water from the well was used for sugar mill 

supply in 1901, but the well was ultimately abandoned (Davis and Yamanaga, 1963). Between 

1899 and 1901 three shafts (Kohala, Alaalae, and Hoea) were sunk to sea level for water for 

sugarcane irrigation; a fourth shaft (Waikane) was constructed in 1920. Well 7347-02 was drilled 

in 1948 for domestic use and the Union Mill #1 well was drilled in 1965 with the intent of using 

the water for irrigating sugarcane. The wells were drilled at altitudes ranging from 310 to 540 ft. 

Finally, the five test holes described by Bowles and others (1974) were drilled in 1964. These holes 

were drilled at altitudes ranging from 170 to 362 ft. The shafts were constructed at low altitudes 

near the coast.

The present source of domestic water in the Kohala area comes primarily from the DWS 

well (0.6 Mgal/d) and three tunnel systems that have been dug to tap spring discharge: the Bond 1 

tunnel, discharging at altitudes of about 1,000 ft and 1,400 ft; the Watt 1 tunnel, discharging at 

1,700 and 1,800 ft; and the Murphy tunnel, discharging at about 1,250 ft. The greatest flows are 

from the Bond and Watt tunnel systems, which have discharges as large as 1.25 Mgal/d each. The 

Murphy tunnel discharges about 0.25 Mgal/d. The greatest untapped springflow is about 0.15 

Mgal/d from the west branch of Halawa Gulch (fig. 4) (State of Hawaii, 1970). The total average 

flow for all tunnels and springs is 4 to 5 Mgal/d with a maximum total flow of 17 to 18 Mgal/d and 

a minimum total flow of about 1 Mgal/d (Stearns and Macdonald, 1946).

Sufficient data are not available to recreate the historical distribution and rates of groundwater development, although the general pattern of this development from 1940 to 1961 is 

discussed by Davis and Yamanaga (1963), and the pattern from 1962 through 1967 is available in 

State of Hawaii (1970). The following summarizes information in those two reports.

From 1940 through 1961 the average pumpage of ground-water from the aquifer was about 

7.5 Mgal/d and most of the water was used to irrigate sugarcane (Davis and Yamanaga, 1963). The 

range in use during this time was 2.2 to 14.2 Mgal/d. Shaft pumpage from 1962 to 1967 was about 

3.2 Mgal/d (State of Hawaii, 1970), about half of that from 1940 through 1961. Davis and 

Yamanaga (1963) indicated that chloride concentrations in one of the shafts averaged about 400 

ppm and at another averaged about 1,500 ppm. Given that the shafts were constructed at low 

altitudes near the coast, and given their rate of pumpage, the relatively high chloride concentrations 

are to be expected. At the time of this study (1989-90) and as of 1995, the DWS well is the only 

well regularly pumped in the basal aquifer.

17
155° 50' 155° 45'

20° 15'

20° 10'

Upolu Point

Union Mill 

(7448-01)

©AUnion Mill #1 

(7448-04)

Union Mill 

(7448-01)

EXPLANATION 

ACTIVE WELL 

INACTIVE WELL 

INACTIVE SHAFT 

HISTORIC TEST HOLE 

TUNNEL

IDENTIFIER-State number in 

parentheses

Area of map

Pacific 

Ocean

Figure 9. Locations of existing and historic wells, existing tunnels, and historic test holes 

in the Kohala area, island of Hawaii.

18
Ground-Water Quality

The quality of water in the basal aquifer was investigated by analyzing water samples from 

five of the eight test-well sites (A, B, E, F, and H in fig. 7) for concentrations of common ions. 

Samples were analyzed at the U.S. Geological Survey Central Laboratory according to standard 

methods (Fishman and Friedman, 1989, Wershaw, and others, 1987). Results show that the ground 

water in Kohala is typical of water from Hawaiian basalt aquifers (Swain, 1973). Dissolved ion 

concentrations were low, with specific conductance values ranging from 165 to 245 uS/cm and 

chloride concentrations ranging from 19 to 36 mg/L (table 1).

At three sites (A, B, and F), water samples were also collected for analysis for 18 dissolved 

metals and as many as 79 organic compounds, including agricultural chemicals, volatile 

constituents of fuel, and solvents (table 2). No dissolved metals or organic compounds were found 

at concentrations exceeding maximum contaminant levels (U.S. Environmental Protection 

Agency, 1991).

Iron and aluminum were the metals found in highest concentrations, as might be expected 

given their abundance in basalt. Iron and aluminum may also have been released into the water 

samples from the iron well casing, metal pump, and aluminum discharge line. The other metals 

were either not detected or detected at low (10 jig/L or less, except for barium) concentrations that 

are typical of ground water from basalt aquifers (Halbig and others, 1986; Eyre, 1994).

Samples were free of significant anthropogenic organic compounds. The only organic 

compounds reported were from well A, where toluene and xylene were found at or near the level 

of detection (0.2 ug/L). This concentration is far below the USEPA limit of 1,000 ug/L (U.S. 

Environmental Protection Agency, 1991). Given these low levels, it is possible that the presence 

of these compounds was the result of sample contamination during collection, false detection in the 

laboratory, or possible local contamination of the water during well drilling. Further sampling at 

this location would be necessary to verify the existence of the compounds.

19
Table 1 . Concentrations of common ions and other water-quality characteristics of water samples from 

the Hawi basal aquifer, Kohala area, island of Hawaii

[mg/L, milligrams per liter; |ig/L, micrograms per liter; <, value shown is less than actual value; |iS/cm, microsiemens per 

centimeter at 25 °C]

State well number

USGS test well (fig. 7)

Latitude

Longitude

Date

Time (24 hour)

Altitude of land surface (feet)

Depth of well, total (feet)

Flow rate, instantaneous (gallons per 

minute)

Specific conductance ([is/cm)

pH (standard units)

Temperature (°C)

Hardness, total (mg/L as CaCO3)

Hardness noncarb. (mg/L as CaCC^)

Calcium, dissolved (mg/L as Ca)

Magnesium, dissolved (mg/L as Mg)

Sodium, dissolved (mg/L as Na)

Potassium, dissolved (mg/L as K)

Alkalinity (mg/L as CaCO3)

Sulfate, dissolved (mg/L as 804)

Chloride, dissolved (mg/L as Cl)

Fluoride, dissolved (mg/L as F)

Silica, dissolved (mg/L as SiO2)

Solids, sum of constituents, dissolved 

(mg/L)

Iron, dissolved (fig/L as Fe)

Manganese (|ig/L as Mn)

7345-04

A

20°13'07"N

155°45'2(rW

7/7/89

1530

396

480

900

175

7

21.5

56

<1

11

6.9

19

1.3

59

6

20

0.1

37

138

55

<1

7449-02

H

20°1428"N

155°49'42"W

8/4/89

800

541

591

475

190

7.7

22

44

<1

7.8

5.9

21

1.9

50

7

21

<0.1

51

148

6

<1

7347-02

E

20°13'52//N

155°47'06"W

8/15/89

1510

342

510

1,000

165

8

22

45

6

8.1

6

18

1.7

39

5

21

0.1

41

18

27

1

7448-07

F

20°14'28//N

155°48'02"W

8/31/89

1300

415

429

185

245

7.8

24.5

58

5

10

7.9

27

2.2

53

10

36

0.1

44

174

41

6

7347-05

B

20°13'19"N

155°47'23/'W

10/20/89

1400

628

720

900

180

7.9

22

47

4

7.9

6.5

16

1.5

43

7

19

0.1

44

133

16

<1

20
Table. 2. Concentrations of trace metal and organic compounds in water samples from the Hawi 

basal aquifer, Kohala area, island of Hawaii

[All values in micrograms per liter; <, actual value is less than value shown]

State well number

USGS test site (fig. 7)

Date

Time

Turbidity (NTU)

7345-04

A

7/7/89

1530

-

7448-07

F

8/31/89

1300

0.1

7347-05

B

10/20/89

1400

0.1

Dissolved metals

Aluminum

Arsenic

Barium

Berylium

Cadmuim

Chromium

Cobalt

Copper

Iron

Lead

Lithium

Manganese

Mercury

Molybdenum

Nickel

Selenium

Silver

Zinc

130

<1

100

<10

<1

2

1

4

180

2

<10

<10

<0.1

<1

6

<1

1

<10

10

<1

<100

<10

<1

1

1

4

50

4

<10

<10

0.1

<1

<1

<1

<1

<10

<10

<1

<100

10

<1

1

1

2

40

1

<10

<10

<0.1

<1

1

<]

