Upward

flow

Downward

flow

Saltwater

Freshwater

Transition

zone

Deep monitor well

Volcanic

rock

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5058

Effects of Groundwater Withdrawal on Borehole Flow

and Salinity Measured in Deep Monitor Wells in Hawaiÿi—

Implications for Groundwater Management 
FRONT COVER

Schematic cross section of deep monitor wells penetrating through the freshwater lens into the underlying saltwater 

in a volcanic-rock aquifer, showing how upward and downward borehole flow affects the salinity distribution with 

depth in the wells.
Effects of Groundwater Withdrawal on Borehole Flow 

and Salinity Measured in Deep Monitor Wells in Hawaiÿi—

Implications for Groundwater Management 

By Kolja Rotzoll

Scientific Investigations Report 2010–5058

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

This report and any updates to it are available online at:

http://pubs.usgs.gov/sir/2010/5058/

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Rotzoll, Kolja, 2010, Effects of groundwater withdrawal on borehole flow and salinity measured in deep monitor wells 

in Hawai‘i—implications for groundwater management: U.S. Geological Survey Scientific Investigations Report 

2010–5058, 42 p.
Vertical salinity profiles measured in deep monitor wells 

commonly are used to estimate the volume of freshwater in 

aquifers in Hawaiÿi. Water-resource managers use information 

from salinity profiles to monitor changes in the freshwater volume over time and determine appropriate groundwater withdrawal rates. Natural or anthropogenic factors can cause water 

in deep monitor wells to flow vertically within the borehole, 

and this may cause the observed salinity distribution in the 

monitor well to be a poor indicator of the salinity distribution 

in the aquifer, resulting in an underestimation or overestimation of the volume of freshwater stored in an aquifer. Thus, a 

better understanding of how borehole flow affects the measured 

vertical distribution of salinity in monitor wells is needed for 

proper analysis and interpretation of vertical salinity profiles 

and sound management of the groundwater resources.

Indicators of Borehole Flow

Salinity profiles from paired shallow and deep monitor wells may differ over the common depth interval of the 

wells. Observed differences can be explained by borehole 

flow moving brackish water upward in the deep well, an 

interpretation supported by flow measurements. Additionally, 

steplike changes in salinity or temperature with depth may 

be indicative of borehole flow over a discrete depth interval, 

and such changes are evident to some extent in all available 

profiles. Vertical flow may affect hundreds of feet of a salinity 

profile, and this occurs commonly in monitor wells located in 

southern Oÿahu. Steplike changes in salinity that do not affect 

long sections (less than 70 ft) of a profile may be related more 

to the presence of low-permeability massive rock layers in the 

aquifer than to borehole flow.

Effects of Withdrawals on Salinity in Boreholes

Although groundwater withdrawals have caused a 

regional thinning of the freshwater lens over the past several 

decades in southern Oÿahu, the apparent midpoint of the 

transition zone between freshwater and saltwater in most deep 

monitor wells has shown only inconsequential depth displacement related to borehole flow caused by short-term variations 

in nearby groundwater withdrawals. In contrast, the apparent 

top of the transition zone, indicated by a specific-conductance 

Executive Summary

value of 1,000 μS/cm, has risen several hundred feet in some 

deep monitor wells because of borehole flow induced by 

withdrawals from nearby production wells (fig. ES1). Some 

deep monitor wells are affected by borehole flow over sections longer than 70 ft, but currently the apparent top of the 

transition zone is not within the longest vertical step in the 

salinity profile (yellow triangles in fig. ES1). The vertical step 

is characterized by nearly constant salinity over the interval 

affected by borehole flow. However, no significant relation 

is evident between proximity of production wells to monitor 

wells and length of vertical step. In some deep monitor wells 

near production wells, the salinity value within a vertical 

step in the measured salinity profile may vary in response to 

withdrawal rate. In deep monitor wells that are distant from 

production wells but within compartmentalized aquifers, the 

salinity value within a vertical step in the measured profile 

may be related to regional withdrawals.

Groundwater Management Implications

Vertical steps in salinity measured in deep monitor wells 

are indicative of borehole flow that can cause the measured 

top of the transition zone in the borehole to rise several 

hundred feet compared to the top of the transition zone in 

the aquifer. This may result in an underestimation of the 

volume of freshwater in groundwater storage. The longest 

vertical steps in salinity occur in monitor wells located in 

southern Oÿahu, coinciding with the most heavily developed 

part of the aquifer. Here, the measured top of the transition 

zone in boreholes may be a poor indicator of the freshwaterlens thickness (fig. ES1). In general, the measured midpoint 

of the transition zone in a borehole is a better proxy for 

freshwater-lens thickness than the measured top. Moreover, 

brackish water flowing upward in deep monitor wells may 

exit the borehole in the upper part of the aquifer containing 

freshwater, which can affect the quality of water pumped by 

nearby production wells.

For deep monitor wells with profiles that are affected by 

nearby withdrawals from production wells, collecting salinity 

profiles under consistent withdrawal conditions will facilitate 

comparisons of profiles over time. Piezometers installed at 

different depths will provide the best information on aquifer 

salinity, because they are unaffected by borehole flow.

iii
0 4 8 MILES

0 84 KILOMETERS

0 25 MILES

0 50 KILOMETERS

158° 157° 156° 155°

21°

20°

19°

Hawaiÿi

Maui

Oÿahu

Molokaÿi

Länaÿi

Kahoÿolawe

Pacific Ocean

"

"

" "

"

"

"

"

"

"

"

"

"

"

"

"

#

#

##

#

#

##

#

#

#

# #

#

#

#

#

#

# # #

# #

#

# #

#

# #

#

#

#

#

#

#

21°20'

21°30'

21°40'

158°10' 158°00' 157°50' 157°40'

EXPLANATION

Maximum length of vertical displacement 

(step) in measured salinity profile, in feet

Longest step is less than 70 feet

Longest step is greater than 70 feet

 Apparent top of transition zone

 is within the largest step

 Apparent top of transition zone is

 currently not within the largest step

#

#

Barrier to horizontal groundwater flow

Valley fill

Other barrier

Mean withdrawal 1996–2005, in million gallons per day

3.1 to 8.0

" 8.1 to 13.5

"

#

##

""

#

##

#

#

Shaded relief generated from U.S. Geological Survey 10‐meter Digital Elevation 

Model, Universal Transverse Mercator projection, zone 4, NAD83 datum. 

Figure ES1. Vertical steps in measured salinity profiles in Hawai‘i and their relation to the apparent top of the freshwater-saltwater 

transition zone, which is defined as having a specific conductance of 1,000 microsiemens per centimeter. Vertical steps longer than 

70 feet can indicate borehole flow that may be related to nearby withdrawal or other factors, including regional drawdown in compartmentalized aquifers and natural hydraulic gradients at depth. Mean groundwater withdrawal rates are for wellfields.

iv
v

Contents

Executive Summary ........................................................................................................................ iii

Indicators of Borehole Flow..................................................................................................... iii

Effects of Withdrawals on Salinity in Boreholes ...................................................................... iii

Groundwater Management Implications ................................................................................. iii

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

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

Purpose and Scope .................................................................................................................2

Previous Investigations ............................................................................................................2

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

Physical Setting................................................................................................................................3

Geography and Geology ..........................................................................................................3

Hydrogeology ...........................................................................................................................4

Flank Lavas......................................................................................................................4

Dike Zones.......................................................................................................................4

Sedimentary Deposits......................................................................................................4

Groundwater Dynamics in the Freshwater Lens......................................................................5

General Characteristics of Salinity Profiles and Deep Monitor Wells...............................................7

Sources of Available Data........................................................................................................7

Historical ..........................................................................................................................7

Recent ..............................................................................................................................9

Data Collection Methods..........................................................................................................9

Analysis of Available Data........................................................................................................9

Well-Construction Information................................................................................................11

Borehole Flow ................................................................................................................................11

Indicators of Vertical Borehole Flow.......................................................................................11

Steplike Change of Salinity with Depth..........................................................................11

Concept..................................................................................................................11

Length of Vertical Step...........................................................................................14

Borehole-Flow Measurements.......................................................................................17

Monitor Wells Near Production Wells.....................................................................17

Monitor Wells Distant from Production Wells.........................................................17

Comparison of Salinity and Temperature Profiles from Shallow and Deep Wells..........18

Effects of Groundwater Withdrawal on Salinity Profiles .........................................................21

Apparent Top of Transition Zone ....................................................................................21

Apparent Top of Transition Zone Not Included Within the Longest 

Vertical Step...............................................................................................22

Apparent Top of Transition Zone Included Within the Longest Vertical Step .........24

Apparent Midpoint of Transition Zone ............................................................................30

Steps at the Apparent Midpoint .............................................................................30

Relation Between Daily Withdrawal Rates and Apparent Midpoint of Transition 

Zone...........................................................................................................30

Case Study: Beretania DMW.................................................................................32
vi

Effects of Withdrawal on Water Quality ..................................................................................33

Deep Monitor Wells Acting as Brackish-Water Conduits ...............................................33

Mass Balance ................................................................................................................36

Groundwater Management Implications ................................................................................36

Summary and Conclusions ............................................................................................................37

References Cited............................................................................................................................38

Figures

ES1. Vertical steps in measured salinity profiles in Hawai‘i......................................................... iv 

 1. Location of deep monitor wells that penetrate through the transition zone ..........................6

 2. Generalized diagram of the groundwater flow pattern in a freshwater-lens system.............7

 3. Locations and names of selected wells mentioned in the text, Island of O‘ahu ...................8

 4. Salinity-depth profile of Kahului Fairgrounds Well, central Maui, Hawai‘i.............................9

 5. Deep monitor wells in Hawai‘i.............................................................................................10

 6. Diagrams of conceptual salinity profiles of different shapes in a borehole.........................13

 7. Salinity profile in South Hälawa DMW on 09/13/2001........................................................14

 8. Maximum length of vertical step in measured salinity profiles in Hawai‘i ...........................15

 9. Specific-conductance, borehole diameter (caliper), and flowmeter measurements...........18

 10. Specific-conductance, borehole diameter (caliper), and flowmeter measurements...........19

 11. Paired specific-conductance and temperature profiles.......................................................20

 12. Salinity (A) and temperature (B) profiles obtained in Punanani DMW ...............................21

 13. Selected profiles in which the longest vertical step in salinity does not include 

the 1,000 microsiemens per centimeter specific-conductance value.............................23

 14. Selected profiles in which the longest vertical step in salinity includes 

the 1,000 microsiemens per centimeter specific-conductance value.............................25

 15. Selected profiles in which the longest vertical step in salinity, induced by nearby or 

regional withdrawal, is persistent....................................................................................27

 16. Maximum length of vertical step in measured salinity profiles from deep monitor wells in 

Hawai‘i.............................................................................................................................28

 17. Changes from 1968 to 2009 in the depths of the apparent top and midpoint 

of the transition zone between fresh and saltwater ........................................................29

 18. Influence of natural and human-induced stresses on the salinity profile in a deep monitor

well..................................................................................................................................31

 19. Observed and estimated specific-conductance changes and relative vertical 

displacement of the apparent midpoint of the transition zone in Beretania DMW..........32

 20. Conceptual cross section between production well and nearby monitor well ....................34

 21. Salinity of water in Punanani production wells....................................................................35

 22. Salinity of water from point piezometers through the freshwater-saltwater 

transition zone in Kawailoa Pump 4................................................................................36
vii

Conversion Factors

Inch/Pound to SI 

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

gallon (gal) 3.785 cubic decimeter (dm3

) 

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

)

Flow rate

foot per minute (ft/min) 0.3048 meter per minute (m/min)

foot per day (ft/d) 0.3048 meter per day (m/d)

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

/d)

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

/s)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Vertical coordinate information is referenced in feet relative to mean sea level. Altitude, as used in this report, refers to 

 distance above the vertical datum.

Specific conductance is given either in microsiemens per centimeter at 25 degrees Celsius (μS/cm) or millisiemens per 

 centimeter at 25 degrees Celsius (mS/cm).

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

Tables

1. Salinity of the top and midpoint of the freshwater-saltwater transition zone..............................7

2. Characteristics of deep monitor wells and summary of available salinity 

and temperature profiles used in this study....................................................................12

3. Specifications of the maximum vertical step of measured salinity........................................................... 16

4. Effect of stresses from ocean tides, barometric pressure, and nearby groundwater 

withdrawals.................................................................................................................................... 33
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Effects of Groundwater Withdrawal on Borehole Flow 

and Salinity Measured in Deep Monitor Wells in Hawaiÿi—

Implications for Groundwater Management

By Kolja Rotzoll1

Abstract 

Water-resource managers in Hawaiÿi rely heavily on salinity profiles from deep monitor wells to estimate the thickness of 

freshwater and the depth to the midpoint of the transition zone 

between freshwater and saltwater in freshwater-lens systems. The 

deep monitor wells are typically open boreholes below the water 

table and extend hundreds of feet below sea level. Because of possible borehole-flow effects, there is concern that salinity profiles 

measured in these wells may not accurately reflect the salinity 

distribution in the aquifer and consequently lead to misinterpretations that adversely affect water-resource management.

Steplike changes in salinity or temperature with depth in 

measured profiles from nonpumped deep monitor wells may be 

indicative of water moving within the well, and such changes 

are evident to some extent in all available profiles. The maximum vertical step length, or displacement, in measured profiles 

ranges from 7 to 644 feet. Vertical steps longer than 70 feet 

exceed the typical thickness of massive lava flows; they therefore cannot be attributed entirely to geologic structure and may 

be indicative of borehole flow. The longest vertical steps occur 

in monitor wells located in southern Oÿahu, coinciding with the 

most heavily developed part of the aquifer.

Although regional groundwater withdrawals have caused a 

thinning of the freshwater lens over the past several decades, the 

measured midpoint of the transition zone in most deep monitor wells has shown only inconsequential depth displacement 

in direct response to short-term variations in withdrawals from 

nearby production wells. For profiles from some deep monitor 

wells, however, the depth of the measured top of the transition zone, indicated by a specific-conductance value of 1,000 

microsiemens per centimeter, has risen several hundred feet 

in response to withdrawals from nearby production wells. For 

these deep monitor wells, monitoring the apparent top of the 

transition zone may not provide an accurate indication of water 

quality in the adjacent aquifer. Hence, the measured midpoint in 

boreholes is a better proxy for freshwater-lens thickness. 

Brackish water transported upward in a deep monitor well 

can exit the borehole in the upper, freshwater part of the aquifer 

and affect the water quality in nearby production wells. Piezometers installed at different depths will provide the best information 

on aquifer salinity because they are unaffected by borehole flow. 

Despite the effects of borehole flow, monitoring the midpoint in 

deep monitor wells is still useful to identify long-term trends in the 

movement of the transition zone.

Introduction

In Hawaiÿi, dozens of deep monitor wells (DMWs) have 

been drilled to provide an indication of how groundwater salinity 

varies with depth. The deep wells penetrate entirely through the 

freshwater zone and partly or entirely through the brackish-water 

transition zone that separates freshwater from underlying higher 

density saltwater in a freshwater-lens system. DMWs typically are 

constructed as open (uncased) boreholes or are cased with slotted 

casing that allows water from the aquifer to readily enter the well. 

Salinity profiles measured in DMWs typically are used to estimate 

the thickness of the freshwater lens in volcanic-rock aquifers in 

Hawaiÿi under the assumption that the measured vertical distribution of salinity in the borehole is representative of salinity in the 

adjacent aquifer. However, vertical flow can displace water inside 

the monitor well so that the observed salinity in the borehole may 

not accurately reflect the salinity distribution in the aquifer. This 

can lead to misinterpretations that adversely affect water-resource 

management. Groundwater provides almost all drinking water and 

more than half of freshwater for other uses in Hawaiÿi (Gingerich 

and Oki, 2000). Decisions related to future infrastructure development and alternate sources of freshwater, including desalinization, will depend on the long-term sustainability of groundwater 

resources. It is therefore critically important, from economic 

and resource perspectives, to have an accurate understanding of 

the volume of fresh groundwater stored in aquifers and how it is 

changing in response to changes in recharge and withdrawal.

The status of fresh groundwater resources in Hawaiÿi 

commonly is assessed through vertical profiles of salinity 

collected from DMWs. The profiles provide information on 1

University of Hawaiÿi at Mänoa, Water Resources Research Center
2 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

the thickness of the freshwater zone at the time the profile 

is collected. Furthermore, monitoring over time provides information on how the freshwater thickness changes as a result of 

changes in groundwater withdrawals or recharge. By monitoring changes in the freshwater thickness in DMWs over time, 

water managers can determine appropriate withdrawal rates 

from production wells. The State of Hawaiÿi Commission on 

Water Resource Management (CWRM) uses the Robust Analytical Model (Mink, 1980, 1981) and the Robust Analytical 

Model 2 (RAM2) (Liu, 2006, 2007) for estimating the amount 

of groundwater that can be withdrawn from aquifer systems in 

the State. RAM2 incorporates information from salinity profiles to estimate sustainable withdrawal rates. Vertical salinity 

profiles also represent valuable data for calibration of numerical groundwater models designed to simulate density-dependent groundwater flow and transport in coastal aquifers (Voss 

and Souza, 1987; Oki and others, 1998). These models provide 

insights into groundwater availability under different recharge 

and withdrawal scenarios (Oki, 2005; Gingerich, 2008) and 

are useful tools for managing groundwater resources.

The underlying assumption in using salinity profiles from 

DMWs is that the measured vertical distribution of salinity 

in the borehole is representative of salinity in the adjacent 

aquifer. However, natural or anthropogenically induced vertical hydraulic gradients in the aquifer can generate flow in the 

borehole. Complex flow patterns may exist in some boreholes 

in Hawaiÿi (Paillet and Hess, 1995). Flow within a monitor 

well causes water that entered the borehole in one interval 

to be present in another interval, and thus the salinity profile 

obtained from a monitor well may not be representative of 

salinity in the adjacent aquifer. For cases in which upward 

flow exists in a borehole, brackish water or saltwater entering 

the borehole from deeper zones can lead to overestimation of 

salinity in the adjacent aquifer in shallower zones. In contrast, 

for cases in which downward flow exists, freshwater entering 

the borehole from shallow zones can lead to underestimation 

of salinity in the adjacent aquifer in deeper zones. All other 

factors being equal, higher rates of borehole flow will have a 

greater effect on the measured salinity profile (Paillet and others, 2002).

Presently (2010), 48 DMWs exist in Hawaiÿi, of which 

40 are monitored on a routine basis by the Honolulu Board 

of Water Supply (HBWS), CWRM, U.S. Geological Survey 

(USGS), National Park Service (NPS), and other private entities. Although various instruments have been used in the past, 

presently most of the profiles are measured using a multiparameter probe that measures fluid electrical conductivity (as a 

proxy for salinity), temperature, and pressure as the probe is 

lowered through the water column.

In 2007, with support from Congress and in collaboration 

with the CWRM, HBWS, and the Water Resources Research 

Center of the University of Hawaiÿi, the USGS initiated an 

investigation to evaluate the effects of borehole flow on 

measured salinity profiles in DMWs. The concern was that 

inaccurate or misleading information on aquifer salinity could 

adversely affect groundwater management. This study focuses 

on the effects of groundwater withdrawal on DMWs. Some 

of the questions addressed by this study include: (1) Does 

borehole flow occur in all DMWs? (2) What are the indicators 

of borehole flow? (3) What are the effects of nearby withdrawal on DMWs? (4) Which DMWs are strongly affected by 

borehole flow? (5) What are the ramifications of borehole-flow 

effects for groundwater management?

Purpose and Scope

This report presents evidence for borehole flow in 

DMWs, explains some of the possible effects of nearby 

groundwater withdrawals on measured salinity profiles, and 

discusses implications for water-resource management in 

Hawaiÿi. This study reviews and evaluates 797 salinity profiles 

from 42 DMWs and reviews vertical flow measurements in 

4 DMWs. The geographic focus of the study is on the most 

developed aquifer systems in Hawaiÿi—the Honolulu-Pearl 

Harbor area on Oÿahu and the Wailuku area on Maui.

