 Oahu
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| The island of Oahu (586
square miles) consists of the eroded remnants of two volcanic mountain
ranges, the Waianae Range and the Koolau Range. Weathering, erosion, and
slope failure have modified the original domed surfaces of the volcanoes,
dissecting the east and west parts of the island into a landscape of deep
valleys and steep interfluvial ridges. A gently sloping saddle, the Schofield
Plateau, lies between the two mountain ranges. The summit of the plateau
is the approximate north-south divide for the extensive surface- and groundwater
flow systems of central Oahu. A coastal plain surrounds much of the island.
Honolulu, one of the 25 largest cities in the nation, is located in southeast
Oahu. |
Geomorphic provinces, Oahu.
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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. |
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. |
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).
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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. |
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).
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Of the total freshwater
used on Oahu, 326 Mgal/d is from ground water 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. |
Ground water 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. |
Honolulu-- an urban area
where runoff affects high-visibility receiving waters that include several
streams, the Ala Wai Canal estuary, and nearshore waters of the major tourism
center, Waikiki (photo by Douglas Peebles).
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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. |
Environmental
Setting
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. |
Map of Oahu showing physiographic provinces.
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Environmental stratification diagram.
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