Ground-Water Quality and its Relation to

Land Use on Oahu, Hawaii, 2000–01

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4305

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Distribution of volatile organic compound mixtures in ground water.
Front cover: Island of Oahu as seen by Landsat satellite (cloud-free mosaic by the Pacific Disaster 

Center) and showing distribution of volatile organic compound mixtures in sampled public-supply 

and monitoring wells, as seen in figure number 32.
Ground-Water Quality and its Relation to Land Use on Oahu, 

Hawaii, 2000–01

By Charles D. Hunt, Jr.

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 03-4305

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Honolulu, Hawaii

2004
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

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

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

For additional information write to: Copies of this report can be purchased 

from:

District Chief U.S. Geological Survey

U.S. Geological Survey Information Services

677 Ala Moana Blvd., Suite 415 Box 25286

Honolulu, HI 96813 Denver, CO 80225-0286

http://hi.water.usgs.gov
FOREWORD

The U.S. Geological Survey (USGS) is committed to providing the Nation with accurate and timely scientific information that 

helps enhance and protect the overall quality of life and that facilitates effective management of water, biological, energy, and 

mineral resources (http://www.usgs.gov/). Information on the quality of the Nation’s water resources is critical to assuring 

the long-term availability of water that is safe for drinking and recreation and suitable for industry, irrigation, and habitat for 

fish and wildlife. Population growth and increasing demands for multiple water uses make water availability, now measured in 

terms of quantity and quality, even more essential to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support national, regional, 

and local information needs and decisions related to water-quality management and policy (http://water.usgs.gov/

nawqa). Shaped by and coordinated with ongoing efforts of other Federal, State, and local agencies, the NAWQA Program is 

designed to answer: What is the condition of our Nation’s streams and ground water? How are the conditions changing over 

time? How do natural features and human activities affect the quality of streams and ground water, and where are those effects 

most pronounced? By combining information on water chemistry, physical characteristics, stream habitat, and aquatic life, the 

NAWQA Program aims to provide science-based insights for current and emerging water issues and priorities. 

From 1991-2001, the NAWQA Program completed interdisciplinary assessments in 51 of the Nation’s major river basins and 

aquifer systems, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.html). Baseline conditions were 

established for comparison to future assessments, and long-term monitoring was initiated in many of the basins. During the 

next decade, 42 of the 51 Study Units will be reassessed so that 10 years of comparable monitoring data will be available to 

determine trends at many of the Nation’s streams and aquifers. The next 10 years of study also will fill in critical gaps in 

characterizing water-quality conditions, enhance understanding of factors that affect water quality, and establish links between 

sources of contaminants, the transport of those contaminants through the hydrologic system, and the potential effects of 

contaminants on humans and aquatic ecosystems.

The USGS aims to disseminate credible, timely, and relevant science information to inform practical and effective waterresource management and strategies that protect and restore water quality. We hope this NAWQA publication will provide you 

with insights and information to meet your needs, and will foster increased citizen awareness and involvement in the protection

and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource issues of interest. 

External coordination at all levels is critical for a fully integrated understanding of watersheds and for cost-effective 

management, regulation, and conservation of our Nation’s water resources. The NAWQA Program, therefore, depends on 

advice and information from other agencies—Federal, State, interstate, Tribal, and local—as well as nongovernmental 

organizations, industry, academia, and other stakeholder groups. Your assistance and suggestions are greatly appreciated.

Robert M. Hirsch

Associate Director for Water
iv Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

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

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

Objectives of the Oahu NAWQA ground-water study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Study area and land use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Chemical use and application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Previous water-quality studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Geohydrologic setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Study design and methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Selection of well networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Sampling and analytical methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Results of laboratory analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Findings and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Most wells contained low concentrations of organic compounds . . . . . . . . . . . . . . . . . . . . 16

Few concentrations exceeded drinking-water regulations. . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Detection rates were highest in the nation for fumigants and among the 

highest for solvents, VOCs, and insecticides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Commonly detected solvents, fumigants, and herbicides have histories of 

intensive use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Gasoline components and insecticides were detected in few wells. . . . . . . . . . . . . . . . . . . 28

Volatile organic compounds were higher in concentration than pesticides . . . . . . . . . . . . 28

Land use, locale, and aquifer vulnerability are main factors in contamination . . . . . . . . . . 29

Ground water carries nutrients and chemicals to streams and coastal waters . . . . . . . . . 37

Well type and depth are less significant factors than land use and locale. . . . . . . . . . . . . . 38

Most ground water appears to be 10 to 50 years old, but ages are problematic. . . . . . . . . 41

Complex mixtures of organic compounds were prevalent. . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Chloroform and CFC contamination were closely associated with 

other compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Organic contamination reflects decades-old releases and former land use . . . . . . . . . . . . 53

Monitoring wells may be useful for early detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Policy implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Summary and conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Water-quality data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Figures 

Maps showing:

1. Geomorphic provinces and land use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. NAWQA ground-water study area, land use, and sampled wells . . . . . . . . . . . . . 4

3. Waianae and Koolau volcanic-rock aquifers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4. Ground-water flow systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5. Ground-water areas and potentiometric surface . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6. Public-supply wells and monitoring wells sampled for the NAWQA study . . . . . 12

Graphs showing:

7. Detection rates for organic compound classes in 30 public-supply wells . . . . . 16

8. Comparison of detection rates of organic compound classes in 

30 public-supply and 15 monitoring wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

CONTENTS
Contents v

Graph showing:

9. Concentrations of organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Maps showing:

10. Distribution of trichloroethene (TCE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

11. Distribution of radon-222 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

12. Distribution of 1,2,3-trichloropropane (TCP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

13. Distribution of 1,2-dibromo-3-chloropropane (DBCP) . . . . . . . . . . . . . . . . . . . . . . 24

14. Distribution of 1,2-dibromoethane (EDB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

15. Distribution of 1,2-dichloropropane (DCP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Graphs showing:

16. Detection rates for volatile organic compounds (VOCs) . . . . . . . . . . . . . . . . . . . . 27

17. Detection rates for pesticide compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Maps showing:

18. Distribution of solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

19. Distribution of fumigants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

20. Distribution of herbicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

21. Distribution of insecticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

22. Distribution of nitrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

23. Distribution of orthophosphate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Graphs showing:

24. Number of organic compounds detected in wells grouped by land use 

and locale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

25. Number of VOCs and pesticides detected in wells grouped by land use and 

locale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

26. Comparison of physical characteristics between public-supply wells and 

monitoring wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

27. Comparison between public-supply wells and monitoring wells for number of 

organic compounds, number of VOCs, and number of pesticides . . . . . . . . . . . 39

Maps showing:

28. Distribution of apparent recharge dates from CFC and SF6 methods . . . . . . . . 42

29. Distribution of tritium concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Graphs showing:

30. Tritium concentration in rain at Hilo, Hawaii, and after accounting for 

radioactive decay of tritium from initial rain concentrations in the 

intervening years to present . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

31. Compound mixtures in public-supply wells and monitoring wells . . . . . . . . . . . 46

Maps showing:

32. Distribution of organic compound mixtures in public-supply wells and 

monitoring wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

33. Distribution of solvent mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

34. Distribution of fumigant mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

35. Distribution of herbicide mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Tables

1. Laboratory methods used to analyze ground-water samples . . . . . . . . . . . . . . . . . . . . . . . . . 14

2. Organic-compound detections in untreated ground water from public-supply 

wells and monitoring wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
vi Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

3. Comparison of drinking-water regulations with organic-compound and 

radon concentrations in public-supply wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4. Wilcoxon rank-sum test of differences between well groups defined on the basis 

of land use and locale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

5. Wilcoxon rank-sum test of differences between public-supply and monitoring wells . . . . 38

6. Spearman rank correlation of selected chemical and physical attributes within 

public-supply and monitoring well networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Water-Quality Data, Provided in Excel Format

Table D1. Organic compounds, nutrients, and environmental tracers in untreated 

ground water from public-supply and monitoring wells, Oahu, Hawaii, 

January 2000 through August 2001

Table D2. Inorganic elements, nutrients, and environmental tracers in untreated 

ground water from public-supply and monitoring wells, Oahu, Hawaii, 

January 2000 through August 2001

Table D3. Pesticide concentrations using method 9002 / 2002 on filtered, untreated 

ground water from public-supply and monitoring wells, Oahu, Hawaii, 

January 2000 through August 2001

Table D4. Pesticide concentrations using method 9060 / 2060 on filtered, untreated 

ground water from public-supply and monitoring wells, Oahu, Hawaii, 

January 2000 through August 2001

Table D5. Concentrations of EDB and DBCP using custom method 8134 on untreated 

ground water from public-supply and monitoring wells, Oahu, Hawaii, 

January 2000 through August 2001

Conversion factors, abbreviations, and datums

Multiply By To obtain

foot (ft) 0.3048 meter

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

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

square foot per day (ft2/d) 0.09290 square meter per day

foot per day per foot (ft/d)/ft 1 meter per day per meter

cubic foot (ft3) 0.02832 cubic meter

cubic foot per second (ft3/s) 0.02832 cubic meter per second

gallon (gal) 0.003785 cubic meter

gallon per day (gal/d) 3.785 liter per day

million gallons (Mgal) 3,785 cubic meter

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

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer

inch (in.) 25.4 millimeter

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

Water temperature is given in degrees Celsius (°C), which can be converted to degrees Fahrenheit 

(°F) by using the equation:

°F = 1.8 × °C + 32
Contents vii

Abbreviations used in water-quality descriptions:

kg, kilograms

mg/L, milligrams per liter

μg/L, micrograms per liter

μS/cm, microsiemens per centimeter at 25° Celsius

pCi/L, picoCuries per liter 

TU, tritium units

Datums

Vertical coordinate information is referenced relative to mean sea level.

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
viii Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01
Abstract 1

ABSTRACT

Water quality in the main drinking-water 

source aquifers of Oahu was assessed by a onetime sampling of untreated ground water from 30 

public-supply wells and 15 monitoring wells. The 

384 square-mile study area, which includes urban 

Honolulu and large tracts of forested, agricultural, 

and suburban residential lands in central Oahu, 

accounts for 93 percent of the island’s groundwater withdrawals. 

Organic compounds were detected in 73 percent of public-supply wells, but mostly at low concentrations below minimum reporting levels. 

Concentrations exceeded drinking-water standards 

in just a few cases: the solvent trichloroethene and 

the radionuclide radon-222 exceeded Federal standards in one public-supply well each, and the 

fumigants 1,2-dibromo-3-chloropropane (DBCP) 

and 1,2,3-trichloropropane (TCP) exceeded State 

standards in three public-supply wells each. Solvents, fumigants, trihalomethanes, and herbicides 

were prevalent (detected in more than 30 percent 

of samples) but gasoline components and insecticides were detected in few wells. Most water samples contained complex mixtures of organic 

compounds: multiple solvents, fumigants, or herbicides, and in some cases compounds from two or 

all three of these classes.

Characteristic suites of chemicals were associated with particular land uses and geographic 

locales. Solvents were associated with central 

Oahu urban-military lands whereas fumigants, 

herbicides, and fertilizer nutrients were associated 

with central Oahu agricultural lands. Somewhat 

unexpectedly, little contamination was detected in 

Honolulu where urban density is highest, most 

likely as a consequence of sound land-use planning, favorable aquifer structure, and less intensive application of chemicals (or of less mobile 

chemicals) over recharge zones in comparison to 

agricultural areas.

For the most part, organic and nutrient contamination appear to reflect decades-old releases 

and former land use. Most ground-water ages were 

decades old, with recharge dates ranging from pre1940 to the present, and with most dates falling 

within the 1950s to 1980s time span. Several 

widely detected compounds were discontinued as 

long ago as the 1970s but have yet to be flushed 

from the ground-water system. Although large 

tracts of land in central Oahu have been converted 

from agriculture to residential urban use since the 

1950s, water quality in the converted areas still 

more closely reflects the former agricultural land. 

It appears to be too early to detect a distinct waterquality signature characteristic of the newer urban 

use, although several urban turfgrass herbicides in 

use for just 10 years or so were detected in monitoring wells and may represent early arrivals of 

urban contaminants at the water table.

INTRODUCTION

The USGS National Water-Quality Assessment 

(NAWQA) is designed to assess the status of and 

trends in the quality of the Nation's water resources and 

to develop an understanding of the natural and human 

factors that affect water quality. A key element of the 

program is the application of standardized methods that 

allow valid comparisons of results from various 

NAWQA Study Units located around the United 

States.

In Hawaii, NAWQA efforts are represented by 

the Oahu Study Unit. Coordinated studies of streams 

Ground-Water Quality and its Relation to Land Use on 

Oahu, Hawaii, 2000–01

By Charles D. Hunt, Jr.
2 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

(surface water), stream ecology, and ground water on 

the island of Oahu were begun in 1997. The studies 

seek to describe the occurrence and distribution of 

inorganic elements, nutrients, organic compounds, 

radionuclides, and species abundance in Oahu's 

streams or ground water. Ground water is especially 

important on Oahu because it is the primary source of 

fresh drinking water. Even minute quantities of certain 

contaminants can render ground water unsuitable for 

drinking and require costly water treatment for purification. 

A fundamental tenet of the NAWQA program is 

that chemical measures of water quality are commonly 

closely tied to surrounding land use, which largely 

governs the types and amounts of point and nonpoint 

chemical releases to the environment. Of particular 

local interest is an ongoing change in land use that is 

taking place in much of the Oahu Study Unit. It entails 

replacement of former large-scale sugarcane plantations that were stable through the 1950s with urban 

development and diversified-crop agriculture. Oahu 

NAWQA studies seek to discern the effect of these 

land-use changes on water quality and stream ecology.

Objectives of the Oahu NAWQA Ground-Water Study

Investigations by the various NAWQA Study 

Units are intended to contribute to the nationwide 

water-quality assessment as well as to provide waterquality information that is relevant to the local community in formulating land- and water-related policies. The Oahu ground-water study was designed to 

meet the following objectives:

(1) Describe the occurrence and distribution of 

inorganic elements and organic compounds in Oahu 

ground water.

(2) Evaluate relationships between ground-water 

quality and land use.

(3) Determine whether land-use changes have 

caused changes in ground-water quality.

(4) Describe how the chemical composition of 

ground water is influenced by flow-system factors 

such as depth below the water table, proximity to 

recharge areas, and presence or absence of confining 

units.

A principal objective of the NAWQA program 

is to assess the quality of drinking-water resources. 

This role complements Federal and State programs 

that administer drinking-water regulations and testing 

at the tap but does not duplicate those efforts. 

NAWQA ground-water studies sample ambient 

ground water within the aquifer, either directly from 

the open interval of a well using a downhole sampling 

pump or from the well head in the case of a highcapacity supply well. Samples are collected prior to 

any water treatment or conditioning that may be 

applied to make the water suitable for distribution and 

consumption. Although the aquifer may provide 

source water for drinking, NAWQA samples are not 

identical to samples of “finished” water taken at the 

tap for drinking-water compliance monitoring.

NAWQA ground-water studies have ecological 

relevance where ground water discharges to streams, 

lakes, or marine waters. Ground water can carry 

excess nutrients that promote nuisance plant or algal 

growth (eutrophication), and can carry other chemical 

loads that may affect the health of stream biota. 

Ground water also may have beneficial effects, for 

example by supplying a base component of flow that 

keeps a stream from going dry or by ensuring a 

steadier stream temperature.

Results of NAWQA studies can be used by policy makers to evaluate whether existing zoning, landuse, chemical-handling, and crop-management practices adequately protect ground water and surface 

water. An important limitation is that most NAWQA 

studies are “occurrence and distribution” studies, and 

results can be used to make only the most general 

inferences regarding specific sources and pathways of 

chemicals unless these aspects were specific elements 

of a particular study. Many chemicals have multiple 

uses, and there may be several potential sources of a 

given chemical within a Study Unit.

Study Area and Land Use

The study area for Oahu NAWQA ground-water 

investigations encompasses 384 mi2 of central Oahu 

between the Waianae and Koolau mountain ranges 

(figs. 1 and 2). The study area coincides with the central Oahu ground-water flow system as defined in Hunt 

(1996). It includes the bulk of ground-water development on Oahu, having accounted for 93 percent of 

Oahu pumpage in the decade 1971–80, or 336 Mgal/d 

(Nichols and others, 1996).
0 5 10 Miles

0 5 10 Kilometers

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

BOUNDARY OF GEOMORPHIC REGION

KOOLAU

RANGE

SCHOFIELD

 PLATEAU

WAIANAE

 RANGE

COASTAL PLAIN

COASTAL PLAIN H o n o l u l u

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

Introduction 3

Land use was mapped as part of the Oahu 

NAWQA study (Klasner and Mikami, 2003) and portrays conditions in about 1998. Only the broadest categories of land use are shown here (fig. 1): urban, 

agriculture, and “other” (which is mostly forested and 

referred to simply as “forested”). Superimposed on the 

present land-use classification are several additional use 

categories that may have affected water quality 

(fig. 2): 

1. United States military installations (fig. 2A). 

2. former pineapple fields (fig. 2B); and

3. present and former sugarcane fields (fig. 2C).