Previous Investigations

Borehole geophysical logging in Hawaiÿi has shown evidence of borehole flow in some cases (Paillet and Hess, 1995; 

Paillet and others, 2002). Although this phenomenon has been 

recognized for more than 20 years (for example, Eyre, 1987; 

Souza and Meyer, 1995; Meyer and Presley, 2001; Oki, 2002), it 

has not been comprehensively addressed in published literature. 

As a result of possible borehole-flow effects, salinity profiles 

from monitor wells may not accurately reflect the salinity 

distribution in the surrounding aquifer (Paillet and Hess, 1995). 

Underwood and others (1995) stated that if upward borehole 

flow exists, salinity profiles would tend to indicate a thinner 

freshwater zone than exists in the aquifer. Voss and Wood (1994) 

concluded that borehole flow may affect local sections of the 

profile that are as much as 66 ft long, although salinity measurements on the scale of the entire profile generally are representative of water quality in the aquifer. Paillet and others (2002) 

found electrical-conductivity profiles of the fluid column generally similar to that of the groundwater in the adjacent formation 

by comparing them to induction logs. Vertical flow was observed 

in the Waipiÿo Mauka DMW (fig. 1), and complex flow patterns 

were observed in boreholes near Waipahu DMW, Oÿahu, and 

the east side of the Island of Hawaiÿi (Paillet and Hess, 1995). 

Borehole flow also was observed in the Kaimukï Station DMW 

(fig. 1) (Paillet and others, 2002). The flow and the salinity in 

the well both increased in the Kaimukï Station DMW when the 

withdrawal rate from the nearby production wellfield doubled. 

Causes of borehole flow and the effects of borehole flow on 

vertical salinity profiles in coastal aquifers in Hawaiÿi and Guam 

have been addressed by Oki and Presley (2008).

Borehole flow can influence the reliability of groundwater 

quality samples collected from monitor wells (Giddings, 

1987). Since the late 1980s, the use of long-screened boreholes 

(hundreds of feet) for groundwater sampling has been 
Physical Setting 3

discouraged in many studies because water samples obtained 

from them are not representative of water in the adjacent 

aquifer (for example, Shosky, 1987; Reilly and others, 1989; 

Robbins, 1989). Vertical borehole flow can occur in a longscreened monitor well because of a natural hydraulic gradient 

with depth (Church and Granato, 1996). This flow also could 

act as a conduit for contaminants (Lacombe and others, 1995). 

In long-screened wells in a sandy aquifer in France, sampling 

for groundwater age at discrete depths was biased by ambient 

vertical flow (Corcho Alvarado and others, 2009). A sand-tank 

model study indicated that a long-screened well can create an 

artifact even in the absence of vertical ambient flow and that 

water-quality stratification in a borehole may not correspond 

to stratification in the aquifer (Britt, 2005). Sukop (2000) used 

a simple solute mass-balance and water-balance method to 

compute the vertical distribution of solute concentration in the 

aquifer using water-quality information from a long-screened 

monitor well. Applying this mass balance, McDonald and Smith 

(2009) found that contaminant stratification in a wellbore is not 

necessarily representative of stratification in the aquifer, and they 

discouraged using passive or micropurge sampling techniques.

Hutchins and Acree (2000) found considerable ambient 

flow in monitoring wells using an electromagnetic flowmeter. 

Izbicki and others (2005) studied temporal changes of chloride 

concentrations and vertical flow in observation wells under 

pumping conditions in California. The combination of flowmeter 

data and depth-dependent water-quality data revealed sources of 

saline water being transported from deeper parts of the aquifer 

into shallower parts. Similar observations showed a monitor 

well forming a connection between the higher and lower 

hydraulic head zones (Elci and others, 2001; Elci and others, 

2003). Konikow and Hornberger (2006) modeled preferential 

groundwater flow through monitor wells and concluded that 

solutes can be redistributed in different aquifers through 

monitor wells. Zinn and Konikow (2007) used a numerical 

model to demonstrate that borehole flow can substantially bias 

groundwater ages determined from water samples, but they 

also indicated that the effects are highly dependent on local or 

regional flow conditions and may change with time.

Hanson and others (2002) attributed temporal differences 

in electromagnetic induction logs to possible seasonal seawater 

intrusion in Monterey, California. Newhouse and Hanson (2002) 

used a prototype borehole acoustic-doppler velocimeter for threedimensional flow measurements in uncased wells in Idaho. Their 

data delineated two independent flow zones with a downward flow 

component and indicated a more complex hydrogeologic setting 

than could be interpreted from water-table maps. Hanson (2005) 

noted that wellbores connect layered aquifers and that vertical 

borehole flow is mainly responsible for the extraction of water 

from or recharge of water to deeper aquifers.

The location of the freshwater-saltwater interface is 

strongly related to the ocean tide in Jeju Island, Korea (Kim 

and others, 2006). Salinity profiles changed depending on the 

stage of the tide. A sharp interface was observed during low 

tide, but the transition zone broadened to 60 ft at high tide 

(Kim and others, 2008; Kim and others, 2009). The change 

was not directly attributed to borehole flow, although the shape 

of the specific-conductance profiles may indicate vertical 

flow in the monitor wells. Salinity profiles in long-screened 

coastal wells in Israel also varied with ocean tides, which was 

attributed to vertical flow in the borehole (Shalev and others, 

2009). Numerical modeling that incorporated an open borehole indicated that vertical flow in the monitor well is three 

orders of magnitude greater than in the aquifer, which indicated that measured salinity profiles do not accurately reflect 

the freshwater-saltwater transition zone in the aquifer (Shalev 

and others, 2009).

Acknowledgments

Glenn Oyama and Darrell Wong of the Honolulu Board 

of Water Supply and Diane England of the State Commission 

on Water Resource Management provided access to numerous 

specific-conductance and temperature profiles, geophysical 

logs, and other data. Paul Eyre, formerly of the U.S. Navy 

Public Works Center, and Joe Mendonca of the Maui Department of Water Supply provided information on pumping rates. 

Constructive comments from Delwyn Oki, William Souza, 

Randall Hanson, and many others from the USGS Pacific 

Islands Water Science Center helped improve the report.

Physical Setting

Geography and Geology

The Islands of Hawaiÿi include six major populated 

islands that were formed by eruption of thousands of lava 

flows as the Pacific Plate of the Earth’s lithosphere moved over 

the Hawaiian hotspot. Each island consists of one or more 

shield volcanoes, from which extrusion of primarily thin basaltic lava flows from central calderas and from fissures along rift 

zones took place between 5 million years ago (Kauaÿi) and the 

present (Hawaiÿi). The young basalts are highly permeable and 

form productive aquifers. Changes in sea level, subsidence, 

and erosion of volcanic rocks contributed to deposition of 

marine and terrestrial sediments on coastal plains and deeply 

incised valley floors (Peterson, 1993).

The aquifers in Hawaiÿi are recharged by rainfall, the 

distribution of which is characterized by steep spatial gradients (Giambelluca and others, 1984). Maximum precipitation 

occurs at high altitudes and along the mountain flanks that are 

exposed to the northeast trade winds. Annual rainfall is less 

than 10 in. in dry leeward areas and can exceed 400 in. in wet 

mountainous areas (Giambelluca and others, 1986). Intense 

rainfall and steep terrain commonly produce flashy stream discharge, although most moderate storms generate little runoff 

and large amounts of infiltration because of the high permeability of the volcanic rocks (Peterson, 1972).

The most densely populated areas, and thus the most 

developed aquifers, in Hawaiÿi are in the Honolulu-Pearl 
4 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

Harbor area on Oÿahu (Oki, 2005) and in the Wailuku-ÿÏao 

area in central Maui (Meyer and Presley, 2001; Gingerich, 

2008). Many areas in Hawaiÿi are undergoing a transition in 

land use from decades of large-scale agriculture (sugarcane 

and pineapple) to small-scale diversified agriculture and 

rapidly expanding urban developments. Changes in recharge 

through different irrigation patterns and increasing water 

demands of a growing population affect the sustainability of 

the groundwater resources (Engott and Vana, 2007). In addition, it is unclear how changes in recharge associated with 

climate change will affect the groundwater resources.

Hydrogeology

More than 80 percent of Hawaiÿi’s DMWs are located 

on Oÿahu, and more than three-quarters of those are concentrated in the Honolulu-Pearl Harbor area (fig. 1). The regional 

hydrogeology of southern Oÿahu is therefore briefly summarized here, although many more detailed descriptions exist (for 

example, Stearns and Vaksvik, 1935; Wentworth, 1951; Visher 

and Mink, 1964; Hunt, 1996; Nichols and others, 1996). For 

a comprehensive overview of Maui’s geology, the reader is 

referred to Stearns and Macdonald (1942).

Flank Lavas

Oÿahu is formed by the eroded remnants of two shield 

volcanoes. Rocks from the older Waiÿanae Volcano were dated 

between 3.7 and 2.9 Ma (Doell and Dalrymple, 1973; Presley 

and others, 1997). The volcanic rocks of the Waiÿanae Volcano 

consist primarily of thin-bedded shield-stage tholeitic flows 

that are on average 40 ft thick and dip about 6 degrees (Stearns 

and Vaksvik, 1935). The lava flows of the shield-stage Lualualei Member of the Waiÿanae Volcanics are 5–75 ft thick, those 

from the transitional or late shield-stage Kamaileÿunu Member 

are 10–120 ft thick, and those from the postshield-stage Pälehua Member are 50–100 ft thick (Stearns, 1940). Rocks from 

the Koÿolau Volcano consist mainly of thin-bedded shieldstage tholeitic flows that have ages between 2.6 and 1.8 Ma 

(Doell and Dalrymple, 1973; Langenheim and Clague, 1987). 

The lava flows of the Koÿolau Volcano range in thickness 

from several to 80 ft, with an average thickness of 10 ft, and 

have dip angles between 3 and 10 degrees. In central Oÿahu, 

where lava flows of the Koÿolau Volcano overlap those of the 

Waiÿanae Volcano, dips of the Koÿolau Basalt are typically 

between 3 and 5 degrees (Stearns and Vaksvik, 1935).

 Openings within lava flows and at contacts between 

flows contribute to the high permeability of the volcanic rocks. 

The bulk hydraulic conductivity of the aquifer formed by flank 

lava flows is controlled by factors including the thickness of 

the flows, the thickness of clinker zones associated with ÿaÿä 

flows, extent of fractures, occurrence of lava tubes associated with pähoehoe flows, and the degree of weathering (Lau 

and Mink, 2006). The effective regional horizontal hydraulic 

conductivity of dike-free flank lavas is on the order of 1,000 

ft/d, although locally the horizontal hydraulic conductivity 

can vary by orders of magnitude (Hunt, 1996; Rotzoll, 2007). 

The basaltic aquifers are heterogeneous and anisotropic, with 

a horizontal hydraulic conductivity that tends to be greater 

parallel to the flow direction of lava flows than perpendicular 

to it (Nichols and others, 1996). The horizontal hydraulic conductivity can be as much as 200 times greater than the vertical 

hydraulic conductivity (Souza and Voss, 1987).

Dike Zones

The Waiÿanae and Koÿolau Volcanoes each have two primary rift zones and a third less pronounced rift zone. The rift 

zones are characterized by numerous nearly vertical intrusive 

dikes (Stearns and Vaksvik, 1935). Dikes are thin, nearvertical sheets of massive, low-permeability rock that formed 

by magma intruding existing rocks and cooling beneath the 

surface. The abundance of dikes can be as high as 1,000 per 

mile in the center of the rift zone and decreases away from 

the center. Dikes impede lateral groundwater flow because of 

their low permeability (Takasaki and Mink, 1985). The overall 

hydraulic conductivity of an entire dike complex can be as low 

as 0.01 ft/d, although the hydraulic conductivity of individual 

dikes may be lower and the conductivity of rocks between 

dikes may be much higher (Meyer and Souza, 1995).

Sedimentary Deposits

The coastal plain of southern Oÿahu comprises terrestrial 

and marine sediments and reef limestone deposits. The coastal 

sedimentary wedge overlying the flank lavas commonly is 

referred to as caprock in literature on Hawaiian hydrology. 

In the area of Pearl Harbor, the caprock ranges in thickness 

from 1,000 ft near the coast to 0 ft about 5 miles inland (see 

fig. 5). The hydraulic conductivity of individual units in the 

caprock spans several orders of magnitude, from 0.01 to 1 ft/d 

for fine-grained muds and saprolite (Wentworth, 1938) to as 

much as several thousand feet per day for coral gravels and 

reef limestone deposits (Oki, 1998). The overall permeability 

of caprock is lower than that of the underlying basalt. The caprock, therefore, impedes groundwater discharge to the ocean 

and acts as a confining unit over the volcanic-rock aquifer 

(Visher and Mink, 1964). As a result, the groundwater levels in 

the Pearl Harbor-Honolulu area are higher than in comparable 

areas without caprock.

Alluvium-filled stream valleys also affect groundwater 

flow. During sea-level lowstands, stream valleys were deeply 

incised and subsequently filled with terrestrial and marine 

sediments. Additionally, beneath the valley-filling sedimentary deposits is a layer of highly weathered volcanic rock. 

The valley fills and underlying weathered rock have lower 

permeability than the surrounding flank lavas and can impede 

lateral groundwater flow across valleys. The effectiveness of 

the barriers formed by the valley-fill deposits is dependent on 

the hydraulic properties of the deposits and depth of valley 
Physical Setting 5

incision (Oki, 2005; Rotzoll and El-Kadi, 2007). Contrasting water levels on either side of a hydrologic barrier are an 

indication of the barrier’s effectiveness at impeding horizontal 

groundwater flow. Valley fills, dike zones, and the coastal 

caprock compartmentalize aquifers in southern Oÿahu, and 

these barriers tend to enhance water-level drawdown caused 

by groundwater withdrawals from pumped wells.

Groundwater Dynamics in the Freshwater Lens

Groundwater in Hawaiÿi principally occurs in the flank 

lavas as a freshwater lens that floats on saltwater and as highlevel water in dike-impounded systems in the rift zones of the 

volcanoes (Mink and Lau, 1990). The latter form of occurrence will not be discussed in detail, because most DMWs 

are located within freshwater-lens systems. Furthermore, the 

distribution of freshwater and saltwater at depth in the dikeimpounded systems remains uncertain because the principles 

of hydrostatic buoyancy may not strictly apply in the rift zones 

(Hunt, 1996). Besides freshwater-lens and dike-impounded 

groundwater systems, groundwater exists on Oÿahu as highlevel water (about 275 ft above sea level) beneath the Schofield Plateau between hydrologic barriers of uncertain origin 

(fig. 1) and locally as perched water bodies over poorly permeable layers (for example, volcanic ash).

The freshwater lens is commonly called a Ghyben-Herzberg lens following the independent descriptions of Badon 

Ghyben (1889) and Herzberg (1901). The occurrence of the 

freshwater lens in Hawaiÿi was described in the early part of 

the 20th century by a number of individuals, including Lindgren (1903), Andrews (1909), Palmer (1927), and Meinzer 

(1930). Understanding the factors that control the occurrence 

and dynamics of a freshwater lens was later improved by 

Stearns and Vaksvik (1935), Stearns and Macdonald (1942), 

Wentworth (1942, 1947, 1948, 1951), Cox (1954), Visher and 

Mink (1964), Lau (1964, 1967), and Chung and others (1982). 

According to the Ghyben-Herzberg principle, which assumes 

hydrostatic conditions, the altitude of the water table in a 

freshwater lens is controlled by the isostatic buoyancy of the 

freshwater floating on saltwater. For freshwater and saltwater 

densities of 1,000 and 1,025 kg/m3, respectively, the GhybenHerzberg principle is simplified to the following relation:

z = -40h, (1)

where z is the depth of the interface between freshwater and 

saltwater in feet below sea level and h is the water level in feet 

above sea level. For every foot of freshwater above mean sea 

level the freshwater lens extends 40 ft below sea level, a relation that has been widely used in Hawaiÿi (Izuka and Gingerich, 1998; Oki and Meyer, 2001; Rotzoll and others, 2010) 

and that will be used in this report. The Ghyben-Herzberg 

principle generally is applicable in homogeneous steady-state 

flow systems in which the transition between freshwater and 

saltwater is sharp (Hubbert, 1940; Glover, 1959). Mixing of 

freshwater and saltwater forms a brackish-water transition 

zone separating the two water bodies. The thickness of the 

brackish-water transition zone may be enhanced by factors 

including ocean tides and variations in barometric pressure, 

recharge, and discharge (Wentworth, 1948; Cooper, 1959).

The freshwater-lens system in southern Oÿahu comprises 

an upper layer of water derived from local rainfall and irrigation over the past decades, a cooler freshwater core containing waters with measured carbon-14 ages of about 1,800 

years, the brackish-water transition zone, and saltwater with 

measured carbon-14 ages ranging from about 9,000 to 6,000 

years (Voss and Wood, 1994). The thickness of the freshwater lens depends on factors including the amount of recharge, 

the hydraulic properties of the aquifer, and the effectiveness 

of the sedimentary caprock that acts as a coastal confining 

unit. Generally, brackish groundwater occurs in areas of low 

recharge without caprock sediments (Stearns, 1941), such as 

the west side of the Island of Hawaiÿi or central Maui. Thin 

freshwater lenses (tens of feet) exist in areas where the coastal 

caprock is thin, such as the Kawailoa area of northern Oÿahu. 

Thick freshwater lenses (hundreds of feet) exist in the Honolulu 

and Pearl Harbor areas of Oÿahu and the ÿÏao area of Maui, 

where coastal caprock effectively confines the volcanic-rock 

aquifer (Meyer and Presley, 2001). Water-table altitudes are 

typically lowest at the coast and increase inland at a rate of 

about 1 ft/mi (Lau and Mink, 2006). However, local variations 

in the hydraulic gradient may be caused by aquifer heterogeneities, discharging springs, heavily pumped wells, and valley-fill 

or other barriers (Oki, 2005).

The amount of fresh groundwater stored in a freshwater 

lens may change over time in response to changes in recharge 

or discharge. Early measurements from about 1880 indicate 

predevelopment water levels between 32 and 42 ft above sea 

level in the Pearl Harbor and Honolulu areas. In 2000, the 

regional water table in the Pearl Harbor and Honolulu areas 

was between 18 and 25 ft above sea level. Relatively wet years 

during 2002–06 resulted in recharge that caused water levels 

to rise by several feet.

Groundwater flow within the freshwater lens of island 

aquifers is mainly horizontal, although groundwater flow 

has a downward component in inland recharge areas and an 

upward component in coastal discharge areas (Tóth, 1962). 

The regional groundwater flow pattern in volcanic-rock 

aquifers can be summarized as follows (after Souza and Voss, 

1987; Oki, 1998): (1) most of the recharge of fresh groundwater from rainfall occurs in the mountainous interior areas, 

and groundwater flow has a downward component in the 

inland recharge areas; (2) groundwater flow is from inland 

recharge areas towards coastal discharge areas and is generally horizontal; (3) in the coastal area, a caprock impedes 

groundwater discharge to the ocean, and the flow of fresh 

and brackish water generally is upward into the caprock; and 

(4) the caprock also impedes the inflow of saltwater into the 

volcanic-rock aquifer (fig. 2).

Evidence for upward flow near the discharge area of the 

flow system is available from the Halekauwila Well located 660 ft 
6 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

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21°30'

21°40'

158°10' 158°00' 157°50' 157°40'

EXPLANATION

Shaded relief generated from U.S. Geological Survey 10-meter Digital Elevation 

Model, Universal Transverse Mercator projection, zone 4, NAD83 datum. 