<]

<10

Organic compounds

Alachlor

Aldrin

Ametryne

Atrazine

Benzene

Bromoform

Carbontetrachloride

Chlordane

Chlorobenzene

Chlorodibromomethane

Chloroethane

2-Chloroethylvinylether

Chloroform

Cyanazine

ODD

DDE

DOT

DBF

Diazinon

1 ,2-Dibromoethane

Dichlorobromomethane

1 ,2-Dichlorobenzene

1,3-Dichlorobenzene

1 ,4-Dichlorobenzene

Dichlorodifluoromethane

1, 2-Dichloroethane

1 , 1 -Dichloroethane

<0.1

<0.01

<0.1

<0.1

<0.2

<0.2

<0.2

<0.1

<0.2

<0.2

<0.2

<0.2

<0.2

<0.1

<0.01

<0.01

<0.01

<0.01

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.1

<0.01

<0.1

<0.1

<0.2

<0.2

<0.2

<0.1

<0.2

<0.2

<0.2

<0.2

<0.2

<0.1

<0.01

<0.01

<0.01

<0.01

<0.01

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

~

<0.01

~

~

<0.2

<0.2

<0.2

<0.1

<0.2

<0.2

<0.2

<0.2

<0.2

~

<0.01

<0.01

<0.01

<0.01

<0.01

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

21
Table. 2. Concentrations of trace metal and organic compounds in water samples from the Hawi 

basal aquifer, Kohala area, island of Hawaii-Continued 

[All values in micrograms per liter; <, actual value is less than value shown]

Organic compounds continued

1 ,2-Transdichloroethene

1 , 1 -Dichloroethylene

1 ,2-Dichloropropane

1 ,3-Dichloropropene

cis- 1 ,3-Dichloropropene

trans- 1 ,3-Dichloropropene

Dieldrin

DiSyston

Endosulfan

Endrin

Ethion

Ethylbenzene

Heptachlor

Heptachlorepoxide

Lindane

Malathion

Methoxychlor

Methylbromide

Methylchloride

Methylenechloride

Methylparathion

Methyltrithion

Metolachlor

Metribuzin

Mirex

Naphthalenes, polychlorinated

Parathion

PCB

Perthane

Phorate

Prometon

Prometryne

Propazine

Silvex

Simazine

Simetryne

Styrene

1 , 1 ,2,2-Tetrachloroethane

Trichloroethylene

Toluene

Trithion

Toxaphene

1,1,1 -Trichloroethane

1 , 1 ,2-Trichloroethane

Tetrachloroethylene

Trichlorofl uoromethane

Trifluralin

Vinylchloride

Xylene

2,4-D

2,4-DP

2,4,5-T

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.01

-

<0.01

<0.01

<0.01

<0.2

<0.01

<0.01

<0.01

<0.01

<0.01

<0.20

<0.20

<0.20

<0.01

<0.01

<0.1

<0.1

<0.01

<0.1

<0.01

<0.1

<0.1

<0.1

<0.1

<0.1

<0.01

<0.1

<0.1

<0.2

<0.2

<0.2

0.3

<0.01

<1

<0.2

<0.2

<0.2

<0.2

<0.1

<0.2

0.2

<0.01

<0.01

<0.01

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.01

<0.01

<0.01

<0.01

<0.01

<0.2

<0.01

<0.01

<0.01

<0.01

<0.01

<0.2

<0.2

<0.2

<0.01

<0.01

<0.1

<0.1

<0.01

<0.1

<0.01

<0.1

<0.1

<0.01

<0.1

<0.1

<0.1

<0.01

<0.1

<0.1

<0.2

<0.2

<0.2

<0.2

<0.01

<1

<0.2

<0.2

<0.2

<0.2

<0.1

<0.2

<0.2

<0.01

<0.01

<0.01

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.01

<0.01

<0.01

<0.01

<0.01

<0.2

<0.01

<0.01

<0.01

<0.01

<0.01

<0.2

<0.2

<0.2

<0.01

<0.01

<0.01

~

<0.01

<0.1

<0.1

<0.01

~

~

-

-

<0.2

<0.2

<0.2

<0.2

<1

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

-

-

22
Vertical Extent of Freshwater

Saltwater underlies freshwater in the basal aquifer. When water is pumped from the basal 

aquifer, the lowering of ground-water levels associated with pumping will result in an upward 

movement of the saltwater. Ultimately, the amount of water that can be developed from the basal 

aquifer is constrained by the salinity of the water. To address the constraint on ground-water 

availability imposed by salinity, two wells (D and I, fig. 7) were designed to measure the vertical 

extent of freshwater and the upper part of the transition zone. Observation well D, located about 

0.4 mi from the coast at an altitude of 108.5 ft, was completed at a depth of 351 ft below sea level. 

Observation well I, located about 0.7 mi from the coast at an altitude of 299.5 ft, was completed at 

a depth of 137 ft below sea level. The wells were open to the aquifer below the water table.

To estimate the thickness of freshwater and the upper transition zone, water samples were 

collected in March 1990 from several depths in each well and analyzed for chloride concentration 

(an indicator of salinity) (fig. 10). For this report, freshwater is defined as ground water containing 

water with a chloride concentration less than 250 mg/L (U.S. Environmental Protection Agency, 

1991). Seawater collected around the Hawaiian islands has a chloride concentration of about 

19,600 mg/L (Wentworth, 1939). The change in salinity between freshwater and seawater at depth 

is not abrupt, rather, freshwater and seawater mix and the change is somewhat gradual through a 

transition zone. The entire transition zone from freshwater to seawater includes all water with a 

chloride concentration between 250 mg/L and about 19,600 mg/L.

Through the transition zone, the density of the mixed water also varies from that of 

freshwater (about 1.000 gm/cm3 ) to that of seawater (about 1.025 gm/cm3). On the basis of this 

density difference, freshwater may be assumed to extend below sea level to a depth 40 times the 

water level above sea level. This is referred to as the Ghyben-Herzberg relation (Bear, 1979, p. 

560). In practice, the depth defined by the Ghyben-Herzberg relation approximates the mid-point 

of the transition zone (Lau, 1962). The mid-point of the transition zone is at a chloride 

concentration of about 9,800 mg/L. Thus, the upper transition zone includes water with a chloride 

concentration between 250 and 9,800 mg/L.

In well D, the depth to a chloride concentration of 250 mg/L and thus the estimated 

thickness of freshwater is about 265 ft. The depth of the mid-point of the transition zone was about 

345 ft below the water table, making the upper transition zone about 80 ft thick (fig. 10). In well I, 

the depth to a chloride concentration of 250 mg/L and thus the estimated thickness of freshwater 

is about 83 ft. No water samples were obtained at or below the mid-point of the transition zone in 

well I. On the basis of an extrapolation of the existing data, the depth of the mid-point of the 

transition zone was about 145 ft below the water table, making the thickness of the upper transition 

zone about 62 ft (fig. 10).

The preceding analysis is based on the assumption that the salinity profiles logged in the 

wells are representative of the salinity of water in the surrounding aquifer. However, each well is

23
50

0

-50

-100

-150

-200

> -250

LJJ

LU -300

Z

UJ -350

^

£. -400

WELLD

FRESHWATER

UPPER TRANSITION ZONE

LU

Lu 50 

CC

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

LU 

LU

LU 

Q

ID

0 

-50

-100

-150

-200

-250

-300

-350

-400

WELL I

FRESHWATER

UPPER TRANSITION ZONE

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

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

12,000

12,000

Figure 10. Chloride concentrations and depth for test wells D and I, Kohala area, 

island of Hawaii.

24
open to the aquifer for the entire depth below the water table, providing an open conduit through 

the pre-existing aquifer layers. If vertical hydraulic gradients exist in the aquifer, water will flow 

vertically within the well bore, entering from aquifer zones with greater hydraulic head and exiting 

to zones with lesser head. If ground-water flow in the aquifer is predominantly horizontal and 

vertical flow in the well bore is inconsequential, the salinity profile in the well can be expected to 

correspond closely to the salinity profile in the aquifer. However, vertical flow in the well could be 

of sufficient magnitude to distort the salinity profile and thickness of the transition zone in the well 

bore from that in the aquifer.