Figure 1. Geomorphic provinces and land use, Oahu, Hawaii (modified from Visher and Mink, 1964; Klasner and Mikami, 2003).
158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

Schofield hofield

Barracks

Honolulu

Honolulu

Kaneohe

Kailua

Makakilo

Waianae

Wheeler AFB

Wahiawa

Mililani

Town

Schofield

Barracks

Waipahu

Kahuku

Haleiwa

Waialua

0 5 10 Miles

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EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

SELECTED MILITARY INSTALLATIONS

NAWQA GROUND-WATER STUDY AREA

PRESENT AND FORMER PINEAPPLE FIELDS

MAJOR URBAN AREAS

PUBLIC-SUPPLY WELL

dMONITORING WELL

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

4 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 2. NAWQA ground-water study area, land use, and sampled wells, Oahu, Hawaii: (A) military installations; (B) present and former pineapple 

fields; (C) former sugarcane fields; (D) land converted from agricultural to urban use, 1950–2000, and mean annual rainfall. Land use is from about 

1998 and is modified from Klasner and Mikami (2003). Pineapple and sugarcane fields are modified from Oki and Brasher (2003). Rainfall is from 

Giambelluca and others (1986).
C E N T R A L P L A T E A U

C E N T R A L P L A T E A U

C O A S T A L P L A I N C O A S T A L P L A I N

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158 ̊10'

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K O O L A U R A N G E 

W A I A N A E R A N G E W A I A N A E

R A N G

E

C E N T R A L P L A T E A U

C O A S T A L P L A I N

K

O

O L A

U

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G E

EXPLANATION

LAND CONVERTED FROM 

 AGRICULTURAL TO URBAN 

 USE FROM 1950 TO 1998

LINE OF EQUAL MEAN ANNUAL 

 RAINFALL, IN INCHES

SELECTED STREAMS

PREVAILING TRADE WINDS

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

FORMER SUGARCANE FIELDS

NAWQA GROUND-WATER 

 STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Introduction 5

Figure 2. NAWQA ground-water study area, land use, and sampled wells, Oahu, Hawaii: (A) military installations; (B) present and former pineapple 

fields; (C) former sugarcane fields; (D) land converted from agricultural to urban use, 1950–2000, and mean annual rainfall. Land use is from about 

1998 and is modified from Klasner and Mikami (2003). Pineapple and sugarcane fields are modified from Oki and Brasher (2003). Rainfall is from 

Giambelluca and others (1986).--Continued
6 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

The pineapple and sugarcane boundaries encompass areas used for those purposes at any time during 

the period 1940–2000 (modified from Oki and Brasher, 

2003). 

The southeastern part of the study area is occupied by the city of Honolulu. The western part of the 

study area includes large tracts of agricultural land that 

cover much of central Oahu. Urbanization has increasingly expanded from the Honolulu urban core to 

occupy former agricultural lands in central Oahu. The 

largest change has been in the southern part of central 

Oahu (fig. 2D), where suburban residential development has supplanted former agricultural land, mostly 

since the 1960s. In contrast, northern Oahu has 

remained largely in agriculture. Conversion to urban 

use accelerated in the 1980s as sugarcane cultivation 

was reduced and ultimately abandoned in the mid1990s. Former sugarcane fields not converted to urban 

use are presently used for pineapple, diversified-crop 

agriculture, or are fallow. Pineapple is still cultivated 

widely on Oahu, although some former pineapple land 

has been converted to urban use. Areas of military 

installations have been mostly stable since the mid1900s World War II era.

Chemical Use and Application

Past use of agricultural and industrial chemicals 

in the study area has provided opportunities for subsequent migration of chemicals to ground water. Some 

chemicals were released as nonpoint-source agricultural applications of fertilizers, soil fumigants, and pesticides. Other chemicals such as fuels and solvents 

likely were released as point-source spills or leaks. 

Recent decades have brought much greater awareness 

of the need for proper handling and disposal, whereas 

in the mid-1900s it was a more common practice to dispose fuels, solvents, and other chemicals on the ground 

or into drains and sumps that leached to the ground. 

The use and application of pesticides on Oahu has been 

summarized as part of the Oahu NAWQA study by Oki 

and Brasher (2003).

Fuels and solvents have been used intensively by 

the United States military dating back to at least the 

1940s. Solvent compounds also are in many household 

and industrial products in the civilian urban environment, for example degreasers, paint thinners, and paint 

strippers. Gas stations, automotive repair shops, and 

dry cleaners—all sources of fuels or solvents—are 

scattered throughout the urban environment. Petroleum 

refining has been restricted to the southwest coast of 

Oahu but several large fuel-storage facilities over 

inland recharge areas have been operated by the military in the mid-1900s.

Fertilizers and pesticides have been applied 

widely for agriculture in central Oahu, with fertilizer 

use dating back to 1900 or earlier and pesticide use dating back to the 1950s. A study of pesticide use for 1977 

(Takahashi, 1982) cited the following Statewide totals: 

4,200,000 kg of pesticides, consisting of:

2,000,000 kg of nematicide fumigants 

(48 percent); 

1,300,000 kg of herbicides (32 percent);

238,000 kg of insecticides (6 percent);

119,000 kg of fungicides (3 percent); and

449,000 kg of wood preservatives (11 percent).

Agricultural use accounted for the largest share 

of 1977 pesticide use: 82 percent of pesticides overall, 

100 percent of nematicide fumigants, 92 percent of herbicides, and 32 percent of insecticides. Nematicide 

fumigants (the largest pesticide class) were applied in 

pineapple cultivation since the 1940s to control nematode rootworms. Herbicides have been applied variously, but most heavily on sugarcane (72 percent 

sugarcane, 15 percent diversified agriculture, 7 percent 

government, 5 percent pineapple, and 1 percent home 

and garden; all 1977 figures from Takahashi, 1982). 

Herbicide use on Hawaiian sugarcane and pineapple 

crops has been particularly intensive, with annual 

application rates as much as five times those of major 

temperate-region field crops (Green and others, 1977). 

In addition to nonpoint application of agricultural 

chemicals, point-source releases may also have 

occurred at pesticide mixing and storage sites or at disposal sites. 

Urban insecticide application in 1977 was 68 percent greater than that for agriculture. Most insecticides 

applied were termiticides that are hydrophobic and 

have limited leaching potential. Urban herbicide use 

(weed and brush control by Federal and local governments, utilities, and commercial and residential property owners) tends to rely more on post-emergence 

herbicides having much less leaching potential than the 

pre-emergence herbicides used for agriculture. Fertilizers have been applied in the urban environment to 
Introduction 7

maintain lawns and gardens, although treated acreage 

is less than in agricultural areas because large percentages of urban land are taken up by buildings and paved 

surfaces.

Previous Water-Quality Studies

Water-quality studies on Oahu progressed 

through several topical themes during the 20th century. 

Early studies dating back to the early 1900s focused on 

the problem of saltwater intrusion. These studies 

reported on salt content and other inorganic chemical 

elements, and were conducted mainly by the Honolulu 

Board of Water Supply (Wentworth, 1951 provides a 

comprehensive summary). The potential for introduced 

contaminants from agricultural and industrial applications became a focus of study in the mid-1900s. Notable reports that touched on this topic in addition to 

saltwater intrusion include those by Visher and Mink 

(1964), Swain (1973), and Takasaki (1977).

Agricultural effects on water quality were studied 

increasingly in the late 1900s. The topic of irrigationreturn recharge received much attention, with most 

studies focusing on nutrients and inorganic elements 

(for example, Tenorio and others, 1969). The use of 

isotopic tracers for tracing and dating ground water 

was also introduced, culminating in a comprehensive 

report by Hufen and others (1980).

Agricultural fumigants were detected in Oahu 

ground water in 1980. Their presence and migration 

were investigated using soil sampling, laboratory analyses, and numerical simulation; notable summaries 

include Lau (1987) and Oki and Giambelluca (1987). 

The other main topic of study from about 1980 to the 

present involved fuels, solvents, and metals. These 

contaminants were investigated by the U.S. military 

and by owners of private facilities under the regulatory 

umbrella of the U.S. Environmental Protection Agency 

Resource Conservation and Recovery Act (USEPA/

RCRA). Results of these investigations are contained 

in numerous reports by environmental consultants and 

military agencies.

Geohydrologic Setting

The island of Oahu consists of two volcanic 

mountain ranges, the Waianae Range and the Koolau 

Range (fig. 1), an intervening saddle between the 

ranges called the Schofield Plateau, and lowland 

coastal plains underlain by sediments. The mountain 

ranges are the eroded remnants of two former shield 

volcanoes built by successive outpourings of basaltic 

lava flows. Subsequent weathering and erosion have 

left a landscape of deep valleys and steep interfluvial 

ridges, with gentler slopes in the central plateau, along 

the floors of the larger valleys, and on the coastal plain. 

Maximum elevations in the Waianae and Koolau 

mountains are 4,025 and 3,150 ft above sea level, 

respectively, and the top of the central plateau is at 

about 900 ft above sea level.

The target of sampling in this study was ground 

water in the volcanic-rock aquifers of central Oahu 

(fig. 3). The Waianae and Koolau aquifers consist of 

highly permeable, fractured and rubbly basaltic lavas 

of the Waianae Volcanics and the Koolau Basalt. The 

aquifers overlap in the western part of the study area 

(fig. 3B) to form a layered, multi-aquifer system with 

an intervening, leaky confining unit of clay-rich soil 

and saprolite (weathered rock) at the top of the 

Waianae Volcanics. Although most NAWQA studies 

restrict sampling to a single aquifer, both aquifers were 

included in this study because their lithologies are similar and both are sources of public drinking-water supplies.

Fresh ground-water flow through the volcanicrock aquifers is driven by topography and rainfall (fig. 

4 and fig. 2D). Rainfall is greatest in the mountains, 

with average annual maxima of 80 and 275 in/yr in the 

Waianae and Koolau Ranges, respectively (Giambelluca and others, 1986). Annual rainfall averages about 

60 in/yr in the central plateau and tapers to 30 in/yr or 

less in the coastal lowlands. Ground water flows 

inward from both mountain ranges to the central plateau and then north and south to the coast. This interior 

drainage of ground water from the Koolau and 

Waianae Ranges has been termed the central Oahu 

ground-water flow system (Hunt, 1996). The central 

flow system encompassed 68 percent of natural, predevelopment recharge on Oahu (521 Mgal/d; Nichols and 

others, 1996), most of it originating in the wetter 

Koolau Range.

Ground-water levels in the volcanic-rock aquifers 

(fig. 5) are smoothly continuous over large areas but 

show abrupt changes at identifiable subsurface barriers 

such as volcanic dikes and weathered valley fills 

(Visher and Mink, 1964; Dale and Takasaki, 1976; 

Hunt, 1996). Ground-water levels are about 280 ft
6,000

3,000

Sea

level

3,000

Pacific

Ocean Pacific

Ocean

Waianae Volcano Koolau Volcano

A A'

Modified from Stearns and Vaksvik, 1935; 

and Presley and others,1997

Feet

VERTICAL EXAGGERATION x 3

alluvium and

marine sediments

Waianae aquifer Koolau aquifer

alluvium

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EXPLANATION

MAJOR VOLCANIC-ROCK AQUIFERS, UNCONFINED

 Koolau aquifer

Waianae aquifer

SEDIMENT OVERLYING AND CONFINING MAJOR 

VOLCANIC-ROCK AQUIFER

A

A'

LINE OF SECTION

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

A A'

WAIANAE

KOOLAU

A

B

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

158 ̊

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8 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 3. Waianae and Koolau volcanic-rock aquifers, Oahu, Hawaii: (A) areal extent of aquifers within depths tapped by wells (modified from Hunt, 1996); 

(B) aquifers in cross section (modified from Stearns and Vaksvik, 1935; Oki and others, 1997; and Presley and others, 1997). Sampled wells are shown for 

reference.
TOPOG RAPHIC

DI VIDE

IANAE

WA

TOPOGRA HP IC

DI V DE I

Kaau rift zone

KOO

UAL

SOUTHEASTERN OAHU

GROUND-WATER

FLOW SYSTEM 

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SOUTHERN OAHU

GROUND-WATER

FLOW SYSTEM 

WESTERN OAHU

 GROUND-WATER

 FLOW SYSTEM 

NORTHERN OAHU GROUNDWATER FLOW SYSTEM EASTERN

 OAHU

 GROUND

 WATER

 FLOW

 SYSTEM 

EXPLANATION

CENTRAL OAHU GROUND-WATER FLOW SYSTEM

MAJOR GEOHYDROLOGIC BOUNDARY

SUBORDINATE GEOHYDROLOGIC BOUNDARY--Queried where uncertain

DISTRICT BOUNDARY

TOPOGRAPHIC DIVIDE

ARROW INDICATES GENERALIZED DIRECTION OF GROUND-WATER FLOW

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

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Introduction 9

Figure 4. Ground-water flow systems, Oahu, Hawaii (Hunt, 1996).
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Kaimuki

North Schofield barrier

Schofield

Kawailoa

Mok

Waialua

uleia

Waianae Rift Zone

Pearl Harbor

Ewa

Koolau

Rift

Zone

Kahuku

Moanalua

Wailupe-Hawaii Kai Waialae

Kalihi

Beretania

South Schofield barrier

CENTRAL OAHU GROUND-WATER FLOW SYSTEM

SUBORDINATE GROUND-WATER AREA

MAJOR GEOHYDROLOGIC BOUNDARY

SUBORDINATE GEOHYDROLOGIC BOUNDARY -- Queried where uncertain

WATER-LEVEL CONTOUR -- Shows altitude of water level in the mid-1950's. Contour interval, in feet, is variable. Datum is mean sea level

POINT OBSERVATION OF ALTITUDE OF WATER LEVEL IN THE MID-1950'S, IN FEET ABOVE MEAN SEA LEVEL

REPRESENTATIVE MID-1950'S WATER LEVEL WHERE WATER TABLE IS TOO FLAT TO BE CONTOURED, IN FEET ABOVE MEAN SEA LEVEL

Ewa

Ewa

EXPLANATION

24

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(165)

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Honolulu Area

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

158 ̊

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10 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 5. Ground-water areas and potentiometric surface in the principal volcanic-rock aquifers, Oahu, Hawaii (Hunt, 1996). Ground water flows from 

interior mountains to the coast, generally perpendicular to potentiometric contour lines (see figure 4 for generalized flow directions).
Study Design and Methods 11

above sea level in the central plateau and about 1,000 

and 1,600 ft in the Koolau and Waianae Ranges, 

respectively. Ground water flows from areas of higher 

water level to areas of lower water level (upgradient to 

downgradient) and ultimately to the coast. At any point 

in the aquifer, ground water flowing from upgradient 

recharge areas is joined at the water table by recharge 

from land directly above. Depth to ground water from 

land surface ranges from zero at low elevations to as 

much as 2,500 ft in the mountains, with depths 

throughout most of the central Oahu plateau ranging 

from 100 to 600 ft.

The volcanic-rock aquifers are unconfined 

throughout most of island’s interior, but are confined 

beneath coastal-plain sediments and larger, sedimentfilled valleys such as those in the Honolulu area (fig. 

3). Soil and saprolite mantle gently sloping parts of the 

landscape to depths that range from about 50 ft at the 

dry north and south ends of the central plateau to 300 ft 

or more in the rainy Koolau uplands (Hunt, 1996). The 

clay-rich saprolite is much less permeable than the 

unweathered basaltic lavas that form the deep aquifers. 

Recharging water infiltrates vertically through the soil 

and saprolite and then through (typically) several hundred to a few thousand feet of unsaturated, unweathered rock before reaching the deep water table. 

Unweathered rock is exposed in cliffs and steep valley 

walls, offering the opportunity to examine bed thickness and the distribution of fractures and other permeable elements in the lavas.

With regard to recharge processes and migration 

of surface contaminants, the Waianae and Koolau aquifers are best classified as “deep unconfined aquifers.” 

Despite the weathered overburden and water-table 

depths of hundreds of feet, the volcanic-rock aquifers 

are as vulnerable to contamination as many shallow 

surficial aquifers in other settings, which are generally 

avoided as sources for municipal drinking-water supplies.

STUDY DESIGN AND METHODS

Sampling networks and laboratory methods were 

selected to fulfill standardized requirements of the 

national NAWQA program as well as local data needs. 

In addition to laboratory methods required nationally, 

several additional pesticide methods were selected for 

local relevance, and a custom method was used to 

achieve lower detection limits for two soil fumigants of 

longstanding interest on Oahu.

Selection of Well Networks

Two networks of wells were selected to represent 

conditions in the Oahu regional aquifer system (fig. 6). 

The first was a group of 30 public-supply wells sampled from January to July 2000. The second was a 

group of 15 monitoring wells sampled the following 

year, from April to August 2001. Because of the 

expense of drilling deep wells in volcanic rock, no new 

wells were drilled for this study; existing wells were 

selected for sampling instead. Depth to water from land 

surface ranged from –3 ft (at an artesian well) to 939 ft, 

with half the wells having depths to water between 89 

and 591 ft.

The 30 public-supply wells comprise a “StudyUnit Survey” (Gilliom and others, 1995). Such studies 

employ a network of supply wells (household or 

public supply) to assess water quality in major aquifer 

systems within a study unit. Wells of a single well type 

(public supply) were selected at random from a list of 

141 candidate wells at 53 sites, using a computerized 

routine by Scott (1990). The wells have pumping 

capacities on the order of 300 to 1,000 gal/min. By 

sampling a subset of public-supply wells, the Oahu 

Study-Unit Survey accurately assesses the drinkingwater resource as it presently is used. However, 

because the wells are used to supply drinking water, 

they likely reflect a favorable or “clean” bias or they 

would be taken out of service. This favorable bias is a 

recognized disadvantage of studies that rely on publicsupply wells (Lapham and others, 1995). In this study, 

however, well selection was truly random, and several 

wells that were out of service because of contamination 

were reactivated for sampling—thus lessening the 

clean bias and better approaching the goal of an unbiased random sampling of aquifer water quality.

Another potential source of clean bias arises from 

well depth. Previous studies showed that ground water 

in central Oahu is distinctly stratified (Visher and 

Mink, 1964; Tenorio and others, 1969; Hufen and others, 1980; Eyre, 1983, 1987; Voss and Wood, 1994). 

Water following deep flowpaths in the aquifer originated as recharge in cool, forested uplands and is older, 

colder, and fresher than shallower water in the aquifer. 

Shallower water in downgradient parts of the aquifer 
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EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL AND MAP NUMBER

MONITORING WELL AND MAP NUMBER

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

158 ̊

158 ̊10'

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21 ̊30'

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12 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 6. Public-supply wells and monitoring wells sampled for the NAWQA study, Oahu, Hawaii, 2000–2001.
Study Design and Methods 13

contains young irrigation-return recharge with elevated 

concentrations of inorganic elements, nutrients, and 

organic compounds. Some public-supply wells have 

been protectively solid-cased through this irrigationreturn layer for about the first 75 to 150 ft below the 

water table and are open only below that. Because of 

this design, they draw deeper water or mixtures of deep 

and shallow water. Recognizing this possible depth 

bias, a supplemental network of 15 monitoring wells 

was selected as a “Special Study” aimed at sampling 

shallower, younger ground water that is closed off from 

some of the public-supply wells (most monitoring 

wells are open at, or just beneath, the water table). The 

monitoring wells supplement the public-supply wells 

to give a more complete picture of water-quality in the 

aquifer. They also provide an early warning for contaminants that have recently arrived at the water table 

and are on their way deeper into the flow system. 