! #

"

"

Agency measuring salinity profiles in deep monitor well

HBWS

CWRM

USGS #

HBWS and CWRM

CWRM and USGS

NPS

#

Groundwater types on Oÿahu

Freshwater lens

High-level, Schofield area

High-level, dike impounded

Schofield Plateau

Ko

Waiÿanae ÿolau Dike Complex

Dike

Complex

Windward Aquifer Systems

Koÿolau Dike Complex

Waiÿanae

North Aquifer Systems

Windward Aquifer Systems

"

"

"

""

""

#

"

#

0 25 MILES

0 50 KILOMETERS

MILES

KILOMETERS

158° 157° 156° 155°

21°

20°

19°

Hawaiÿi

Maui

Oÿahu

Molokaÿi

Länaÿi

Kahoÿolawe

Pacific

Ocean

Waipiÿo Mauka

Waipahu Waipahu

Waiola

Poliwai

Kunia

Mauka

Kunia Middle

Honouliuli

Kunia T41

Waimano

Helemano Twin Bridge

Kawailoa

Kahuku

Läÿie

Hauÿula

Punaluÿu

Kaluanui 2

Waiawa

Mänana

Punanani

Waimalu

Newtown

Kaamilo

Hälawa

South Hälawa

Moanalua Kalihi Shaft

Waiÿalae

Kaimukï Station

Kaimukï HS

Beretania

Punchbowl Kalihi Station

Jonathan

Springs

Waÿahila

Kualapuÿu ÿÏao

Waiehu

Lahaina

Häwï Makai I Hapuÿu Bay D

Kahaluÿu

Kahului Fairgrounds

T-133

T-67

Kohanaiki Keöpü

Barbers Point

Figure 1. Location of deep monitor wells that penetrate through the transition zone between freshwater and saltwater in 

Hawai‘i, and areas with freshwater-lens systems and high-level groundwater on O‘ahu, Hawai‘i (modified from Mink and Lau, 

1990). Symbols indicate the agency that collected most of the salinity profiles from the indicated well: Honolulu Board of Water 

Supply (HBWS), Commission on Water Resource Management (CWRM), U.S. Geological Survey (USGS), and National Park 

Service (NPS). The dashed line separates the Pearl Harbor aquifer systems from the Honolulu aquifer systems.
General Characteristics of Salinity Profiles and Deep Monitor Wells 7

(1) (2)

(3)

(4)

Saltwater

Freshwater

Volcanic rock

Ocean

Transition zone

Caprock

Figure 2. Generalized diagram of the groundwater flow pattern 

in a freshwater-lens system in volcanic rocks in Hawai‘i: (1) 

downward flow in recharge areas, (2) horizontal flow, (3) upward 

flow in coastal discharge areas, and (4) discharge through the 

coastal sediments.

inland near the coastal discharge zone of the Honolulu area (fig. 3). 

This artesian well has two piezometers open to the Koÿolau Basalt 

aquifer at altitudes of -988 and -1,043 ft. Generally, the equivalent 

freshwater head (elevation of freshwater in the well needed to balance the pressure outside of the well screen created by the denser 

brackish water and saltwater) in the deeper piezometer is about 

1.5 ft higher than in the shallower piezometer, indicating an 

upward vertical head gradient of 0.027 ft/ft (140 ft/mi). Upward 

flow in the aquifer is consistent with the regional aquifer flow concept at this location. Upward flow also occurs near the Helemano 

area of northern Oÿahu. Two exploratory wells located 

4,820 ft inland and 20 ft apart (fig. 3) are open to the Koÿolau 

Basalt aquifer, one from altitudes of -68.5 to -73.5 ft and the other 

from -271 to -291 ft. During 1995, artesian heads measured in the 

deep well were 0.23 ft higher than in the shallow well (Presley and 

Oki, 1996). The smaller vertical head gradient of 0.001 ft/ft (5 ft/

mi) associated with the Helemano wells compared to the head 

gradient of 0.027 ft/ft from the Halekauwila Well may be related 

to the greater distance to the discharge zone associated with the 

Helemano wells.

General Characteristics of Salinity 

Profiles and Deep Monitor Wells

Deep monitor wells in Hawaiÿi generally penetrate 

through the freshwater lens and the transition zone into saltwater, although some wells terminate in the brackish-water transition zone. Because the amount of water that can be developed for potable use from a freshwater lens is constrained 

by its salinity, the altitudes of the top and midpoint of the 

transition zone are monitored in Hawaiÿi (table 1). A change in 

thickness of the upper part of the transition zone can be identified by monitoring the water level and the top and midpoint of 

[μS/cm, microsiemens per centimeter; mg/L, milligram per liter; ppm, parts 

per million; %, percent; ~, approximately] 

Physical

property Units

Top of the 

transition 

zone

Midpoint of 

the transition zoneSaltwater

Specific

conductance μS/cm 1,000 25,000 55,000

Chloride mg/L 250 9,800 19,600

Salinity ppm 0.7 17.9 35.7

Seawater

salinity % ~2 50 100

Table 1. Salinity of the top and midpoint of the freshwatersaltwater transition zone compared to saltwater salinity.

the transition zone over time. By convention, the depth of the 

midpoint between freshwater and saltwater is the depth where 

the fluid specific conductance equals 25,000 microsiemens 

per centimeter (μS/cm). However, this may not represent the 

true 50-percent point of seawater salinity, because the measured specific conductance at the bottom of several DMWs 

exceeds 50,000 μS/cm. For example, the maximum specific 

conductance in South Hälawa DMW and Waiehu DMW is 

51,500 μS/cm, and the maximum specific conductance in 

Punaluÿu DMW is 54,250 μS/cm. For the purposes of this 

report, saltwater is defined as having a specific conductance of 

55,000 μS/cm, although the definition of the midpoint is kept 

at the conventional specific conductance of 25,000 μS/cm. The 

top of the transition zone is represented by a salinity of about 

2 percent of the seawater salinity, corresponding to a specific 

conductance of 1,000 μS/cm, which is approximately equivalent to the advisory secondary drinking water standard of 250 

milligrams per liter (mg/L) for chloride, based on aesthetic 

concerns (U.S. Environmental Protection Agency, 2006).

Sources of Available Data

Historical

Information from DMWs in Hawaiÿi has contributed 

tremendously to the understanding of water quality in freshwater-lens systems. Palmer (1927) reported a progressive 

increase in salinity with depth in several wells below -500 ft, 

indicating a broad transition zone. The first salinity measurements with depth on Maui were made in 1935 (Stearns and 

Macdonald, 1942, p. 117). Chloride concentrations of water 

samples collected at selected depths in a well increased from 

500 to 18,000 mg/L between -116 and -139 ft relative to mean 

sea level (fig. 4). The observed water table was at 3.24 ft above 

sea level, and the midpoint of the salinity profile was at -124.2 ft. 

Using the Ghyben-Herzberg ratio of 1:40, the estimated 

midpoint is at -129.6 ft, which is consistent with the observed 

midpoint depth from the salinity profile.
8 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

In the 1950s, several specific-conductance logs were 

collected by the Hawaiian Sugar Planters’ Association using 

a bridge and weighted cell to measure capacitance variability 

between the two elements (Lao and others, 1969). In 1960, 

wells at Kaimukï and Beretania pumping stations were drilled 

1,500 ft deep into the basalt aquifer (Lao, 2005). Visher and 

Mink (1964) observed a thick transition (1,200 ft) between 

freshwater and saltwater in well T-67, which is located near 

the Pearl Harbor shoreline (fig. 1). Well T-133 (fig. 1) was 

drilled in 1965 near Oÿahu’s south coast to a depth of -1,070 

ft relative to mean sea level. The well penetrates the transition 

zone into the saltwater (Cox and Lao, 1967). These early wells 

were used to make preliminary observations of the freshwaterlens thickness. Over time, additional monitor wells were 

drilled, and salinity profiles in these wells were measured on 

a routine basis. Starting in 1966, the applicability of electrical 

geophysical logging (Lao and others, 1969; Peterson and Lao, 

1970) and neutron logging techniques (Peterson and Seghal, 

1974) were tested in several wells on Oÿahu and Kauaÿi by the 

HBWS and the University of Hawaiÿi. Beginning in 1968, as 

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158°10' 158°00' 157°50' 157°40'

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0 4 8 KILOMETERS

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0 25 MILES

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158° 157° 156° 155°

21°

20°

19°

Hawaiÿi

Maui

Oÿahu

Molokaÿi

Länaÿi

Kahoÿolawe

Pacific Ocean

! Deep monitor well

Other monitor well

Pumped well

Nested piezometers

EXPLANATION

"

#

"

Shaded relief generated from U.S. Geological Survey 10-meter Digital Elevation

Model, Universal Transverse Mercator projection, zone 4, NAD83 datum. 

Red

Hill Shaft

Kawailoa Pump 4

Halekauwila

Mänana 1 Kalauao

Punanani P1-6

Helemano

Exploratory

Wells I and II

Helemano

Exploratory

Wells I and II

Punaluÿu II

 Red Hill monitor wells

T-86

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Figure 3. Locations and names of selected wells mentioned in the text, Island of O‘ahu.
General Characteristics of Salinity Profiles and Deep Monitor Wells 9

Measured

Interpolated

0 20 40 60 80 100 ‐160

‐140

‐120

‐100

‐80 Kahului Fairgrounds Well

SALINITY, IN PERCENT OF SEAWATER SALINITY

ALTITUDE, IN FEET

RELATIVE TO MEAN SEA LEVEL

Figure 4. Salinity-depth profile of Kahului Fairgrounds Well, 

central Maui, Hawai‘i. Water samples were taken at 1-foot intervals 

on May 22, 1936, and analyzed for salt content (Stearns and 

Macdonald, 1942).

part of the testing of techniques, the HBWS regularly logged 

four DMWs (T-86, Beretania DMW, Punanani DMW, and 

Punaluÿu DMW) with analog specific-conductance and temperature sensors. In the 1980s, the logging efforts were expanded 

to include additional DMWs (Kaimukï Station DMW, Waipahu 

DMW, and Waipiÿo Mauka DMW on Oÿahu and Waiehu DMW 

on Maui). Because the monitor well T-86 at Kaimukï pumping 

station was not plumb, which made monitoring at depth difficult, it was replaced with Kaimukï Station DMW in 1986.

Recent

In the past 10 years, additional DMWs have been drilled 

and are frequently logged to monitor the freshwater lens. For 

example, during 1999–2002, 19 DMWs were drilled. Currently, 40 DMWs regularly are logged in Hawaiÿi (fig. 5). 

Altogether, a dense network of monitor wells exists in the 

Pearl Harbor and Honolulu areas. Several agencies collect 

salinity profiles on a routine basis, and some DMWs are monitored by more than one agency. The majority of wells (30) are 

monitored by the HBWS, 11 are monitored by the CWRM, 

and 2 are monitored by the U.S. Geological Survey (USGS) 

(table 2). Recently (2008), a DMW (Kohanaiki DMW) was 

completed near the western coast on the Island of Hawaiÿi, and 

this well is monitored by the National Park Service (NPS). The 

797 salinity profiles analyzed for this study include profiles 

measured occasionally in Twin Bridge DMW, Häwï Makai I 

DMW, Hapuÿu Bay D DMW, and profiles from monitor wells 

that regularly are logged (fig. 5), excluding Kohanaiki DMW 

on the Island of Hawaiÿi.

Data Collection Methods

Salinity-profile data were collected using several methods. An early method involved collecting water samples at 

discrete depths and analyzing the water samples for salt content. This method is still used by the USGS at Waiehu DMW 

on Maui. Early profiles were collected by the HBWS with 

analog specific-conductance and temperature sensors and were 

recorded on paper charts at three different sensitivity scales. 

Data handling and archiving improved in 1998, when the 

CWRM began using a digital conductivity, temperature, and 

depth sensor (CTD) to collect data. The HBWS began collecting data with this type of sensor in 1999. The HBWS uses a 

winch equipped with a rotation-counting wheel for automatic 

measurements at every 0.33 ft. The CWRM and USGS use 

recorded pressure data and convert pressure to depth. The 

USGS corrects for errors in pressure measurements using 

a calibrated steel cable to measure depths. The HBWS, the 

CWRM, and the USGS currently log the monitor wells 

2–4 times per year, although some data gaps exist when equipment was malfunctioning or unavailable.

Analysis of Available Data

Proper comparison of data from different sources requires 

consideration of differences in how instruments record fluid 

conductivity. The analog recorder of the HBWS compensated 

for temperature internally and recorded specific conductance 

at a reference temperature of 25oC. Selected profiles collected 

by the HBWS with the analog recorder were digitized from 

the paper charts for this analysis. The electrical conductivity 

measured by the HBWS with a digital sensor since 1999, and 

all electrical-conductivity profiles collected by the CWRM and 

USGS, required conversion from electrical conductivity under 

ambient temperatures to specific conductance at 25oC. The 

following equation was used for postprocessing (Clesceri and 

others, 1998; Hayashi, 2004):

SC = EC

1 + 0.0191(t – 25) (2) ,

where SC is the specific conductance, in microsiemens 

per centimeter at 25oC, EC is the measured electrical conductivity, in microsiemens per centimeter, and t is the measured 

temperature, in degrees Celsius.

The USGS integrates regular accuracy checks of the conductivity sensor and annual accuracy checks of the temperature sensor in their data-collection routine. The USGS checks 

the accuracy of the conductivity sensor with four potassium 

chloride standard solutions (1, 10, 25, and 50 mS/cm) before 

each use. Differences between specific conductance of each 

standard and the corresponding measured specific conductance 

are recorded and archived for quality control. The sensor is 

considered properly calibrated if the error is within 5 percent 
10 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

(U.S. Geological Survey, variously dated). The accuracy of the 

pressure sensor of the instrument is checked during each log 

by comparing the recorded pressure to known depths from the 

calibrated steel cable.

The CWRM did not check the accuracy of their conductivity sensor before each deployment, although the sensor 

was sent back to the manufacturer annually for calibration. 

Between March 2007 and August 2008, negative conductivity 

Figure 5. Deep monitor wells in Hawai‘i, with the time period when routine logging of salinity profiles in each began. 

Also shown are the State of Hawai‘i aquifer systems and sectors for the freshwater-lens systems on O‘ahu (modified 

from Mink and Lau, 1990) and the surficial extension of capping sediments on O‘ahu.

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Pearl Harbor

Honolulu

North

Windward

Pearl Harbor

Honolulu

ÿEwa-Kunia

Waipahu-Waiawa

Waimalu

North

Windward

Koÿolauloa

Kawailoa

Waialua

Moanalua

Kalihi

Nuÿuanu

Pälolo

Waiÿalae West Waiÿalae West

0 4 8 MILES

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21°30'

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158°10' 158°00' 157°50' 157°40'

EXPLANATION

!

Year when regular logging of deep monitor wells started 

1968

1972 to 1988

1989 to 1998

1999 to 2002

2003 to 2008

!

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Sediment overlying volcanic-rock aquifers

Aquifer-sector boundary

Aquifer-system boundary

0 25 MILES

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21°

20°

19°

Hawaiÿi

Maui

Oÿahu

Länaÿi

Kahoÿolawe

Pacific

Ocean

! !!

!

!

!

Molokaÿi

Shaded relief generated from U.S. Geological Survey 10‐meter Digital Elevation 

Model, Universal Transverse Mercator projection, zone 4, NAD83 datum. 
Borehole Flow 11

values were measured in the shallow part of profiles using the 

CWRM sensor. On March 27, 2008, the USGS and CWRM 

checked the accuracy of the CWRM sensor using standard solutions and found the following errors: -137 percent at 1 mS/cm; 

-15.4 percent at 10 mS/cm; -7 percent at 25 mS/cm; and 

-4.5 percent at 50 mS/cm. Because of uncertainty related to the 

accuracy of the CWRM sensor, all profiles collected using the 

CWRM sensor between March 2007 and August 2008 were 

excluded from the analysis. The CWRM acquired a new sensor in June 2008. The HBWS also did not check the calibration of the conductivity sensor before each use, although they 

sent their digital sensor back at least once to the manufacturer 

for calibration in June 2008.

To enable meaningful comparisons of salinity profiles 

collected by different agencies and instruments, consistent 

quality-assurance (protocol) and quality-control (independent 

check) measures are needed. Regular accuracy checks of the 

conductivity, temperature, and pressure sensors are necessary 

to ensure proper data quality. In addition, review procedures 

conducted by personnel not directly involved in the data collection are part of a sound approach to data collection.

Well-Construction Information

The monitor wells used in this study generally penetrate 

through the transition zone into saltwater and have bottom 

altitudes ranging from -137 to -1,881 ft relative to mean 

sea level. The shallower wells are located on the Island of 

Hawaiÿi, whereas the deepest wells are in the Pearl Harbor 

and Honolulu areas of Oÿahu. The wells generally are cased 

above the water table and are uncased at depths below several 

tens to a few hundreds of feet below mean sea level (table 2). 

The Honouliuli DMW, Twin Bridge DMW, Kawailoa DMW, 

Kualapuÿu DMW, and Kahaluÿu DMW are cased to bottom 

depth with slotted casing. Casing diameters range from 4 to 

14 in. The uncased holes have uneven diameters, because the 

volcanic rocks penetrated have varying degrees of competence 

and voids. Caliper logs indicate that it is not uncommon for 

the hole diameter to vary between 10 and 20 in. over a vertical 

distance of several feet (see fig. 9).

Borehole Flow

Indicators of Vertical Borehole Flow

Data from salinity profiles generally have been used under 

the assumption that the observed vertical distribution of salinity 

in the borehole is representative of the salinity in the surrounding aquifer. However, natural or anthropogenically induced 

vertical hydraulic gradients in the aquifer can generate vertical 

flow that is greater in the borehole than in the aquifer, causing 

salinity inside the well to be displaced with respect to the aquifer (Oki and Presley, 2008). Indicators of vertical borehole flow 

include steplike changes in specific conductance or temperature 

with depth, flow measurements in monitor wells, and differences in observed salinities in paired shallow and deep wells.

Steplike Change of Salinity with Depth

Concept

Vertical borehole flow commonly is evident in measured 

salinity profiles as steplike changes in specific conductance 

with depth. These steps indicate where water may be either 

entering or exiting the borehole (Eyre, 1987). Flow may be 

upward, downward, converging, or diverging (fig. 6) (Oki 

and Presley, 2008). Temperature profiles have been used to 

estimate groundwater flow rates (Stallman, 1963; Bredehoeft 

and Papadopulos, 1965; Cartwright, 1970; Sorey, 1971), and 

analysis of thermal anomalies allows a quantitative estimate 

of water flow within a borehole or between a borehole and 

intersecting fractures (Drury and Jessop, 1982; Drury, 1984). 

Steplike changes in available temperature profiles in Hawaiÿi 

commonly occur at the same depths as those observed in salinity profiles. However, the temperature distribution with depth 

in the aquifer, the temperature near the bottom of the DMW, 

and the exact geothermal gradient are unknown, which makes 

it difficult to interpret deviations in temperature profiles. The 

analyses in this study therefore focus on steps in salinity profiles, because the general shape of the profile and the specific 

conductance of saltwater are known.

Upward Flow.—Suppose water flows into a borehole at 

a depth where a conductive layer is present and flows upward 

within the borehole, driven by an upward vertical hydraulic 

gradient. As the water moves up within the borehole through a 

massive, low-permeability layer, little or no inflow or outflow 

occurs. Thus, the salinity of the water in this zone remains 

nearly uniform and equal to the salinity of the inflowing water. 

Water flows out of the borehole into a shallower conductive 

zone where the horizontal hydraulic gradient is directed away 

from the borehole. Water that exits the borehole mixes with 

water of lower salinity in the aquifer. The section over which 

upward flow occurs appears as a vertical line in the measured 

specific-conductance profile, and the horizontal line shallower 

in the profile is indicative of the outflow location. The deviation from the assumed aquifer salinity appears as an upwardflow step (fig. 6A). 

