USGS

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

Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4305

 

By Charles D. Hunt, Jr.

 

Prepared in cooperation with the National Water-Quality Assessment Program

 

This report is also available as a pdf.

 

Abstract

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. 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 ground-water 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 pre-1940 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 water-quality 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.

Contents

ABSTRACT

INTRODUCTION

Objectives of the Oahu NAWQA ground-water study

Study area and land use

Chemical use and application

Previous water-quality studies

Geohydrologic setting

STUDY DESIGN AND METHODS

Selection of well networks

Sampling and analytical methods

Results of laboratory analyses

FINDINGS AND INTERPRETATIONS

Most wells contained low concentrations of organic compounds

Few concentrations exceeded drinking-water regulations

Detection rates were highest in the nation for fumigants and among the highest for solvents, VOCs, and insecticides

Commonly detected solvents, fumigants, and herbicides have histories of intensive use

Gasoline components and insecticides were detected in few wells

Volatile organic compounds were higher in concentration than pesticides

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

Ground water carries nutrients and chemicals to streams and coastal waters

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

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

Complex mixtures of organic compounds were prevalent

Chloroform and CFC contamination were closely associated with other compounds

Organic contamination reflects decades-old releases and former land use

Monitoring wells may be useful for early detection

POLICY IMPLICATIONS

SUMMARY AND CONCLUSIONS

REFERENCES CITED

WATER-QUALITY DATA

DOCUMENT TABLES PROVIDED IN EXCEL FORMAT:

Table 2

Table 3

Table 4

Table 5

Table 6

APPENDIX TABLES PROVIDED IN EXCEL FORMAT:

Table D1

Table D2

Table D3

Table D4

Table D5


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Suggested citation:

 

Citation: Hunt, C.D., Jr., 2004, Ground-Water Quality and its Relation to Land Use on Oahu, Hawaii, 2000-01: U.S. Geological Survey Water-Resources Investigations Report 03-4305, 76 p.

 

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Download the text of the report (PDF, 65.3 MB)

 

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For more information about USGS activities in Hawaii, visit the USGS Hawaii District home page.

 

 

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