Although some of the selected monitoring wells originally were installed to investigate point-source contamination, only upgradient or “far-field” wells were 

selected so that results reflect regional water quality 

and not that of the point sources. Unlike the random 

selection of the public-supply wells, the monitoring 

wells were selected arbitrarily, from a much smaller 

candidate list. 

Sampling and Analytical Methods

Wells were sampled according to standard waterquality protocols of the U.S. Geological Survey (Koterba and others, 1995; U.S. Geological Survey, 1997). 

These protocols entail “clean-environment” sampling 

achieved by collecting and preserving water samples 

within a portable collection chamber (a transparent 

polyethylene bag stretched over a plastic pipe frame 

that approximates a portable “glove-box”). Powderfree vinyl gloves were worn at all sensitive stages of 

sample and container handling, and gloves were 

changed several times in the sampling process as 

instructed by protocols. Sampling took place inside a 

mobile laboratory van, providing additional shelter 

from wind and dust. Water was delivered from the well 

head to the van by Teflon tubing so that water was not 

exposed to the atmosphere until inside the collection 

chamber. A second sampling stream was diverted from 

the well head to an independent sampling apparatus 

where trace-gas samples were collected under a nitrogen atmosphere, again without air contact. Prior to 

sampling, physical water-quality characteristics (temperature, pH, dissolved oxygen, specific conductance) 

were measured during a 30 to 60 minute well-purging 

period to ensure that a representative sample of aquifer 

water was being withdrawn from the well under stable 

geochemical conditions. Following sampling, all tubing and accessory equipment was cleaned according to 

protocols and placed in sealed plastic bags and containers in preparation for the next well.

Public-supply wells typically had high-capacities 

(300 to 1,200 gal/min); most had line-shaft turbine 

pumps, a few had submersible pumps, and a few were 

sampled under positive artesian pressure. Supply wells 

were sampled by connecting Teflon tubing to a gardenhose valve using a throw-away Teflon adapter. A few 

monitoring wells had low-capacity pumps (5 to 

25 gal/min) and were sampled in similar fashion. 

Monitoring wells without installed pumps were sampled using a portable downhole, positive-displacement 

sampling pump operating at a maximum pumping rate 

of about 2 gal/min. At all wells, the actual flow rate in 

the sample-collection chamber was restricted by valve 

to about 0.5 L/min.

Upon completion of sampling, ultrapure preservatives were added as necessary and samples were 

placed on ice and shipped via overnight courier to the 

USGS National Water Quality Laboratory (NWQL) in 

Denver, Colorado. Samples were analyzed at NWQL 

using certified, quality-assured methods. A limited 

number of field quality-assurance (QA) samples 

(blanks, replicates, and spiked replicates) also were 

collected and analyzed. The QA results indicated only 

one systematic problem, the presence of toluene at low 

concentrations in equipment blanks run on the portable 

downhole sampling pump. Based on these results, toluene detections for several monitoring wells sampled by 

that pump were disregarded (not considered to be true 

detections in the environmental sample).

Laboratory methods used to analyze groundwater samples are listed in table 1. The methods 

include the analysis of 283 organic compounds and 55 

inorganic constituents. Most analyses were conducted 

at the NWQL or at USGS laboratories in Reston, Virginia. Tritium and helium isotopes were analyzed at 

Lamont-Doherty Earth Observatory at Columbia University, New York, under contract with USGS NWQL.

Three methods were in provisional or custom status during the sampling and analysis period: two 
14 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

pesticide methods under development (9002 and 9060) 

and later approved as official NWQL methods 2002 

and 2060; and a custom method (8134) for low-level 

detection of the soil fumigants EDB and DBCP (1,2-

dibromoethane and 1,2-dibromo-3-chloropropane).

Method 9002/2002 analyzes moderate-use pesticides and degradates by gas chromatography/mass 

spectrometry (GC/MS) and is described in Sandstrom 

and others (2001). Method 9060/2060 analyzes polar 

pesticides and metabolites by high-performance liquid 

chromatography/mass spectrometry (HPLC/MS) and 

is described in Furlong and others (2001). Both methods were approved in June 2001. All drinking-well 

samples and the first one-third of monitoring-well 

samples were analyzed while the methods were still in 

provisional status.

Custom method 8134 for low-level EDB and 

DBCP uses gas chromatography/mass spectrometry 

(GC/MS) as described in Connor and others (1998) 

with the exception of operating the mass spectrometer 

in the selected ion monitoring (SIM) mode to increase 

sensitivity. Aside from the SIM modification, it is 

identical to method 2020 for volatile organic compounds (VOCs).

Several trace atmospheric gases were employed 

as water tracers to estimate the age of ground water. 

Chlorofluorocarbons (CFCs) are anthropogenic compounds that have been released to the atmosphere over 

decades as a result of their use as refrigerants, propellants, solvents, and foam-blowing agents. Sulfur 

hexafluoride (SF6) is manufactured for use in electrical components but also has natural sources in volcanic fluids and fluid inclusions in rocks (Busenberg and 

Plummer, 1997). Both dating methods work by comparing concentrations in a water sample with 

Table 1. Laboratory methods used to analyze ground-water samples, Oahu, Hawaii, 2000–2001

[Drinking-water wells were sampled January-July 2000; monitoring wells were sampled April-August 2001; method numbers are those of the U.S. Geological 

Survey National Water Quality Laboratory (NWQL); Reston lab, analyzed at USGS laboratory in Reston, Virginia; EDB, 1,2-Dibromoethane; DBCP, 1,2-Dibromo-3-

chloropropane; CFC, chlorofluorocarbon; SF6, sulfur hexafluoride; methods 9002 and 9060 were in provisional status until June 2001, when they were officially 

approved; “permanent” gases are nitrogen, argon, oxygen, carbon dioxide, and methane]

Category

USGS

method

Drinking-water

wells

Monitoring 

wells Remarks

INORGANICS

Major ions 2750 Yes Yes --

Nutrients 2752 Yes Yes --

Trace elements 2710 Yes Yes --

ORGANICS

Volatile organic compounds (VOCs) 2020 Yes Yes --

Dissolved organic carbon (DOC) 2612 Yes Yes --

EDB/DBCP, custom low-level 8134 Yes Yes Custom

Pesticides:

Pesticides 2001 Yes Yes --

Moderate use pesticides and degradates 9002/2002 Yes Yes Provisional

Polar pesticides and metabolites 9060/2060 Yes Yes Provisional

Organochlorine pesticides 1324 Yes No --

RADIONUCLIDES

Radium-226, -228 1262 Yes No --

Radon-222 1369 Yes No --

ENVIRONMENTAL TRACERS AND DATING

CFC dating, deuterium/oxygen isotopes 1173 Yes Yes --

SF6 dating Reston lab No Yes --

Dissolved permanent gases Reston lab Yes Yes --

Helium isotopes for tritium/helium dating 1033 Yes Yes --

Tritium isotope for tritium/helium dating 2112 Yes Yes --

Unsaturated-zone air:

CFC dating Reston lab No Yes --

SF6 dating Reston lab No Yes --

Permanent gases in air Reston lab No Yes --
Study Design and Methods 15

atmospheric concentration histories after accounting 

for how much of the gases would have dissolved from 

air into water (Plummer and Busenberg, 1999; Busenberg and Plummer, 2000). The analytical methods are 

extremely sensitive and measure trace concentrations 

several orders of magnitude lower than reporting levels for other organic-compound methods used in this 

study.

Results of Laboratory Analyses

Analytical results from Oahu NAWQA groundwater studies are presented in several forms:

1. Condensed summaries of results are included 

here to aid interpretation of the data (tables D1 and D2 

in the “Water-Quality Data” section at the end of this 

report).

2. Complete results for approved methods, 

including reporting levels for all constituents, are available on the internet from the USGS Hawaii District 

website at URL: http://hi.water.usgs.gov/nawqa/

gw.html

3. Complete results for provisional and custom 

methods are included here as the initial public release 

of these data (tables D3–D5 in the “Water-Quality 

Data” section at the end of this report). USGS policy 

requires that provisional data be publicly released in a 

published report that contains descriptions of the laboratory methods. Provisional data include results of pesticide methods 9002 and 9060, and custom method 

8134 for low-level detection of EDB and DBCP (1,2-

dibromoethane and 1,2-dibromo-3-chloropropane).

Tables D1 and D2 are condensed summaries of 

analytical results for the ground-water samples. These 

tables show only a subset of all results: only those 

organic compounds that were detected are listed, and 

non-detections are shown as blank entries instead of 

“less than” values (for example <.05, where the value 

is the minimum reporting level for the constituent). The 

“blank-entries” format has the advantage of making 

detections easily discernible in the tables.

Values preceded by the letter “E” are estimated 

values that are outside the calibration limits of the laboratory methods. Typically, these values are lower than 

the minimum reporting level of the method. In these 

cases the constituent is known to be present (confirmed 

by mass spectrometer, for example) but the exact concentration is known with less certainty than concentrations within the calibration range (Childress and others, 

1999). Similarly, table entries of “M” signify that the 

constituent is present but the concentration is so low 

that no quantification can be reported with confidence. 

In these cases, the estimated concentration would 

round to zero if rounded to the same precision as the 

reporting level; typically, the estimated concentration 

is lower than the reporting level by an order of magnitude or more.

Wells are listed sequentially in table D2, whereas 

in table D1 they are grouped by land use and geographic locale because these factors largely determine 

patterns of use and release of chemicals to the environment. Group selection was not wholly objective: measurements of present and former land use were 

combined with subjective consideration of upgradient 

land use and even of the results themselves. However, 

the groups do make clusters of related compounds evident, as well as associations of particular compounds 

with particular land uses. For example, solvent compounds are associated with urban-military lands in central Oahu, and fumigant and herbicide compounds are 

associated with agricultural land use.

Constituents are grouped into classes on the basis 

of chemical properties and predominant uses as defined 

by the NAWQA program at the national level. Organic 

compounds are divided into two main categories, volatile organic compounds (VOCs) and pesticides. Pesticides are subdivided into herbicides and insecticides. 

Subdivision of VOCs is somewhat problematic, 

however, because many of these compounds have multiple uses or are components in various end products. 

They can also arise as intermediates or byproducts during organic synthesis of other VOC compounds or pesticides. Actual use in a given study area can also 

complicate matters. For example, fumigants are classified as VOCs on the basis of their chemical composition. They are used in many locales to fumigate already 

harvested crops in warehouses, but on Oahu they are 

used fundamentally as pesticides, to kill rootworms via 

soil fumigation. Another class of VOCs, the trihalomethanes, are commonly viewed as disinfection byproducts because they are known to form when water is 

chlorinated. But they also are produced as solvents or 

during organic synthesis, and they have natural sources 

in soil and the marine environment. Regardless of the 

classification scheme, an important principle is that 

some organic compounds have multiple uses in the 
16 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

study area, and so the probable origin or source of 

detections in ground water may be unavoidably ambiguous.

The CFC (chlorofluorocarbon) and SF6 (sulfur 

hexafluoride) methods were used to estimate water age 

or recharge date. They also provided another useful 

piece of information in this study: the identification of 

“excess” CFCs or SF6 in some samples (tables D1 and 

D2). “Excess” in this context means “at concentrations 

greater than possible by dissolution from the atmosphere (air-water equilibrium).” Excess amounts of 

these trace gases are of interest in that they indicate 

CFC or SF6 contamination that may have originated 

with other VOC or pesticide contamination, or may 

have followed similar environmental pathways as other 

compounds.

FINDINGS AND INTERPRETATIONS

Most Wells Contained Low Concentrations of Organic 

Compounds

Organic compounds were detected in most water 

samples, although compound concentrations were generally low. Out of 283 organic compounds analyzed 

(85 VOCs, 197 pesticides, and caffeine) 43 compounds 

were detected (26 VOCs, 16 pesticides, and caffeine). 

Organic compounds were detected in 73 percent 

of public-supply wells and 100 percent of monitoring 

wells (table 2 and figs. 7 and 8). Supply-well detection 

rates for VOCs and pesticides were identical, at 63 percent. Detection rates for VOC subclasses in supply 

wells were: solvents and fumigants, both 40 percent; 

refrigerants, zero; excess CFCs and trihalomethanes, 

both 47 percent; and gasoline components, 13 percent. 

Supply-well detection rates for pesticide subclasses 

were 57 percent for herbicides and 13 percent for 

insecticides. Detection rates were higher in monitoring 

wells than in supply wells for all classes of organic 

compounds except gasoline components and insecticides (fig. 8).

Detection rates for refrigerants shown in figures 7 

and 8 are from VOC method 2020 (table 1). Refrigerants were actually detected in 97 percent of supply 

wells and 100 percent of monitoring wells (table 2) 

using method 1173, which estimates ground-water age 

by measuring CFCs that have dissolved from the atmosphere. These high detection rates are not shown in 

figures 7 and 8, however, because method 1173 is 

ultrasensitive and not readily comparable to the other 

methods reported here (its minimum reporting levels 

are several orders of magnitude lower). What is shown 

in figures 7 and 8 from method 1173 are detection rates 

of “excess CFCs.” Excess CFCs are refrigerant concentrations that exceed possible contributions from the 

atmosphere via air-water equilibrium and thereby indicate some source of CFC contamination.

Organic Compounds in Public-Supply Wells

0 10 20 30 40 50 60 70 80

Organic compounds

VOCs

Pesticides

VOCs and pesticides

Solvents

Fumigants

Refrigerants

Excess CFCs

Trihalomethanes

Gasoline components

Herbicides

Insecticides

PERCENTAGE OF WELLS WITH DETECTIONS

NONE DETECTED

Figure 7. Detection rates for organic compound classes in 30 

public-supply wells, Oahu, Hawaii, 2000–2001. Refrigerants were not 

detected using standard VOC method 2020 but were detected in all but 

one well using ultrasensitive method 1173 (CFC dating).

Organic Compounds in Public-Supply and Monitoring Wells

0 10 20 30 40 50 60 70 80 90 100

Organic compounds

VOCs

Pesticides

VOCs and pesticides

Solvents

Fumigants

Refrigerants

Excess CFCs

Trihalomethanes

Gasoline components

Herbicides

Insecticides

PERCENTAGE OF WELLS WITH DETECTIONS

SUPPLY

MONITOR

Figure 8. Comparison of detection rates for organic compound classes 

in 30 public-supply wells and 15 monitoring wells, Oahu, Hawaii, 

2000–2001. Refrigerants were not detected in public-supply wells 

using standard VOC method 2020 but were detected in one monitoring 

well.
Findings and Interpretations 17

Public 

supply

MonitoringPublic 

supply

MonitoringPublic 

supply 

Monitoring

Organic compounds

At least one organic compound detected 22 15 73 100 30 15

Two or more organic compounds detected 19 14 63 93 30 15

At least one VOC and one pesticide detected 16 12 53 80 30 15

Volatile organic compounds (VOCs)

At least one VOC detected 19 13 63 87 30 15

Two or more VOCs detected 16 12 53 80 30 15

Solvents, organic synthesis 12 9 40 60 30 15

Tetrachloromethane (Carbon tetrachloride) 5 6 17 40 30 15 2020 56-23-5

 Dichloromethane (Methylene chloride) 2 7 30 15 2020 75-09-2

Tetrachloroethene (PCE, Perchloroethene) 5 6 17 40 30 15 2020 127-18-4

 Trichloroethene (TCE) 9 8 30 53 30 15 2020 79-01-6

 cis-1,2-Dichloroethene 3 5 10 33 30 15 2020 156-59-2

 trans-1,2-Dichloroethene 3 3 10 20 30 15 2020 156-60-5

1,1,2-Trichloroethane (Vinyl trichloride) 3 2 10 13 30 15 2020 79-00-5

1,1,1,2-Tetrachloroethane (1,1,1,2-TCA) 3 1 10 7 30 15 2020 630-20-6

1,1-Dichloroethene (Vinylidene chloride) 1 3 30 15 2020 75-35-4

1,2-Dichloroethane (Ethylidene dichloride) 1 7 30 15 2020 107-06-2

Chlorobenzene (Monochlorobenzene) 1 3 30 15 2020 108-90-7

Fumigants 12 8 40 53 30 15

1,2-Dibromoethane (EDB, Ethylene dibromide) 4 13 30 15 8134 106-93-4

1,2-Dibromo-3-chloropropane (DBCP) 5 3 17 20 30 15 8134 96-12-8

1,2,3-Trichloropropane (TCP) 10 8 33 53 30 15 2020 96-18-4

1,2-Dichloropropane (DCP) 12 7 40 47 30 15 2020 78-87-5

1,1-Dichloropropene (1,1-DCPe) 1 3 30 15 2020 563-58-6

Refrigerants (chlorofluorocarbons) 29 15 97 100 30 15 1173

Excess CFCs (CFC-11,-12,-113) 14 10 47 67 30 15 1173

Trichlorofluoromethane (CFC-11) 1 7 30 15 2020 75-69-4

1,1,2-Trichloro-1,2,2-trifluoroethane (CFC-113) 1 7 30 15 2020 76-13-1

Trihalomethanes 14 11 47 73 30 15

Tribromomethane (Bromoform) 1 3 30 15 2020 75-25-2

Dibromochloromethane 1 3 30 15 2020 124-48-1

Trichloromethane (Chloroform) 14 11 47 73 30 15 2020 67-66-3

Bromodichloromethane 21 77 30 15 2020 75-27-4

Gasoline components 4 13 30 15

1,2,4-Trimethylbenzene (Psudocumene) 3 10 30 15 2020 95-63-6

Ethyl tert-butyl ether (ETBE) 1 3 30 15 2020 637-92-3

Methylbenzene (Toluene) 1 3 30 15 2020 108-88-3

Pesticides

At least one pesticide detected 19 14 63 93 30 15

Two or more pesticides detected 11 11 37 73 30 15

Herbicides 17 14 57 93 30 15

Atrazine 5 6 17 40 30 15 2001,9060 1912-24-9

 Deethylatrazine 7 7 23 47 30 15 2001,9060 6190-65-4

 Deethyldeisopropylatrazine 2 1 7 7 29 15 9060 3397-62-4

 Deisopropylatrazine 1 3 29 15 2060 1007-28-9

 2-Hydroxyatrazine 2 2 7 13 29 15 9060 2163-68-0

Diuron 5 6 17 40 29 15 9060 330-54-1

 3(4-Chlorophenyl)-1-methyl urea 8 5 28 33 29 15 9060 5352-88-5

 3,4-Dichloroaniline 7 1 24 7 29 15 9002 95-76-1

Bromacil 12 7 41 47 29 15 9060 314-40-9

Hexazinone 2 4 7 27 29 15 9002 51235-04-2

2-Chloro-2,6-diethylacetanilide 1 7 29 15 9002 6967-29-9

Bentazon 1 7 29 15 9060 25057-89-0

Imazaquin 1 7 29 15 9060 81335-37-7

Metsulfuron methyl 1 7 29 15 9060 74223-64-6

Insecticides 4 13 30 15

Dieldrin 3 10 29 15 2001 60-57-1

p,p'-DDE 1 3 29 15 2001 72-55-9

Caffeine 4 27 29 15 9060 58-08-2

Number of compounds detected Public 

supply

Monitoring All wells

Organic compounds 36 30 42

VOCs 24 16 25

Pesticides 12 13 16

Table 2. Organic compound detections in untreated ground water from public-supply and monitoring wells, Oahu, Hawaii, 