Downward Flow.—Suppose water flows into a borehole 

at a depth where a conductive layer is present and downward 

within the borehole, driven by a downward vertical hydraulic 

gradient. As the water flows down in the borehole through a 

massive, low-permeability layer where no mixing with the water 

outside the borehole occurs, the salinity of the water in this 

zone remains nearly equal to the salinity of the inflowing water. 

Water flows out of the borehole into a deeper conductive zone 

where the horizontal hydraulic gradient is directed away from the 

borehole and mixes with water of higher salinity in the aquifer. 

The section over which downward flow occurs appears as a 

vertical line in the measured specific-conductance profile, 
12 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

Well name Island Logging 

agency

Type of 

profile

Date of first 

available 

profile

1

Date of last 

available 

profile

Number of 

available 

profiles

2

Casing 

diameter, in 

inches

2

Bottom of 

well, in ft

2

Bottom of 

solid casing, in ft

Beretania Oÿahu HBWS C, T 02/16/1970 10/16/2008 28 6 -1,486 -427

Hälawa Oÿahu HBWS C, T 12/05/2005 10/31/2008 9 8 -955 0

Hapuÿu Bay D Hawaiÿi USGS C, T 04/05/1989 03/22/1990 3 8 -351 -73

Hauÿula Oÿahu HBWS C, T 03/13/2001 03/16/2009 19 8 -893 7

Häwï Makai I Hawaiÿi USGS C, T 04/03/1989 03/21/1990 3 10 -137 169

Helemano Oÿahu HBWS C, T 02/02/2004 09/02/2008 15 8 -694 6

Honouliuli Oÿahu CWRM C, T 05/10/1998 03/02/2005 7 4 -756 19

ÿÏao Maui CWRM C, T 04/06/2006 09/19/2007 7 8 -1,118 -21

Jonathan Springs Oÿahu HBWS C, T 11/18/2003 10/17/2008 16 14 -1,119 -19

Kaamilo Oÿahu HBWS C, T 05/25/2001 11/05/2008 19 8 -1,126 11

Kahaluÿu Hawaiÿi CWRM C, T 09/07/2000 11/14/2006 13 8 -430 -15

Kahuku Oÿahu HBWS C, T 11/21/2003 12/08/2008 15 8 -838 42

Kaimukï HS Oÿahu HBWS C, T 03/07/1975 10/08/2008 20 8 -897 -291

Kaimukï Station Oÿahu HBWS C, T 05/14/1991 10/13/2008 26 9 -1,177 -97

Kalihi Shaft Oÿahu HBWS C, T 02/18/2000 10/23/2008 24 8 -1,881 4

Kalihi Station Oÿahu HBWS C, T 08/31/2000 10/20/2008 20 8 -1,179 -239

Kaluanui 2 Oÿahu HBWS C, T 07/29/2003 08/14/2008 13 14 -917 -44

Kawailoa Oÿahu HBWS C, T 05/17/1994 08/01/2008 17 4 -392 10

Kualapuÿu Molokaÿi USGS C, T 04/03/2001 07/19/2007 12 4 -603 52

Kunia Mauka Oÿahu CWRM C, T 12/28/2004 03/12/2007 7 8 -873 -13

Kunia Middle Oÿahu CWRM C, T 10/23/2002 09/11/2007 15 8 -705 -15

Kunia T41 Oÿahu HBWS C, T 08/07/2000 11/17/2008 22 12 -1,029 -8

Lahaina Maui CWRM C, T 08/22/2001 09/19/2007 11 8 -599 -11

Läÿie Oÿahu HBWS C, T 04/07/2004 12/09/2008 14 10 -837 -197

Mänana Oÿahu HBWS C, T 06/02/2000 11/12/2008 22 8 -1,196 -6

Moanalua Oÿahu HBWS C, T 01/16/2002 10/30/2008 18 12 -1,218 -30

Newtown Oÿahu HBWS C, T 09/12/2000 09/28/2008 18 12 -1,022 -22

Poliwai Oÿahu HBWS C, T 03/12/2001 08/28/2006 13 8 -1,283 25

Punaluÿu Oÿahu HBWS C, T 08/29/1968 12/15/2008 26 5 -1,001 -140

Punanani Oÿahu HBWS C, T 11/26/1968 02/04/2009 29 12 -1,303 -30

Punchbowl Oÿahu HBWS C, T 12/18/2001 10/14/2008 16 8 -1,325 -75

South Hälawa Oÿahu CWRM C, T 08/15/2000 07/11/2007 17 8 -1,350 -25

Twin Bridge Oÿahu HBWS C, T 05/18/1994 05/18/1994 1 4 -613 23

Waÿahila Oÿahu HBWS C, T 12/02/1999 10/07/2008 22 12 -1,238 5

Waiÿalae Oÿahu HBWS C, T 10/10/2000 10/06/2008 23 10 -644 14

Waiawa Oÿahu HBWS C, T 02/05/2002 11/10/2008 18 10 -994 6

Waiehu Maui CWRM C, T 01/02/1998 06/24/2008 20 10 -1,020 -20

Waiehu Maui USGS Chloride 08/22/1985 07/01/2008 92 -- -- --

[HBWS, Honolulu Board of Water Supply; CWRM, Hawaiÿi Commission on Water Resource Management; USGS, U.S. Geological Survey; C, Fluid electrical 

conductivity; T, Temperature; ft, feet relative to mean sea level; --, not applicable]

Table 2. Characteristics of deep monitor wells and summary of available salinity and temperature profiles (as of March 

2009) used in this study.
Borehole Flow 13

and the horizontal line deeper in the profile is indicative of 

the outflow location. The deviation from the assumed aquifer 

salinity appears as a downward-flow step (fig. 6B). 

Because aquifer salinity is unknown, it is not possible to determine unequivocally whether an upward or 

downward flow condition exists solely by inspection of 

a measured salinity profile. This is illustrated in the next 

cases (figs. 6C and 6D), in which the same step can be 

reasonably interpreted to be indicative of flow in either 

direction. Diffuse inflow of water from the aquifer at a 

shallow depth interval with downward flow results in a 

measured salinity profile that is slightly curved above the 

zone of high inflow (fig. 6C). The depth of higher inflow 

of water that is saltier than the water in the well is the 

horizontal line in the observed profile. The section over 

which the salinity remains constant can be associated with 

massive layers from which there is little or no inflow to the 

well. In contrast, the deeper section with downward flow 

is characterized by diffuse inflow and outflow, resulting 

in a curved salinity profile (fig. 6C). What was interpreted 

as an upward flow step in figure 6A can also be caused 

by downward flow in the borehole (fig. 6C). Similarly, 

what appears to be a downward flow step (fig. 6B) may be 

caused by upward flow in the borehole (fig. 6D).

Converging and Diverging Flow.—Converging flow 

may occur near an outflow zone as water flows both 

upward and downward within the borehole toward the 

outflow zone (fig. 6E). Diverging flow may occur near an 

inflow zone where water enters the borehole and flows 

both upward and downward within the borehole away from 

the inflow zone (fig. 6F). Because the aquifer salinity is 

unknown, it is not possible to determine unequivocally 

whether a converging or diverging flow condition exists 

solely by inspection of a measured salinity profile.

1

Last available profile refers to at the time of this study.

2

Information from CWRM.

[HBWS, Honolulu Board of Water Supply; CWRM, Hawaiÿi Commission on Water Resource Management; USGS, U.S. Geological Survey; C, Fluid electrical 

conductivity; T, Temperature; ft, feet relative to mean sea level; --, not applicable]

Table 2. Characteristics of deep monitor wells and summary of available salinity and temperature profiles (as of March 

2009) used in this study. —Continued.

Well name Island Logging 

agency

Type of 

profile

Date of first 

available 

profile

1

Date of last 

available 

profile

Number of 

available 

profiles

2

Casing 

diameter, in 

inches

2

Bottom of 

well, in ft

2

Bottom of 

solid casing, in ft

Waimalu Oÿahu CWRM C, T 01/09/2005 09/30/2008 10 10 -975 -45

Waimano Oÿahu HBWS C, T 01/17/2002 11/06/2008 14 14 -1,032 2

Waiola Oÿahu HBWS C, T 02/15/2000 01/06/2004 9 8 -982 13

Waipahu Oÿahu HBWS C, T 11/13/1986 11/13/2008 20 12 -1,074 -12

Waipahu Oÿahu CWRM C, T 01/16/1998 03/24/2009 24 -- -- --

Waipiÿo Mauka Oÿahu HBWS C, T 04/23/1987 05/27/2004 9 9 -1,377 0 

Waipiÿo Mauka Oÿahu CWRM C, T 01/16/1998 07/12/2007 21 -- -- --

# #

# #

# #

SALINITY

DEPTH

Upward Flow

Inflow

Outflow

Measured

salinity in

borehole

Aquifer

salinity

SALINITY

DEPTH

Downward Flow

SALINITY

DEPTH

Converging Flow

SALINITY

DEPTH

Diverging Flow

A

C

E F

Inflow

Inflow

Outflow

Inflow

SALINITY

DEPTH

Upward Flow

SALINITY

DEPTH

Downward Flow

B

D

Figure 6. Diagrams of conceptual salinity profiles of different 

shapes in a borehole (modified from Oki and Presley, 2008). A, 

Upward flow. B, Downward flow. C, Downward flow. D, Upward 

flow. E, Converging flow. F, Diverging flow. 
14 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

Length of Vertical Step

Available salinity profiles for a given well indicate consistent steplike changes in salinity at the same depths under 

repeated profiling over several years. These steps likely reflect 

alternating hydraulically conductive and massive (relatively 

less hydraulically conductive) zones and vertical flow within 

the borehole. The heterogeneous and anisotropic nature of 

basalt aquifers in Hawaiÿi would tend to cause deviations from 

a smooth increase of salinity with depth in measured salinity 

profiles. Steps of different magnitude occur in all DMWs. Voss 

and Wood (1994) indicate, on the basis of borehole-flow measurements in southern Oÿahu, that water enters and leaves open 

boreholes through conductive zones and that these boreholeflow zones generally are limited in scale to lengths of less than 

about 66 ft. This length scale generally is consistent with the 

thickness of lava flows. Available data were insufficient to confirm whether inflows and outflows corresponded to conductive 

zones and whether zones of uniform salinity in measured profiles corresponded to massive low-permeability zones. Available 

data examined included caliper, spontaneous potential, resistivity, neutron, and gamma logs from various DMWs (unpub. data, 

HBWS and CWRM data files). 

Regardless of the interpretation of borehole-flow direction, the vertical distance over which physical properties, such 

as salinity and temperature, remain uniform can be uniquely 

determined for each well. The longest vertical step or apparent 

displacement of salinity within the borehole (vertical distance 

over which salinity remains uniform) in each monitor well, 

considering all available profiles for that well, was used as an 

indicator of borehole flow in the well. A vertical step is defined 

by a zone of nearly uniform salinity, within a tolerance of plus 

or minus 0.1 percent seawater salinity, which is equivalent to a 

specific conductance of 55 μS/cm. For each profile, every data 

point was compared to all other data points in the profile and 

the longest continuous depth interval in which salinity varies 

by less than plus or minus 0.1 percent of seawater salinity was 

identified (fig. 7). For some DMWs, only a segment of the entire 

salinity profile was analyzed to identify vertical steps, because 

vertical sections of near-uniform salinity in the profiles occur 

naturally in the freshwater and saltwater range that are not considered vertical steps. Exclusion zones were defined to exclude 

sections in some DMWs that were not interpreted to be vertical 

steps associated with borehole flow (table 3). The longest continuous interval from all profiles was designated as the single 

longest vertical step in each DMW (table 3). This method identifies only the single maximum vertical step, although additional 

smaller vertical steps may exist in the same profile (for example, 

Kaimukï Station DMW, Kalihi Station DMW, Moanalua DMW, 

or South Hälawa DMW). The length of the vertical step can be 

extended if the 55 μS/cm tolerance is increased. Profiles from 

the Punanani DMW between -340 and -450 ft (see fig. 15A) 

have specific-conductance increases of several hundred μS/cm 

in multiple smaller steps.

The longest vertical steps in measured salinity profiles in 

Hawaiÿi have lengths between several feet and several hundreds 

of feet (table 3). The longest individual step known is 644 ft 

in Kaamilo DMW, and the shortest is 7 ft in Kahaluÿu DMW. 

Shorter steps may not indicate borehole flow but simply reflect 

the heterogeneity of the basalt aquifer; longer steps greater than 

70 ft likely are related to borehole flow because the step length 

exceeds the typical thickness of lava flows. Spatially, a cluster 

Maximum length

of vertical step

‐0.5 ‐0.3 0 0.1 0.3 0.5

‐700

‐600

‐500

‐400

‐300

‐200

‐100

RELATIVE SALINITY,

IN PERCENT SEAWATER SALINITY

ALTITUDE, IN FEET

RELATIVE TO MEAN SEA LEVEL

4,400 4,500 4,600 4,700 4,800

SPECIFIC CONDUCTANCE,

 IN MICROSIEMENS PER CENTIMETER

10,000 20,000 30,000 40,000

0

ALTITUDE, IN FEET RELATIVE 

TO MEAN SEA LEVEL

A

B

South Hälawa DMW

SPECIFIC CONDUCTANCE,

 IN MICROSIEMENS PER CENTIMETER

‐1,000

‐600

‐400

‐200

‐800

0 50,000

‐0.1

‐1,200

Figure 7. Salinity profile in South Hälawa DMW on 09/13/2001. 

A, Entire salinity profile. B, Closeup of the longest vertical step 

(outlined by gray box), with upper boundary at -102 feet and 

lower boundary at -647 feet relative to mean sea level, within 

which salinity varies by less than 0.1 percent of seawater salinity.
Borehole Flow 15

of wells with long steps spans from northeast of Pearl Harbor 

to Honolulu (fig. 8). Other locations with long steps are in the 

Punaluÿu area on the northeast coast of Oÿahu and the Waiehu 

area on Maui. Not surprisingly, areas of long vertical salinity 

steps in boreholes coincide with areas where monitor wells 

are located near large production wells. However, the distance 

between the production well and monitor well is not the only 

factor influencing the step length. The linear relation between 

distance of the closest major production well and maximum 

length of vertical step of salinity has a correlation coefficient of 

-0.25 and is statistically not significant at the 95-percent confidence level (p = 0.11). Although DMWs with vertical steps 

"

"

" "

"

"

"

"

"

"

"

"

"

"

"

"

!! !

! ! !

!!

!

!

! ! !

!

!

!

! ! ! ! ! !

!

!

!

! !

!

!

! !

!

!

!

!

21°20'

21°30'

21°40'

158°10' 158°00' 157°50' 157°40'

EXPLANATION

Shaded relief generated from U.S. Geological Survey 10‐meter Digital Elevation 

Model, Universal Transverse Mercator projection, zone 4, NAD83 datum. 

Maximum length of vertical step in measured salinity profile, in feet

6 to 25

26 to 70

71 to 150

151 to 300

301 to 644

!

!

!

!

!

Mean withdrawal 1996–2005, in million gallons per day 

3.1 to 8.0

" 8.1 to 13.5

"

Valley fill

Other barrier

Barrier to horizontal groundwater flow

!

!

!

!!

""!!

0 4 8 MILES

0 84 KILOMETERS

0 25 MILES

0 50 KILOMETERS

158° 157° 156° 155°

21°

20°

19°

Hawaiÿi

Maui

Oÿahu

Molokaÿi

Länaÿi

Kahoÿolawe

Figure 8. Maximum length of vertical step in measured salinity profiles in Hawai‘i and mean groundwater 

withdrawal rates during 1996 to 2005 from selected production wells and wellfields. The barriers to 

groundwater flow are modified from Oki (2005) and Rotzoll and El-Kadi (2007).
16 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

Vertical 

displacement

Displacement of 

top of 

transition 

zone

Well name

Bottom 

of top 

exclusion 

zone, 

in ft

Top of 

bottom 

exclusion 

zone, in ft

Dimensions of maximum 

single vertical step

Specific

conductance of 

vertical step in all 

profiles

Correlation between mean

withdrawal rates and

conductance of step 

around the top of 

the transition zone

depth of the 

midpoint of the 

transition zone

Top,

in ft

Bottom, 

in ft

Length, 

in ft

Minimum1,

in μS/cm

Maximum,

in μS/cm R p R p

<70 ft

No

Hapuÿu Bay D -150 -- -207 -262 55 360 700 -- -- -- --

Häwï Makai I -75 -- -104 -118 14 7,000 8,500 -- -- -- --

Honouliuli -240 -- -282 -324 42 1,050 1,370 -- -- -- --

ÿÏao -520 -- -696 -764 68 2,100 2,900 -- -- -- --

Kahaluÿu -15 -80 -17 -24 7 2,500 6,200 -- -- -- --

Kunia Mauka -440 -- -792 -820 27 38,500 42,000 -- -- -- --

Lahaina -70 -190 -88 -106 18 1,100 4,000 -- -- -- --

Läÿie -400 -680 -423 -448 25 1,600 2,550 -- -- -- --

Twin Bridge -300 -- -312 -325 13 500 650 -- -- -- --

Waiÿalae -40 -450 -337 -352 15 25,500 27,500 -- -- -- --

Waiola -350 -- -431 -484 53 11,200 18,200 -- -- -- --

Yes

Hauÿula -565 -740 -573 -584 11 600 2,650 -- -- -- --

Helemano -385 -600 -393 -403 10 650 2,700 -- -- -- --

Kahuku -375 -600 -399 -420 21 500 1,150 -- -- -- --

Kaimukï HS -330 -- -346 -374 28 800 1,800 -0.11 0.708 -0.37 0.194

Kawailoa -- -- -11 -70 59 600 1,300 -- -- -- --

Kualapuÿu -220 -- -322 -379 57 550 2,500 -- -- -- --

Kunia Middle -300 -- -329 -360 31 800 1,250 -- -- -- --

Kunia T41 -340 -- -354 -397 43 650 1,020 -- -- -- --

Poliwai -570 -1070 -684 -753 69 640 1,640 -- -- -- --

Punchbowl -825 -1100 -842 -855 13 700 1,750 -- -- -- --

Waiawa -450 -- -519 -582 63 860 1,250 -- -- -- --

Waipahu -370 -- -415 -456 41 800 1,600 -- -- -- --

Waipiÿo Mauka -- -- -703 -731 28 650 2,900 -- -- -- --

>70 ft

No

Kalihi Station -239 -- -239 -455 216 475 950 0.96 0.000 0.00 0.998

Kaluanui 2 -- -- -26 -469 443 100 950 0.76 0.019 0.35 0.359

Mänana -- -- -1007 -1149 142 46,150 48,150 -- -- -- --

Moanalua -- -- -20 -335 315 420 950 0.80 0.000 -0.12 0.647

Waÿahila -200 -900 -222 -375 153 400 850 0.06 0.961 0.69 0.514

Waiehu -350 -800 -396 -604 208 70 650 -0.07 0.913 0.32 0.603

Waimano -320 -- -424 -627 203 200 800 -- -- -- --

Yes

Beretania -660 -- -716 -859 143 270 3,700 0.54 0.022 0.05 0.852

Hälawa -- -- 14 -93 107 850 1,300 -- -- -- --

Jonathan Springs -300 -- -447 -605 158 550 1,250 0.80 0.006 -0.38 0.281

Kaamilo -- -- -87 -731 644 200 9,100 0.87 0.005 0.64 0.026

Kaimukï Station -100 -- -147 -301 154 500 1,400 0.87 0.000 0.43 0.148

Kalihi Shaft -- -- 11 -559 570 500 3,380 0.37 0.081 0.11 0.629

Newtown -- -- -264 -664 400 350 6,100 -0.81 0.000 -0.27 0.314

Punanani -- -- -107 -334 227 250 4,600 0.97 0.000 0.55 0.008

Punaluÿu -140 -- -230 -559 329 450 1,430 -0.76 0.000 0.24 0.306

South Hälawa -- -- -102 -647 545 200 5,000 0.35 0.239 0.46 0.117

Waimalu -600 -- -713 -816 103 200 32,250 -0.35 0.650 0.25 0.754

[ft, feet relative to mean sea level; μS/cm, microsiemens per centimeter; R, correlation coefficient between two random variables; p, calculated by transforming the correlation to create a t statistic having n-2 degrees of freedom, where n is the number of data points. If p < 0.05, the hypothesis of no correlation is 

rejected at the 95% confidence level; --, not applicable; bold indicates statistically significant correlation at the 95% confidence level]

1

Minimum refers to specific conductance immediately above the horizontal part of the step. 