Test wells D and I are located between 0.4 and 0.7 mi from the shore, and here the dominant 

direction of ground-water movement is expected to be horizontal. As a result, vertical flow in the 

aquifer, and thus in the test wells would not be significant. If vertical flow does occur, it would 

most likely be upward because of the wells proximity to the shoreline where freshwater begins to 

discharge upward into the ocean. Upward flow within the well bore would tend to bring saltwater 

to higher altitudes in the well bore than exist in the aquifer. Accordingly, the salinity profile as 

logged in the well bore would be displaced upward by some unknown amount. Thus the well bore 

salinity profile would indicate that the top of the transition zone is shallower than it actually is in 

the aquifer.

25
HYDRAULIC CONDUCTIVITY OF THE BASAL AQUIFER

Estimates of the horizontal hydraulic conductivity of the basal aquifer were obtained from 

aquifer tests at five wells: A, E, F, H, and Union Mill #1 (fig. 11). Aquifer tests were done in wells 

drilled during the present study, and also in several wells drilled prior to this study. Similarly, 

aquifer-test data were mostly from tests done during the present study but also from data from 

several earlier tests (not done by USGS) that were reanalyzed as part of this study. A summary of 

well characteristics is shown in table 3.

Each test site consisted of a pumped well and an observation well. Each observation well 

was installed so that the depth of penetration into the aquifer was about the same as that of the 

corresponding pumped well. None of the wells fully penetrated the aquifer, the thickness of which 

is unknown. For an aquifer test, it is desirable to have the deepest possible well penetration that 

allows sufficient withdrawal without inducing an unacceptable saltwater upconing. As a result, the 

penetration of each pumped well drilled for this study is about one third of the estimated freshwater 

thickness, as approximated from the Ghyben-Herzberg relation. By avoiding the withdrawal of 

salty or brackish water, the complicating effects of variable-density fluid flow on hydraulic 

analysis were avoided.

Two types of aquifer tests were done at each site: a step-drawdown test and a 12-hour 

continuous pumping-rate test. The step-drawdown test involved measuring the drawdown induced 

in the pumped well at each of three to five successively higher pumping rates. Each step lasted 

about 1 hour. The sustained 12-hour test involved pumping the well at a steady rate for 12 hours 

and measuring the drawdown induced in both the pumped and the observation well over this time. 

Tests done during this study included step-drawdown tests at sites A, E, and F, and sustained 

pumping tests at site A, E, F, and H. Earlier tests included a step-drawdown test done in 1975 at 

site H by the County of Hawaii Department of Water Supply (unpub. data in files of U.S. 

Geological Survey, Honolulu), and a step-drawdown test and a sustained pumping test at Union 

Mill #1 in 1964 (Akinaka and others, 1975).

The step-drawdown data were analyzed to estimate the two components of drawdown in 

the pumped well: (1) the hydraulic head loss in the aquifer, (2) the hydraulic head loss from friction 

and turbulence within and near the well column, and (3) well entrance losses at the screen. An 

empirical relation between drawdown and pumping rate developed by Jacob (1947) allows a well 

efficiency at a desired pumping rate to be calculated. The measured drawdown in the well is then 

adjusted on the basis of the efficiency, and the resulting value represents the drawdown due only 

to hydraulic head loss in the aquifer. This adjusted value of drawdown is then used in subsequent 

equations of aquifer analysis.

26
155° 50' 155° 45'

20° 15'

20° 10'

910- 

790-

E

EXPLANATION

HYDRAULIC CONDUCTIVITY, 

IN FEET PER DAY

 Elliptical equation analysis 

~~ Theim equation analysis

WELL AND IDENTIFIER

Area of map

Pacific 

Ocean

Figure 11 . Aquifer-test values for horizontal hydraulic conductivity, Kohala area, 

island of Hawaii.

27
Table 3. Summary of characteristics of wells used for exploration and aquifer testing of the Hawi basal aquifer, Kohala area, island of

Hawaii[Includes exploratory wells drilled for this study and pre-existing wells used in the aquifer tests. Well number refers to the State of Hawaii numbering system."Test pumping" wells were pumped during aquifer tests only and are not permanent withdrawal wells; --, no data]

Test site 

or well 

(figs. 7 and 11)

AA

BB

D

EE

F

F

H

H

I

JJ

Union Mill #1

Union Mill #1

State well 

number

7345-03

7345-047347-04

7347-05

7445-017847-027347-037448-06

7448-07

7449-02

7449-037549-037451-01

7451-02

7448-037448-04

Well use

observation (water level)

test pumping

observation (water level)

test pumping

observation (salinity profile)

withdrawal

observation (water level)observation (water level)

test pumping

withdrawal

observation (water level)

observation (salinity profile)

observation (water level)

test pumping

observation (water level)

withdrawal

Land surface 

altitude 

(feet)

395.3

395.9

630.4628.2

108.5

344

340.5411.6

414.8

541.4541.2

299.5

566.6566.8

306.3

312

Depth 

drilled 

(feet)

440

495

730720

460

505

405440429

591585

436

632632402412

Water-table 

altitude 

(feet)

10.210.3

11.4

11.4

7.5-9.8

8.08.1-7.0

2.3

4.24.84.9- Altitude of open interval (feet) 175 to -45 -44 to -99 530 to -100 18 to -102 74 to -352 -8 to -161 261 to -64 289 to -28 8 to -22 7 to -50 451 to -44 169 to -137 467 to -65 7 to -65 306 to -96 0 to -100 Type of open interval open hole open hole open hole perforated steel/open open hole open hole open hole open hole steel perforated perforated steel/open open hole open hole open hole perforated steel/open open hole perforated steel/open hole hole hole hole
The adjusted drawdown data from the step-drawdown test were then analyzed using 

equation 1 (Harr, 1962; Polubarinova-Kochina, 1962):

(1)

where: K is horizontal hydraulic conductivity, in feet per day;

Q is pumping rate, in cubic feet per day;

In is natural logarithm (base e = 2.718);

L is length of the open interval of the well, in feet;

71 is the number pi, equal to 3.1415; 

rw is radius of the well, in feet; and 

sw is drawdown in the pumped well, in feet, adjusted for well loss.

Equation 1 describes steady-state flow to a pumped well that partially penetrates an aquifer that is 

much thicker than the depth of the well, a condition met by the wells in this study. The geometry 

of flow is such that lines of equal drawdown in the aquifer take the shape of ellipses; thus the 

equation is referred to herein as the elliptical equation. This equation was solved for values of K 

using drawdown data from the step-drawdown tests, adjusted for well loss as described above.

Drawdown data from the 12-hour sustained pumping-rate tests were analyzed by the Thiem 

equation, which assumes steady, radial flow and requires measurements of drawdown in two 

observation wells. Differences in well construction required use of both the confined and 

unconfined forms of the Thiem equation (Todd, 1980):

( r2\

Qln\ ^

K = ,,' , (confined); (2) 2nb (s, s9 )

and:

r2\ GIV

K = f o V\ (unconfined), (3)

where: K is horizontal hydraulic conductivity, in feet per day; 

Q is pumping rate, in cubic feet per day; 

In is natural logarithm (base e = 2.718);

ri is radial distance to observation well 1 (closer to the pumped well), in feet; 

r2 is radial distance to observation well 2, in feet; 

71 is the number pi, equal to 3.1415; 

b is aquifer thickness, in feet;

29
51 is drawdown in observation well 1, in feet;

52 is drawdown in observation well 2, in feet;

hi is thickness of flow at observation well 1, in feet; and

/i2 is thickness of flow at observation well 2, in feet.

Several qualifications regarding application of the Thiem equation are pertinent. First, the 

pumped well was considered to be observation well 1, and the radius of the pumped well and the 

adjusted drawdown (computed from the step-drawdown analysis) were used. Next, because the 

thickness of the aquifer in the Kohala area is unknown, the submerged open length of the pumped 

well was substituted for aquifer thickness, b (flow is presumed to be horizontal and radial within 

the depth interval corresponding to the open length of the pumped well). The confined form of the 

Thiem equation was used for sites E, H, and Union Mill #1, sites where there was no drawdowninduced dewatering of the pumped interval because solid casings in the pumped wells extend 

below the level of drawdown. The unconfined form of the Thiem equation was used for sites A and 

F, sites where the pumped wells are open at the water table and drawdown caused dewatering of 

the pumped interval. Here, drawdown was subtracted from the pre-pumping submerged open 

length of the well to obtain the thickness of flow, h. Lastly, the Thiem equation assumes that 

steady-state, or equilibrium, flow has been attained. Typically, this constraint is closely 

approximated within a few hours in the permeable lavas of Hawaii at the short radial distances of 

these tests. Also relevant is the work of Bennett and others (1967), who conducted pumping tests 

in the Punjab region of West Pakistan. Using drawdown measurements in observation wells both 

within and outside the pumping interval, they demonstrated that the slope of the cone of depression 

within a few hundred feet of the pumped well stabilized after a few hours of pumping even though 

the cone of depression continued to expand. It was concluded that the flow to the pumped well was 

dominantly horizontal within the depth range of the open interval of the pumped well at radial 

distances of a few open-interval lengths, once the hydraulic gradient stabilized within this region. 