January 2000 through August 2001

[Compounds are grouped by use, with breakdown products indented beneath parent compounds; Number of samples was one per well, but one

sample for each pesticide method was ruined in preparation and affects some percentages; Method, USGS laboratory method, bold where custom or

provisional; CAS, Chemical Abstract Service Number. Refrigerants were detected in all but one well using ultrasensitive method 1173; excess CFCs

(chlorofluorocarbons) indicate contamination in excess of air-water equilibrium; CFC-11 and CFC-113 were detected on VOC method 2020]

Compound class, compound, and synonym (in 

parentheses)

Number of wells 

with detections

Percentage of 

wells with 

detections Method CAS

Number of 

samples
18 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Few Concentrations Exceeded Drinking-Water 

Regulations

PUBLIC-SUPPLY WELLS (30)

CONCENTRATION, IN MICROGRAMS PER LITER

0.001 0.01 0.1 1 10 100

MONITOR WELLS (15)

0.001 0.01 0.1 1 10 100

Volatile Organic Compounds (VOCs) Pesticides

MAINLY URBAN USE

MAINLY AGRICULTURAL USE

SOLVENTS

FUMIGANTS

REFRIGERANTS

TRIHALOMETHANES

GASOLINE

COMPOUNDS

HERBICIDES

INSECTICIDES

CAFFEINE

USEPA

MCL=5

USEPA U.S. Environmental Protection Agency

Hawaii State of Hawaii drinking-water regulations

MCL Maximum Contaminant Level

Hawaii

MCL=0.04

Hawaii

MCL=0.8

Hawaii

MCL=0.04

Hawaii

MCL=0.8

USEPA

MCL=5

Tetrachloromethane (carbon tetrachloride)

Dichloromethane

Tetrachloroethene (PCE)

Trichloroethene (TCE)

cis-1,2-Dichloroethene

trans-1,2-Dichloroethene

1,1,2-Trichloroethane

1,1,1,2-Tetrachloroethane

1,1-Dichloroethene

1,2-Dichloroethane

Chlorobenzene

1,2-Dibromoethane (EDB)

1,2-Dibromo-3-chloropropane (DBCP)

1,2,3-Trichloropropane (TCP)

1,2-Dichloropropane (DCP)

1,1-Dichloropropene

Trichlorofluoromethane (CFC-11)

1,1,2-Trichloro-1,2,2-trifluoroethane (CFC-113)

Tribromomethane (bromoform)

Dibromochloromethane

Trichloromethane (chloroform)

Bromodichloromethane

1,2,4-Trimethylbenzene

Ethyl tert-butyl ether (ETBE)

Methylbenzene (toluene)

Atrazine

Deethylatrazine

Deethyldeisopropylatrazine

Deisopropylatrazine

2-Hydroxyatrazine

Diuron

3,4-Dichloroaniline

3(4-Chlorophenyl)-1-methyl urea

Bromacil

Hexazinone

2-Chloro-2,6-diethylacetanilide

Bentazon

Imazaquin

Metsulfuron methyl

Dieldrin

p,p'-DDE

Caffeine

Although various compounds and elements were 

detected, few concentrations exceeded drinking-water 

regulations (for those constituents for which regulations have been established). These are shown as 

shaded entries in tables D1 and D2. Concentrations of 

organic compounds for all public-supply and monitoring wells that were sampled for the study are shown in 

figure 9. Comparison with Federal and State DrinkingWater Regulations (table 3 and discussion below) 

focuses on public-supply wells that produce drinking 

water, although concentrations in a few monitoring 

wells also exceeded regulations.

Only 2 of 30 public-supply wells (7 percent) had 

concentrations exceeding USEPA Maximum Contaminant Levels (U.S. Environmental Protection Agency, 

2002). One central Oahu supply well (fig. 10) contained the solvent TCE (trichloroethene) at 20.4 μg/L, 

four times greater than the USEPA MCL of 5 μg/L. 

One Honolulu supply well (fig. 11) contained radon at 

397 pCi/L, slightly above the proposed USEPA MCL 

of 300 pCi/L. The proposed MCL for radon is based on 

inhalation risk rather than a risk from drinking. Radon222 is a radioactive gas that forms naturally by decay 

of radium-226; both are intermediate steps in the decay 

of uranium-238 to lead-206.

Figure 9. Concentrations of organic compounds in sampled public-supply and monitoring wells, Oahu, Hawaii, 2000–2001.
Minimum Median Maximum EPA

MCL

Hawaii 

MCL RSD5 HAL EPA MCL

Hawaii 

MCL

Volatile organic compounds (VOCs) Nsamples = 30

Solvents, organic synthesis

Tetrachloromethane (Carbon tetrachloride) 56-23-5 5 .06 E.04 <.04 .64 5

 Dichloromethane (Methylene chloride) 75-09-2 2 .38 E.008 <.008 E.01 5

Tetrachloroethene (PCE, Perchloroethene) 127-18-4 5 .10 E.01 <.01 .6 5

 Trichloroethene (TCE) 79-01-6 9 .038 E.01 <.01 20.4 5 1

 cis-1,2-Dichloroethene 156-59-2 3 .038 E.02 <.02 E.05 70

 trans-1,2-Dichloroethene 156-60-5 3 .032 E.01 <.01 E.04 100

1,1,2-Trichloroethane (Vinyl trichloride) 79-00-5 3 .06 E.02 <.02 .16 5

1,1,1,2-Tetrachloroethane (1,1,1,2-TCA) 630-20-6 3 .030 E.01 <.01 E.02 10

1,1-Dichloroethene (Vinylidene chloride) 75-35-4 1 .04 E.02 <.02 E.02 7

1,2-Dichloroethane (Ethylidene dichloride) 107-06-2 0 .13 <.13 <.13 <.13 5

Chlorobenzene (Monochlorobenzene) 108-90-7 1 .028 E.004 <.004 E.004 100

Fumigants

1,2-Dibromoethane (EDB, Ethylene dibromide) 106-93-4 4 .0037 .0065 <.0037 E.0128 .05 .04

1,2-Dibromo-3-chloropropane (DBCP) 96-12-8 5 .0077 .018 <.0077 .146 .2 .04 3

1,2,3-Trichloropropane (TCP) 96-18-4 10 .16 E.1 <.1 2.7 .8 40 3

1,2-Dichloropropane (DCP) 78-87-5 12 .068 E.01 <.01 .41 5

1,1-Dichloropropene (1,1-DCPe) 563-58-6 1 .026 E.03 <.03 E.03

Refrigerants (chlorofluorocarbons)

Trichlorofluoromethane (CFC-11) 75-69-4 0 .09 <.09 <.09 <.09 2000

1,1,2-Trichloro-1,2,2-trifluoroethane (CFC-113) 76-13-1 0 .06 <.06 <.06 <.06

Trihalomethanes

Tribromomethane (Bromoform) 75-25-2 1 .06 <.06 <.06 <.06 80

Dibromochloromethane 124-48-1 1 .18 E.1 <.1 E.1 80

Trichloromethane (Chloroform) 67-66-3 14 .052 E.01 <.01 .20 80

Bromodichloromethane 75-27-4 2 .048 E.01 <.01 E.02 80

Gasoline components

1,2,4-Trimethylbenzene (Psudocumene) 95-63-6 3 .056 E.02 <.02 .34

Ethyl tert-butyl ether (ETBE) 637-92-3 1 .054 E.01 <.01 E.01

Methylbenzene (Toluene) 108-88-3 1 .05 E.01 <.01 E.01 1000

Pesticides and caffeine Nsamples = 29

Atrazine 1912-24-9 5 .001 .005 <.001 .035 3

 Deethylatrazine 6190-65-4 7 .002 E.005 <.002 E.044

 Deethyldeisopropylatrazine 3397-62-4 2 .0599 E.01 <.01 E.02

 Deisopropylatrazine 1007-28-9 1 .0737 E.003 <.003 E.003

 2-Hydroxyatrazine 2163-68-0 2 .1927 E.005 <.005 E.006

Diuron 330-54-1 5 .0793 .010 <.01 .05 10

 3(4-Chlorophenyl)-1-methyl urea 5352-88-5 8 .0915 .01 <.01 .01

 3,4-Dichloroaniline 95-76-1 7 .005 E.002 <.002 E.006

Bromacil 314-40-9 12 .0807 E.003 <.003 E1.08 90

Hexazinone 51235-04-2 2 .008 .05 <.008 .08

2-Chloro-2,6-diethylacetanilide 6967-29-9 0 .008 <.008 <.008 <.008

Bentazon 25057-89-0 0 .0193 <.0193 <.0193 <.0193 200

Imazaquin 81335-37-7 0 .103 <.103 <.103 <.103

Metsulfuron methyl 74223-64-6 0 .1138 <.1138 <.1138 <.1138

Insecticides

Dieldrin 60-57-1 3 .001 E.003 <.001 .015 .02

p,p'-DDE 72-55-9 1 .006 E.001 <.001 E.001 1

Caffeine 58-08-2 0 .0805 <.0805 <.0805 <.0805

Radon Nsamples = 30

Radon (pCi/L) 14859-67-7 30 26 40 104 397 300 1

Table 3. Comparison of drinking-water regulations with organic-compound and radon concentrations in untreated ground water 

from public-supply wells, Oahu, Hawaii, January-July 2000

[Concentrations in micrograms per liter (μg/L), except radon in picoCuries per liter (pCi/L); concentrations exceeding drinking-water regulations

are bold and shaded; compounds are arranged by use, with breakdown products indented beneath parent compounds; CAS, Chemical

Abstract Service Number; MRL, Minimum Reporting Level; Median, concentration at the 50th percentile (where fewer than half of samples

were detections, this value is less than the MRL or the minimum detected concentration, whichever is smaller); EPA MCL, U.S. Environmental

Protection Agency Maximum Contaminant Level; Hawaii MCL, State of Hawaii MCL, only shown where different from EPA MCL; RSD5, RiskSpecific Dose at 10-5 cancer risk level for suspected carcinogens; HAL, Lifetime Health Advisory Level for non-carcinogens; Nsamples, number of

samples analyzed; E, estimated value]

Number 

of detections

Drinking-water regulations 

(μg/L)

Number of 

concentrations 

exceeding

Concentration range 

Compound class, compound, and synonym (in (μg/L)

parentheses) CAS MRL 

(μg/L)

Findings and Interpretations 19
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

SELECTED MILITARY INSTALLATIONS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

TRICHLOROETHENE (TCE) CONCENTRATION IN GROUND WATER,

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level = 0.038

 Low - less than 0.1

 Medium - 0.1 to 1

 High - 1 to 5

 Exceeds U.S. Environmental Protection Agency

 maximum contaminant level of 5

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

20 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 10. Distribution of trichloroethene (TCE), Oahu, Hawaii. One public supply well in central Oahu had a concentration exceeding the U.S. EPA 

Maximum Contaminant Level.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

RADON-222 CONCENTRATION IN GROUND WATER, 

IN PICOCURIES PER LITER

 Low - 40 to 100

 Medium - 100 to 200

 High - 200 to 300

 Exceeds U.S. Environmental Protection Agency

 proposed maximum contaminant level of 300

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Findings and Interpretations 21

Figure 11. Distribution of radon-222, Oahu, Hawaii. One public-supply well in Honolulu had a concentration exceeding the proposed U.S. EPA 

Maximum Contaminant Level.
22 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Four of 30 public-supply wells (13 percent) 

exceeded State MCLs for fumigants, which are lower 

than USEPA MCLs (State of Hawaii, 1999). State 

MCLs were exceeded in three central Oahu supply 

wells each for 1,2,3-trichloropropane (TCP; fig. 12) 

and 1,2-dibromo-3-chloropropane (DBCP; fig. 13). 

Two of these wells contained both compounds (table 

D1), and two more wells contained one compound 

each, for a total of four public-supply wells in which 

fumigants exceeded state MCLs. Maps for 1,2-dibromoethane (EDB) and 1,2-dichloropropane (DCP) 

are included here because of longstanding local interest 

in fumigant contamination in central Oahu (figs. 14 and 

15).

Two public-supply wells had inorganic concentrations (table D2) that exceeded USEPA Secondary 

Drinking Water Regulations, which are non-enforceable guidelines concerned mainly with aesthetic qualities such as taste and odor of the water. One well had a 

chloride concentration of 263 mg/L, slightly above the 

maximum guideline of 250 mg/L, and one well had pH 

lower than the minimum guideline of 6.5.

Constituents of prominent human-health concern 

nationally appear to pose little problem on Oahu. 

Nitrate was below drinking-water limits at all wells, 

with the maximum nitrate concentration of 5.24 mg/L 

as N (table D2) being about half the USEPA drinkingwater MCL of 10 mg/L. All arsenic concentrations 

were less than the 0.9 μg/L minimum reporting limit 

and therefore well below the USEPA drinking-water 

MCL of 10 μg/L. The gasoline oxygenate MTBE 

(methyl tert-butyl ether), which has caused much concern in some parts of the continental United States, is 

not currently used as a gasoline additive in Hawaii and 

was not detected by Oahu NAWQA sampling.

Detection Rates Were Highest in the Nation for 

Fumigants and Among the Highest for Solvents, VOCs, 

and Insecticides

Out of over 80 NAWQA Study-Unit Surveys 

nationwide that sampled for VOCs, Oahu ranked first 

in fumigant detections, third in solvent detections, and 

fourth in overall VOC detections. These rankings used 

data screened to report detections only above a common assessment level of 0.2 μg/L, so that results from 

early NAWQA study units with higher reporting limits 

could be included in the comparison. The screened 

detection rates in Oahu public-supply wells were 30, 

20, and 47 percent for fumigants, solvents, and VOCs, 

compared to 40, 40, and 63 percent for the raw, 

unscreened detection rates.

Herbicide detections in Oahu public-supply wells 

ranked 51st and insecticide detections 12th out of over 

90 Study-Unit Surveys that sampled for pesticides. 

These rankings were based on unscreened data. Notably, the pesticide rankings do not reflect two supplemental pesticide methods used in many 1997 Study 

Units but not in earlier studies begun in 1991 and 1994. 

Over 75 percent of Oahu herbicide detections were 

from these supplemental methods (methods 9002 and 

9060, table D1) and are not factored into the national 

herbicide rankings.

Commonly Detected Solvents, Fumigants, and 

Herbicides Have Histories of Intensive Use

Organic compounds detected most commonly 

and in highest concentrations have been in use on Oahu 

from the mid-1900s or earlier. Trichloromethane (chloroform) was the most prevalently detected VOC, with a 

detection rate of 47 percent in public-supply wells 

(table 2 and fig. 16). High concentrations of chloroform occurred together with other solvents and appear 

to have originated primarily from solvent use or breakdown of solvent compounds. Lower concentrations of 

chloroform may have originated as impurities or breakdown of other VOCs or pesticides, or as a disinfection 

byproduct of water chlorination.

Other solvents commonly detected were trichloroethene (TCE), tetrachloroethene (PCE), and tetrachloromethane (carbon tetrachloride), which were 

detected in 30, 17, and 17 percent of supply wells, 

respectively (fig. 16). These are among the leading solvents used historically in the United States for cleaning, degreasing, and textile dry-cleaning. Solvents were 

used intensively at military installations in central 

Oahu dating back to at least the 1940s, and various 

potential sources of solvents have been investigated, 

including aircraft and automotive maintenance and 

engine rebuilding shops, drum-disposal and burn areas, 

and laundries (Harding Lawson Associates, 1995). 