 

Table 3. Specifications of the maximum vertical step of measured salinity, exclusion zones to identify the flow step, range 

of specific conductance above and within the vertical part of the step, and correlation coefficients and p values from the 

regression analysis for the mean specific conductance within the vertical flow step near the apparent top of the transition zone 

versus nearby withdrawal rates and the depth of the apparent midpoint of the transition zone versus nearby withdrawal rates.
Borehole Flow 17

of less than 70 ft consistently are located greater than 1,000 ft 

from a production well, the opposite is not true. Monitor wells 

with long vertical steps in salinity profiles are located both 

close to production wells, for example Kalihi Shaft DMW (at 

170 ft), and far from production wells, for example Newtown 

DMW or South Hälawa DMW. The latter DMWs are greater 

than 3,600 ft from major wellfields; other DMWs at similar 

distances have maximum vertical steps that are an order of 

magnitude shorter.

Borehole-Flow Measurements

Flow measurements within boreholes are available for 

several wells in Hawaiÿi. Paillet and Hess (1995) measured 

flow with a heat-pulse flowmeter in Waipiÿo Mauka DMW, 

and Paillet and others (2002) used an electromagnetic flowmeter in Kaimukï Station DMW. Additional flow measurements with the electromagnetic flowmeter used by Paillet and 

others (2002) are available from Waiÿalae DMW and Punanani 

DMW (D.S. Oki, U.S. Geological Survey, written commun., 

2008). Data from each well are in the form of both trolling and 

stationary flow measurements. The stationary flow measurements were used to calibrate the continuous flow measurements collected by trolling. Although both yield comparable 

results (Newhouse and others, 2005), the stationary flow 

measurements are considered more reliable. Flow measurements are available from two monitor wells near production 

wells (Kaimukï Station DMW and Punanani DMW) and two 

monitor wells distant from production wells (Waiÿalae DMW 

and Waipiÿo Mauka DMW).

Monitor Wells Near Production Wells

The flow in monitor wells near production wells may 

be controlled directly by nearby groundwater withdrawals, 

which could obscure borehole flow caused by natural regional 

vertical hydraulic gradients in the aquifer. Flow was measured in Kaimukï Station DMW under two withdrawal rates 

from an adjacent production wellfield (3.8 Mgal/d on May 

22, 2001, and 7 Mgal/d on May 17, 2001). The flowmeter 

logs indicate high upward flow from -130 to -380 ft (fig. 9A). 

Highest upward flow occurs between -200 and -300 ft, which 

overlaps the slotted interval of the production wells. In the 

shallowest parts of the monitor well, around -100 ft, the flow 

is downward, indicating that water is converging towards 

the slotted interval of the production wells. Measured flow 

in the monitor well over the interval from -130 to -380 ft 

increased in response to an increase in withdrawal rate from 

the nearby wellfield. The measured flow pattern is consistent with steps observed in the specific-conductance logs 

(see fig. 14A). Between -530 and -585 ft, a step consistent 

with diverging flow is observed in the specific-conductance 

profile in figure 9A. The flowmeter log indicates low upward 

flow near -550 ft and downward flow near -590 ft, which is 

generally consistent with the steps in the conductance profile. 

Additionally, between -650 and -690 ft a step consistent with 

downward flow in the specific-conductance profile coincides 

with downward flow near -700 ft in the flowmeter log. Other 

local spikes in the trolled flowmeter log over several tens of 

feet that can be attributed to changes in borehole diameter, 

for example at -825 ft, are not reflected by steps in specificconductance profiles. 

Flow measurements in Punanani DMW were taken 

when nearby groundwater withdrawal was 16.7 Mgal/d. The 

flowmeter data indicate a similar flow pattern as in Kaimukï 

Station DMW. Flow is downward between -40 and -100 ft, 

upward between -130 and -450 ft, and low below -460 ft 

(fig. 9B). The observed upward flow pattern is consistent 

with the observed step in the specific-conductance profile 

over the same depth interval. Between -560 and -600 ft, 

a step in the specific-conductance profile coincides with 

downward flow between -530 and -600 ft in the flowmeter 

log. The high variability in measured flow indicated by 

the continuous flowmeter log is attributed to variations in 

borehole diameter, as shown in the caliper log. Adjusting 

flow rates according to estimated borehole diameter did not 

entirely reduce the variability—the correction removed the 

scatter over one section but amplified it over other sections. Borehole flow can range from laminar to turbulent 

flow, which limits the utility of a well designed to monitor 

aquifer-salinity properties with depth.

Monitor Wells Distant from Production Wells

The magnitude of flow in monitor wells distant from 

production wells is expected to be lower than in monitor 

wells located near production wells, all other factors being 

equal. Reilly and others (1989) concluded that small vertical 

head gradients of 6.8 × 10-5 ft/ft (0.4 ft/mi) in the aquifer 

can cause borehole flow in long-screened wells. This could 

constrain the utility of the borehole for groundwater-quality 

monitoring, unless a more sophisticated approach is utilized 

by sampling well-bore flow and depth-dependent waterquality data (Izbicki and others, 1999). In Waipiÿo Mauka 

DMW, the flow measurements indicate a complex flow pattern with depth, alternating between upward and downward 

flow at intervals of 10–50 ft (fig. 10A). The stationary measurements indicate downward flow between -150 and -280 

ft and between -450 and -680 ft. Upward flow is indicated 

between -300 and -400 ft and between -720 and -820 ft. At 

some depths, the stationary flow measurements are inconsistent with the continuous flowmeter measurements collected 

by trolling. Overall, the magnitude of the measured flow and 

the complex pattern of upward and downward flow indicate 

low overall flow in this monitor well.

Variations from upward to downward flow are less pronounced in Waiÿalae DMW than in Waipiÿo Mauka DMW. In 

Waiÿalae DMW, all the flow above -450 ft is directed downward, an observation consistent with the regional flow concept for recharge areas (fig. 10B). The specific-conductance 
18 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

profile is characterized by an absence of long steps. A short 

step between -335 and -350 ft corresponds with downward 

flow in the stationary flow measurements. 

Comparison of Salinity and Temperature Profiles 

from Shallow and Deep Wells

Comparing specific-conductance and temperature profiles 

from a pair of wells, one shallow and one deep, provides an 

indication of borehole flow in the deep wells. A shallow well that 

extends only into freshwater is less prone to borehole-flow effects 

because of its limited vertical extent. In the absence of borehole 

flow, the upper part of a specific-conductance profile from a DMW 

should be similar to the profile from a nearby shallow monitor 

well. Dissimilar salinity (or temperature) profiles are an indication 

of different flow conditions in the monitor wells. Temperature in 

wells unaffected by borehole flow generally decreases by several 

degrees Celsius with depth in the upper 50 ft of the water column, 

remains nearly uniform over the next few hundred feet, and below 

that increases nearly linearly with depth following the geothermal 

ALTITUDE, IN FEET RELATIVE TO MEAN SEA LEVEL

Slotted interval

of Punanani

production wells 

0 10 20 30 40 ‐900

‐800

‐700

‐600

‐500

‐400

‐300

‐200

0

8 12 16 20

Punanani DMW

‐0.05 0 0.05

‐100

Kaimukï Station DMW

SPECIFIC

CONDUCTANCE,

IN MILLISIEMENS

PER CENTIMETER

0 10 20 30 40 8 12 16 20

CALIPER ARM

EXTENSION,

IN INCHES

‐0.05 0 0.05

FLOW RATE, IN

MILLION GALLONS

PER DAY (Mgal/d)

Slotted interval

of Kaimukï

production wells

Bottom of Casing

Bottom of Casing

downflow upflow

downflow upflow

SPECIFIC

CONDUCTANCE,

IN MILLISIEMENS

PER CENTIMETER

CALIPER ARM

EXTENSION,

IN INCHES

FLOW RATE, IN

MILLION GALLONS

PER DAY (Mgal/d)

A B

12/11/2001 01/30/2001

Nearby withdrawal rate 7.0 Mgal/d:

 Trolled log 5/17/2001

 Stationary 5/17/2001

Nearby withdrawal rate 3.8 Mgal/d:

 Trolled log 5/22/2001

 Stationary 5/22/2001

Nearby withdrawal rate 16.7 Mgal/d:

 Trolled log 5/24/2001

 Stationary 5/24/2001

Figure 9. Specific-conductance, borehole diameter (caliper), and flowmeter measurements. A, Kaimukï Station DMW 

(modified from Paillet and others, 2002). B, Punanani DMW (unpub. data, USGS data files). The different pumping rates at 

nearby production wells at the times of the flow measurements are indicated.
Borehole Flow 19

ALTITUDE, IN FEET RELATIVE TO MEAN SEA LEVEL

0 10 20 30 40 ‐1,200

‐800

‐600

‐400

‐200

0

8 12 16 20

Waiÿalae DMW

‐0.01 0 0.01

Waipiÿo Mauka DMW

0 10 20 30 40 8 12 16 20 ‐0.01 0 0.01

‐1,000

downflow upflow

downflow upflow

SPECIFIC

CONDUCTANCE,

IN MILLISIEMENS

PER CENTIMETER

CALIPER ARM

EXTENSION,

IN INCHES

FLOW RATE, IN

MILLION GALLONS

PER DAY

SPECIFIC

CONDUCTANCE,

IN MILLISIEMENS

PER CENTIMETER

CALIPER ARM

EXTENSION,

IN INCHES

FLOW RATE, IN

MILLION GALLONS

PER DAY

A B

Trolled log 03/13/1989

Stationary: Mean of

several measurements

between 06/1987 and

03/1989

Trolled log

5/22/2001

Stationary 

5/22/2001

05/27/2004 01/25/2001

Figure 10. Specific-conductance, borehole diameter (caliper), and flowmeter measurements. A, Waipi‘o Mauka 

DMW (modified from Paillet and Hess, 1995). B, Wa‘alae DMW (unpub. data, USGS data files).

gradient. Salinity and temperature profiles from paired shallow and 

deep wells are available for monitor wells both near and distant 

from production wells (fig. 11).

Mänana DMW is located 1,200 ft from Mänana 1 (fig. 3), 

and the measured specific-conductance profiles from these 

wells generally are similar over the interval of the shallow 

well. The profiles were measured within 3 weeks of each 

other, during a period when no major changes from regional 

aquifer flow are expected. Small changes in specific conductance commonly are measured near the bottom of the 

solid casing of a monitor well. The specific-conductance 

profiles indicate freshwater in both wells to a depth of -140 ft 

(fig. 11A). The temperature is 0.25°C lower in the shallow 

well, but the shapes of the temperature profiles in the two 

wells are similar. The comparison of profiles from Mänana 

DMW and Mänana 1 indicates the absence of significant borehole flow in the upper part of the deep well, although vertical 

steps exist even above -140 ft. This pair of shallow and deep 

wells is more than a mile away from a large wellfield.

Punanani DMW and two nearby shallow production wells, Punanani P1 and P2 located 100 and 25 ft from 

Punanani DMW, were logged on the same day (fig. 11B). 
20 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

The shallow production wells were not in service during a 

pump–replacement period, though other wells in the Punanani 

wellfield were producing 6 Mgal/d. The profiles from the 

shallow wells indicate freshwater over the entire lengths of 

the boreholes down to -300 ft. However, the profile from the 

DMW shows a sharp specific-conductance increase at -100 ft 

to 2,100 μS/cm, which remains nearly uniform to -350 ft. The 

temperature in the shallow wells below about -100 ft is 0.25°C 

lower than in the deep well, indicating that warmer water from 

-450 ft is moving up the deep borehole and exiting at -100 ft. 

Thus, comparison of salinity and temperature profiles from the 

deep and shallow wells indicates that borehole flow is affecting 

the profiles from the deep well. In Punanani DMW the salinity 

profile is shifted to a more brackish profile, and the temperature 

profile is shifted to a warmer profile from the upward flow.

The same phenomenon is observed in Kaamilo DMW 

and the shallow Kalauao well, which is located 0.6 mi from 

the Kaamilo DMW and within a production wellfield (fig. 3). 

The specific-conductance profile from the shallow well 

indicates freshwater, whereas the profile from the deep well 

0 2,000 4,000 6,000 8,000

‐400

‐300

‐200

‐100

0

ALTITUDE, IN FEET RELATIVE TO MEAN SEA LEVEL

20 21 22

0 2,000 4,000 6,000 8,000 ‐500

‐400

‐300

‐200

‐100

0

20 21 22

0 2,000 4,000 6,000 8,000

‐800

‐600

‐400

‐200

0

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

20 21 22

TEMPERATURE,

IN DEGREES CELSIUS

Bottom of casing (deep well)

Bottom of casing (shallow well)

Bottom of casing (deep well)

Bottom of casing (shallow well)

Bottom of casing (deep well)

Bottom of casing (shallow well)

Mänana DMW

Punanani DMW

Kaamilo DMW

A

B

C

Mänana DMW 02/22/2003

Mänana 1 01/13/2003

Punanani DMW 12/18/06

Punanani P1 12/18/2006

Punanani P2 12/18/2006

Kaamilo DMW 02/08/2003

Kalauao 01/07/2003

Distance to DMW: 1,200 feet

Distance P1 to DMW: 25 feet

Distance P2 to DMW: 100 feet

Distance to DMW: 3,100 feet

Figure 11. Paired 

specific-conductance 

and temperature 

profiles in shallow and 

deep monitor wells 

(DMW). A, Mänana 

DMW and Mänana 1. 

B, Punanani DMW 

and Punanani P1 and 

P2. C, Kaamilo DMW 

and Kalauao.
Borehole Flow 21

indicates a salinity increase to 7,000 μS/cm between -80 and 

-730 ft, which corresponds to brackish water (fig. 11C). The 

temperature profiles from the wells deviate below -80 ft, where 

the temperature in the shallow well drops to 20.3°C and the 

temperature in the deep well remains nearly constant at 21.5°C 

through -730 ft. The shift in the profiles of the deep well to 

brackish and warmer water is another indicator of upward 

flow. Comparisons of profiles from shallow and deep monitor wells indicate that the measured salinities in the Punanani 

DMW and Kaamilo DMW are not representative of salinities 

in the surrounding aquifer.

Effects of Groundwater Withdrawal on Salinity 

Profiles

Regional groundwater withdrawals in Hawaiÿi have 

caused a thinning of the freshwater lens over recent decades, 

reflected in salinity profiles by a rise of the apparent top and 

midpoint of the transition zone by several feet per year in 

many monitor wells (for example, Oki 2005; Gingerich, 2008). 

Salinity profiles in monitor wells also have been affected on 

shorter time scales by daily variations in pumping rates from 

nearby production wells. An example of how salinity and 

temperature profiles differ under nearby pumping and nonpumping conditions is available from logging done before and 

after a constant-rate aquifer test was conducted at Punanani 

wellfield (fig. 12). In this test, all pumping from production 

Measured 48 hours after pumping was 

curtailed (09/26/2008)

Measured during constant withdrawal 

rate of 12 million gallons per day, 84 

hours after withdrawal resumed 

(09/30/2008)

0 10,000 20,000

‐600

‐500

‐400

‐300

‐200

‐100

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER 

ALTITUDE, IN FEET RELATIVE TO MEAN SEA LEVEL

20 20.5 21 21.5 22

TEMPERATURE,

IN DEGREES CELSIUS

0 A Punanani DMW, Salinity B Punanani DMW, Temperature

Figure 12. Salinity 

(A) and temperature 

(B) profiles obtained 

in Punanani DMW 

after pumping in the 

adjacent wellfield was 

curtailed for 48 hours 

and after pumping was 

resumed for 84 hours 

at a constant rate of 

12 million gallons 

per day.

wells at the adjacent wellfield was curtailed for two days. 

The DMW was logged at the end of the water-level recovery 

phase and after pumping was resumed at the wellfield for 

3.5 days at 12 Mgal/d. At the end of the recovery phase, the 

step evident in the upper part of the salinity profile during 

pumping conditions (figs. 11B, 12A) is absent (fig. 12A). Specific conductance gradually increases from 360 μS/cm at -150 ft 

to 2,000 μS/cm at -360 ft (fig. 12A). Similarly, the step evident 

in the temperature profile under pumping conditions is absent 

at the end of the recovery phase (figs. 11B, 12B). Temperature 

gradually decreases with depth to -300 ft and increases nearly 

linearly with depth below -450 ft as a result of geothermal heating. After the 3.5 days of pumping, both salinity and temperature profiles indicate upward flow, because brackish and warmer 

water from about -430 ft is moved up the wellbore to -125 ft, 

evident from the uniform specific conductance and isothermal 

conditions in this interval. Two days of recovery from pumping 

may not be long enough for the water column in the DMW to 

equilibrate to nonpumping conditions; however, the salinity and 

temperature profiles provide an indication of how borehole flow 

is affected by nearby groundwater withdrawals. 

Apparent Top of Transition Zone

Steps in the upper part of specific-conductance profiles 

are compared to nearby withdrawal rates to investigate the 

influence of nearby groundwater withdrawals on the shape 
22 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

of salinity profiles and the measured depth of the apparent 

top of the transition zone. The upward-flow step (fig. 6A and 

figs. 11B,C) can be conceptually separated into horizontal 

and vertical parts. The vertical part, characterized by a nearly 

uniform specific-conductance value, is the section over which 

borehole flow occurs, and the horizontal part is where substantial outflow of water from the monitor well occurs. Steps in a 

profile can be characterized by (1) the depth of outflow, (2) the 

mean specific-conductance value of the vertical part, (3) the 

vertical distance over which borehole flow occurs, and (4) the 

change in specific conductance of the vertical part in different profiles. The change in specific conductance for upward 

flow is determined from the difference between maximum 

specific conductance of the vertical step and the specific 

conductance immediately above the horizontal part of the step 

(table 3). The difference represents the increase in salinity in 

the vertical-flow zone. The conductance above the flow step 

and maximum conductance of the step also define whether 

the 1,000 μS/cm specific-conductance value (corresponding 

to the top of the transition zone) is included in the step. For 

most days that a specific-conductance profile was available 

from a DMW, mean daily pumping rates from large nearby 

production wellfields were obtained to evaluate whether the 

average specific-conductance value of the vertical step in the 

profiles was significantly correlated to pumping rate. Profiles 

with evident vertical steps of salinity are separated into two 

categories—whether the apparent top of the transition zone 

(1,000 μS/cm specific-conductance value) is included in the 

vertical step in the profile or not. 

Apparent Top of Transition Zone Not Included Within the 

Longest Vertical Step

Salinity profiles from wells in this category have a vertical step of specific conductance that is longer than 70 ft, but 

the maximum specific conductance within the longest vertical 

step is less than 1,000 μS/cm. However, this does not preclude 

the possibility that borehole flow may affect monitoring of 

the apparent top of the transition zone sometime in the future, 

when the specific-conductance value of the longest vertical 

step exceeds 1,000 μS/cm, or even currently for a shorter vertical step occurring below the longer one.