Consequently, the Thiem equation could be used to analyze this data.

Values of hydraulic conductivity(/T) obtained from the aquifer tests ranged from 610 to 

6,400 ft/day (table 4, fig. 11). Differences between values obtained by the two analytical methods 

at a given site ranged from 0 to 44 percent of the mean of the two values. Within the area of the 

tests (fig. 11), hydraulic conductivity seems to increase progressively in the northwest direction.

30
Table 4. Summary of aquifer-test data and results, Kohala area, island of Hawaii

[ft/d, feet per day; r, radius from center of pumped well; K, horizontal hydraulic conductivity of aquifer]

Sustained pumping test

Test well 

site or 

well 

(fig. 11)

AEFHUnion

Mill #l b

Pumped 

well

7345-047347-02

7448-07

7449-027448-04

Length 

of 

pumping 

interval 

(feet)

55

153

2257

100

Distance at

observation 

well from 

pumped well 

(feet)

50

40

50

5573

 ir^aver;

Kmed:

Step drawdown test

Number 

of steps

65

44a

3

age "'""""

K,

calculated 

using 

elliptical equation

(ft/d)

610910940

4,300

1,800

1,700

940

Pumping 

rate, Q 

(gal/min)

900

1,240

400

495

2,000

Duration 

of 

pumping 

(hours)

71

14

16

72

365

Observed 

in 

pumped 

well

14.5

7.5 -3.1

8.7

Drawdown

(feet)

Adjusted 

in 

pumped 

well

4.5

1.6

2.60.32

1.9

K

K

Observed 

in observation well

0.290.31

0.13

0.130.8

calculated 

using 

Theim 

equation, 

(ft/d)

610780

1,100

6,400

2,700

2,300

1,100

a Step-drawdown test was done at time of completion of well construction in 1975; data reported by Hawaii County Department of Water Supply (1975). b Union mill #1 aquifer tests were done in 1965-66; data used in analysis reported in Akinaka and others (1975).
NUMERICAL SIMULATION OF THE BASAL AQUIFER

Ground-water flow in the basal aquifer was simulated using the two-dimensional areal 

ground-water model AQUIFEM-SALT (Voss, 1984), which is a finite-element numerical model 

that simulates an aquifer containing freshwater that floats on saltwater of greater density. The 

model simulates water-levels and movement of the freshwater only. The saltwater is assumed not 

to flow and to have a hydrostatic pressure distribution. It is assumed that the boundary between the 

freshwater and saltwater is a sharp interface and that the two waters do not mix. It is further 

assumed that the location of the interface in the model represents the mid-point of the transition 

zone of the aquifer. The position of the interface in the model is determined so that the hydrostatic 

pressure in both freshwater and saltwater are equal on both sides of the interface.

Description of the Model

Boundary conditions.---The inland extent of the basal aquifer terminates at the rift zones of 

Kohala Mountain. Although water moves through the rift zone into the basal aquifer, hydraulic 

conductivity of the dikes in the rift zone is orders of magnitude less than that of the basal aquifer. 

As a result, the rift zone was not included as part of the modeled area, but the discharge of water 

from it into the basal aquifer was accounted for in the model by specifying additional recharge to 

the model elements adjacent to the rift zone (fig. 12).

Discharge of ground water along the shoreline of the modeled area is simulated using a 

head-dependent flux boundary. Ground-water discharge into the ocean through the ocean floor 

occurs in model elements along of the shoreline (fig. 12). Such discharge occurs because there is 

a hydraulic connection between the basal aquifer and the ocean, and the freshwater head in the 

aquifer is higher than the head at the ocean floor. The rate at which water discharges is controlled 

by the difference between the freshwater head in the aquifer, the equivalent freshwater head at the 

ocean floor, and the thickness and vertical hydraulic conductivity of the basalt and the weathered 

zones in the basalt, if the latter exist on the ocean floor.

The model was extended beyond the primary study area to include the entire Pololu 

drainage area, because simulated pumpage in the Hawi basal aquifer was close to this area and 

drawdowns induced by pumping would be expected to extend into it and possibly beyond it. 

Because the geohydrologic setting of the area east of Pololu Stream is highly complex and not 

completely understood, the model was not extended into this area. Given the lack of detailed 

knowledge concerning the relation between the Waimanu basal aquifer and Pololu Stream, the 

stream was not simulated on the model. Model-simulated drawdowns at the mouth of Pololu 

Stream resulting from pumping 20 Mgal/d in the Hawi basal aquifer are discussed, however, in 

order to qualitatively evaluate the possible effect of pumping on this stream.

32
155° 50' 155° 45'

20U 15'

20° 10'

EXPLANATION

HEAD-DEPENDENT FLUX ELEMENT (Coastal discharge area) 

ELEMENTS WITH AUGMENTED RECHARGE (Rift zone) 

AUGMENTED RECHARGE (Kohala Ditch) 

NO-FLOW BOUNDARY 

HAWI INJECTION NODE 

PUMPING NODE (7449-02)

Area of map

Pacific 

Ocean

Figure 12. Model grid and boundaries used in the numerical simulation, Kohala area, 

island of Hawaii.

33
The basal aquifer is made up of hundreds of thin permeable lava flows that, because of 

subsidence of the island, now extend from the land surface to well below sea level. The depth of 

the bottom of the aquifer is not known. However, it is likely that the permeable lava flows extend 

several thousand feet below sea level because most Hawaiian volcanoes have subsided 6,000 to 

12,000 ft (Moore, 1987). As a result, the model uses a single layer with a total thickness of 3,000 

ft. This is not to indicate that freshwater extends to this depth. This depth represents only the 

estimated depth of permeable rock and conceptually, the total aquifer thickness includes both 

freshwater and saltwater. In the model, however, freshwater flows only in the interval between the 

water table and the freshwater-saltwater interface. The freshwater-saltwater interface as calculated 

by the model defines the bottom of the aquifer and is a no-flow boundary at a depth that is a 

function only of the local head in the aquifer.

Recharge. Water is input into the model as ground-water recharge distributed areally over 

the modeled area using the distribution of mean annual recharge rates estimated by Shade (in 

press). Total recharge to the model is 78.4 Mgal/d. Recharge to the model from direct infiltration 

of rainfall on the basal aquifer, 60.3 Mgal/d, was distributed on the basis of values shown in figure 

6. Recharge to the basal aquifer derived from that part of the rift zone between the topographic 

divide at the crest of Kohala Mountain and the boundary of the modeled area (8.1 Mgal/d) is 

accounted for by augmenting the recharge in the elements along the model boundary adjacent to 

the rift zone (fig. 12). Recharge to the basal aquifer from the Hawi hydroelectric plant injection 

wells (8 Mgal/d) was simulated as a constant flux to the model node where the injection wells are 

located. The estimated seepage loss from Kohala ditch (2 Mgal/d) was modeled as augmented 

recharge in elements along the ditch (fig. 12).

Ground-water pumpage. lihe withdrawal of ground water at the DWS well (7449-02) was 

simulated as a constant rate of withdrawal equal to 0.6 Mgal/d. The location of the pumping node 

is shown in figure 12.

Hydraulic conductivity.--Values of hydraulic conductivity of the basal aquifer calculated 

during this study provided a range from 610 to 6,400 ft/d (table 4). The average value of hydraulic 

conductivity calculated from the aquifer test using the Thiem method is 2,300 ft/d; the median 

value is 1,100 ft/d. However, among the aquifer-test sites, higher values of hydraulic conductivity 

were obtained in the northwestern part of the basal aquifer than in the southeastern part (fig. 11). 

This variability was investigated and is discussed in the calibration section of the report. The 

hydraulic properties of the basal aquifer in Pololu Valley are not known, so that the hydraulic 

properties of the basal aquifer in this area were assumed to be the same as the southeastern part of 

the Hawi basal aquifer. In addition it is possible that the regular, tabular structure of basalt lava 

flows may cause the hydraulic conductivity to be greater along the lava flow direction than across 

it, making the aquifer areally anisotropic.