However, specific source sites responsible for solvent 

contamination of public-supply wells in central Oahu 

were not found in more than a decade of investigations 

(U.S. Environmental Protection Agency, 2000). Potential civilian-sector sources of solvents in central Oahu 

include commercial automotive repair and painting 

shops, dry cleaners, and industrial or automotive
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

PRESENT AND FORMER PINEAPPLE FIELDS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

1, 2, 3-TRICHLOROPROPANE (TCP) CONCENTRATION IN GROUND WATER, 

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level = 0.16

 Low - less than 0.1 

 Medium - 0.1 to 0.8

 Exceeds State of Hawaii maximum contaminant level of 0.8

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Findings and Interpretations 23

Figure 12. Distribution of 1,2,3-trichloropropane (TCP), Oahu, Hawaii. Three public-supply wells in central Oahu had concentrations exceeding the State 

of Hawaii Maximum Contaminant Level.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

PRESENT AND FORMER PINEAPPLE FIELDS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

1, 2-DIBROMO-3-CHLOROPROPANE (DBCP) CONCENTRATION IN GROUND WATER, 

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level = 0.0077

 Low - less than 0.04

 Exceeds State of Hawaii maximum contaminant level of 0.04

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

24 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 13. Distribution of 1,2-dibromo-3-chloropropane (DBCP), Oahu, Hawaii. Three public-supply wells in central Oahu had concentrations

exceeding the State of Hawaii Maximum Contaminant Level.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

PRESENT AND FORMER PINEAPPLE FIELDS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

1, 2-DIBROMOETHANE (EDB) CONCENTRATION IN GROUND WATER, 

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level = 0.0037

 Low - less than 0.04

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Findings and Interpretations 25

Figure 14. Distribution of 1,2-dibromoethane (EDB), Oahu, Hawaii. EDB was only detected in the southern part of central Oahu.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

PRESENT AND FORMER PINEAPPLE FIELDS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

1,2-DICHLOROPROPANE (DCP) CONCENTRATION IN GROUND WATER, 

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level = 0.068

 Low - less than 0.1

 Medium - 0.1 to 1

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

26 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 15. Distribution of 1,2-dichloropropane (DCP), Oahu, Hawaii. Concentrations were greatest in central Oahu, in a pattern similar to that of 1,2,3-

trichloropropane (TCP).
Volatile Organic Compounds in 30 Public-Supply and 15 Monitoring Wells

0 10 20 30 40 50 60 70 80

Trichloromethane (Chloroform)

Excess CFCs

1,2-Dichloropropane (DCP)

1,2,3-Trichloropropane (TCP)

Trichloroethene (TCE)

Tetrachloromethane (Carbon tetrachloride)

Tetrachloroethene (PCE, Perchloroethene)

1,2-Dibromo-3-chloropropane (DBCP)

1,2-Dibromoethane (EDB, Ethylene dibromide)

cis-1,2-Dichloroethene

trans-1,2-Dichloroethene

1,1,2-Trichloroethane (Vinyl trichloride)

1,1,1,2-Tetrachloroethane (1,1,1,2-TCA)

1,2,4-Trimethylbenzene (Pseudocumene)

Bromodichloromethane

Dichloromethane (Methylene chloride)

1,1-Dichloroethene (Vinylidene chloride)

Chlorobenzene (Monochlorobenzene)

1,1-Dichloropropene (1,1-DCPe)

Tribromomethane (Bromoform)

Dibromochloromethane

Methylbenzene (Toluene)

Ethyl tert-butyl ether (ETBE)

1,2-Dichloroethane (Ethylidene dichloride)

Trichlorofluoromethane (CFC-11)

1,1,2-Trichloro-1,2,2-trifluoroethane (CFC-113)

PERCENTAGE OF WELLS WITH DETECTIONS

SUPPLY

MONITOR

Findings and Interpretations 27

maintenance incidental to mechanized agriculture 

in the area.

The VOC fumigant compounds 1,2-dichloropropane (DCP) and 1,2,3-trichloropropane (TCP) were 

detected in 40 and 33 percent of supply wells, respectively (fig. 16). Detected less frequently were 1,2-

dibromo-3-chloropropane (DBCP) and 1,2-dibromoethane (EDB), in 17 and 13 percent of supply wells. 

Several fumigant formulations used on Oahu pineapple 

fields contained DCP, including the fumigant DD (a 

dichloropropane-dichloropropene mixture that was 

used from 1943 to 1977), Telone, and Telone II. The 

DD formulation also contained TCP as an impurity of 

as much as 7 percent by weight (Oki and Giambelluca, 

1987; Orr and Lau, 1987). EDB was used on Oahu 

from 1948 to 1983, whereas DBCP was used from 

1959 to 1977. The greater number of detections and 

higher concentrations of DCP and TCP (in comparison 

to EDB and DBCP) may reflect greater persistence 

(lesser tendency to volatilize or break down; Oki and 

Giambelluca, 1987), the earlier introduction of DD 

fumigant (J. Mink, personal commun., 1985, as cited in 

Oki and Giambelluca, 1987), or the fact that DD was 

applied at rates 4 to 20 times greater per acre than EDB 

and DBCP, respectively (Yim and Dugan, 1975).

Fourteen herbicides or herbicide breakdown 

products were detected (table 2 and fig. 17). Bromacil 

was the most prevalent, detected in 41 percent of 

public-supply wells, followed by diuron and atrazine at 

17 percent, and hexazinone at 7 percent. All four compounds have been used in pineapple cultivation and all 

but bromacil were used for sugarcane. Much greater 

amounts of herbicide have been applied to sugarcane 

than to pineapple, and diuron and atrazine have been 

used in greater amounts than bromacil or hexazinone 

(State of Hawaii, 1969; Pacific Biomedical Research 

Center, 1975; Takahashi, 1982). Dates of introduction 

on Oahu are not available but diuron was probably 

applied first, after becoming available in 1954, followed by atrazine in 1958, bromacil in 1962, and 

hexazinone in 1976 (Oki and Brasher, 2003).

Herbicide breakdown products were detected, 

several of them more commonly than their parent compounds. Two diuron breakdown products were 

detected: 3(4-chlorophenyl)-1-methyl urea and 3,4-

dichloroanaline, in 28 and 24 percent of supply wells, 

both more than the parent compound. Four atrazine 

breakdown products were detected, the most common 

being deethylatrazine in 23 percent of supply wells 

(again, more than the parent compound).

Figure 16. Detection rates for volatile organic compounds (VOCs), Oahu, Hawaii.
28 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Gasoline Components and Insecticides Were Detected 

in Few Wells

Gasoline components were unexpectedly scarce 

(table 2 and figs. 7 and 8), given the heavily urbanized 

character of much of the study area. Methylbenzene 

(toluene) and ethyl-tert-butyl-ether (ETBE, a gasoline 

oxygenate) were detected in one Honolulu publicsupply well (table D1 and fig. 16), and three supply 

wells contained 1,2,4-trimethylbenzene (one in Honolulu and two in central Oahu). The presence of both toluene and ETBE together at the one well is strongly 

suggestive of gasoline contamination. But 1,2,4-trimethylbenzene is also common in solvents and paint 

thinners so its presence alone, without additional gasoline components, cannot be attributed unequivocally to 

gasoline. Similarly, the presence of 1,2-dibromoethane 

(EDB) in four central Oahu supply wells is not strongly 

suggestive of gasoline contamination without the presence of other gasoline compounds. In addition to its use 

as a nematicide fumigant in central Oahu, EDB also 

was used historically in leaded gasoline, as a component of the anti-knock additive tetraethyl lead. Some 

investigators have reasoned that a possible source of 

EDB from known gasoline leaks in central Oahu cannot be ruled out (Oki and Giambelluca, 1987). The four 

supply wells in which EDB was detected in this study 

also contained 2 to 3 other fumigants but no additional 

gasoline components (table D1).

Only two insecticides were detected in ground 

water (table 2 and fig. 17). Dieldrin was detected in 

three public-supply wells in urban Honolulu (table D1) 

and p,p'-DDE was detected in one supply well in central Oahu. Dieldrin has been used on Oahu to control 

termites and also is a breakdown product of the termiticide aldrin (Brasher and Anthony, 2000). NAWQA 

ecology studies have detected dieldrin in fish tissue and 

in bed sediment from Oahu streams. The second compound, p,p'-DDE is a breakdown product of p,p'-DDT, 

which was used for insect control in pineapple cultivation on Oahu and probably for other purposes as well. 

DDT was widely used in the United States until it was 

banned in 1972, and is highly persistent in the environment. Despite three decades of non-use, DDT and its 

breakdown products were detected in bed sediment and 

fish tissue by Oahu NAWQA ecology studies (Brasher 

and Anthony, 2000).

Volatile Organic Compounds Were Higher in 

Concentration Than Pesticides

Pesticides in 30 Public-Supply and 15 Monitoring Wells

0 5 10 15 20 25 30 35 40 45 50

Bromacil

3(4-Chlorophenyl)-1-methyl urea

3,4-Dichloroaniline

Deethylatrazine

Atrazine

Diuron

Dieldrin

Hexazinone

2-Hydroxyatrazine

Deethyldeisopropylatrazine

Deisopropylatrazine

p,p'-DDE

Caffeine

2-Chloro-2,6-diethylacetanilide

Bentazon

Imazaquin

Metsulfuron methyl

PERCENTAGE OF WELLS WITH DETECTIONS

SUPPLY

MONITOR

Maximum VOC concentrations were more than 

an order of magnitude greater than maximum pesticide 

Figure 17. Detection rates for pesticide compounds, Oahu, Hawaii.
Findings and Interpretations 29

concentrations (fig. 9). Furthermore, whereas only one 

pesticide concentration (bromacil) was above 1 μg/L, 

twelve VOC concentrations exceeded that value: six 

from trichloroethene, two from tetrachloromethane, 

and four from 1,2,3-trichloropropane (table D1).

Several possible reasons could explain why VOC 

concentrations were higher than pesticide concentrations. Perhaps the simplest is application rate. Of the 

compounds applied for agriculture, about five times 

more fumigants than herbicides have been applied by 

weight (State of Hawaii, 1969; Pacific Biomedical 

Research Center, 1975; Takahashi, 1982). Another reason may be that some VOC contamination originated 

from concentrated point-source spills or disposal. This 

mode of release is likely for solvents, which had the 

highest concentrations of any compounds. In contrast, 

agricultural chemicals were released widely in nonpoint-source applications that exposed them to dilution 

and degradation over vastly greater surface areas. 

Finally, some of the VOC compounds may be more 

resistant to degradation than pesticides and hence more 

persistent in the environment.

Land Use, Locale, and Aquifer Vulnerability are Main 

Factors in Contamination

Organic compounds and nutrients were detected 

with greater frequency and at higher concentrations in 

central Oahu than in urban Honolulu or near upland 

forests. Furthermore, characteristic suites of chemicals 

were associated with particular land uses and locales 

(table D1).

1. Solvents were mainly associated with urban 

and military lands in central Oahu (fig. 18).

2. Fumigants and herbicides were associated with 

agricultural lands in central Oahu, including former 

agricultural lands that have been converted to residential urban use (figs. 19 and 20).

3. Insecticides (fig. 21) were detected in urban 

Honolulu and in one well in central Oahu.

4. Nutrient concentrations were highest beneath 

agricultural lands in central Oahu, following much the 

same pattern as fumigants and herbicides (figs. 22 and 

23).

These associations were tested statistically (table 

4) and are shown in figures 24 and 25. Tests were performed for differences in median values between pairs 

of well groups defined in table D1 (1=forest; 2=urban 

Honolulu; 3=urban-military central Oahu; 4=agricultural central Oahu). Regarding organic compounds 

(fig. 24), the median number of compounds was not 

significantly different at the 95-percent confidence 

level between forest and urban Honolulu well groups, 

but these two groups were significantly different from 

the urban-military and agricultural central Oahu groups 

(table 4). The two central Oahu groups did not differ by 

median number of compounds, but when compounds 

were divided into VOCs and pesticides (fig. 25), urban 

and agricultural central Oahu groups differed statistically from each other and from the forest and urban 

Honolulu groups as well. In these tests, fumigants were 

excluded from the VOC class and included with pesticides because the fumigants are applied agriculturally. 

Median nutrient concentrations also showed important 

differences between paired groups, and urban Honolulu 

differed from central Oahu in tritium concentration 

(table 4).

The strong correspondence between water quality 

and land use is an expected outcome and a main focus 

of the NAWQA program. Land use is a principal determinant of chemical release to the environment, whether 

by nonpoint-source agricultural application of fumigants, herbicides, and fertilizers; by nonpoint leaching 

of urban termiticides; or by point-source releases of 

solvents or fuels. Also unsurprising is that wells in and 

near the upland forest contained few organic compounds (table D1), given that there is little opportunity 

for chemical release to the environment in these areas.

Besides land use, aquifer vulnerability and rapid 

ground-water flow rates also play a role in contamination on Oahu. Most of the central Oahu area where 

organic compounds are prevalent is a recharge zone, 

with no confining units between land surface and the 

deep unconfined aquifer. Much of the earth material in 

the intervening unsaturated zone is highly permeable 

and this makes the deep aquifer vulnerable to contamination, even where depth to the water table is as much 

as 500 to 1,000 ft. Apparent recharge dates from CFCs 

and SF6 were “young” for almost all samples, with 

only one pre-1940 date out of 45 wells (table D1). The 

young apparent ages and the prevalence of organic 

compounds highlight the vulnerability of Hawaiian 

unconfined basalt aquifers to contamination: water 

recharges from land surface to the deep water table on 

a timescale of a few decades or less and carries anthropogenic chemicals with it.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

SELECTED MILITARY INSTALLATIONS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

SOLVENT CONCENTRATION IN GROUND WATER, 

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level varied by compound

 Low - less than 0.1

 Medium - 0.1 to 1

 High - greater than 1

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

30 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 18. Distribution of solvents, Oahu, Hawaii. Concentrations were highest in and downgradient from urban-military lands in central Oahu.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

PRESENT AND FORMER PINEAPPLE FIELDS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

FUMIGANT CONCENTRATION IN GROUND WATER, 

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level varied by compound

 Low - less than 0.1

 Medium - 0.1 to 1

 High - greater than 1

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Findings and Interpretations 31

Figure 19. Distribution of fumigants, Oahu, Hawaii. Concentrations were highest in and downgradient from present and former pineapple fields in 

central Oahu.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

PRESENT AND FORMER PINEAPPLE FIELDS

FORMER SUGARCANE FIELDS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

HERBICIDE CONCENTRATION IN GROUND WATER, 

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level varied by compound

 Low - less than 0.1

 Medium - 0.1 to 1

 High - greater than 1

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

32 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 20. Distribution of herbicides, Oahu, Hawaii. Concentrations were highest in present and former agricultural lands in central Oahu.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

INSECTICIDE CONCENTRATION IN GROUND WATER, 

IN MICROGRAMS PER LITER

 Not detected - minimum reporting level varied by compound

 Low - less than 0.1

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Findings and Interpretations 33

Figure 21. Distribution of insecticides, Oahu, Hawaii. Insecticides were detected at three wells in Honolulu (dieldrin) and one well in central Oahu 

(p,p'-DDE). 
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

PRESENT AND FORMER PINEAPPLE FIELDS

FORMER SUGARCANE FIELDS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

NITRATE CONCENTRATION IN GROUND WATER, 

IN MILLIGRAMS PER LITER AS N

 Natural background - 0.1 to 1

 Low - 1 to 2

 Medium - 2 to 3

 High - 3 to 5.5

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

34 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 22. Distribution of nitrate, Oahu, Hawaii. Concentrations were highest in and downgradient from present and former agricultural lands in central 

Oahu.
0 5 10 Miles

0 5 10 Kilometers

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

PRESENT AND FORMER PINEAPPLE FIELDS

FORMER SUGARCANE FIELDS

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

ORTHOPHOSPHATE CONCENTRATION IN GROUND WATER, 

IN MILLIGRAMS PER LITER AS P

 Natural background - 0.02 to 0.1

 Low - 0.1 to 0.2

 Medium - 0.2 to 0.25

 High - 0.25 to 0.4

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Findings and Interpretations 35

Figure 23. Distribution of orthophosphate, Oahu, Hawaii. Concentrations were highest in and downgradient from present and former agricultural lands 

in central Oahu.
NUMBER OF ORGANIC COMPOUNDS

FOREST

URBAN HONOLULU

URBAN-MILITARY

CENTRAL OAHU

AGRICULTURAL

CENTRAL OAHU

0

2

4

6

8

10

12

14

16 5 8 10 22

22

o

EXPLANATION 

o

75th percentile

median

25th percentile

value greater than 90th

percentile

number of values

90th percentile

10th percentile

36 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 24. Number of organic compounds detected in wells 

grouped by land use and locale, Oahu, Hawaii. Numbers of 

compounds were greater in urban and agricultural central Oahu 

well groups than in forest and urban Honolulu well groups. 

In contrast to central Oahu, few organic compounds were detected in the deep aquifer beneath urban 

Honolulu despite the high urban density there. Several 

factors probably contribute to the lack of organic contamination. One factor is aquifer vulnerability: most of 

urban Honolulu lies downgradient from mountain 

recharge zones and is instead in the zone of aquifer discharge near the coast. Much of the city is underlain by 

sedimentary confining units that retard vertical recharge 

and contaminant transport and divert ground-water flow 

laterally to the shore through shallow sedimentary aquifers that are not used as drinking-water sources. Landuse planning is another factor: urban Honolulu lands 

that do lie over upland recharge zones are mainly residential, with little commercial, industrial, or military 

use. This reflects nearly a century of sound urban 

planning and watershed protection by State and County 

agencies that restricted facilities with high contamination potential to coastal areas and away from upland 

recharge zones. A final factor is that urban residential 

lands in the Honolulu uplands likely receive much less 

chemical loading than urban-military and agricultural 

lands in central Oahu, and typically less mobile chemicals would be used in urban residential areas. This is 

certainly true compared to fumigant and herbicide application over agricultural lands (Takahashi, 1982; Green

Number of 

organic 

compounds

Number of 

VOCs 

(excluding 

fumigants)

Number of 

pesticides and 

fumigants

Nitrate 

concentration

Orthophosphate 

concentration

Tritium 

concentration

Between Group 1-Forest and:

 Group 2-Urban Honolulu No No No No Yes No

 Group 3-Urban-military central Oahu Yes Yes Yes No Yes No

 Group 4-Agricultural central Oahu Yes No Yes Yes Yes No

Between Group 2-Urban Honolulu and:

 Group 3-Urban-military central Oahu Yes Yes Yes No * Yes Yes

 Group 4-Agricultural central Oahu Yes Yes Yes Yes Yes Yes

Between Group 3-Urban-military central Oahu and:

 Group 4-Agricultural central Oahu No Yes Yes Yes No No

P-values from Wilcoxon rank-sum test from which the above answers were determined: 

Between Group 1-Forest and:

 Group 2-Urban Honolulu 0.38 0.79 0.087 0.94 0.030 0.81

 Group 3-Urban-military central Oahu 0.0023 0.0046 0.011 0.099 0.0007 0.13

 Group 4-Agricultural central Oahu 0.0026 0.15 0.0011 0.0010 0.0030 0.38

Between Group 2--Urban Honolulu and:

 Group 3-Urban-military central Oahu 0.0004 0.0004 0.031 0.0505 * 0.0000 0.0044

 Group 4-Agricultural central Oahu 0.0004 0.046 0.0002 0.0001 0.0000 0.022

Between Group 3-Urban-military central Oahu and:

 Group 4-Agricultural central Oahu 0.92 0.0000 0.0009 0.0014 0.92 0.17

Chemical attributes

Well groups

[Two-sided Wilcoxon test is a non-parametric test for difference between two groups of data; P-value is the probability that the group medians are the

same, and values less than 0.05 (bold and shaded) indicate with greater than 95% confidence that the group medians do indeed differ; *, value is close

to 95% confidence level (within 2.5%); VOC, volatile organic compound. See table D1 for detailed information on the well groups]

Table 4. Wilcoxon rank-sum test of differences between well groups defined on the basis of land use and locale, Oahu,

Hawaii, January 2000 through August 2001

Do median values for pairs of well groups differ with 95% or greater confidence for these chemical attributes ?
Findings and Interpretations 37

and others, 1977) and probably true for nutrients as well. 