Figures 13, 14, and 15 all have a similar layout as 

described here. In the large main plot for a particular well, 

selected specific-conductance profiles are color-coded to show 

extreme (minimum and maximum specific conductance) cases 

of the vertical step (green and red) and intermediate cases 

(yellow and orange). The gray box defines the common depth 

interval of all salinity profiles over which the nearly constant 

specific-conductance values of the flow step are averaged for 

analysis in the lower plot. The black bracketed line indicates 

the slotted interval of nearby production wells. The slotted 

intervals of the production wells were vertically shifted to 

account for dip of the lava flows along their principal flow 

direction (Oÿahu 3° and west Maui 8°) and the distance to

 the DMW. The vertical dashed line corresponds to the 

1,000 μS/cm specific-conductance value, indicative of the top 

of the transition zone. In the lower plot for each well, the mean 

specific conductance of the vertical flow step is plotted relative 

to the daily mean withdrawal rate. Data points corresponding 

to profiles shown in the plot immediately above are coded with 

the same color as the profiles, whereas other available profiles, 

not shown in the upper plot of profiles, are shown as blue dots. 

Withdrawal rates were not available for all days when profiles 

were collected. The black line in the lower plot is a leastsquare fit line for all data points, and the gray lines are the 

95-percent confidence intervals of one standard deviation of 

the regression. The correlation coefficient, R, is shown in the 

corner of the plot. Additionally, to test for statistical significance of the correlation, a p value is provided (table 3). The 

p value is calculated by transforming the correlation between 

two parameters to create a t statistic having n-2 degrees of 

freedom, where n is the number of data points (Helsel and 

Hirsch, 1992). For correlations with a p value smaller than 

0.05, the hypothesis of no correlation between the parameters 

has to be rejected at the 95-percent confidence level. Lower 

p values correspond to greater confidence in the correlation 

between the two parameters.

Three examples of DMWs are described below in which 

the longest vertical step in salinity profiles currently does 

not affect monitoring the apparent top of the transition zone. 

The first example is from Kalihi Station DMW (fig. 13A). A 

uniform specific conductance between about -240 and -450 ft 

is consistent with upward flow over about 200 ft. The upward 

flow terminates when it reaches the bottom of the solid casing 

of the monitor well at -239 ft. The mean specific conductance 

of the vertical section of the step varies temporally between 

475 and 950 μS/cm, and the specific conductance and nearby 

withdrawal rates are strongly correlated (correlation coefficient of 0.96 and statistically significant p value) (table 

3). Although the longest vertical step does not include the 

apparent top of the transition zone, a second flow step between 

about -450 and -485 ft does. The depth difference of the apparent top of the transition zone in the green and the red profiles 

is 80 ft (fig. 13A).

Salinity profiles from the Moanalua DMW are affected 

by nearby groundwater withdrawals (fig. 13B). The longest 

flow step stretches vertically over 315 ft, and two more steps 

affect 90 ft of the profile below the main step. The mean 

specific conductance of the vertical section of the longest 

step varies temporally between 420 and 800 μS/cm, although 

one profile from 2003, not shown in fig. 13B because the 

withdrawal rate was unavailable, is at 950 μS/cm. The correlation coefficient of the linear relation between mean specific 

conductance of the vertical step and daily mean withdrawal 

rate is lower (0.80) than that associated with Kalihi Station 

DMW, but it is statistically significant at the 95-percent level. 

The main vertical step does not affect the apparent top of the 

transition zone. However, the second and third step below -340 

ft do affect the 1,000-μS/cm threshold. The apparent top of 

transition zone differs by 80 ft between the green and the red 

profile (fig. 13B).
0 500 1,000 1,500 ‐550

‐500‐450‐400‐350‐300‐250‐200‐150‐100

Kalihi Station DMW

ALTITUDE, IN FEET RELATIVE TO MEAN SEA LEVEL

4 6 8 10 0

250500750

1,000

0 500 1,000 1,500 ‐450

‐400‐350‐300‐250‐200‐150‐100

‐50

0

Moanalua DMW

2 3 4 5 0

250500750

1,000

MEAN DAILY WITHDRAWAL RATE, IN MILLION GALLONS PER DAY (Mgal/d) 

0 500 1

‐650‐600‐550‐500‐450‐400‐350‐300‐250‐200

Waiehu D

0 

0

250500750

1,000

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

MEAN SPECIFIC 

CONDUCTANCE OF

VERTICAL STEP,

IN MICROSIEMENS 

PER CENTIMETER 

A

B

C

Vertical‐flow

zone common in

available profiles

to determine the

mean specific

conductance used

in the lower plot

Slotted interval ofWaiheÿe productionwells, distance to DMW: 1,600 feet

Slotted intervalof Kalihi Stationproduction wells,distance to DMW: 210 feet

Slotted intervalof Moanalua Stationproduction wells,distance to DMW: 290 feet

12/27/2001 4.2 Mgal/d10/25/2006 6.8 Mgal/d10/20/2005 7.9 Mgal/d08/19/2008 9.7 Mgal/d

R = 0.96 R = 0.80 

11/02/20062.2 Mgal/d08/18/20083.7 Mgal/d07/28/20054.9 Mgal/d02/09/20035.2 Mgal/d

95% confidenceinterval

Linear

regression

Figure 13. Selected profiles in which the longest vertical step in salinity does not include the 1,000 microsiemens per centimeter specific-con

A, Kalihi Station DMW. 

B, Moanalua DMW. 

C, Waiehu DMW. See text for full explanation of plots and symbols.
24 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

The third example is Waiehu DMW on Maui (fig. 13C). 

The observed specific-conductance values of the flow step 

show poor correlation with the available daily withdrawal 

rates. The upward flow step affects at least 100 ft in most of 

the profiles, but currently not the apparent top of transition 

zone, although that depth in the red profile is 40 ft shallower 

than in the green profile. The depth of the horizontal part of 

the step generally increases as the specific conductance of 

the vertical step increases. This phenomenon also occurs in 

other wells, for example in Kaamilo DMW and Newtown 

DMW. The Kalihi Station and Moanalua DMWs are less than 

300 ft from production wells, whereas the Waiehu DMW is 

five times farther from production wells (Kaamilo DMW and 

Newtown DMW are more than ten times farther). Short-term 

variations in withdrawal rates from production wells may not 

affect borehole flow in distant DMWs. The observed flow step 

in profiles from the Waiehu DMW could be a result of regional 

drawdown of the upper part of the freshwater lens over longer 

periods (several years to decades).

Scatter around the regression lines in the bottom plots 

in figures 13A,B may be related to limitations of using daily 

mean withdrawal rates for the analysis. Because groundwater 

withdrawal generally is unequally distributed throughout the 

day, the mean value could underestimate or overestimate the 

withdrawal rate at the time the monitor well was logged. In 

addition, the withdrawal rate from nearby production wells 

may change while the monitor well is being logged. A few 

profiles may have been affected by changing withdrawal rates. 

These profiles indicate changes of specific-conductance values 

with depth over the part of the profile that otherwise has a 

uniform specific-conductance value in most other available 

profiles. Detailed data on withdrawal rates during the day were 

not available to evaluate whether the withdrawal rate changed 

during the periods when wells were logged. Nevertheless, 

profiles without a pronounced vertical step in specific 

conductance were excluded from the regression analysis. 

Correlating specific-conductance values of vertical steps 

in profiles with daily mean withdrawal rates also may have 

limitations for monitor wells located distant from production 

wells. Variations in salinity profiles may not be related to the 

amount of withdrawal from a distant production well on the 

day the monitor well was logged. Instead, variations in salinity 

profiles may be affected by withdrawals on a regional scale. 

Regional drawdown may be enhanced in the aquifer systems 

of Honolulu and east Pearl Harbor because of compartmentalization by low-permeability valley fills (fig. 8), inland dikes, 

and the coastal caprock (fig. 5).

Although the apparent top of the transition zone is not 

included in the longest step for wells in this category, the 

maximum specific conductance of the longest step is commonly only slightly less than 1,000 μS/cm. The longest vertical step may include the apparent top of the transition zone 

in the future because of increased withdrawals or decreased 

recharge, or because the salinity at the inflow position of 

the step increases because of a shrinking freshwater lens. 

Moreover, monitoring the apparent top of the transition zone 

using profiles from Kalihi Station DMW, Moanalua DMW, 

and Waiehu DMW may not be appropriate because shorter 

steps below the longest one already include the apparent top of 

the transition zone.

Apparent Top of Transition Zone Included Within the 

Longest Vertical Step

For all available salinity profiles from wells in this category, 

the specific conductance above the longest vertical step is less 

than 1,000 μS/cm and the maximum observed mean specific 

conductance within the step is greater than 1,000 μS/cm. Monitoring the top of the transition zone using profiles from these 

wells may not be appropriate. Examples of monitor wells 

with profiles that have vertical steps of salinity induced by 

nearby or regional withdrawals that include the apparent top 

of transition zone salinity, depending on the withdrawal rate, 

include Kaimukï Station DMW, Jonathan Springs DMW, and 

Punaluÿu DMW.

Salinity profiles from the Kaimukï Station DMW indicate a vertical step with mean specific conductance greater than 

1,000 μS/cm for high withdrawal rates from nearby production 

wells (fig. 14A). The longest step stretches vertically over 150 ft, 

although it could be extended over 200 ft if the shorter step 

above it is included. Another nearly vertical step between -300 

and -370 ft is present in some profiles (yellow, orange, and red 

profiles in fig. 14A) but not in others that were measured under 

relatively lower withdrawal rates (green profile in fig. 14A). 

The specific conductance of the vertical step ranges from 500 

to 1,400 μS/cm in available profiles. The correlation between 

mean specific conductance of the step and nearby withdrawal 

rate is statistically significant. Tracking the depth of the 1,000-

μS/cm specific-conductance value in profiles from this well 

may not provide a meaningful indicator of water-quality 

changes in the aquifer. For the profiles measured under low 

nearby withdrawal rates (less than 4 Mgal/d), the apparent 

top of the transition zone is between -325 and -300 ft. For 

withdrawal rates greater than 4 Mgal/d, the depth of the 

1,000 μS/cm specific-conductance value occurs at a much 

shallower altitude near -110 ft, within the horizontal part of 

the step in the profile. The change in the depth of the apparent 

top of the transition zone is not caused by upconing (upward 

movement of brackish water beneath an active production 

well) and does not reflect an overall change of salinity in the 

adjacent aquifer. Instead, it can be attributed to upward flow in 

the monitor well induced by nearby withdrawals from production wells located only 70 ft away.

The apparent top of the transition zone in profiles from 

Jonathan Springs DMW also is affected by withdrawals (fig. 

14B). The vertical step between -480 and -580 ft is not as 

pronounced as in profiles from other wells, and several smaller 

steps occur in the upper part of the specific-conductance 

profile. The monitor well is 2,300 ft from the nearest large 

production wells. All other factors being equal, the farther a 

monitor well is from production wells, the smaller is the effect 
Figure 14. Selected profiles in which the longest vertical step in salinity includes the 1,000 microsiemens per centimeter specific-conductance value

withdrawal rate from nearby or regional withdrawals. 

A, Kaimukï Station DMW. 

B, Jonathan Springs DMW. 

C, Punalu‘u DMW. See text for full explana

ALTITUDE, IN FEET RELATIVE TO MEAN SEA LEVEL

0 500 1,000 1,500 2,000

‐600‐500‐400‐300‐200‐100

0

Kaimukï Station DMW

2 4 6 8

500

1,0001,500

0 500 1,000 1,500 2,000

‐700‐600‐500‐400‐300‐200‐100 Jonathan Springs DMW

4 6 8 10

500

1,0001,500

0 500 1,000 1,50

‐600‐500‐400‐300‐200‐100

0

Punaluÿu DM

0 

500

1,0001,500

2,500 2,500 

Slotted Interval using 10° dip

MEAN DAILY WITHDRAWAL RATE, IN MILLION GALLONS PER DAY (Mgal/d) 

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

A

B

C

MEAN SPECIFIC 

CONDUCTANCE OF

VERTICAL STEP,

IN MICROSIEMENS 

PER CENTIMETER 

Vertical‐flow zone

common in available

profiles to determine

 the mean specific

conductance used

in the lower plot

04/17/20002.6 Mgal/d01/05/20063.1 Mgal/d07/06/20004.0 Mgal/d04/17/20086.0 Mgal/d

R = 0.87 R = 0.80 R = -0.76

04/01/20054.5 Mgal/d01/16/20076.9 Mgal/d07/17/20067.2 Mgal/d05/01/20088.8 Mgal/d

95% confidenceinterval

Linear

regression

Slotted interval of Kaimukï Station production wellsdistance to DMW: 70 feet

Slotted interval of Punaluÿu wellfdistance to DMW: 2,000 feet

Slotted interval of Kalihi Station production wellsdistance to DMW: 2,300 feet
26 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

of withdrawals on measured salinity profiles. However, the 

mean specific-conductance values of the step in the profiles are 

significantly correlated with daily mean withdrawal rates from 

the nearest production wells. The specific-conductance value 

of the step ranges from 830 to 1,250 μS/cm, and the depth of 

the apparent top of the transition zone varies by 160 ft in profiles from this monitor well, depending on the withdrawal rate 

of the production wells.

The longest vertical step in some salinity profiles from 

Punaluÿu DMW is between -150 and -400 ft and between -250 

and -560 ft in other profiles. The mean specific-conductance 

values of the extensive step are negatively correlated with 

daily mean withdrawal rates from nearby production wells 

(fig. 14C). The significant negative correlation (R = -0.76) 

indicates that higher withdrawal rates from the production 

wells cause lower specific conductance in the vertical steps of 

the profiles and a deeper apparent top of the transition zone. 

In fact, the highest specific-conductance value of the longest 

step in the profile is associated with the lowest withdrawal 

rate (0.9 Mgal/d), and the lowest specific-conductance value is 

associated with the highest withdrawal rate (8.7 Mgal/d). The 

shape of the profiles may indicate upward flow (see fig. 6A) 

from about -400 to -150 ft associated with the orange and red 

profiles, but downward flow (see fig. 6B) from about -350 

to -560 ft associated with the yellow and green profiles (fig. 

14C). It appears that under high withdrawal rates downward 

flow is induced in the borehole. Punaluÿu II wellfield is located 

inland from the monitor well (fig. 3), and the slotted interval 

of the production wells may therefore affect a deeper section in the monitor well because of the dip of the lava flows. 

However, the assumed dip of 3° is near the lower end of the 

range of reported angles. If the maximum reported dip of 10° 

is assumed, the slotted interval of the production wells would 

affect a deeper part of the monitor well, between -330 and 

-620 ft, and this is consistent with the observed depth of outflow from the profiles with a downward-flow step. The monitor well is located 420 ft from the coast, where upward flow 

driven by the regional flow system is expected. Thus, during 

periods of low groundwater withdrawal, the natural hydraulic 

gradient in the aquifer may cause upward flow in the borehole, 

whereas during periods of high withdrawal downward borehole flow may be induced.

In the examples presented above, the apparent top of transition zone changed hundreds of feet depending on the withdrawal rate, and the specific-conductance value of the vertical 

step in the profile exceeded 1,000 μS/cm for some withdrawal 

rates. Cases also exist in which the 1,000-μS/cm depth in 

profiles is included in the longest vertical step for all available 

(nonzero) withdrawal rates. Three examples (Punanani DMW, 

Kaamilo DMW, and Kalihi Shaft DMW) are described below.

Specific conductance along the vertical part of the step 

in profiles from the Punanani DMW (fig. 15A) ranges from 

300 (zero withdrawal from nearby production wells) to 

4,600 μS/cm in available profiles and stretches over 230 ft 

in some profiles. The depths of outflow from the upwardflow steps coincide with the slotted interval of the nearby 

production wells. The withdrawal of groundwater from the 

Punanani production wells started in 1972. The specificconductance profile from 1968 is therefore representative of 

the conditions in the monitor well before nearby groundwater 

withdrawal started. The profile from 1968 shows an apparent top of transition zone at -560 ft and no clear indication 

of an upward-flow step. A similar profile was available from 

September 2008, when pumping at the Punanani wellfield 

was curtailed for two days (fig. 12). Available data from the 

Punanani DMW indicate that upward borehole flow is directly 

induced by nearby withdrawals, and if nearby withdrawals 

cease then the upward borehole flow mainly is eliminated. 

The significant correlation (R = 0.97) between daily mean 

withdrawal rate and specific conductance of the vertical step 

is supported by little scatter around the regression (table 3). 

The slope of the regression line indicates that for a 1 Mgal/d 

increase in withdrawal rate the specific-conductance value of 

the step in the profile between -160 and -320 ft increases by 

275 μS/cm. The six production wells at the Punanani wellfield 

are located in close proximity (between 25 and 140 ft) to the 

monitor well, and withdrawals from these wells induce upward 

flow in Punanani DMW, which drastically shifts the depth of 

the apparent top of the transition zone upward and misrepresents its position in the aquifer. Despite this, monitoring the 

midpoint of the transition zone is still valuable.

A longer upward-flow step is observed at Kaamilo DMW. 

In some profiles, the step stretches over 644 ft, and the specific-conductance values increase to 9,100 μS/cm (fig. 15B). 

A profile taken under no or low withdrawal conditions does 

not exist to verify whether the step disappears under those 

conditions. The depth of outflow corresponds to the slotted 

intervals of Kalauao and Kaamilo production wells. Although 

the major production wells at Kalauao are 0.6 mi from the 

monitor well, a significant relation exists between daily mean 

withdrawal rate and specific conductance of the vertical step 

in the profile from the monitor well. The aquifer between the 

monitor well and production wells may be hydraulically well 

connected, for example by a lava tube, or the aquifer may be 

compartmentalized to an extent that causes regional drawdown 

to extend to the monitor well. Available data appear to fall 

into two separate groups (see small plot fig. 15B). The data 

collected during 2001 to 2003 indicates a separate relation 

relative to the data collected since 2005, although the relations 

have similar slopes. The separate relations may be associated 

with separate factors, including nearby withdrawal, regional 

drawdown, or changes in recharge. Regional pumping effects 

in compartmentalized aquifers can cause a hydraulic gradient 

that induces upward flow in a distant monitor well. The apparent midpoint of the transition zone between freshwater and 

saltwater has risen several feet per year in the Kaamilo DMW 

because of long-term regional withdrawals and low recharge 

(Rotzoll and others, 2010). Hence, the uniform specific conductance of the flow step increases over time, because of a rise 

of the transition zone in the aquifer and because the inflow 

at the bottom of the flow step remains at the same altitude 

because of geologic constraints.
ALTITUDE, IN FEET RELATIVE TO MEAN SEA LEVEL

0 2,000 4,000 6,000 8,000 ‐800

‐700‐600‐500‐400‐300‐200‐100

0

Punanani DMW

0 5 10 15 0

2,5005,000

0 2,000 4,000 6,000 8,000

Kaamilo DMW

4 7 10 13 5,000

7,500

10,000

0 2,000 4,000 6,000 8,00

Kalihi Shaft DMW

7 7.5 8 

0

2,5005,000

MEAN DAILY WITHDRAWAL RATE, IN MILLION GALLONS PER DAY (Mgal/d) 

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

A

B

C

MEAN SPECIFIC 

CONDUCTANCE OF

VERTICAL STEP,

IN MICROSIEMENS 

PER CENTIMETER 

11/26/19680.0 Mgal/d12/18/20066.2 Mgal/d11/07/200510.4 Mgal/d01/30/200114.3 Mgal/d

R = 0.97

R = 0.94

05/25/20017.1 Mgal/d11/26/200313.2 Mgal/d04/22/20054.6 Mgal/d08/02/200610.2 Mgal/d

02/187.7 M06/197.0 M10/278.3 M05/158.0 M

95% confidenceinterval

Linear

regression

R = 0.87

Slotted intervalof Kalauaoproduction wellsdistance to DMW:3,100 feet

Infiltration tunnelof Kalihi Shaftdistance to DMW:170 feet

Slotted interval of

Punanani production

wells, distance

to DMW: 50 feet

Vertical‐flow zone com

available profiles to det

the mean specific condu

used in the low

Figure 15. Selected profiles in which the longest vertical step in salinity, induced by nearby or regional withdrawal, is persistent and includes the 1

siemens per centimeter specific-conductance value. 