34
Shoreline discharge. One of the most important considerations in boundary conditions is 

the nature of the discharge of freshwater from the basalt aquifer through the ocean floor. This 

discharge is impeded because, near the shoreline, ground-water movement is predominantly 

vertically upward and across the layering of the basalt flows. There is resistance to vertical 

movement because the freshwater must flow through the dense, less permeable interior part of 

individual lava flows. There would be additional resistance to vertical movement if the basalt 

layers are weathered or there are ocean bottom sediments overlying the basalt. Also, freshwater 

must discharge against the saltwater head at the ocean floor. In the model, the saltwater head would 

be expressed as an equivalent freshwater head, fy = (1.025z - z) for z equal to the depth below sea 

level to the ocean floor. Because AQUIFEM-SALT does not distinguish between freshwater and 

saltwater, the model will use the ocean as a source of freshwater to the aquifer if the calculated 

freshwater equivalent head at the ocean floor in the discharge area of the model (fig. 12) exceeds 

the head in the underlying basalt aquifer. To avoid this, the freshwater equivalent head of the ocean 

was set to zero in the discharge area. Thus, in any simulation, the aquifer head would be greater 

than the head at the modeled discharge area. The effect on the simulation is a small error in the 

simulated hydraulic connection between the ocean and the basal aquifer. This error would tend to 

make the model overestimate water-level declines associated with pumpage and thus make the 

model more conservative when estimating ground-water availability.

Near the shoreline, ground water moves upward from the basal aquifer to discharge into the 

ocean. This movement is resisted by the vertical hydraulic conductivity (K1) of the basalts and by 

the weathered zone at the top of the basalt if it exists. Discharge is proportional to the effective 

vertical hydraulic conductivity of the basalt and weathered zone (assuming it exists) divided by the 

thickness of the zone of discharge (m'). As discussed previously, the vertical hydraulic 

conductivity of basalts in Hawaii is assumed to fall within a range of about one tenth to one 

thousandth of the horizontal hydraulic conductivity. On the basis of results of the aquifer tests for 

the Hawi basal aquifer, this would result in a range of values for the vertical hydraulic conductivity 

of the basalt between 0.6 to 640 ft/d. The thickness of the zone of discharge can be approximated 

by estimating the freshwater thickness at the shoreline. Water levels near the shoreline are about 5 

ft, indicating a freshwater thickness of about 200 ft. For modeling purposes, it can be assumed the 

vertical distance over which discharge occurs is one-half this distance, or 100 ft. Assuming that the 

weathered zone does not exist, the value for K'/m' would fall between 0.006 and 6.4 ft/d/d (d" 1 ). 

The ratio K'/m' is referred to as the hydraulic connection between the basal aquifer and the ocean.

35
Calibration of the Steady-State Model

The model was calibrated by comparison of model-calculated water levels with water 

levels measured in the basal aquifer at the test wells (fig. 7) on March 22, 1990. Two parameters 

were varied during the calibration procedure; the horizontal hydraulic conductivity, K, of the basal 

aquifer, and the hydraulic connection of the basal aquifer with the ocean (K'/m1). Despite being 

varied, the simulated values for the horizontal hydraulic conductivity of the aquifer were kept 

within the general range of values obtained from the aquifer tests and ultimately varied spatially in 

the same manner as indicated by the test results. Thus, the major uncertainty in modeled parameters 

was the hydraulic connection of the aquifer with the ocean.

The first step in the calibration analysis was to make a series of simulations where the 

simulated K was held constant while the simulated value of K'/m' was varied over a range from 

0.0005 to 10.0 ft/d/ft. The model was run to steady state for each value of K'/m', and the values of 

model-calculated water levels at the test wells were recorded. Three sets of simulations were made 

using simulated values for K of 500; 1,000; and 5,000 ft/d. Results of this series of simulations are 

shown in figure 13.

The relation between the model-calculated water levels and water levels observed at the test 

wells varied spatially, falling into two groups (fig. 13). The plot of model-calculated water levels 

compared with the simulated value of K'/m' shown for test well E (fig. 13) is representative for 

wells E, A B, D, and F, all of which are located in the eastern part of the basal aquifer. A similar 

plot for test well H (fig. 13) is representative for wells H, I, and J, all of which are in the western 

part of the basal aquifer.

As shown in figure 13, it was possible to match water levels observed at test wells E, A, B, 

D, and F for all simulated values of K, although the value for K'/m' ranged from about 0.004 to 0.1 

ft/d/ft. On the other hand, as shown in figure 13, values of K greater than 1,000 ft/d are necessary 

in order to match water levels observed in test holes H, I, and J regardless of the simulated value 

for K'/m'. At test well H, a match between model-calculated and observed water levels was 

obtained for K equal to 5,000 ft/d and the K'/m' equal to about 0.006 ft/d/ft.

Several conclusions are possible from the above information. First, larger values for K are 

required in the model for the western part of the basal aquifer compared with the eastern part. This 

corresponds to the distribution of hydraulic conductivity indicated by the aquifer tests. Second, the 

range in values of K'/m' that resulted in model-calculated water levels matching observed (0.004 

to 0.1 ft/d/ft) are within the general range of values previously calculated for this parameter (0.006 

to 6.4 ft/d/ft) assuming vertical movement of ground water is resisted only by the vertical hydraulic 

conductivity (A") of the basalt flows.

The spatial variability of K of the basal aquifer necessary to allow a closer match between 

model-calculated and observed water levels was investigated further, initially by using the results

36
70

60 

_i 

LU 

> 

LU

< 50

LiJ

CO

^ 40 

LiJ

O

CO

< 30

LJJ 

LJJ

^ 20 

_T

LJJ 

>

LJJ

CC 

LJJ

10

WELLE

Observed water level

Horizontal hydraulic conductivity (K)

------ 500 feet per day

- 1,000 feet per day

- - - 5,000 feet per day

0

0.0001

i i i i i i 11___i i i i i i 111___i i i i i i 111___i i i i i i 111___i i i i i i 111___i i i i i i 11

0.001 0.01 0.1 1

K'/m'

10 100

LJJ 

LJJ

LJJ 

CO

70

60

50

LJJ 40

LJJ

O

CO

< 30

LJJ 

LJJ 

LL

? 20

_f

LJJ

LJJ

CC 

LJJ

10

I I I I I I I 11 I I I I I I I 11 I I I I I I I 11 I I I I I I I 11 I I I I I I I 11 i I I I I I

WELLH

Horizontal hydraulic conductivity (K)

500 feet per day

 1,000 feet per day

 5,000 feet per day

Observed water level

0

0.0001

i i i i i i i 11___i i i i i i i M i i i i i M 11___i i i i i i i i i i i i

0.001 0.01 0.1 1 

K'/m'

10 100

Figure 13. Model-calculated water levels at test wells E and H, for selected values of horizontal 

hydraulic conductivity and hydraulic connection, K'/m1, between the basal aquifer and the 

ocean, Kohala area, island of Hawaii (location of wells shown in figure 7).

37
of three of the model simulations from the initial series. These three simulations used a value for 

K of 500; 1,000; and 5,000 ft/d and a value for K'/m' equal to 0.01 ft/d/ft. Model-calculated and 

observed water levels were plotted on a scatter diagram (fig. 14A). The scatter diagram indicates 

any bias or trends in model-calculated water levels, such as all of the model-calculated water levels 

being higher or lower than observed.

The simulated value for K'/m' used for the above models (0.01 ft/d/ft) was selected for 

several reasons. First, as shown in figure 13, model-calculated water levels for all the wells-are 

relatively insensitive to values of K'/m' greater than about 0.1 ft/d/ft, and model-calculated water 

levels become sensitive to values lower than 0.1 ft/d/ft. The model becomes increasingly sensitive 

to values of K'/m' less than about 0.01 ft/d/ft and for values lower than about 0.004 ft/d/ft, modelcalculated water levels rise sharply beyond observed water levels for all values of simulated K. 

Second, when pumping is simulated, lower values of K'/m' result in greater model-simulated 

drawdowns and freshwater-saltwater interface rise for a given distribution and rate of ground-water 

pumpage, other things being equal. Therefore, for lower values of K'/m', the model becomes more 

conservative for estimation of ground-water availability. Finally, the value used for this analysis 

(0.01 ft/d/ft) also fell within the range of values (0.004 to 0.1 ft/d/ft) that resulted in modelcalculated water levels matching observed water levels.