Residential lands in the Honolulu uplands also have fewer 

potential VOC sources than do the urban-military lands of 

central Oahu or the urban commercial and industrial lands 

along the south coast. Insecticides are the only chemical class 

having higher loading in urban residential areas, most notably 

the termiticides. An alternate explanation for the lack of 

contaminants in Honolulu wells is the age of ground water 

there. Most wells had CFC dates in the 1960s (table D1) and it 

is possible that contaminants are on their way but have not yet 

been detected.

Ground Water Carries Nutrients and Chemicals to 

Streams and Coastal Waters

Elevated concentrations of nitrate and phosphorus 

were associated with agricultural lands in central Oahu 

(figs. 22 and 23). Natural background concentrations in the 

study area are less than about 1 mg/L for nitrate as N 

(Visher and Mink, 1964) and 0.1 mg/L for orthophosphate 

as P. Concentrations higher than these are anthropogenic, 

resulting from over a century of agricultural fertilizer application. 

Many orthophosphate concentrations (fig. 23) were 

greater than the USEPA recommended goal of 0.1 mg/L for 

avoiding nuisance plant growth (eutrophication) in moving 

waters. Nutrient concentrations were found to be higher in 

stream base flow than in storm flow by NAWQA surfacewater studies, and the higher concentrations were attributed 

to discharge of nutrient-laden ground water (Tomlinson and 

Miller, in press). The organic compounds 1,2-dibromoethane (EDB), bromacil, atrazine, and de-ethylatrazine also 

were higher in base flow than in storm flow, raising concerns about possible effects of organic compounds on 

aquatic organisms.

Besides streams, discharging ground water also carries nutrients and chemicals into estuaries and nearshore 

marine waters. Nutrients are a particular concern with 

respect to coral reefs. The coral-reef ecosystem is highly 

nutrient-sensitive and can be degraded where excess nutrients promote excessive algal growth that displaces or interferes with the growth of healthy corals. Another concern is 

the proliferation of invasive species. Several species of nuisance seaweed have plagued Hawaii in recent years, and 

elevated nutrient concentrations are a suspected factor in 

their proliferation.

NUMBER OF VOCs OR PESTICIDES

0

2

4

6

8

10

12

5 8 10 22

o

VOCs

(excluding fumigants)

PESTICIDES AND FUMIGANTS

FOREST

URBAN HONOLULU

URBAN-MILITARY

CENTRAL OAHU

AGRICULTURAL

CENTRAL OAHU

FOREST

URBAN HONOLULU

URBAN-MILITARY

CENTRAL OAHU

AGRICULTURAL

CENTRAL OAHU

5 8 10 22 EXPLANATION 

o

75th percentile

median

25th percentile

value greater than 90th percentile

90th percentile

10th percentile

22 number of values

Figure 25. Number of volatile organic compounds (VOCs) and pesticides detected in wells 

grouped by land use and locale, Oahu, Hawaii. The number of VOCs (excluding fumigants) was 

greatest in the urban-military central Oahu well group. The number of pesticides and fumigants 

was greatest in the agricultural central Oahu well group.
38 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Well Type and Depth are Less Significant Factors Than 

Land Use and Locale

The principal reason for sampling monitoring wells 

was to gain a more complete distribution of sampling 

depths by sampling shallower ground water that is cased 

off from most public-supply wells. However, cursory 

analysis of the two well networks revealed few patterns 

unequivocally related to well type or depth that cannot be 

explained equally or more so by the location of wells and 

nearby land use. Put another way, location and land-use 

factors would have to be removed sufficiently to permit 

valid tests on well-type and depth influences. 

Monitoring wells did differ statistically from supply wells in depth to open interval at a 95-percent confidence level (table 5 and figure 26). Median depth below 

water to the top of the open interval was 0 ft for monitoring wells and 60 ft for supply wells. The two well types 

did not differ significantly by depth to water from land 

surface. 

Detection rates were higher in monitoring wells 

than in supply wells for all classes of organic compounds 

except gasoline components and insecticides (fig. 8). 

Although the median number of compounds in monitoring wells (8) was twice that in supply wells (4), this difference was not significant at the 95-percent confidence 

level but was nearly so, at 94-percent (table 5, p-value of 

0.062). Differences in median VOC and pesticide concentrations also were not significant at 95-percent confidence but would be significant if confidence levels were 

relaxed to about 90 percent (p-values were 0.11 and 

0.10). Figure 27 aids in understanding these findings: 

although median values are quite different between well 

groups for number of compounds, VOCs, and pesticides, 

the paired distributions overlap enough to reduce the statistical significance of the differences. The small sample 

sizes of the data sets also reduce statistical significance, 

particularly the 15-well sample count for monitoring 

wells.

Several factors besides depth may contribute to differences between the two well types, or may diminish 

statistical differences. Perhaps foremost among them is 

the biased spatial distribution of the monitoring wells. In 

contrast to the widespread, randomly selected spatial distribution of the public-supply wells, the monitoring wells 

are clustered in central Oahu where the predominance of 

chemicals were detected. Because this area has seen 

intensive use or application of solvents, fumigants, and 

herbicides, the monitoring-well network is biased 

towards detection of these compounds. The few compounds detected more commonly in public-supply wells 

(gasoline components and insecticides) were detected in

Chemical or physical attribute

Do median values for 

supply wells and 

monitoring wells differ 

with 95% or greater 

confidence ?

P-value

Median value out 

of 30 public-supply 

wells

Median value out 

of 15 monitoring 

wells

Organic compounds and nutrients

 Number of organic compounds No * 0.062 * 4 8

 Number of VOCs (excluding fumigants) No 0.11 1 2

 Number of pesticides (plus fumigants) No 0.10 2 4

 Nitrate concentration (mg/L) No 0.215 0.92 1.9

 Orthophosphate concentration (mg/L) Yes 0.014 0.1 0.2

Land use within 500 meters of well

 Percentage of urban land use Yes 0.013 54 15

 Percentage of agricultural land use No 0.18 1 34

 Percentage of former agricultural land use No 0.60 55 44

Well geometry

 Depth to top of open interval of well below water (feet) Yes 0.0000 60 0

 Depth to water from land surface (feet) No 0.99 320 340

Water age and tracers

 CFC (chlorofluorocarbon) recharge date Yes 0.013 1966 1976

 Tritium concentration (tritium units) Yes 0.038 0.26 0.63

 Oxygen-18 isotopic ratio (per mill) No 0.53 -3.2 -3.1

Table 5. Wilcoxon rank-sum test of differences between public-supply and monitoring wells, Oahu, Hawaii, January

2000 through August 2001

[Two-sided Wilcoxon test is a non-parametric test for difference between two groups of data; P-value is the probability that the group

medians are the same; values less than 0.05 (bold and shaded) indicate with greater than 95% confidence that the group medians do

indeed differ; *, value is close to 95% confidence level (within 2.5%); VOC, volatile organic compound; mg/L, milligrams per liter]
DEPTH, IN FEET

PUBLIC-SUPPLY WELLS

MONITORING WELLS

PUBLIC-SUPPLY WELLS

MONITORING WELLS

-200

0

200

400

600

800

1000

1200

o

o

o

o

DEPTH TO

WATER FROM

LAND SURFACE

DEPTH OF TOP OF

OPEN INTERVAL

OF WELL BELOW

WATER TABLE

30 15 EXPLANATION 

o

75th percentile

median

25th percentile

30

30

15

number of

values

value greater

than 90th

percentile

90th percentile

10th percentile

Findings and Interpretations 39

Figure 26. Comparison of physical characteristics between public-supply wells and monitoring 

wells, Oahu, Hawaii. Depth below water table to the top of the well’s open interval was less for 

monitoring wells than supply wells. Depth to water from land surface did not differ statistically 

between the two types of wells. NUMBER OF ORGANIC COMPOUNDS

0

2

4

6

8

10

12

14

16

PUBLIC-SUPPLY WELLS

MONITORING WELLS

PUBLIC-SUPPLY WELLS

MONITORING WELLS

PUBLIC-SUPPLY WELLS

MONITORING WELLS

ORGANIC

COMPOUNDS

30 15

o

o

30 15

VOLATILE

ORGANIC

COMPOUNDS

(VOCs)

PESTICIDES

EXPLANATION 

o value greater thaqn 90th percentile

75th percentile

median

25th percentile

90th percentile

10th percentile

30

30 15

number of

values

Figure 27. Comparisons between public-supply wells and monitoring wells for number of organic 

compounds, number of volatile organic compounds (VOCs), and number of pesticides, Oahu, Hawaii. 

Apparent differences between the two types of wells were not statistically significant at the 95-

percent confidence level, but were nearly so.
Correlation 

coefficient P-value Correlation 

coefficient P-value

Correlation with percent urban land use within 500 meters of well

 Number of organic compounds and urban use 30, 15 -0.27 0.15 -0.17 0.52

 Number of VOCs (excluding fumigants) and urban use 30, 15 -0.18 0.32 0.32 0.24

 Number of pesticides (plus fumigants) and urban use 30, 15 -0.22 0.23 -0.40 0.13

 Nitrate concentration and urban use 30, 15 -0.37 0.045 -0.41 0.12

Correlation with percent agricultural land use within 500 meters of well

 Number of organic compounds and agricultural use 30, 15 0.49 0.0085 0.33 0.22

 Number of organic compounds and former agricultural use 30, 15 0.50 0.0075 0.27 0.32

 Number of VOCs (excluding fumigants) and agricultural use 30, 15 0.32 0.081 -0.40 0.13

 Number of VOCs (excluding fumigants) and former agricultural use 30, 15 0.19 0.31 -0.54 0.044

 Number of pesticides (plus fumigants) and agricultural use 30, 15 0.46 0.014 0.61 0.023

 Number of pesticides (plus fumigants) and former agricultural use 30, 15 0.63 0.0007 0.74 0.006

 Nitrate and agricultural use 30, 15 0.57 0.0021 0.68 0.011

 Nitrate and former agricultural use 30, 15 0.65 0.0005 0.84 0.0018

Correlation with nitrate concentration

 Number of organic compounds and nitrate 30, 15 0.83 0.0000 0.41 0.13

 Number of VOCs (excluding fumigants) and nitrate 30, 15 0.60 0.0011 -0.21 0.42

 Number of pesticides (plus fumigants) and nitrate 30, 15 0.88 0.0000 0.74 0.0058

Correlation with depth to water from land surface

 Number of organic compounds and depth to water 30, 15 0.19 0.31 -0.01 0.95

 Number of VOCs (excluding fumigants) and depth to water 30, 15 0.15 0.43 0.65 0.015

 Number of pesticides (plus fumigants) and depth to water 30, 15 0.17 0.35 -0.66 0.013

 Nitrate and depth to water 30, 15 0.20 0.28 -0.46 0.086

 CFC recharge date and depth to water 29, 15 -0.36 0.056 * -0.05 0.83

 Tritium and depth to open water 29, 15 0.29 0.13 0.10 0.72

Correlation with depth to open interval of well below water

 Number of organic compounds and depth to open interval 30, 15 -0.22 0.24 -0.13 0.63

 Number of VOCs (excluding fumigants) and depth to open interval 30, 15 -0.13 0.48 -0.37 0.17

 Number of pesticides (plus fumigants) and depth to open interval 30, 15 -0.22 0.24 0.34 0.21

 Nitrate and depth to open interval 30, 15 -0.18 0.32 0.22 0.42

 CFC recharge date and depth to open interval 29, 15 -0.11 0.56 0.31 0.25

 Tritium and depth to open interval 29, 15 -0.27 0.16 -0.30 0.26

Correlation with CFC recharge date (larger date=younger)

 Number of organic compounds and CFC recharge date 29, 15 0.51 0.0075 0.18 0.50

 Number of VOCs (excluding fumigants) and CFC recharge date 29, 15 0.50 0.0077 -0.02 0.93

 Number of pesticides (plus fumigants) and CFC recharge date 29, 15 0.44 0.020 0.46 0.086

 Nitrate and CFC recharge date 29, 15 0.34 0.075 * 0.49 0.066 *

 Orthophosphate and CFC recharge date 29, 15 0.25 0.18 0.32 0.23

 Tritium and CFC recharge date 28, 15 0.33 0.085 0.17 0.52

Correlation with tritium "water age" (larger concentration=younger)

 Number of organic compounds and tritium 29, 15 0.52 0.0057 0.42 0.12

 Number of VOCs (excluding fumigants) and tritium 29, 15 0.44 0.019 0.43 0.11

 Number of pesticides (plus fumigants) and tritium 29, 15 0.43 0.024 0.08 0.76

 Nitrate and tritium 29, 15 0.34 0.074 * 0.15 0.58

 Orthophosphate and tritium 29, 15 0.56 0.003 0.64 0.018

 Delta 18-oxygen/16-oxygen ratio and tritium 29, 15 0.29 0.13 0.12 0.66

[Correlation coefficient is non-parametric Spearman rho coefficient computed on rank-transformed data; larger magnitudes indicate stronger

correlations; P-value is probability that the null hypothesis is true (no correlation); values less than 0.05 (bold and shaded) indicate correlations

that are significant at greater than 95% confidence level; *. value is close to 95% confidence level (within 2.5%); nominal number of data pairs was

30 for public-supply wells and 15 for monitoring wells; lesser numbers are due to missing data; VOC, volatile organic compound; CFC,

chlorofluorocarbon, N, number of wells]

Table 6. Spearman rank correlation of selected chemical and physical attributes within public-supply and monitoring well

networks, Oahu, Hawaii, January 2000 through August 2001

Public-supply wells, 

N=30

Monitoring wells, 

N=15

Chemical and physical attributes, paired Number of 

data pairs

40 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

urban Honolulu, again probably reflecting a difference 

in locale more than a difference due to well type or 

depth.

Various land-use and well attributes were subjected to correlation analysis to determine influences 

on water-quality results by well type (table 6). Strongest correlations were with agricultural land use and 

included the number of organic compounds, VOCs, 

and pesticides, as well as nitrate concentration (orthophosphate had similar results as nitrate). Most of these 

correlations were significant for supply wells and for 

monitoring wells. Organic compounds, VOCs, and pesticides were also strongly correlated with nitrate concentration (though this appears again to simply reflect 

the influence of agricultural land use) and with younger 

waters as indicated by apparent CFC recharge date and 

tritium concentration. 
Findings and Interpretations 41

There may be important differences between 

monitoring and supply wells that are not obvious by 

comparing the two groups in their entirety. For example, three “recent-use” herbicides (bentazon, imazaquin, and metsulfuron methyl) were detected only in 

monitoring wells (table D1). The three compounds have 

been used on Oahu only since about 1990 and their 

detection may reflect early arrival at the water table and 

preferential detection in monitoring wells with shallow 

open intervals. Additional depth-related patterns in 

water quality might be discerned by exhaustive comparison of specific pairs of adjacent wells and careful consideration of compound breakdown products.

Most Ground Water Appears to be 10 to 50 Years Old, 

But Ages are Problematic

Water samples from all 45 wells sampled had 

some component of young water less than 50 years old, 

as indicated by CFC or SF6 dates within this time span 

or by the presence of post-1953 bomb tritium (table D1 

and figs. 28 and 29). Tritium-helium dating also was 

attempted but results are not reported here because of 

interference by natural terrigenic helium from the volcanic rocks.

Each dating method has limitations and potential 

interferences. The CFC (chlorofluorocarbon) and SF6

(sulfur hexafluoride) methods date waters recharged 

since about 1941 and 1970, respectively, by comparing 

concentrations of these trace atmospheric gases dissolved in ground water with histories of atmospheric 

concentrations (Plummer and Busenberg, 1999). However, apparent ages can be biased by old air in the 

unsaturated zone and by contamination or degradation 

processes. The tritium method measures radioactive 

hydrogen-3 isotope (3H) incorporated in the water molecule and truly tracks water through the hydrologic 

cycle. Tritium forms naturally in the upper atmosphere 

and is incorporated in rainfall at low background concentrations. It was also added to the atmosphere by 

nuclear weapons testing starting in 1953 and peaking in 

1962 at concentrations several hundred-fold above 

background in rain. Tritium can be used to date ground 

water in only the most general sense on Oahu, by distinguishing “pre-bomb” (pre-1953) water from 

younger water containing higher concentrations of 

bomb tritium.

Tritium concentrations generally were lowest 

(0–0.3 tritium units, or TU) in Honolulu, in the 

Waianae aquifer, and in the eastern side of the central 

plateau (table D1 and fig. 29). These samples include 

possible pre-bomb waters (concentrations less than 0.1 

TU) and probable mixtures of old water with small to 

moderate fractions of post-bomb water (0.1–0.3 TU). 

Highest tritium concentrations (0.4–1.8 TU) were scattered throughout the rest of the central plateau and at a 

few wells in the Koolau uplands. Values from 0.4 to 1 

TU likely signify post-bomb water or mixtures of prebomb and post-bomb waters. Values greater than 1 TU 

also could mark mixtures of old and young waters, but 

are not readily distinguishable from entirely young 

waters recharged since about 1980. These interpretations are based on measurements of tritium in Hawaiian rain (fig. 30A) and concentrations representing 

radioactive decay of those initial tritium amounts (fig. 

30B) up to 2001, when the second set of Oahu wells 

was sampled. The rain concentrations are annual values 

from measurements at Hilo, Hawaii from 1952 to 1990, 

with other years estimated by correlation with stations 

at Ottawa, Canada, and Midway Island (data and decay 

calculations are from M.A. Scholl, U.S. Geological 

Survey, written commun., 2003). Scholl’s assumed 

pre-bomb background concentration of 1.0 TU has 

been replaced here by a value of 0.6 TU (average of 

five Oahu rain samples from 1953; Von Buttlar and 

Libby, 1955 as cited in Hufen and others, 1980).