A, Punanani DMW. 

B, Kaamilo DMW. 

C, Kalihi Shaft DMW. See text for full explanation of plots 
28 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

0 4 8

0 84

0 25 MILES

0 50 KILOMETERS

MILES

KILOMETERS

158° 157° 156° 155°

21°

20°

19°

Hawaiÿi

Maui

Oÿahu

Molokaÿi

Länaÿi

Kahoÿolawe

Pacific

Ocean

#

#

##

#

#

##

#

#

#

# #

#

#

#

#

#

# # #

# #

#

#

#

#

# #

#

#

#

#

#

#

21°20'

21°30'

21°40'

158°10' 158°00' 157°50' 157°40'

EXPLANATION

Maximum length of vertical displacement

(step) in measured salinity profile, in feet

Longest step is less than 70 feet

Longest step is greater than 70 feet

 Apparent top of transition zone

 is within the largest step

 Apparent top of transition zone is

 currently not within the largest step

#

#

Barrier to horizontal groundwater flow

Valley fill

Other barrier

#

##

#

##

#

#

Shaded relief generated from U.S. Geological Survey 10‐meter Digital Elevation 

Model, Universal Transverse Mercator projection, zone 4, NAD83 datum. 

Figure 16. Maximum length of vertical step in measured salinity profiles from deep monitor wells in Hawai‘i. Steps longer than 70 

feet may be related to nearby withdrawal or other factors, including regional drawdown in compartmentalized aquifers and natural 

hydraulic gradients at depth. The top of the transition zone is defined as having a specific conductance of 1,000 microsiemens per 

centimeter. The barriers to groundwater flow are modified from Oki (2005) and Rotzoll and El-Kadi (2007).
Borehole Flow 29

Profiles from the Kalihi Shaft DMW indicate persistent 

vertical steps in salinity profiles that include the apparent 

top of the transition zone. Vertical steps in the Kalihi Shaft 

DMW are almost 600 ft long (fig. 15C). The depth of outflow 

from Kalihi Shaft DMW coincides with the infiltration tunnel 

of Kalihi Shaft. The uniform specific conductance of the flow 

step increases over time, partly because withdrawal rates 

are consistently between 7 and 8.5 Mgal/d and for similar 

reasons to those stated for Kaamilo DMW above. A profile 

taken under no or low withdrawal conditions does not exist 

to verify whether the step disappears under those conditions. 

By collecting salinity profiles under consistent withdrawal 

rates, changes in specific conductance over time may be 

more readily detectable in wells that are affected by nearby 

or regional withdrawals.

Profiles from some wells indicate changes in specific conductance of the vertical flow step that are correlated significantly 

to daily mean withdrawal rates at nearby production wells. However, profiles from other wells may indicate changes in specific 

conductance of the flow step that are not correlated significantly 

to daily mean withdrawal rates (table 3). For these latter cases, 

the changes in specific conductance may be related to regional 

withdrawals or natural changes in the vertical hydraulic gradient. 

Vertical steps less than 70 ft long that include the 1,000 μS/cm 

specific-conductance value exist in 13 DMWs (table 3). A vertical step less than 70 ft may be related more to the presence of a 

massive lava flow than to high rates of borehole flow, because 

the step may not exceed the thickness of the massive flow. 

Vertical steps less than 70 ft commonly are persistent, which is 

consistent with a step that is related mainly to the local geology. Nevertheless, monitoring the top of the transition zone in 

these wells may not be appropriate.

In general, wells with profiles having flow steps that 

exceed massive lava-flow thicknesses of 70 ft and that include 

the 1,000-μS/cm specific-conductance value are concentrated 

from east Pearl Harbor to Honolulu (fig. 16). Additional wells 

with profiles having flow steps that exceed 70 ft exist in the 

Punaluÿu area and near Waiehu on Maui. Although the longest 

vertical step in profiles from some wells may not include the 

1,000 μS/cm specific-conductance value, the maximum specific 

conductance of the flow step may exceed 1,000 μS/cm in the 

future or the depth of that value is currently already included 

in shorter steps below the longest one. Because salinity 

profiles without steps are not available for many wells, it is 

difficult to determine where the 1,000-μS/cm depth is in the 

aquifer and how much the monitored apparent top of the transition zone in the well differs from the true location of the top 

of the transition zone in the aquifer. 

Low-permeability valley fills, dike zones, and the caprock 

impede horizontal flow in aquifers in Honolulu and east Pearl 

Harbor. The locations of wells with profiles that have long 

vertical steps not directly related to changes in daily withdrawal rates coincide with the aforementioned area of aquifers 

with barriers to horizontal flow. Profiles from these wells may 

be affected by regional withdrawals in the compartmentalized 

aquifers and provide a poor estimate of the top of transition 

zone in the aquifer. Withdrawal from production wells more 

than 0.5 mi from a monitor well may affect salinity profiles in 

the monitor well (for example, South Hälawa DMW and Newtown DMW), although not necessarily on a short-term, daily 

time scale. Profiles from a few monitor wells (Punchbowl 

DMW and Kaimukï HS DMW) in compartmentalized aquifers 

have short vertical steps. Low-permeability feeder dikes 

associated with a rejuvenated-stage volcanic vent (Punchbowl 

1970 1990 2010

‐800

‐700

‐600

‐500

‐400

‐300

‐200

‐100

0

Kaimukï Station DMW

1970 1990 2010

‐1,200

‐1,100

‐1,000

‐900

‐800

‐700

‐600

‐500

‐400

Beretania DMW

1970 1990 2010

‐800

‐700

‐600

‐500

‐400

‐300

‐200

‐100

0

Punanani DMW

1970 1990 2010

‐800

‐700

‐600

‐500

‐400

‐300

‐200

‐100

0

Punaluÿu DMW

ALTITUDE, IN FEET

RELATIVE TO MEAN SEA LEVEL

Maximum length

of vertical step

Top of transition zone

 Midpoint of

transition zone

A B C D

Figure 17. Changes from 1968 to 2009 in the depths of the apparent top and midpoint of the transition zone between 

fresh and saltwater measured in deep monitor wells, and longest vertical step (gray bands). A, Punanani DMW. B, 

Beretania DMW. C, Kaimukï Station DMW. D, Punalu‘u DMW.
30 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

Crater) intrude the aquifer near the Punchbowl DMW, and 

these dikes may impede regional drawdown from extending to 

the monitor well. Vertical steps that include the 1,000 μS/cm 

specific-conductance value exist in profiles from Kaimukï HS 

DMW, although the maximum vertical extension of the steps 

is only 28 ft. Monitor wells located outside the compartmentalized aquifers of Honolulu and the east Pearl Harbor area and 

far from any major production wells generally have profiles 

with vertical steps of salinity less than 70 ft or that are largely 

unaffected by changes in withdrawal rates.

Time series of the measured depths of the apparent top 

and midpoint of the transition zone illustrate the potential 

problem of misinterpreting aquifer water-quality changes from 

profiles (fig. 17). The longest vertical step in salinity profiles 

(table 3) is shown as a gray box on figure 17. The production 

wells at Punanani were not completed and used before 1971. 

Three of the six production wells are slotted between -70 and 

-220 ft, and the others are slotted between -150 ft and -310 ft. 

After 1972, groundwater was withdrawn from the Punanani 

production wells continuously (see fig. 21A). The effect of the 

vertical step is evident when the measured depth of the apparent top of the transition zone changes from -560 ft to -100 ft 

within 4 years from 1969 to 1973 (fig. 17A). The apparent midpoint depth rose several feet per year, whereas the depth of the 

apparent top of the transition zone rose rapidly after 1972 and 

then fluctuated between -80 and -156 ft, depending on whether 

the shallow or the deep production wells were pumped. The 

apparent top of the transition zone in the Beretania DMW rose 

slowly until the mid 1980s, because the step in the profiles did 

not include the 1,000-μS/cm specific-conductance value before 

then. In about 1985, the apparent top of the transition zone 

rose rapidly from -850 ft to -500 ft (fig. 17B). Since 1998, the 

depth of the apparent top of the transition zone has remained 

nearly stationary at about -680 ft, although the apparent midpoint has risen continuously over the past 40 years. Both these 

examples show that the measured depth of the apparent top 

of the transition zone may shift rapidly to a shallower depth 

when the horizontal part of the step includes the 1,000-μS/cm 

specific-conductance value. Because of upward borehole flow, 

monitoring the depth of the apparent top of the transition zone 

in these monitor wells may not accurately reflect conditions in 

the aquifer, because it underestimates the 1,000-μS/cm specificconductance depth in the aquifer.

In the next two examples (figs. 17C,D), nearby withdrawals affect borehole flow and the depth of the apparent top of 

the transition zone (see also fig. 14). The depth of the apparent 

top of the transition zone (1,000-μS/cm specific conductance) 

in Kaimukï Station DMW is near -260 to -375 ft when nearby 

withdrawal from Kaimukï Station is less than 4 Mgal/d and 

near -100 to -150 ft when withdrawal rates are higher (fig 

17C). Thus, when nearby groundwater withdrawals are high, 

monitoring salinity profiles from this DMW underestimates 

the top of transition zone. In Punaluÿu DMW, nearby withdrawal induces downward flow in the monitor well, which 

counteracts the natural upward flow associated with the discharge zone of the aquifer. Under low withdrawal rates, the 

apparent top of the transition zone is near a depth of 

-170 ft, whereas when withdrawal rates are high it is below 

-500 ft (fig 17D). The top of the transition zone in the aquifer 

is most likely between these extremes, but salinity profiles 

underestimate or overestimate the measured depth, depending on the nearby withdrawal rates. Both examples show a 

progressively shallower apparent midpoint depth over time but 

large fluctuations of the depth of the apparent top of the transition zone because of borehole flow induced by nearby groundwater withdrawal. Because of upward and downward borehole 

flow (fig. 6), monitoring the depth of the apparent top of the 

transition zone in these monitor wells also may not accurately 

reflect conditions in the aquifer.

Apparent Midpoint of Transition Zone

Steps at the Apparent Midpoint

Vertical steps at the apparent midpoint of the transition zone (defined as the level at which specific conductance 

equals 25,000 μS/cm) occur in several DMWs. The longest 

vertical step of 120 ft at the apparent midpoint occurs in 

Waimalu DMW. Steps at the apparent midpoint in other monitor wells range in length from a few tens of feet in Jonathan 

Springs DMW, Newtown DMW, and Poliwai DMW to 65 ft 

in Waimano DMW. However, all steps are persistent over time 

because they are evident in all available profiles at the same 

interval. The steps may be related to the presence of massive 

lava flows rather than to high rates of borehole flow. Nevertheless, the measured apparent midpoint of the transition zone in 

these boreholes may consistently underestimate the midpoint 

depth in the aquifer.

Relation Between Daily Withdrawal Rates and Apparent 

Midpoint of Transition Zone 

The depth of the apparent midpoint of the transition 

zone between freshwater and saltwater in salinity profiles 

was compared to daily mean withdrawal rates from the largest production wells near the DMW. The rise of the apparent 

midpoint depth between 1999 and 2009 (Rotzoll and others, 

2010) was removed, assuming a linear trend that was determined using the nonparametric Mann-Kendall test (Helsel and 

Hirsch, 1992). Although it is recognized that the movement of 

the transition zone is transient, available data of the apparent 

midpoint depth show almost no response to seasonal or interannual recharge and discharge variations in the past 10 years 

(fig. 17). Thus, using a simplified linear trend was justified.

The depth of the apparent midpoint of the transition 

zone and daily mean withdrawal rate generally do not show a 

statistically significant correlation, indicated by p values much 

larger than 0.1 (table 3). The effects of nearby withdrawals 

apparently are more pronounced in upper levels of the monitor wells. However, two cases show a statistically significant 

relation (p values <0.03) between withdrawal rate and depth of 

the apparent midpoint. The first case is Punanani DMW, with 
Borehole Flow 31

a correlation coefficient of 0.55. The apparent midpoint depth 

in Punanani DMW shows the largest fluctuations (compared 

to apparent midpoint depths that rise more smoothly in other 

wells; see fig. 17A), which may indicate that withdrawal also 

affects the apparent midpoint depth. The slope of the regression equation between depth of the apparent midpoint and 

daily mean withdrawal rate is 1.5 ft/Mgal/d. Considering 

that withdrawal rates can be as high as 16 Mgal/d, the apparent midpoint could theoretically be displaced more than 22 ft 

compared to zero-withdrawal conditions. However, comparing 

two salinity profiles under nonpumping and pumping conditions (fig 12A), the apparent midpoint at zero withdrawal is 

only 4 ft deeper compared to the apparent midpoint under 

12 Mgal/d withdrawal (not 18 ft, as indicated by the slope of 

the regression). The second case is Kaamilo DMW, with a 

correlation coefficient of 0.64 and a slope of the regression 

between apparent midpoint depth and nearby withdrawals of 

0.65 ft/Mgal/d. This slope translates into an 8.5-ft displacement 

under high withdrawal conditions of 13 Mgal/d at the nearby 

wellfield. The effects of withdrawals on the apparent midpoint 

depth are reasonable, given the very large flow step extending 

over 686 ft. The bottom of the step and recent apparent midpoint depths are only 85 ft apart, and nearby withdrawal may 

plausibly affect more than the apparent top of transition zone. 

The relation between apparent midpoint depth and withdrawal 

rate can be verified by installing a CTD sensor near the depth 

of the apparent midpoint and monitoring salinity under variable 

withdrawal rates. For all other monitor wells, no significant 

relation exists between daily mean withdrawal rate from nearby 

production wells and apparent midpoint depth.

‐0.5

0.0

0.5

1.0

1.5

OCEAN TIDE, IN

FEET RELATIVE

TO MEAN

SEA LEVEL 

33.95

34.00

34.05

34.10 BAROMETRIC

PRESSURE,

IN FEET

OF WATER

5

0

10 BERETANIA

PUMPAGE, IN

MILLION GALLONS

PER DAY

23.0

23.1

WATER LEVEL, IN 23.2

FEET RELATIVE

TO MEAN

SEA LEVEL

6/10/2005 6/11/2005 6/12/2005 6/13/2005 25.3

25.4

25.5

SPECIFIC

CONDUCTANCE,

IN MILLISIEMENS

PER CENTIMETER

A

B

C

E

D

Figure 18. Influence of natural and human-induced stresses on the salinity profile in a deep monitor well. 

A, Honolulu ocean tide. B, Honolulu Airport barometric-pressure oscillations. C, Groundwater withdrawal 

rates at Beretania pumping center. D, Water level in Beretania Deep Monitor Well (DMW). E, Specific 

conductance near the apparent midpoint of the transition zone in Beretania DMW.
32 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

Case Study: Beretania DMW

The following case study illustrates small effects of 

nearby withdrawals in the Beretania DMW near the apparent midpoint depth of the transition zone. Beretania DMW 

is located 3,600 ft inland in the Nuÿuanu area (fig. 1), 270 ft 

from the Beretania production wells. A CTD data logger installed in the DMW near the altitude of the apparent 

midpoint at -982 ft measured and recorded fluid specific 

conductance, temperature, and water level every second 

for more than 4.5 days starting June 10, 2005. Ocean tides, 

barometric pressure, and nearby withdrawals affect water 

levels and water quality in the DMW (fig. 18). The ocean 

tide was recorded at the Honolulu Harbor tide gage, and the 

atmospheric pressure was monitored at the Honolulu airport. 

The head fluctuations in Beretania DMW indicate a tidal 

efficiency (ratio of tidal amplitude in well to tidal amplitude 

in ocean) of only 3.6 percent and a tidal lag of 0.5 hour. The 

amplitude of the tidal response is greatly attenuated compared to the response in other wells in Hawaiÿi at comparable distances inland (Oki, 1997; Rotzoll and others, 2008), 

mainly because of the thick low-permeability sedimentary 

coastal caprock confining the highly permeable basalt aquifer 

(Li and others, 2007; Rotzoll and others, 2008). Barometric 

efficiency (ratio of barometric-pressure amplitude in well to 

atmospheric-pressure amplitude), as indicated by the response 

-50

0

50

-50

0

-25

-50

0

50

6/10/2005 6/11/2005 6/12/2005 6/13/2005

-50

0

50

RELATIVE SPECIFIC CONDUCTANCE AT -982 FEET,

 IN MICROSIEMENS PER CENTIMETER

0.25

0.00

-0.25

RELATIVE VERTICAL DISPLACEMENT OF THE APPARENT MIDPOINT, IN FEET 

0.25

0.00

-0.25

0.25

0.00

-0.25

0.25

0.00

-0.25

50

25

-25

25

-25

25

-25

25

B

A

C

D

Filtered conductance without barometric pressure and pumping oscillations

Scaled ocean tides

Filtered conductance without tidal and pumping oscillations

Scaled and inverted barometric pressure

Filtered conductance without tidal and barometric pressure oscillations

Scaled water-level displacement from pumping

Filtered conductance without tidal, barometric pressure, and pumping oscillations

Observed specific conductance

Figure 19. Observed and estimated specific-conductance changes and relative vertical displacement of 

the apparent midpoint of the transition zone in Beretania DMW attributed to (A) ocean tides, (B) barometricpressure oscillations, (C) nearby groundwater withdrawal, and (D) all three stresses combined.
Borehole Flow 33

Stress

Time lag 

of specific 

conductance 

to stress, in 

hours

Specific 

conductance 

change at 

midpoint of 

transition 

zone, in μS/cm

Vertical 

displacement 

of midpoint of 

transition zone 

in response to 

stress, in ft

Ocean tide 1.5 70 0.36

Barometric

pressure

0.0 30 0.15

Withdrawal at 

4.7 Mgal/d 0.5 20 0.10

Withdrawal at 

7.1 Mgal/d 0.5 41 0.21

Withdrawal at 

10.2 Mgal/d 0.5 83 0.42

Withdrawal at 

12.3 Mgal/d 0.5 105 0.54

[Mgal/d, million gallons per day; μS/cm, microsiemens per centimeter; ft, feet]

Table 4. Effect of stresses from ocean tides, barometric 

pressure, and nearby groundwater withdrawals on specific 

conductance recorded at the apparent midpoint of the 

transition zone in Beretania Deep Monitor Well.

of the water level in the DMW, is 66 percent, and drawdown 

in the monitor well did not exceed 0.25 ft (during the period 

of measurement) for a withdrawal rate of 12.3 Mgal/d at the 

Beretania production wells.

Each of the three stresses—ocean tides, barometric 

pressure, and nearby groundwater withdrawals—is isolated by regressing the two other stresses onto the observed 

specific-conductance time series, including the time lag for 

each stress by cross correlation, and subtracting both time 

series from the observed one. The filtering procedure is 

outlined as follows (see, for example Rotzoll and El-Kadi, 

2008): (1) estimate the time lag of specific conductance to 

each stress (table 4) and shift the stress time series accordingly, (2) subtract the mean value of each stress from each 

stress time series, (3) estimate the scaling coefficient of each 

stress by linear regression of the stress onto the specificconductance time series and multiply each stress time series 

by its coefficient, and (4) subtract the time-lag-corrected, 

zero mean, and scaled stress time series from the observed 

specific-conductance time series. The result is a filtered time 

series that shows the computed specific-conductance variations attributed to each of the three stresses (fig. 19). The 

filtered time series contains small-scale fluctuations caused 

by incomplete accounting of all factors that may affect the 

observed variations in specific conductance. The removal 

of withdrawal effects assumes instantaneous response with 

change in discharge and does not account for the transient 

response of drawdown and recovery. Furthermore, the 

removal of specific-conductance variations caused by barometric pressure could be improved by accounting for air flow 

in the unsaturated zone and vertical flow in the aquifer (Oki, 

1997; Spane, 2002). 