As expected, these simulations did not produce an acceptable match to observed water 

levels. For K equal to 500 ft/d, all of the model-calculated water levels were significantly higher 

than observed water levels. For K equal to 1,000 ft/d, a strong bias existed in the distribution of 

water levels; model-calculated water levels were significantly higher than observed on the western 

side of the aquifer (wells I, J, and H; fig. 14A) and very close to observed water levels on the eastern 

side (wells E, F, D, A, and B). For K equal to 5,000 ft/d, model-calculated water levels were 

somewhat higher than observed on the western side of the aquifer and significantly lower than 

observed on the eastern side. These trends in the model-calculated water levels indicate that to 

match observed water levels a relatively low K is required in the eastern part of the model and a 

relatively high K is required in the western part.

On the basis of the above analyses, the value of K of the aquifer was then varied spatially 

on the model with higher values in the western part of the model and lower values on the eastern 

part of the aquifer while K'/m' was held constant at 0.01 ft/d/ft. Various ranges and distributions of 

K (ranging from 250 to 8,000 ft/d across the model) were tested and nearly all of these simulations 

improved the model. The best match in this series of simulations resulted in a model with K varying 

from 650 to 5,000 ft/d (fig. 145). The average difference between model-calculated and observed 

water levels for this distribution was 0.8 ft. Water levels calculated by the model were still 

relatively high at test wells I and J. To improve the match, K'/m' was increased from 0.01 to 0.1 

ft/d/ft in the western part of the model (fig. 15); this resulted in a much better fit between modelcalculated and observed water levels (fig. 145). The average difference between model-calculated

38
16

14

LU

-> 12 HI '^

LU

>

LU

CO 10

01 

LU

LU

IE w

CO LL 4

WELL 

HDF EA B

10 12 14

OBSERVED WATER LEVEL, 

IN FEET ABOVE MEAN SEA LEVEL

16

14

LU

-> 12 _j LU l^

LU _J

LU S

-1 CO 10

EXPLANATION

K 

(feet per day)

K'/m' 

(feet per day per foot)

IS

CO LL

o 650 to 5,000

650; 2,000; and 5,000 

(see fig. 15)

0.01

0.01 and 0.1 

(see fig. 15)

16

B

EXPLANATION

K K'/m' 

(feet per day) (feet per day per foot)

500

1,000

5,000

0.01

0.01

0.01

WELL

HDF EA B

10 12 14 16

OBSERVED WATER LEVEL, 

IN FEET ABOVE MEAN SEA LEVEL

Figure 14. Model-calculated water levels and observed water levels for selected values 

of horizontal hydraulic conductivity (K) and the hydraulic connection between aquifer 

and ocean (K'/m1 ), Kohala area, island of Hawaii.

39
155° 50' 155° 45'

20° 15'

20° 10'

EXPLANATION

MODEL-CALCULATED HYDRAULIC CONDUCTIVITY (/O 

5,000 feet per day 

2,000 feet per day 

650 feet per day

MODEL-CALCULATED HYDRAULIC CONNECTION 

BETWEEN AQUIFER AND OCEAN (K'/mr)

0.1 feet per day per foot 

0.01 feet per day per foot

NO-FLOW BOUNDARY

Area of map

Pacific 

Ocean

Figure 15. Distribution of horizontal hydraulic conductivity and hydraulic connection 

between the aquifer and ocean as modeled, Kohala area, island of Hawaii.

40
and observed water levels was about 0.5 ft and this version of the model was accepted as the best 

representation of the aquifer based on available data. Increasing K'/m' in the western part of the 

model was believed to be justified because of the higher value of K of this area as compared with 

the eastern part of the model. Model-calculated water-levels were compared with those from field 

data and are shown in figure 16. Model-calculated depth of the freshwater-saltwater interface is 

shown in figure 17.

The sources and discharges characterizing the steady-state simulation described above are 

summarized in table 5. The magnitude of the flow components shown in the table represent mean 

annual conditions. Shade (in press) has shown that these conditions vary significantly during an 

average year, but even so, the time response of the aquifer, in terms of changes in water levels is 

small (1 ft or less) relative to seasonal changes in the magnitude of the individual components of 

the water budget.

Table 5. Steady-state ground-water budget (existing pumpage) for the numerical model, Kohala 

area, island of Hawaii

Ground-water sources Million gallons per day

Direct infiltration of precipitation 60.3

High-level discharge into the basal aquifer 8.1

Kohala ditch seepage 2.0

Hydroelectric plant injection wells 8.0

Total........... 78.4

Ground-water discharges

Into ocean 77.8

Existing pumpage 0.6

Total........... 78.4

41
155° 50' 155° 45'

20° 15'

20° 10'

I I 

0 1 2 KILOMETERS

10.1

EXPLANATION

LINE OF EQUAL MODEL-CALCULATED 

GROUND-WATER LEVEL-Interval 2 feet. 

Datum is mean sea level

WELL LOCATION AND OBSERVED 

GROUND-WATER LEVEL. IN FEET 

ABOVE MEAN SEA LEVEL

Area of map

Pacific 

Ocean

Figure 16. Model-calculated and observed ground-water levels, Kohala area, 

island of Hawaii.

42
155° 50' 155° 45'

20° 15'

20° 10'

EXPLANATION

-200 LINE OF EQUAL MODEL-CALCULATED

DEPTH OF THE FRESHWATER-SALTWATER 

INTERFACE-Interval 50 feet Datum is mean 

sea level

Area of map

Pacific 

Ocean

Figure 17. Model-calculated depth of the freshwater-saltwater interface, Kohala area, 

island of Hawaii.

43
Response of the Basal Aquifer to Ground-Water Pumping

The response of the basal aquifer to pumping 20 Mgal/d above that currently being 

withdrawn from the aquifer was simulated for two pumping scenarios. In each scenario, there are 

six pumping sites aligned roughly parallel to the coast, with the four inside sites each pumping 4 

Mgal/d and two outside sites each pumping 2 Mgal/d. The location of pumping sites and pumping 

rates are shown in figure 18. The only difference between the two scenarios is the areal distribution 

of pumping. In the first scenario, the six pumping sites were located over a distance of 5 mi with 

the four sites pumping 4 Mgal/d in the middle 3 mi. In the second scenario, the six pumping sites 

were located over a distance of 3 mi with the four sites pumping 4 Mgal/d concentrated in the 

middle 1.5 mi. In general, the simulations show that increasing ground-water pumping results in 

(1) a decrease of discharge to coastal discharge boundary equal to the amount of pumpage (see 

tables 5 and 7); (2) a decline in water-table altitude (see figs. 19 and 20); and (3) a decrease in the 

depth to the freshwater-saltwater interface (see figs. 17, 21, and 22).

In analyzing the model results in terms of water-level declines and the resulting rise in the 

freshwater-saltwater interface, a distinction needs to be made between model-calculated 

drawdown for a model node and the actual drawdown that would occur in a well or wells. The 

model-calculated drawdown represents the average drawdown in an area around the model node 

(see fig. 12) and in all cases drawdown in a well or wells will be greater. The actual difference 

between the two will depend on the number of wells that the withdrawal in a given node represents, 

the pumping rate of individual wells, and the construction details of the well. The model assumes 

full penetration of the pumped well and for a pumping rate of 1.0 Mgal/d per well, application of 

an equation developed by Prickett (1967) indicates that drawdown in a well would be about 0.3 ft 

greater than the model-calculated drawdown at a node. Wells in basal aquifers are typically 

constructed to depth ranging from 50 to 200 ft below sea level rather than penetrating the entire 

freshwater thickness of the aquifer, however, and the effects of partial penetration would increase 

actual drawdown in a well above values calculated from Prickett's equation. These considerations 

indicate that the model cannot directly address the subject of individual well yields compared with 

well depth and that this information must be gained from actual field experience or from more 

detailed "local scale" model analysis.

Model-calculated changes in water level.--1\\& decline in ground-water levels estimated 

by the model for scenarios 1 and 2 are shown in figures 19 and 20, respectively. Water-level 

declines for the first scenario range from about 1 to 3.5 ft at the pumping centers and to about 0.1 

ft near the extreme northwestern edge of the model. Water-level declines in the extended study area 

are about 1 ft.

Water-level declines for the second model scenario are similar to those of scenario 1 except 

that drawdown at the pumping centers ranges from about 2 to almost 5 ft, an increase of as much 

as 1.5 ft over the first scenario. The increase in drawdown is a result of more localized pumping.