Apparent recharge dates from CFC and SF6

methods were post-1950 with two exceptions: a pre1940s date and a 1947 date, both in the Koolau uplands 

of central Oahu (table D1 and fig. 28). Most dates were 

in the 1950s to 1980s time span. Older dates (1940s1960s) were generally clustered in Honolulu, in the 

Waianae aquifer, and more toward the uplands in central Oahu. Younger dates (1970s-1990s) were generally 

clustered in central Oahu, with mostly 1970s dates near 

the top of the central plateau, and 1980s-1990s dates at 

lower elevations to the north and south. This general 

pattern of older dates in the uplands and younger dates 

downgradient is opposite the usual pattern expected in 

a ground-water flow system. Usually, younger dates 

mark newly recharged water in more active upland 

recharge zones, and older dates downgradient reflect 

travel time and aging of the water along flow paths. 

Interpretation of the Oahu pattern requires detailed 

consideration of recharge processes and timing as well 

as possible artifacts.

The CFC and SF6 recharge dates are termed 

“apparent” because they can differ from true dates of 

recharge for a number of reasons. One distinction is 

that the dates reported here assume simple piston-flow 

displacement of older water by younger water in the 

hydrologic system, with no mixing of waters of different ages. In reality, mixed-age samples are expected as 

the rule and not the exception because most of the 

wells have open intervals tens to hundreds of feet long 

and will not obtain “point” samples of aquifer water. 
0 5 10 Miles

0 5 10 Kilometers

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL SAMPLED FOR CFCs

MONITORING WELL SAMPLED FOR CFCs AND SF6

APPARENT RECHARGE DATE OF GROUND WATER, 

IN CALENDAR YEARS

Pre-1940

 1940s

 1950s

 1960s

 1970s

 1980s

 1990s

 2000s

 No Result (Excess CFC or SF6 contamination at 

 concentrations greater than possible by

 atmospheric deposition)

CFC

SF6

WAIANAE

KOOLAU

AQUIFER

AQUIFER

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

42 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 28. Distribution of apparent recharge dates from CFC (chlorofluorocarbon) and SF6 (sulfur hexafluoride) methods, Oahu, Hawaii. Younger dates 

were clustered mostly in central Oahu.
Base modified from U.S. Geological Survey digital data,

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

0 5 10 Miles

0 5 10 Kilometers

WAIANAE

KOOLAU

AQUIFER

AQUIFER

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

NAWQA GROUND-WATER STUDY AREA

PUBLIC-SUPPLY WELL

MONITORING WELL

TRITIUM CONCENTRATION IN GROUND WATER, 

IN TRITIUM UNITS

 Less than 0.1 – Probable pre-bomb (pre-1953) water or mixture of old water with small fraction of post-bomb water

 0.1 to 0.2 – Probable mixture of old water with small fraction of post-bomb water

 0.2 to 0.3 – Probable mixture of old water with moderate fraction of post-bomb water

 0.4 to 1 – Probable mixture of old water with large fraction of post-bomb water

 Greater than 1 – Possible mixture of old water with large fraction of post-bomb water, but indistinguishable from entirely

 young water recharged since about 1980

 No result

Findings and Interpretations 43

Figure 29. Distribution of tritium concentrations, Oahu, Hawaii. Low concentrations were clustered mainly in the Waianae aquifer, in Honolulu, and 

in a north-south band in south-central Oahu.
0

20

40

60

80

100

120

140

160

180

200

220

1950 1960 1970 1980 1990 2000

TRITIUM CONCENTRATION IN RAINFALL AT

HILO, HAWAII, IN TRITIUM UNITS

A

0

1

2

3

4

5

20

21

22

23

24

25

1950 1960 1970 1980 1990 2000

PRESENT TRITIUM CONCENTRATION (ACCOUNTING FOR DECAY), IN TRITIUM UNITS

B

Vertical scale break

YEAR

44 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 30. Tritium concentration in rain at Hilo, Hawaii (A) and after 

accounting for radioactive decay of tritium from the initial rain 

concentrations in the intervening years to present (B). Data and decay 

calculations are by Martha Scholl (USGS, written commun., 2003).

This appears to be borne out by the tritium data, 

which suggest mixed waters for many samples (fig. 

29). “Unmixing” calculations are possible to some 

extent, but are complex and have not been attempted as 

of this writing.

Another distinction is that CFC and SF6 dates 

mark the time when recharge was sequestered below 

the water table and no longer in contact with air, not the 

time when water first infiltrated below ground. Dissolved CFC and SF6 concentrations are imparted under 

equilibrium with the last air contacted, which is unstaturated-zone air just above the water table and not the 

atmosphere at land surface. A time lag will be introduced as atmospheric air penetrates into the unsaturated zone. Air can migrate by diffusion (a slow 

process) through soil and weathered overburden and by 

advection (a faster process) through fractures and other 

permeable pathways in response to barometric pumping, wind, and thermal influences. Unsaturated-zone air 

will be older than atmospheric air and will impart an 

apparent date that is older than the true date of recharge 

(Plummer and Busenberg, 1999; Busenberg and Plummer, 2000). This old bias is most pronounced in thick 

unsaturated zones such as on Oahu, although it would 

be lessened where there is rapid advection of air 

through high-permeability pathways in the basaltic 

lavas. At 60 m (200 feet) depth to water in Snake River 

basalts beneath playa lake sediments, CFC ages were 

biased old by 20-23 years because of slow air diffusion 

(Plummer and Busenberg, 1999). 

Degradation of CFCs also can impart an old bias. 

Chlorofluorocarbons degrade in anaerobic environments under sulfate-reducing or methanogenic conditions (Plummer and Busenberg, 1999). Degradation 

does not appear probable in Oahu saturated aquifers 

because all ground-water samples contained dissolved 

oxygen and no methane. However, CFCs may degrade 

in parts of the unsaturated zone. Some exposures of red 

saprolite in the study area contain greenish-gray bands 

where iron has been reduced from its ferric state (red) 

to its ferrous state (green). This suggests that anaerobic 

reducing conditions existed in the saprolite, most likely 

within perched water bodies. 

Additional sources of CFCs or SF6 besides the 

atmosphere can make apparent ages younger than true 

ages. Many water samples had concentrations in excess 

of possible atmosphere-water equilibrium concentrations (table D1). The excess concentrations could indicate anthropogenic contamination of CFCs or SF6, or a 

natural terrigenic source of SF6 from rocks or igneous 

fluids (Busenberg and Plummer, 2000). Only a few 

samples had excess concentrations sufficient to prevent 

dating, but other samples could have a young bias if 

partly contaminated.
Findings and Interpretations 45

Because CFC or SF6 ages are set at the water 

table, they should not reflect infiltration time through 

the unsaturated zone. Instead, ages should correspond 

to the time it takes for water to move deeper into the 

aquifer once it has reached the water table, and to flow 

laterally downgradient to intercepting wells. Estimates 

of lateral pore velocity in the study area include 4 ft/d 

by Orr and Lau (1987) and 5-15 ft/d by Eyre (1987), a 

range of 0.28 to 1.0 mi/yr. Estimates for piston-flow 

infiltration time through the unsaturated zone in central 

Oahu include 3 years (Eyre, 1987), 8-10 years (also 

Eyre, 1987), and 15-20 years (Orr and Lau, 1987). The 

3-year estimate was for infiltration through 50 feet of 

saprolite and 100 feet of unsaturated basalt at a dry, 

low-elevation site. The 8-year estimate was apportioned as 7 years for infiltration through 100 feet of 

saprolite and 1 year through 400 feet of unsaturated 

basalt. The 15-20 year estimate cited 13.5 years 

through 120 feet of saprolite and 5.25 years through 

700 feet of unsaturated basalt. Eyre (1987) reviewed 

several rapid water-level responses to rainfall, on the 

order of months to a year, and inferred that these 

reflected piston-flow displacements of water from the 

bottom of the saprolite moisture profile rather than 

contributions of new water through the entire unsaturated zone via fractures, macropores, and other rapid 

pathways. He did equate the fast responses to infiltration time through unweathered basalt beneath the 

saprolite.

The unexpected pattern of older CFC dates in the 

uplands and younger dates downgradient (fig. 28) 

probably reflects artifacts as well as actual recharge 

processes. Some of the older CFC recharge dates 

appear to be biased by old unsaturated zone air, the best 

examples being the 1947 and pre-1940 dates at wells 

W01 and W24 in the Koolau uplands. These ages are 

contradicted by moderate to high tritium concentrations that indicate appreciable components of younger 

post-bomb (post-1953) water in the samples, and by the 

presence at W24 of fumigants and herbicides not introduce until the 1940’s and 1950s (table D1 and fig. 29). 

Furthermore, wells W01 and W24 lie 4.5 and 6 miles 

from the probable ground-water divide near the Koolau 

mountain crest. Using the range of lateral travel times 

cited earlier (0.28-1.0 mi/yr), maximum travel times 

over these distances would be 18-21 years at the slower 

rate and 5-6 years at the faster rate. These are much 

shorter durations than the apparent CFC ages of 53 and 

60+ years.

The Koolau uplands are mantled by a thick 

(100-300 feet) layer of clay-rich saprolite that is much 

less permeable than the unweathered basaltic lavas that 

comprise the deep aquifers. The saprolite thins at lower 

elevations, to about 50 feet or less at the north and 

south ends of the central Oahu plateau, and is also thinner in the drier Waianae uplands than in the wetter 

Koolau uplands. Vertical diffusion of air through the 

saprolite would be much slower where it is thickest, 

resulting in older unsaturated-zone air and older apparent CFC and SF6 ages in the Koolau uplands compared 

to other areas. Even water recharging within the last 

decade or so could be tagged with an older apparent 

CFC age by equilibrating with old unsaturated-zone air 

at the water table. Alternately, CFC degradation could 

be more prevalent in the thicker, wetter saprolite of the 

Koolau uplands than in drier areas.

Apparent CFC and SF6 dates may be less biased 

and closer to true recharge dates in other areas where 

saprolite is thinner. Furthermore, stream valleys at the 

dry north and south ends of the central plateau have 

vertical walls as much as 150 feet high that expose layers of permeable, unweathered basaltic lavas, and valley walls in Honolulu expose partly weathered rock. 

Unweathered valley walls should allow advective ventilation of young unsaturated-zone air that would 

impart truer apparent recharge ages. Notably, apparent 

CFC and SF6 recharge dates in the Waianae aquifer 

and in Honolulu are somewhat more in line with tritium data than in the Koolau uplands.

Irrigation may have contributed to young CFC 

and SF6 ages in downgradient areas of central Oahu by 

supplying large volumes of young recharge. This 

would represent an actual hydrologic process rather 

than an artifact. High-rate furrow irrigation of sugarcane prevailed until conversion to low-rate drip irrigation in the mid-1980s. Return recharge from furrow 

irrigation in central Oahu was estimated at 84-97 in/yr, 

far exceeding natural recharge recharge of 4-5 in/yr in 

those areas and surpassing rates in much wetter mountainous areas (Giambelluca, 1986). Volumetrically, 

much of the upper layer of ground water beneath 

former sugarcane fields may have originated as pre1985 irrigation-return recharge, with comparatively little recharge from rainfall or drip irrigation having been 
46 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

added in later years. Clusters of apparent dates in the 

1970s and 1980s could reflect a large influx of irrigation-return recharge before irrigation rates were 

reduced sharply. If "old" ground water was pumped 

and applied to fields, this might explain waters in the 

southeast corner of central Oahu that have low tritium 

concentrations but CFC and SF6 dates in the 1970s1990s (figs. 28 and 29). The CFC and SF6 signatures 

would be "reset" younger by exposure with the atmosphere and unsaturated-zone air, whereas the tritium 

dates would not be reset; the water would maintain an 

"old" tritium signature. 

Irrigation may also have contributed to organic 

and nutrient contamination in central Oahu. Application of water in excess of plant needs would tend to 

leach or flush contaminants from the soil to deep aquifers rather than allowing them to stay in the soil and 

degrade more fully or be removed by plant uptake. 

Leaching would tend to be greater where more irrigation water was applied

Complex Mixtures of Organic Compounds Were 

Prevalent

Mixtures of organic compounds in ground water 

are of particular interest for human-health reasons. Toxicologists have evaluated health risks and established 

drinking-water regulations for many single compounds 

but risks associated with compound mixtures are far less 

known and may be greater than those of single compounds in some cases (Carpenter and others, 1998; 

Bartsch and others, 1998). NAWQA datasets provide a 

recent opportunity for evaluating the occurrence of compound mixtures in the environment as, for example, in 

Stackleberg and others (2001) and Squillace and others 

(2002).

Compound mixtures were prevalent in Oahu 

ground-water samples (fig. 31). Multiple organic compounds were detected in 63 percent of public-supply 

wells and 93 percent of monitoring wells (table 2). Mixtures of VOCs and pesticides were detected in 53 percent 

of public-supply wells and 80 percent of monitoring 

wells. Wells having higher concentrations tended to contain greater numbers of compounds (table D1). Many 

wells in central Oahu contained several classes of compounds such as solvents, fumigants, and pesticides (fig. 

32). Samples with high solvent concentrations contained 

multiple solvents (fig. 33), as many as ten. Samples with 

high fumigant concentrations contained three to four 

fumigants (fig. 34) as well as herbicides and solvents, 

mostly at trace concentrations below reporting levels. 

Herbicides also tended to occur together (fig. 35), with as 

many as ten herbicides and breakdown products detected 

in one sample.

MULTIPLE ORGANIC COMPOUNDS IN 30 PUBLIC-SUPPLY WELLS

0 5 10 15 20 25 30

13

12

11

10

9

8

7

6

5

4

3

2

1

0

NUMBER OF ORGANIC COMPOUNDS

PERCENTAGE OF WELLS

MULTIPLE ORGANIC COMPOUNDS IN 15 MONITORING WELLS

0 5 10 15 20 25 30

PERCENTAGE OF WELLS

Figure 31. Compound mixtures in public-supply and monitoring wells, Oahu, Hawaii. Most wells contained two or 

more organic compounds. As many as 13 compounds were detected in one public-supply well.
0 5 10 Miles

0 5 10 Kilometers

Solvents Fumigants

Excess CFCs (chlorofluorocarbons) Pesticides

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

NAWQA GROUND-WATER STUDY AREA

ORGANIC COMPOUND CLASSES IN GROUND WATER

Public-supply well

 Monitoring well

CONCENTRATIONS, IN MICROGRAMS PER LITER

 Solvents, fumigants, and pesticides

 Not Detected (minimum reporting level varied by compound)

 Low - less than 0.1

 Medium - 0.1 to 1

 High - greater than 1

 Excess CFCs

 No excess CFCs

 1 of 3 CFCs exceeded possible air-water equilibrium

 2 of 3 CFCs exceeded possible air-water equilibrium

 All 3 CFCs exceeded possible air-water equilibrium

 

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Findings and Interpretations 47

Figure 32. Distribution of organic compound mixtures in public-supply and monitoring wells, Oahu, Hawaii. Wells in urban-military and agricultural lands 

in central Oahu had the highest concentrations and greatest number of compound classes.
0 5 10 Miles

0 5 10 Kilometers

Trichloroethene (TCE) Tetrachloroethene (PCE)

Tetrachloromethane

(carbon tetrachloride)

Trichloromethane

(chloroform)

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

SELECTED MILITARY INSTALLATIONS

NAWQA GROUND-WATER STUDY AREA

SOLVENTS IN GROUND WATER

Public-supply well

 Monitoring well

CONCENTRATION, IN MICROGRAMS PER LITER

 Not Detected

 Low - less than 0.1

 Medium - 0.1 to 1

 High - greater than 1 

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

48 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 33. Distribution of solvent mixtures, Oahu, Hawaii. Wells in urban-military lands in central Oahu had the highest concentrations and 

greatest number of solvent compounds.
0 5 10 Miles

0 5 10 Kilometers

1, 2-Dibromoethane (EDB) 1, 2-Dibromo-3-chloropropane (DBCP)

1, 2-Dichloropropane (DCP) 1, 2, 3-Trichloropropane (TCP)

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

NAWQA GROUND-WATER STUDY AREA

PRESENT AND FORMER PINEAPPLE FIELDS

FUMIGANT CONCENTRATIONS IN GROUND WATER

Public-supply well

 Monitoring well

CONCENTRATION, IN MICROGRAMS PER LITER

 Not Detected (minimum reported level varied by compound)

 Low - less than 0.1

 Medium - 0.1 to 1

 High - greater than 1 

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

Findings and Interpretations 49

Figure 34. Distribution of fumigant mixtures, Oahu, Hawaii. Wells in or downgradient from present and former pineapple fields in central Oahu had the highest 

concentrations and greatest number of fumigant compounds.
0 5 10 Miles

0 5 10 Kilometers

Atrazine Diuron

Bromacil Hexazinone

158 ̊

158 ̊10'

21 ̊40'

21 ̊30'

21 ̊20'

157 ̊50'

157 ̊40'

EXPLANATION

LAND USE

 Agriculture

 Urban

 Other (mostly forest)

NAWQA GROUND-WATER STUDY AREA

PRESENT AND FORMER PINEAPPLE FIELDS

FORMER SUGARCANE FIELDS

HERBICIDE CONCENTRATIONS IN GROUND WATER

Public-supply well 

 Monitoring well 

CONCENTRATION, IN MICROGRAMS PER LITER

 No result

 Not Detected (minimum reporting level varied by compound)

 Low - less than 0.1

 Medium - 0.1 to 1

 High - greater than 1 

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

1:24,000, 1983 & 1999, Albers equal area projection, standard

parallels 21°19'40" and 21°38'20", central meridian 157°58'.

Relief from U.S. Geological Survey digital elevation models,

1:24,000

50 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

Figure 35. Distribution of herbicide mixtures, Oahu, Hawaii. Wells in present and former agricultural lands in central Oahu had the highest concentrations 

and greatest number of herbicide compounds. 
Findings and Interpretations 51

Some possible explanations for the widespread 

detection of compound mixtures are obvious, others are 

less so. Composition and use of particular product formulations are probable factors. For example, co-occurrence of multiple solvent compounds in a sample could 

reflect the use and release of several individual solvents, solvent blends, or manufacturing impurities or 

intermediates. Impurities found in commercial trichloroethene products include chloroform, carbon 

tetrachloride, tetrachloroethene, cis-1,2-dichloroethene, trans-1,2-dichloroethene, 1,1,2-trichloroethane, 1,1,1,2-tetrachloroethane, 1,2-dichloroethane, 

1,1-dichloroethene, bromodichloromethane, bromodichloroethene, 1,1,1-trichloroethane, 1,1,2,2-tetrachloroethane, pentachloroethane, and benzene (U.S. 

