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OAHU NAWQA

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Oahu NAWQA: Study Unit Description

Map of the geomorphic provinces, Oahu.In 1991 the U.S. Geological Survey initiated the National Water-Quality Assessment (NAWQA) Program to assess the status and trends in the quality of freshwater streams and aquifers, and to provide a sound understanding of the natural and human factors that affect the quality of these resources. As part of the program, investigations were conducted in 51 areas-- called "study units" -- throughout the Nation to provide a framework for national and regional water-quality assessment. Together, these areas account for 60 to 70 percent of the Nation's water use and population served by public water supplies, and cover about one-half of the land area of the Nation.

The subtropical climate of the study unit is characterized by mild temperatures, moderate humidity, prevailing northeasterly tradewinds, and extreme variation in rainfall over short distances. Median annual rainfall is about 280 in/yr near the crest of the Koolau Range, 80 in/yr at the summit of the Waianae Range, and 15-30 in/yr in the rain-shadowed coastal lowlands of western, north-central, and south-central Oahu.

Windward Oahu. Photo by Douglas Peebles.

Windward Oahu -- a mostly rural area where stream-water quality is critical for instream habitat of endemic species and for cultivation of taro under Native Hawaiian water rights (photo by Douglas Peebles).

Oahu streams are important habitats for numerous species of endemic fish and invertebrates. The streams are short, with steep gradients and small drainage areas. Runoff occurs rapidly, and permeable upland soils permit rapid infiltration of water to underlying aquifers. As a result, streamflow is characteristically flashy, with high flood peaks. Several large bays and estuaries receive runoff from Oahu's streams, including Pearl Harbor, Mamala Bay, Kailua Bay, Kaneohe Bay, and Waialua-Kaiaka Bay.

Land use on Oahu is classified as about 41% conservation (forest), 30% agriculture, and 29% urban. The principal industry is tourism, followed by military activities and agriculture. Principal agricultural crops are pineapple and, until recently, sugarcane. Major land-use changes are occurring following a period of about 100 years during which plantation agriculture dominated central Oahu. In particular, the past several decades have seen increasing curtailment of agricultural acreage and incursion of suburban development in central Oahu. The population of Oahu, which has more than doubled since 1950, was about 836,000 in 1990.

Of the total freshwater used on Oahu, 326 Mgal/d is from groundwater and 71 Mgal/d is from surface water. Irrigation and public supply are roughly equal as the two largest freshwater uses (about 170 Mgal/d each), but a major shift continues as sugarcane cultivation is replaced with diversified agriculture and urban development.

Central Oahu. Photo by Douglas Peebles.

Central Oahu-- an area of rapidly changing agricultural and suburban land use. The area is underlain by the island's principal drinking water aquifer, part of which was designated a "sole source aquifer" by the USEPA in 1987 (photo by Douglas Peebles).

Groundwater provides essentially all municipal and domestic water on Oahu. Most of the water is derived from extensive volcanic aquifers of thin-bedded basalts beneath central and southern Oahu. Although the depth to water is as great as 600 to 1000 ft in the island's interior, the aquifers are unconfined and are essentially "surficial" aquifers. The deep water table is overlain by a thick unsaturated zone of highly permeable basalts and lateritic soils, resulting in aquifers that are intrinsically susceptible to contamination. Vulnerability to contamination has been confirmed by the widespread detection of pesticides and herbicides in the aquifers beneath agricultural fields, and the more localized presence of VOCs beneath sites of known use or spillage.

Although surface water does not supply much consumptive use, its instream uses are important. Streams play a vital ecological role as wildlife habitat and as conveyors of runoff to estuaries, bays, and nearshore coastal waters. Stream waters have recreational and aesthetic value to residents and visitors alike, and a steady supply of cool stream water is necessary for cultivation of taro, the traditional staple of Native Hawaiians.

Central Oahu. hoto by Douglas Peebles. Central Oahu-- an area of rapidly changing agricultural and suburban land use. The area is underlain by the island's principal drinking water aquifer, part of which was designated a "sole source aquifer" by the USEPA in 1987 (photo by Douglas Peebles).

Environmental Setting

Map of Oahu showing physiographic provinces.All NAWQA study units have developed an environmental framework that is used to compare and contrast findings on water quality within and among study units in relation to causative factors and ultimately, to develop inferences about water quality in areas that have not been sampled. This framework is based on common natural and human-related factors, such as geology and land use, that affect the sources, behavior, and effects of contaminants and water quality conditions.

The environmental framework for Oahu is based on physiographic zones (see map) and land use. The primary physiographic zones are windward and leeward which relate to the exposure of these areas to the trade winds and orographic rainfall. In general, the windward area has smaller watersheds, higher rainfall, and perennial streams; in contrast the leeward area has larger watersheds, lower rainfall, and intermittent streamflow. The leeward area is further subdivided into Honolulu, central, and Waianae areas. The distribution of major land uses (urban, agriculture, and forest) among these physiographic zones results in 11 major sub-areas of the study unit.

The Oahu NAWQA study assessed water-quality conditions within 6 of 11 sub-areas of the study unit. The Waianae area, which includes 3 sub-areas, was not be assessed as it has little or no streamflow.

Environmental stratification diagram

Environmental stratification diagram.

For More Information

If you have questions and comments related to the Oahu NAWQA study, contact:

Stephen Anthony (santhony@usgs.gov)
or write:
Stephen Anthony
Oahu NAWQA Project Chief
U.S. Geological Survey
677 Ala Moana Blvd., Suite 415
Honolulu, HI 96813

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