Other stresses that are not considered here may influence the water level and specific conductance at the apparent 

midpoint depth. However, for identifying the general effects 

of the three main stresses, the technique used in this study 

is satisfactory. If the effects of all stresses are completely 

removed, the filtered time series should be a horizontal line 

(fig. 19D). Because the filtering method did not completely 

remove the effects of all stresses, the filtered time series is 

not a horizontal line and represents the residual or part of the 

specific-conductance variations unexplained by or improperly 

attributed to tidal, barometric-pressure, and nearby groundwater-withdrawal variations. Nevertheless, variations of specific 

conductance at the apparent midpoint depth in the profile 

caused by the three stresses can be evaluated.

The stationary data logger at the apparent midpoint of the 

transition zone reveals the effects of ocean tides, barometric 

pressure, and nearby withdrawals. Table 4 lists the maximum 

variation of specific conductance caused by the individual 

stresses. The specific-conductance changes are converted 

to vertical displacements by using the slope of specificconductance profiles at the apparent midpoint. The average 

slope of three salinity profiles in Beretania DMW from 2005 

at the apparent midpoint (between 23,000 and 27,000 μS/cm) 

is 0.0051 ft/μS/cm. The vertical displacement caused by any 

single stress does not exceed 0.5 ft. For an extreme scenario of 

coinciding high tides, low barometric pressure, and withdrawal 

of 12.3 Mgal/d, the vertical displacement would be about 1 

ft. Therefore, these stresses cause only small specific-conductance fluctuations near the apparent midpoint of the profile 

in Beretania DMW. Although this response may be relevant 

only to the Beretania DMW, a similar observation was made 

in Kaimukï Station DMW. Paillet and others (2002) compared 

fluid-column resistivity profiles taken on consecutive days 

in 2001 under different nearby withdrawal rates of 3.8 and 7 

Mgal/d. The apparent midpoint was located near -690 ft in 

both profiles, although the upper part of the profiles (above 

-470 ft) changed in response to nearby withdrawals (fig. 9A). 

This indicates that nearby withdrawals (70 ft distant) may 

mainly affect the upper part of the salinity profiles from the 

Kaimukï Station DMW.

Effects of Withdrawal on Water Quality 

Deep Monitor Wells Acting as Brackish-Water 

Conduits

Deep wells were drilled close to production wells to 

monitor upconing caused by the pumped wellfield. However, 

the DMW may provide a conduit for brackish water to flow 

upwards into parts of the aquifer from which production wells 

withdraw groundwater (fig. 20). Drawdown near a pumped 

well propagates horizontally more readily than vertically in 

the anisotropic aquifers in Hawaiÿi. Conceptually, therefore, 
34 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

drawdown near the Punanani production wells extends farther 

in a horizontal direction than a vertical direction because of 

the aquifer anisotropy. Water from deeper parts of the aquifer 

moves upward inside the borehole and exits the well at depths 

corresponding to the slotted interval of the production wells. 

Water flows vertically with less resistance inside an open borehole than through the layered basalt aquifer. This is evident 

by the observed upward flow (fig. 9B) and the steps in the 

specific-conductance and temperature profiles between -140 

and -450 ft (fig. 11B).

Because of upward borehole flow in a DMW, the 

quality of water discharged at nearby production wells 

may be impaired by brackish water from deeper parts of 

the aquifer. The Punanani DMW is located within the 

compound of the Punanani production wells (fig. 3). Water 

samples from individual production wells indicate higher 

#

0 10 20 30 40 50

SPECIFIC CONDUCTANCE, IN

MILLISIEMENS PER CENTIMETER

ALTITUDE, IN FEET RELATIVE TO MEAN SEA LEVEL

SALTWATER

FRESHWATER

TRANSITION

ZONE

VOLCANIC-ROCK

AQUIFER

Pumped well Deep monitor well 

3

21.5

1.25

1

0.1

0.01 -1,000 

-800 

-600 

-400 

-200 

0

-1,000 

-800 

-600 

-400 

-200 

0

0 5 10

FEET

Estimated drawdown, in feet

1

Slotted interval

of Punanani

production wells

Water table

Figure 20. Conceptual cross section between production well and nearby monitor well and specific-conductance 

profile in Punanani DMW. The contours represent conceptual drawdown in an anisotropic basalt aquifer caused 

by groundwater withdrawals that induce upward flow of brackish water in the upper part of the monitor well. The 

arrows are principal pathways of groundwater flowing towards the production well. 

chloride concentrations and specific-conductance values 

from wells closest to the monitor well, such as P1 and P2 

(fig. 21). Wells P1 and P2 are slotted between -150 ft and 

-310 ft, deeper than the slotted interval of P3, P4, and P5. 

The chloride concentrations of water samples from P1 are 

higher than those from P2 (fig. 21A), although P2 is located 

closer to the monitor well. P1 may withdraw higher salinity 

water from deeper in the aquifer than P2. P6 is slotted 

at the same interval as P1 and P2 but is farther from the 

monitor well. Water from P6 has the lowest chloride and 

specific-conductance values, indicating that water quality 

in P6 may not be affected much by upward flow of brackish 

water in the DMW.

Because water quality from P1 did not meet the EPA 

standard for drinking water, P1 has been used rarely since 

May 2001. Overall withdrawal rates at the wellfield declined 
Borehole Flow 35

Figure 21. Salinity of water in Punanani production wells. A, Time series of chloride concentration of water samples 

and Punanani pumping rates. B, Time series of specific conductance of water samples. C, Map view of shallow 

(circles) and deep (squares) slotted production wells adjacent to Punanani DMW and mean specific conductance of 

samples between August 2006 and February 2009.

!

! 

!

#

"

"

"

"

!

N

Punanani

DMW

P1 

523 μS/cm

P2 

678 μS/cm

 P3

407 μS/cm

P4

339 μS/cm

P5

310 μS/cm

P6

308 μS/cm

0 100 FEET

METERS

50

0 15 30

1973 1978 1983 1988 1993 1998 2003 2008

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

0

50

100

150

200

250

300

EPA secondary drinking water standards

2007

200

300

400

500

600

700

800

900

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER CENTIMETER

2008 2009

PUNANANI PUMPAGE, IN

MILLION GALLONS PER DAY

0

5

10

15

20

A

B C

 -70 to -220 feet

-160 to -310 feet

Slotted interval

Punanani

Pumpage

Specific conductance, in microsiemens per centimeter (μS/cm)

Principal groundwater

flow direction

P1

P2

P6

P5

P4

P3

P1

P2

P6

P5

P4

P3

from 14 Mgal/d to 10 Mgal/d between 2001 and 2007, 

resulting in a decrease of chloride concentrations in water 

from all remaining wells except P4. When P2 is pumped, the 

depth of outflow from the upward-flow step in the monitor 

well is between -125 and -150 ft. When P2 is not pumped, 

the depth of outflow is shallower at -100 ft. In addition, 

the conductance of water from P4 increases when P2 is not 

in use. All specific-conductance values from P4 that are 

greater than 400 μS/cm were sampled on days that P2 was 

not sampled and therefore not pumped (D. Wong, Honolulu 

Board of Water Supply, written commun., 2008) (fig. 21B). 

This may indicate that brackish water flowing out of the 

monitor well, which usually flows to P2, is extracted by 

P4. Water from the production wells downgradient of the 

monitor well and closer to the ocean has higher salinity 

(fig. 21C). Another source of brackish water in the Pearl 

Harbor aquifer is return water from irrigation (Eyre, 1983, 

1987). The effects of nutrient-rich irrigation water on water 

quality in the aquifer generally are reduced after several 

years (Eyre, 1983), and because sugarcane agriculture 

upgradient from Punanani ceased in the mid-1970s (Oki, 

2005), irrigation return water cannot explain the recent 

elevated salinity in the aquifer near Punanani.

Water sampled from wells near the South Hälawa DMW 

also has elevated salinities that may be related to borehole flow 

in the DMW. Water samples collected in two Red Hill monitor 

wells in 2005 (fig. 3) at the same altitude as South Hälawa 

DMW have mean specific-conductance values of 744 μS/cm 

and 701 μS/cm. For comparison, water from a monitor well 

upgradient of South Hälawa DMW had a specific conductance 

of 461 μS/cm, and water from a well downgradient had a 

specific conductance of 561 μS/cm (TEC Inc., 2007). Brackish 
36 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

water transported through the DMW may be the source for the 

elevated salinities in the monitor wells close to the DMW.

Mass Balance

A simple mass balance based on proportional mixing is 

used to estimate the contribution of brackish water from Punanani monitor well to the production wells, according to the 

following relation (Izbicki and others, 1999; Sukop, 2000):

Ctotal = CAqQAq + CMWQMW

Qtotal

(3) ,

where Ctotal and Qtotal are the solute concentration of water 

that was withdrawn by the production well and total rate of 

withdrawal by the production well, CAq and QAq are the background concentration in the surrounding aquifer and the part 

of total withdrawal by the production well contributed by the 

surrounding aquifer, and CMW and QMW are the solute concentration of water from the monitor well and the part of total 

withdrawal by the production well contributed by the monitor 

well, respectively. The calculation assumes conservative mixing and conservation of mass.

On June 10, 2008, four production wells (P2, P3, P5, and 

P6, fig. 21C) were being pumped while Punanani DMW was 

logged. The capacity of each pump is 2.7 Mgal/d. The ambient background specific-conductance level in the Punanani 

area is 300 μS/cm, which is taken from the specific-conductance profile before withdrawals began in 1968. On June 10, 

2008, the specific conductance of water within the interval of 

the upward-flow step in the monitor well was 4,100 μS/cm, 

and in P2 it was 684 μS/cm. Flow measurements from 2001 

indicate upward flow in the monitor well that did not exceed 

0.053 Mgal/d. Using equation 3 and assuming all water exiting the monitor well was extracted by the closest production 

well, the elevated specific conductance in the monitor well 

would only raise the specific conductance of the pumped 

water from 300 to 376 μS/cm. To account for the observed 

684 μS/cm in P2, the contribution from the monitor well must 

be 10 percent of the total pumped water and the necessary 

upflow in the monitor well would have to be five times higher 

(0.27 Mgal/d) than observed. Because other production wells 

also may capture water from the monitor well and not all 

water exiting the monitor well is captured by the production 

wells, the required flow is even greater.

Upward borehole flow in the DMW may not fully 

account for the enhanced salinity in the nearby production 

wells from mass-balance considerations because (1) available flow measurements may underestimate the flow in the 

monitor well, (2) upconing of the transition zone under the 

production wells may contribute to elevated salinity in the 

production wells, and (3) additional salinity may be contributed from wellbore flow in unused production wells of the 

wellfield. Because water from the production wells closest to 

the monitor well have the highest specific-conductance values 

and water from the farthest production well (slotted at the 

Figure 22. Salinity of water from point piezometers through 

the freshwater-saltwater transition zone in Kawailoa Pump 4, 

northern O‘ahu, Hawai‘i, measured on March 12, 1956 (unpub. 

data, USGS data files).

same interval) has specific-conductance values as low as the 

background salinity, brackish water pumped by some wells 

may originate from the monitor well. However, upconing 

beneath the production wells and borehole flow in the unused 

production wells also may contribute to enhanced salinity of 

water pumped by some wells.

Groundwater Management Implications

Borehole flow affects some DMWs, but may not affect 

all wells in the same way. Vertical steps in salinity profiles 

are indicative of borehole flow (fig. 6), and the longest 

steps occur in monitor wells located in south-central 

Oÿahu (fig. 8), which coincides with the most heavily 

developed part of the aquifer, and in monitor wells located 

close to production wells or within compartmentalized 

aquifers. Hence, in many DMWs in the Honolulu and 

east Pearl Harbor areas, the salinity profiles provide poor 

estimates of the top of the transition zone in the aquifer 

(fig. 16). Withdrawals may cause a rise of the apparent 

top of the transition zone of several hundred feet in some 

wells (table 3), which is not representative of the top of 

the transition zone in the aquifer. Monitoring salinity 

profiles in these wells can be improved by logging under 

consistent withdrawal conditions or deploying CTD sensors 

at multiple depths with variable nearby groundwater 

withdrawals to estimate the direct influence of pumping on 

measured water quality near the apparent top and midpoint 

Measured

Interpolated

0 20 40 60 80 100

‐250

‐200

‐150

‐100

‐50

Kawailoa Pump 4

SALINITY, IN PERCENT OF SEAWATER SALINITY

ALTITUDE, IN FEET 

RELATIVE TO MEAN SEA LEVEL

0
Summary and Conclusions 37

of the transition zone. In contrast to the apparent top of 

the transition zone, the effects of nearby withdrawals may 

be inconsequential at the measured midpoint. Monitoring 

the apparent midpoint depth as a proxy for freshwaterlens thickness therefore may be more useful for evaluating 

movement of the transition zone and shrinking or 

expanding of the freshwater lens.

Quality assurance and quality control are important 

to enable meaningful comparisons of salinity profiles from 

successively logged monitor wells, particularly for data 

compiled from different sources. Piezometers installed 

at different depths will provide the best information 

on aquifer salinity, because they are not affected by 

borehole flow. Piezometers are open to the aquifer over 

short depth intervals and water samples collected from 

the piezometer tube represent water from the aquifer at 

the slotted interval. Generally, several piezometers at 

different depths facilitate monitoring the freshwater lens. 

Ten piezometers (plastic tubes) were installed through the 

transition zone near Kawailoa Pump 4 of northern Oÿahu 

(fig. 3). Water quality at discrete depths was used to define 

a profile (unpub. data, USGS data files), and the apparent 

midpoint of the transition zone between freshwater and 

saltwater was determined by interpolation (fig. 22). The 

apparent midpoint is estimated to be at -150 ft using a 

piecewise cubic Hermite interpolating polynomial (Fritsch 

and Carlson, 1980). The water level was 3.8 ft and the 

theoretical midpoint according to the Ghyben-Herzberg 

relation should be at -152 ft, which agrees with the 

observed midpoint. Repeated salinity measurements in the 

piezometers allow tracking the movement of the transition 

zone over time, without any borehole-flow effects.

Using misrepresentative indicators of the position of 

the transition zone between freshwater and saltwater has 

implications for groundwater management. Most DMWs that 

are affected by borehole flow have vertical steps in measured 

salinity profiles that include the apparent top of the transition 

zone. In these cases, the indicators, if followed, could lead 

to overly conservative groundwater-management decisions 

because the top of the transition zone is deeper in the aquifer 

than measured in the DMW—a thicker freshwater lens exists 

in the aquifer than indicated by the salinity profile. One well 

(Punaluÿu DMW) is partly affected by downward borehole 

flow. In this case, misinterpretations are consequential for 

groundwater management, because a thinner freshwater 

lens exists in the aquifer than indicated by the salinity 

profile. Although the depth of the apparent top of the 

transition zone from DMWs may not accurately reflect 

the top of the transition zone in the aquifer, monitoring is 

still valuable because the measured apparent midpoint of 

the transition zone may provide a useful indicator of the 

freshwater-lens thickness.

It would be useful to identify deep zones in existing 

boreholes that contribute salinity to the freshwater resource 

and target these zones for sealing. The shallower parts of 

these boreholes may be used for future production. This 

could be an economical way to reduce contamination by 

brackish water and improve production.

Summary and Conclusions

Vertical flow in a deep monitor well (DMW) may cause 

measured salinity profiles to reflect inaccurately the water 

quality in the adjacent aquifer. Such borehole flow is caused 

by natural or anthropogenically induced head gradients. 

Indicators of borehole flow include differences in salinity and 

temperature between adjacent shallow and deep monitor wells, 

flow measurements, and stepwise increases of salinity and 

temperature with depth. The vertical part of steps may represent depth intervals over which water is transported vertically 

in a well, and salinity and temperature of the water remain 

nearly uniform over that section. Because of borehole flow and 

the vertical heterogeneity and anisotropy of layered Hawaiian 

basalt aquifers, many steps of different magnitude occur in 

salinity profiles for different reasons. Persistent steps that are 

shorter than 70 ft may be related more to the presence of massive low-permeability layers and less to substantial borehole 

flow. Vertical steps longer than 70 ft, which exceeds the typical 

thickness of dense lava flows, are indicative of borehole flow. 

The longest vertical step in salinity in one borehole is 644 ft. 

Substantial borehole flow is induced locally by nearby groundwater withdrawals or through effects of regional withdrawals 

within compartmentalized aquifers near Honolulu and the east 

Pearl Harbor area, and it may affect long sections of a salinity 

profile that could result in underestimation of the freshwater 

storage in the aquifer by several hundred feet.

The effect of groundwater withdrawal on measured 

salinity is commonly observed near the apparent top of the 

transition zone (taken to be at 1,000 μS/cm specific conductance). Nearby groundwater withdrawal may induce mostly 

upward borehole flow in DMWs and cause an increase in 

specific conductance in the upper part of the profile, which 

commonly is positively correlated with increases in withdrawal rates. The vertical steps that are evident in salinity 

profiles are consistent with both steps in temperature profiles 

and flow measurements in monitor wells near production 

wells. The depth of the apparent top of the transition zone in a 

DMW may differ from the depth of the top of the transition 

zone in the aquifer by several hundred feet. Most of the 

monitor wells with profiles having vertical steps that include 

the apparent top of the transition zone are located in the 

Honolulu and east Pearl Harbor areas. No significant correlation exists between the proximity of monitor wells to production wells and the length of vertical steps of salinity in 

profiles. However, borehole flow in monitor wells near 

production wells commonly is affected by daily variations in 

withdrawals. Profiles from monitor wells located farther from 

production wells within compartmentalized aquifers indicate 

that changes in specific conductance may be controlled by 

regional drawdown. DMWs located in compartmentalized 
38 Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi

aquifers or near large groundwater withdrawals may provide a 

poor estimate of the top of transition zone. In contrast, the 

apparent midpoint of the transition zone (25,000 μS/cm specific 

conductance) in DMWs is less affected by daily variations in 

nearby withdrawal rates. Water quality measured for several 

days near the apparent midpoint in one monitor well indicated 

inconsequential depth displacements of less than 1 ft in response 

to barometric pressure and tidal fluctuations and to withdrawal 

from the nearby wellfield. Hence, the apparent midpoint depth 

may be more useful than the apparent top of transition zone to 

monitor changes in freshwater-lens thickness.

Production wells that withdraw water with elevated 

chloride concentrations and specific-conductance values and 

that are near a DMW may be affected by upward flow of 

brackish water within the monitor well. A DMW may act as a 

conduit that transports brackish water from deeper to shallower 

zones in an aquifer, which may impair the water quality in 

nearby production wells. A simple mass-balance computation 

associated with brackish water upflow in Punanani DMW could 

not account for the entire increase in salinity in pumped water 

relative to background concentrations. Additional salinity may 

be contributed from upconing beneath production wells and 

potential borehole flow in unused production wells of the 

wellfield. The longest vertical steps in salinity occur in profiles 

from monitor wells located in south-central Oÿahu, which 

coincides with the most heavily developed part of the aquifer, 

near production wells, and within compartmentalized aquifers.

Piezometers at different depths can provide the best 

information on aquifer salinity because they are not affected by 

borehole flow, although they are a more expensive alternative to 

uncased DMWs. Despite the effects of borehole flow, salinity 

measurements in DMWs may be useful for observing long-term 

trends in thickness of the upper part of the transition zone by 

monitoring the depth of the apparent midpoint of the transition 

zone between freshwater and saltwater. An understanding of 

borehole flow is critically important for proper interpretation of 

salinity profiles and evaluation of the amount of fresh groundwater that can be developed for future needs.

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Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, March 15, 2010

Text edited by Peter H. Stauffer

Layout and design by Jeanne S. DiLeo
Rotzoll–Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep Monitor Wells in Hawaiÿi–Scientific Investigations Report 2010–5058