Dept. of Health and Human Services, 2002). Notably, 

the first 10 of these 15 compounds were present in central Oahu water samples having the highest trichloroethene concentrations (table D1). Similarly, 1,2-dichloropropane (DCP) and 1,2,3-trichloropropane 

(TCP) are both components in DD fumigant and tended 

to be detected together (fig. 34).

Degradation is a likely explanation for the presence of some compounds. Several herbicide breakdown products were detected along with their parent 

compounds (table D1), and some solvent compounds 

are known breakdown products of others. For example, 

tetrachloromethane (carbon tetrachloride) breaks down 

into trichloromethane (chloroform) and dichloromethane. Breakdown products of tetrachloroethene 

(PCE) include trichloroethene (TCE), 1,1-dichloroethene, cis-1,2-dichloroethene, and trans-1,2-dichloroethene.

Mixed land use and coalescing contaminant 

plumes may have contributed mixtures of disparate 

compounds (for example, agricultural chemicals and

industrial solvents). Although land use near Oahu 

NAWQA wells was characterized (table D1), each 

well's results need not correspond only to land use 

directly above it. Complicating factors include the 

length and depth of the well’s open interval and the 

pumping rate. The longer the open interval and the 

deeper it is below the water table, the more likely that 

the well will intercept ground water originating farther 

upgradient in the watershed than water recharging from 

directly above. Even if the well is open at the water 

table, a long open interval will draw a mixture of 

waters recharged both locally and upgradient. And the 

greater the pumping rate, the more water will be drawn 

from farther away. As a result, contributing recharge 

areas of many wells encompass a mix of land uses. 

This is more true of public-supply wells than the monitoring wells, which were pumped at much lower rates 

and generally had shorter open intervals. In central 

Oahu, the end result of these factors is that wells are 

likely to contain a mix of agricultural fumigants and 

herbicides from local recharge and industrial solvents 

carried in dilute ground-water plumes from upgradient 

urban-military areas.

Application practices may have contributed to 

detections of compound mixtures. Fumigant-herbicide 

mixtures may simply reflect that both have been 

applied on agricultural lands. Co-occurrence of multiple fumigants could reflect sequential application of 

single fumigants or application of fumigant mixtures; 

for example 1,2-dibromo-3-chloropropane (DBCP) 

typically was applied together with DD fumigant in the 

study area (Yim and Dugan, 1975). Detection of herbicide mixtures could similarly reflect application of 

multiple herbicides, either in succession or in combination.

Although solvent-class compounds might not be 

expected in ground water beneath rural or agricultural 

lands, they are commonly being detected by the 

NAWQA program nationwide (Squillace and others, 

1999, 2002). Co-occurrence of trace-level solvents 

with fumigants and herbicides in central Oahu could 

reflect mixing of multiple urban and agricultural contaminant plumes. However, application practices and 

composition of fumigant and herbicide formulations 

may also be contributing factors. Regarding application 

practices, fumigants were sometimes mixed with diesel 

fuel, paint thinner, or white gas on Oahu (Orr and Lau, 

1987; U.S. Department of Health and Human Services, 

1995). Solvent-class compounds are not main components of diesel fuel or white gas, but "paint thinner" is a 

general term encompassing formulations that do contain solvent compounds. Regarding composition, many 

pesticide formulations contain "adjuvants," a general 

term for additives that act as carrier solvents, dispersal 

or wetting agents, or in a host of other capacities. Solvent-class compounds and CFCs are known adjuvants 

or manufacturing intermediates in pesticide synthesis. 

Pesticide labeling laws do not require disclosure of 

compounds other than the main active ingredient, however, and so other components are lumped together 

under the term "inert ingredients" despite the fact that 

they may comprise as much as 95 percent of the formu-
52 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

lation by weight and are not necessarily "inert" with 

respect to human health or the environment. Additives 

or intermediates in product formulations provide a 

plausible explanation for widespread co-occurrence of 

trace-level solvents with fumigants and herbicides in 

central Oahu.

Chloroform And CFC Contamination Were Closely 

Associated With Other Compounds

Widespread detections of trichloromethane (chloroform) and excess CFCs do not correspond to any 

explicit, known use in the study area. Co-occurrence of 

chloroform and excess CFCs with other solvents, fumigants, and pesticides suggests that they may have originated mainly as intermediates or impurities in other 

products, or by degradation of parent compounds in the 

environment.

Chloroform was the most commonly detected 

organic compound in this study, being present in 

47 percent of public-supply wells and 73 percent of 

monitoring wells (table 2 and fig. 16). Chloroform is 

one of a class of compounds known as trihalomethanes 

that form when water containing organic compounds is 

disinfected with chlorine. Recharge of chlorinated 

water from leaking distribution pipes, lawn watering, 

or wastewater effluent is thought to contribute chloroform to ground water in many areas (Squillace and others, 1999; Bender and others, 2000). But chloroform is 

a common solvent itself, a feedstock or intermediate in 

organic synthesis of other VOCs and pesticides, and a 

breakdown product of tetrachloromethane (carbon tetrachloride). Complicating the situation further, chloroform has natural sources in the terrestrial and marine 

environments, being produced in soil (Laturnus and 

others, 2000) and by macroalgae that also produce 

other trihalomethanes (Nightingale and others, 1995). 

These natural sources can account for only the lowest, 

trace-level concentrations observed in the NAWQA 

sampling, however.

Most water samples in which chloroform was 

detected also contained other solvents, fumigants, or 

pesticides. Chloroform was at highest concentration 

where solvents such as trichloroethene or carbon tetrachloride also were highest (table D1) and was at trace 

concentrations below 0.1 μg/L where it otherwise cooccurred with fumigants, pesticides, or trace concentrations of solvents. These relations suggest that most 

chloroform contamination in the study area originated 

together with other compounds (especially solvents) or 

by their breakdown. A disinfection origin cannot be 

ruled out for some detections, however, particularly for 

trace concentrations and where chloroform was the 

only compound detected (or one of only a few compounds detected). It is unlikely that chloroform forms 

pervasively in soil in the study area because it was 

mostly undetected in wells that reflect "background" 

conditions, such as those in forested areas and urban 

Honolulu (table D1). A disinfection origin for trihalomethanes is strongly suggested for only one well, 

W05, which contains all four trihalomethanes (table 

D1). If disinfection is the origin, it probably was a sampling artifact rather than a true measure of trihalomethanes in the aquifer because there is no known 

disposal of chlorinated water upgradient from the well. 

Water is chlorinated at the wellhead and, although it 

was disconnected during sampling, residual concentrations of trihalomethanes may have somehow contaminated the sample. Alternately, all four trihalomethanes 

could truly be present in ground water from a marine 

algae source of the compounds. Well W05 is located 

where prevailing winds blow ocean spray onto the land 

most days of the year. The likelihood of this source carrying trihalomethanes to ground water at the observed 

concentrations could be clarified by analysis of rainwater samples from the area.

Excess CFCs (chlorofluorocarbon refrigerants) 

were present in 47 percent of public-supply wells and 

67 percent of monitoring wells (table 2 and fig. 16). 

Most of these samples also contained solvents, fumigants, or pesticides (table D1). CFCs are known manufacturing intermediates or additives in other VOCs and 

pesticides, and this may explain the co-occurrence of 

excess CFCs with these other compounds. If not 

already present in other chemical products, CFCs may 

partition preferentially from the atmosphere into highly 

concentrated VOCs or pesticides in the environment 

(such as in shallow soil) and thence into water at higher 

concentrations than would be possible by single-component, air-water equilibrium (Cook and Solomon, 

1997). Notably, a common application practice for 

fumigants and some pesticides is to cover the application with plastic sheeting (a "vapor barrier") to enhance 

pesticide potency by inhibiting evaporation and maintaining a high concentration in the soil. This practice 

may provide a favorable environment for enhanced 

partitioning of CFCs from the atmosphere into the 
Policy Implications 53

fumigant or pesticide within the shallow soil. A localized spill or disposal of concentrated solvent to shallow 

soils could represent a similarly favorable environment 

for enhanced partitioning of CFCs from the atmosphere 

to the solvent and thence to ground water.

Organic Contamination Reflects Decades-Old 

Releases And Former Land Use

Apparent recharge dates from CFC and SF6

methods ranged from pre-1940s to present, with most 

dates in the 1950s to 1980s time span (table D1 and fig. 

28). Although there is uncertainty in some of the dates, 

if they are roughly representative then most organic 

and nutrient contamination in central Oahu probably 

originated from applications or spills 20-50 years old 

or older. Where land use has changed during this 

period, water quality more likely reflects former land 

use than present land use.

If it takes several decades for water to flow 

through the hydrologic system, flushing of organic 

compounds will take even longer, on the order of 

decades to a century. Transport of organic compounds 

is delayed by retardation (the tendency for a compound 

to adsorb to soil particles or natural organic carbon in 

the soil and desorb later) and dispersion (solute spreading). Retardation delays the time of peak concentration 

of a compound in ground water beyond the average 

travel time of water alone. And because of dispersion, 

once the peak concentration has passed a certain point 

in the aquifer, concentration will decline gradually in a 

long, tapering tail. For example, using computer simulation, Orr and Lau (1987) estimated that DBCP concentrations in a central Oahu well would increase from 

1983 through 2000 before beginning a long decline 

thereafter.

For the most part, it is probably too soon for 

chemicals associated with recent land-use changes 

(such as the residential urbanizition of central Oahu) to 

have been detected in most wells. It may also be possible that urban chemicals have migrated to ground water 

already but that a "recent-urban signature" is difficult 

to recognize against the background of older contamination.

Monitoring Wells May Be Useful For Early Detection

Three "recent-use" herbicides were detected in 

monitoring wells: bentazon, imazaquin, and metsulfuron methyl (table D1 and fig. 17). The compounds 

were detected in only one well each. These herbicides 

are used primarily for turf grass maintenance and have 

been used on Oahu only since about 1990 (Robert 

Boesch, Hawaii Department of Agriculture, oral commun., 2002). Their detection indicates that they have 

leached to deep ground water within a decade or so, 

which is reasonable because they were detected in a 

dry area where saprolite is thin.

Monitoring wells that are open only a short depth 

below the water table have the best chance of characterizing recent contaminant transport associated with 

present land use. Although it takes some time for 

recharge to make its way from land surface to the deep 

aquifer, monitoring wells offer the best opportunity for 

collecting an isolated sample of the youngest water 

presently arriving at the water table. This type of "early 

warning" role for monitoring wells may be useful for 

evaluating whether newly introduced chemicals leach 

to deep ground-water before fully degrading. There 

also may be a role for direct monitoring at the deep 

water table where wastewater is reused via land application. Wastewater indicators (inorganic major and 

trace elements; pharmaceuticals and personal care 

products; caffeine) could be diagnostic in identifying 

or quantifying the wastewater fraction in recharge 

arriving at the water table.

POLICY IMPLICATIONS

Although the USGS NAWQA program has no 

regulatory role, results from the program may be useful 

to policy makers and others involved in land-use planning, engineering, chemical handling and application, 

and human health.

Most organic and nutrient contamination in 

Oahu's deep aquifers appears to reflect decades-old 

releases and close association with agricultural and 

military land uses. To a large degree, most of the 

observed contamination appears to be an "old" problem 

associated with outmoded or banned practices. Chemical application and handling have improved markedly 

in recent decades in comparison to standards prevailing 

in the mid-1900s when most of the observed contamination probably originated. Pesticides and soil 
54 Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01

fumigants having high toxicity or environmental persistence have been discontinued and replaced with 

compounds that are less toxic or degrade quicker. 

Requirements for the use, storage, and disposal of solvents and fuels are much more stringent than in the 

1940s and 1950s, when common practice was for disposal directly on the ground, to sumps that drained to 

the shallow subsurface, or to landfills in drums or other 

containers. Present users of large volumes of chemicals, (such as agriculture, industry, and the military) 

follow strict handling and disposal procedures that 

were yet to be developed a half century ago. And compared to previous decades, there is now a much greater 

awareness among the general public of the need for 

proper disposal of household solvents, paint thinners, 

and pesticides.

Recent land-use changes on Oahu may bring new 

issues or new compounds to greater prominence in the 

future. A notable change is the ongoing large-scale 

conversion of former agricultural lands in central Oahu 

to residential suburban use. On the basis of results from 

this study, it appears to be too early to discern a distinct 

water-quality signature that may accompany the newer 

suburban land use. However, the newer urban land use 

likely will contribute much less chemical loading to 

soil and ground water than former agricultural use, in 

which fertilizers and pesticides were applied intensively and high-rate irrigation enhanced leaching.

Another notable land-use change is the ongoing 

decentralization of former mono-crop sugarcane cultivation to diversified-crop agriculture. Several issues 

accompany this change: the attendant decentralization 

of pesticide use and oversight; use of a greater variety 

of pesticides than the few used by large-scale plantations; differences in pesticide and fertilizer leaching 

associated with a diversity of crops and irrigation 

regimes. Again, results from the 2000-2001 NAWQA 

ground-water samples have little direct bearing on 

these mechanisms because their effects have yet to 

reach the deep water table to any large degree. However, the detection of several recent-use herbicides in 

this study seems to illustrate the utility of shallow-penetration monitoring wells for gaining an early warning 

of anticipated water-quality changes.

Re-use of treated wastewater by land application 

is being considered in Hawaii, and a pilot program is 

already underway in central Oahu. The method has 

several desirable aspects, one being that it provides 

additional recharge to aquifers and another being the 

environmental and cost advantages of not discharging 

the effluent to streams or piping it to the coast for offshore disposal. Effects on ground-water quality are not 

fully known, however, and detailed studies of effects 

will likely accompany any large-scale reuse/application progam. Direct monitoring of ground water would 

appear to be a likely component in such studies, using 

shallow-penetration monitoring wells to sample deep 

ground water, and even shallower monitoring wells or 

lysimeters in soil and saprolite to obtain younger water 

samples that would show earlier effects. New laboratory methods are available to detect compounds such as 

pharmaceuticals and personal care products that are 

present in wastewater, and stable isotopes and inorganic major and trace elements also may show particular wastewater signatures. Similar approaches would be 

useful in evaluating the leaching potential of new pesticides under full-scale field conditions.

Finally, the health of nearshore marine waters is a 

focus of increasing interest nationally and in Hawaii. 

Most concern is with surface-water loads of sediment 

and nutrients delivered to coastal waters by streams. 

However, the role of ground water in carrying nutrient 

and other chemical loads to nearshore waters cannot be 

overlooked. Stream contributions are large in magnitude but are episodic, whereas ground-water contributions are steady and ongoing every day of the year. 

Best management practices that reduce nutrient runoff 

and leaching can help reduce nutrient loads to nearshore waters. Perhaps foremost in importance are the 

need to avoid overapplication of fertilizers and to time 

applications so as to avoid high-intensity rainfall 

events that cause surface runoff and enhance leaching 

through soils.

SUMMARY AND CONCLUSIONS

Water quality in the main drinking-water source 

aquifers of Oahu was assessed by a one-time sampling 

of untreated ground water from 30 public-supply wells 

and 15 monitoring wells. Organic compounds were 

detected in 73 percent of public-supply wells, but 

mostly at low concentrations below drinking-water 

regulations and even below minimum laboratory 

reporting levels. Of 43 organic compounds detected, 26 

were volatile organic compounds (VOCs), 16 were 

pesticides, and the remaining compound was caffeine. 

Subclasses of VOCs that were prevalent throughout the 

area included solvents, fumigants, and trihalomethanes. 
References Cited 55

In contrast, gasoline components and refrigerants were 

detected in very few wells using standard VOC methods. Of the two pesticide subclasses, herbicides were 

prevalent and insecticides were scarce. 

Characteristic suites of chemicals were associated with particular land uses. In central Oahu, solvents 

were associated with urban and military lands whereas 

fumigants, herbicides, and fertilizer components nitrate 

and phosphorus were associated with agricultural 

lands. Somewhat unexpectedly, very little contamination was detected in areas of highest urban density in 

Honolulu, most likely as a consequence of sound landuse planning, favorable aquifer structure, and less 

intensive application of chemicals (or of less mobile 

chemicals) compared to agricultural areas. 

Water quality was within drinking-water regulations in all but a few cases. Concentrations exceeding 

USEPA Maximum Contaminant Levels (MCLs) 

included the solvent trichloroethene and the radionuclide Radon-222 in one public-supply well each. Concentrations of soil fumigants 1,2,3-trichloropropane 

(TCP) and 1,2-dibromo-3-chloropropane (DBCP) 

exceeded State of Hawaii MCLs (which are lower than 

Federal MCLs) in four supply wells. Complex mixtures 

of organic compounds were common: most water samples contained multiple solvents, fumigants, or herbicides, and many samples contained compounds from 

two or all three of these classes. With regard to aquatichealth quidelines, phosphorus concentrations in much 

of central Oahu were greater than the USEPA recommended goal for avoiding nuisance plant growth in 

moving waters. Nutrients, VOCs, and pesticides carried to streams and coastal waters by ground water may 

affect stream ecology or the health of coral reefs.

Most organic and nutrient contamination appears 

to reflect decades-old releases and former land use. 

Apparent recharge dates for ground-water samples 

ranged from pre-1940 to the present, with most dates 

falling in the 1950s to 1980s time span. Several widely 

detected compounds were discontinued from use as 

long ago as the 1970s but have yet to be flushed from 

the ground-water system. Although large tracts of central Oahu have been converted from sugarcane to residential urban use since the 1950s, water quality more 

closely reflects the former agricultural land use, and it 

appears to be too early to detect a water-quality signature characteristic of the newer urban use.

Emerging local issues in ground-water quality 

include ongoing organic and nutrient leaching, proposed land application of treated wastewater effluent, 

and the ongoing decentralization of former mono-crop 

sugarcane cultivation to diversified-crop agriculture, 

with attendant decentralization of oversight in pesticide 

application and handling. Shallow-penetration monitoring wells could be a valuable component in ongoing 

water-quality monitoring efforts for these and other 

issues. 

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