44
155° 50' 155° 45'

20° 15'

20° 10'

P-7

EXPLANATION

WELL LOCATION AND PUMPING RATE, IN MILLION 

GALLONS PER DAY, FOR PUMPING SCENARIO 1

WELL LOCATION AND PUMPING RATE, IN MILLION 

GALLONS PER DAY, FOR PUMPING SCENARIO 2

PUMPED WELL IDENTIFIER

Area of map

Pacific 

Ocean

Figure 18. Pumping rates used for the two simulated pumping scenarios, 

Kohala area, island of Hawaii.

45
155° 50' 155° 45'

20° 15'

20° 10'

P-7.

EXPLANATION

LINE OF EQUAL MODEL-CALCULATED 

DRAWDOWN-Interval 0.5 feet

PUMPED WELL AND IDENTIFIER

Area of map

Pacific 

Ocean

Figure 19. Model-calculated drawdown for pumping scenario 1, Kohala area, 

island of Hawaii.

46
155° 50' 155° 45'

20° 151

20° 101

EXPLANATION

LINE OF EQUAL MODEL-CALCULATED 

DRAWDOWN-Interval 0.5 feet

PUMPED WELL AND IDENTIFIER

Area of map

Pacific 

Ocean

Figure 20. Model-calculated drawdown for pumping scenario 2, Kohala area 

island of Hawaii.

47
Once again, model-calculated decline in water-level in Pololu drainage area is about 1 ft. The 

termination of the model at the eastern drainage divide of Pololu Stream precludes the induced 

movement of ground water from the excluded area into the modeled area, which would be 

presumed to occur under actual pumping conditions. The exclusion of this induced movement of 

ground water toward the pumping centers results in the model over-estimating water-level declines 

by about 0.5 ft. Over-estimating water-level declines makes model calculations of ground-water 

availability conservative.

Model-calculated changes in depth to freshwater-saltwater interface.--As a result of 

pumping and subsequent decline of water levels, the freshwater-saltwater interface moves upward. 

The location of the interface is important because it is the best indicator of the limits on available 

water at the pumping sites. If the freshwater-saltwater interface rises near or into wells, the salinity 

of water pumped by the well may increase to levels unacceptable for domestic uses.

The pre-pumping position of the model-calculated freshwater-saltwater interface near the 

pumping sites is shown in figure 17 and the model-calculated position of the interface resulting 

from scenario 1 is shown in figure 21. The model-calculated position of the interface resulting from 

scenario 2 is shown in figure 22. Information on pre-pumping and pumping water levels and 

model-calculated depth to the freshwater-saltwater interface below sea level are given for each of 

the simulated pumping nodes in table 6 for scenarios 1 and 2.

The greatest model-calculated decline in water level for scenario 1 is about 3.5 ft at 

pumping node P-7 and the resulting model-calculated rise in the interface is about 140 ft. Depth to 

the freshwater-saltwater interface is least (227 ft below sea level) at pumping node P-9. If the 

thickness of the brackish zone above the interface is assumed to be about 80 ft as indicated at well 

D, the range in freshwater thickness at the pumping nodes would be about 153 to 252 ft.

The greatest model-calculated decline in water levels for scenario 2 is about 4.7 ft at 

pumping node P-6 and the resulting model-calculated rise in the interface is about 188 ft. Depth to 

the freshwater-saltwater interface is least (228 ft below sea level) at pumping node P-7. Using a 

thickness for the brackish zone equal to 80 ft above the interface as for scenario 1, the resulting 

range in freshwater thickness from is about 154 to 234 ft.

As discussed previously, wells in basal aquifers are typically constructed to depths ranging 

from about 50 to 200 feet below sea level. The above results indicate that if ground water is 

withdrawn in the areas simulated in the two model scenarios, deeper wells (about 200 ft below sea 

level) would likely experience saltwater intrusion at least at some locations, while shallower wells 

would tend to maintain a greater buffer of freshwater between the wells and the transition zone. 

The amount of freshwater buffer would depend on the actual depth, spacing, and pumping rates of 

these wells. Results also indicate that, as would be expected, the potential for saltwater intrusion 

increases as pumpage is concentrated.

48
155° 50' 155° 45'

20° 15'

20° 10'

EXPLANATION

 -300 LINE OF EQUAL DEPTH OF MODEL-CALCULATED 

FRESHWATER-SALTWATER INTERFACE-Interval 

50 feet. Datum is mean sea level

P"7 PUMPED WELL AND IDENTIFIER

Area of map

Pacific 

Ocean

Figure 21 . Depth of model-calculated freshwater-saltwater interface for pumping 

scenario 1, Kohala area island of Hawaii.

49
155°45'

20° 15'

20° 10' I -

-300 

P-7

EXPLANATION

LINE OF EQUAL DEPTH OF MODEL-CALCULATED 

FRESHWATER-SALTWATERINTERFACE-Interval 

50 feet. Datum is mean sea level

PUMPED WELL AND IDENTIFIER

Area of map

Pacific 

Ocean

Figure 22. Depth of model-calculated freshwater-saltwater interface for pumping 

scenario 2, Kohala, Hawaii.

50
Table 6. Water levels and interface locations at simulated pumping sites, Kohala area, island of 

Hawaii

Non-pumping Pumping

Simulated . 

 * T «. -e Approximate pumping Water Interface thickness of

site level depth . . AC ION /f* u /AU i freshwater (fig. 18) (ft above (ft below , , & , IN i IN below water 

sea level) sea level) , , ,_ . table (feet)

Water Interface ... _ Water- 

, , , thickness of , , level depth . . level 

,r i ,r i i freshwater (ft above (ft below , , change i IN i IN below water ,_ ° 

sea level) sea level) , , ... . (feet) ' ' table (feet) '

Scenario 1

P-l 10.3 413

P-3 10.3 412

P-5 10.9 436

P-7 10.1 405

P-8 9.0 360

P-9 7.0 281

343

342

367

335

289

208

8.1

7.0

7.5

6.6

5.9

5.7

324

281

300

266

236

227

252

208

228

193

162

153

2.2

3.3

3.4

3.5

3.1

1.3

Scenario 2

P-2 10.3 414

P-4 10.1 406

P-5 10.9 436

P-6 10.5 422

P-7 10.1 405

P-8 9.0 360

344

336

367

352

335

289

Table 7. Steady-state ground-water budget (existing 

model, Kohala area, island of Hawaii

Sources

Direct infiltration of precipitation

High-level discharge into the basal aquifer

Kohala ditch seepage

Hydroelectric plant injection wells

Ground-water discharges

Into ocean

Existing pumpage

Proposed pumpage

7.6

5.8

6.4

5.8

5.7

6.6

306

232

255

234

228

264

234

158

181

160

154

191

plus simulated pumpage) for the

Total

Total..

Million gallons per day

60.3

8.1

2.0

8.0

.......... 78.4

57.8

0.6

20.0

.......... 78.4

2.7

4.3

4.5

4.7

4.4

2.4

numerical

51
Model results. The main emphasis of this study was to determine if 20 Mgal/d of ground 

water can be withdrawn from the Hawi basal aquifer in addition to the existing use of 0.6 Mgal/d. 

The previous discussion indicates that this withdrawal is feasible, but spacing, depth, and pumping 

rates of individual wells are an important considerations in planning ground-water development. It 

is desirable to maintain as thick a body of freshwater beneath the wells as possible.

For existing conditions, water levels range from about 10 ft to 7 ft (see fig. 7) from east to 

west in the area of simulated pumpage shown in figure 18. Model-calculated steady-state water 

levels resulting from pumping an additional 20 Mgal/d for scenarios 1 and 2 are shown in figures 

23 and 24. As can be seen, water levels are still highest (about 8 ft) in the eastern part of the area 

of simulated pumpage. Water levels in the western part of the area of simulated pumpage are about 

6ft.

Table 7 shows the components of the steady-state ground-water budget for the modeled 

area following the simulation of the 20 Mgal/d of additional pumpage for both scenarios. 

Comparison of table 7 with table 5 indicates that the source of water to the additional pumpage is 

from reduced ground-water discharge to the ocean by an amount equal to pumpage. Other 

components of the water budget are the same.

For both pumping scenarios, the model calculates a water-level decline of slightly more 

than 1 ft near the mouth of Pololu Stream where the stream discharges into the ocean. In this area, 

it is possible that Pololu Stream is hydraulically connected to the basal aquifer. As a result, a 

decline in water level near the mouth of Pololu Stream would be sufficient to cause a reduction in 

streamflow. Because the model terminates along the Pololu watershed boundary, the model would 

be expected to over-estimate the water-level decline in the area of Pololu Stream, perhaps by as 

much as 0.5 ft; but even so, the stream would still be affected assuming a hydraulic connection 

exists with the basal ground-water system.

52
155° 50' 155° 45'

20° 15'

20° 10' 

EXPLANATION

LINE OF EQUAL MODEL-CALCULATED 

GROUND-WATER LEVEL-Interval 2 feet. 

Datum is mean sea level

PUMPED WELL AND IDENTIFIER

Area of map

Pacific 

Ocean

Figure 23. Model-calculated ground-water levels for pumping scenario 1, 

Kohala area, island of Hawaii.

53
155° 50' 155° 45'

20° 15'

20° 10'

P-7.

EXPLANATION

LINE OF EQUAL MODEL-CALCULATED 

GROUND-WATER LEVEL-Interval 2 feet. 

Datum is mean sea level

PUMPED WELL AND IDENTIFIER

Area of map

Pacific 

Ocean

Figure 24. Model-calculated ground-water levels for pumping scenario 2, 

Kohala area, island of Hawaii.

54
SUMMARY AND CONCLUSIONS

Model results indicate that ground-water withdrawal of 20 Mgal/d above the existing 0.6 

Mgal/d withdrawal at the Hawaii County Department of Water Supply well is feasible from the 

Hawi basal aquifer, but that spacing, depth, and pumping rates of individual wells are important. 

If pumping is concentrated, the likelihood of saltwater intrusion is increased. Model results 

indicate that concentrating as much as 16 Mgal/d in a 1.5 mile stretch roughly parallel to the coast 

on the eastern side of the basal aquifer, and withdrawing an additional 2 Mgal/d within a mile on 

either side of this would provide a freshwater thickness of 154 to 234 ft below the areas of 

withdrawal. In this case, wells with depths of greater than 200 ft below sea level would likely 

experience saltwater intrusion. If the area of concentrated pumpage is increased from 1.5 to 3 miles 

and another 2 Mgal/d withdrawn on either side within a distance of about three-quarters of a mile 

from the area of concentrated pumpage, the freshwater thickness in the area of withdrawal would 

increase slightly to between 153 and 252 ft.

Under existing conditions, water levels in the basal aquifer range from about 10 to 7 ft 

above sea level in the area of simulated pumpage. Model results indicate that water levels would 

range from about 8 to 6 ft above sea level after water-level declines induced by the additional 20 

Mgal/d of pumpage stabilized.

The model cannot directly address the subject of individual well yields compared with 

depth, and this information must be gained from field experience. Even so, it is clear from model 

results that 20 Mgal/d of additional ground-water withdrawal from the basal part of the Hawi 

aquifer is possible if pumping centers are spaced adequately apart and well depths are limited.

The withdrawal of 20 Mgal/d would result in a reduction of ground-water discharge to the 

ocean by an amount equal to pumpage. It is possible that pumping could cause some reduction of 

streamflow near the mouth of Pololu Stream, but because of a lack of field data concerning the 

hydraulic connection of this stream with the basal aquifer, the magnitude of the reduction cannot 

be addressed at this time.

55
REFERENCES CITED

Akinaka and Associates, Bowles, S.P., and Mink, J.F., 1975, Kohala water resources management 

and development plan: phase III, Honolulu, Hawaii, 77 p.

Bear, Jacob, 1979, Hydraulics of groundwater: New York, McGraw-Hill, 569 p.

Bennett, G.D., Rehman, A., Sheikh, LA., and Ali, Sabir, 1967, Analysis of Pumping Tests in the 

Punjab region of West Pakistan: U.S. Geological Survey Water-Supply Paper 1608-G, 56 p.

Bowles, S.P., Mink, J.F., Akinaka and Associates, and May, C. S., 1974, Kohala water resources 

management and development plan: phase II, Honolulu, Hawaii, 144 p.

Davis, D. A. and Yamanaga, G., 1963, Preliminary report on the water resources of Kohala

Mountain and Mauna Kea, Hawaii, State of Hawaii Circular C14, Honolulu, Hawaii, 44 p.

Eyre, P.R., 1994, Ground-water quality reconnaissance, Tutuila, American Samoa, 1989, U. S. 

Geological Survey Water-Resources Investigations Report 94-4142, 15 pp.

Fishman, M.J., and Friedman, L.C., eds., 1989, Methods for determination of inorganic substances 

in water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 5, chap. Al, 545 p.

George A.L. Yuen and Associates, Inc., 1992, State water resources protection plan, volumes I and 

II: prepared for State of Hawaii Department of Land and Natural Resources, 313 p.

Giambelluca, T. W., Nullet, M. A., and Schroeder, T. A., 1986, Rainfall atlas of Hawaii: State of 

Hawaii, Department of Land and Natural Resources, Division of Water and Land 

Development, Report R76, 267 p.

Halbig, J.B., Barnard, W.M., Bartlett, S.A., Overfield R.W., and Abbott, L.L, 1986, A baseline 

study of ground water geochemistry in the Kawaihae and Hilo areas of the Island of Hawaii, 

State of Hawaii Department of Planning and Economic Development, 74 p.

Harr, M.E., 1962, Groundwater and seepage: New York, McGraw-Hill, 315 p.

Jacob, C.E., 1947, Drawdown test to determine effective radius of artesian wells, American 

Society of Civil Engineering, Transactions, v. 112, p. 1047-1071.

Langenheim, V.A.M. and Clague, D.A., 1987, Stratigraphic framework of volcanic rocks of the 

Hawaiian islands, chap. 1 of Decker, R.W., Wright, T.L., and Stauffer, P.H., eds., Volcanism 

in Hawaii: U.S. Geological Survey Professional Paper 1350, v. 1, p. 55-73.

Lau, L.S., 1962, Water development of Kalauao basal springs hydraulic model studies: Board of 

Water Supply, City and County of Honolulu, 102 p.

Martin, W. F., and Pierce, C.H., 1913, Water resources of Hawaii, 1909-1911: U.S. Geological 

Survey Water-Supply Paper 318, 552 p.

Moore, J.G.,1987, Subsidence of the Hawaiian Ridge, chap. 2 of Decker, R.W., Wright, T.L., and 

Stauffer, P.H., eds., Volcanism in Hawaii: U.S. Geological Survey Professional Paper 1350, 

v. 1, p. 85-100.

56
Polubarinova-Kochina, P. Ya., 1962, Theory of ground water movement: Princeton, N.J., 

Princeton University Press, 613 p.

Prickett, T.A., 1967, Designing pumped well characteristics into electric-analog models: Ground 

Water, v. 5, no 4, p. 38-46.

Shade, P.J., in press, Water budget for the Kohala area, island of Hawaii: U.S. Geological Survey 

Water-Resources Investigations Report 95-4114.

Souza, W.R. and Voss, C.I., 1989, Assessment of potable groundwater in a freshwater lens using 

variable-density flow and solute transport simulation, in Proceedings of National Water 

Works Association Conference on Solving Ground Water Problems with Models, Feb. 7-9, 

1989, Indianapolis, Indiana.

State of Hawaii, 1970, Basic water resources data: island of Hawaii, Department of Land and 

Natural Resources, Division of Water and Land Development Report R34, 188 p.

Stearns, H.T. and Macdonald, G.A., 1946, Geology and groundwater resources of the island of 

Hawaii, Hawaii Division of Hydrography Bulletin 9, 362 p.

Swain, L.A., 1973, Chemical quality of ground water in Hawaii: State of Hawaii Department of 

Land and Natural Resources, Division of Water and Land Development, Report R48, 54 p.

Thompson, W.Y., 1988, Feasibility study for a Kohala coastal water system, Department of Water 

Supply, County of Hawaii, 21 p.

Todd, D.K., 1980, Groundwater hydrology (2nd ed.): New York, Wiley, 535 p.

U.S. Environmental Protection Agency, 1991, Drinking water regulations and health advisories: 

U.S. Environmental Protection Agency, Office of Water, lip.

Voss, C.I., 1984, AQUIFEM-SALT: A finite element model for aquifers containing a seawater 

interface: U.S. Geological Survey Water-Resources Investigations Report 84-4263, 37 p.

Wentworth, C.K., 1939, The specific gravity of sea water and the Ghyben-Herzberg ratio at 

Honolulu: University of Hawaii Bulletin, v. 18. no. 8, 24 p.

Wershaw, R.L., Fishman, M.J., Grabbe, R.R., and Lowe, L.E., eds., 1987, Methods for

determination of organic substances in water and fluvial sediments: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 5, chap. A3, 80 p.

GPO 686-022/39174 57