OAHU REGIONAL AQUIFER SYSTEM
Oahu is the most developed island in the State of Hawaii and contains the largest population. Because of its varied geohydrologic framework and intense ground-water development, Oahu was chosen for a Regional Aquifer-System Analysis (RASA) study. Analysis of the hydrologic and geohydrologic characteristics of Oahu has transfer value to the other main islands of Hawaii.
Volcanic rocks ranging in age from Pliocene to Holocene, make up most of Oahu and compose the most important aquifers. Quaternary-age consolidated sedimentary deposits, which are principally coralline limestone, form productive aquifers in lowlands and nearshore areas but generally contain brackish water or saltwater. These deposits are underlain by and interbedded with Quaternary-age low-permeability sedimentary deposits that form confining units that impede the discharge of freshwater into the ocean and, thus, allow the freshwater lens to build up to a greater thickness than would be possible in an unconfined setting. A synopsis of the water-yielding characteristics of the sedimentary deposits and volcanic rocks of Oahu is shown in table 3.
Sedimentary deposits (table 3) include coralline limestone, dunes, sand, lagoonal deposits, and alluvium. Permeability of the deposits ranges from very low to very high. Where the coralline limestone has been partially dissolved, it contains large secondary openings and can have a hydraulic conductivity of 20,000 feet per day, which is the highest known for any aquifer in the Hawaiian islands. The limestone mostly contains saltwater or brackish water except where locally recharged by irrigation water or by upward-flowing freshwater from the underlying volcanic-rock aquifer. Consolidated dunes and sand have low permeability, and unconsolidated dunes and sand have low to moderate permeability. The dunes and sand mostly contain brackish water or saltwater. The lagoonal deposits range from mud with very low permeability to sand with low to moderate permeability. The lagoonal deposits are near the shore and mostly contain brackish water or saltwater. The unconsolidated younger alluvium generally is in stream valleys and is unsaturated. The older alluvium generally is highly weathered and has low permeability.
Oahu is formed by two volcanoes-Waianae on the west and Koolau on the east (fig. 54). Major rift zones in these volcanoes intersect near the center of each volcano. The caldera complex near the center of each volcano occupies a large area and is underlain by rocks that commonly have low permeability.
The Waianae Volcanics forms the most important aquifer in western Oahu and the Koolau Basalt is overall the most important aquifer on Oahu. Permeability ranges from low in the dike complexes to very high in dike-free lava flows (table 3). The level of ground water in the dike complexes generally is much higher than the level of the regional water table in the freshwater lenses. The Honolulu Volcanics in eastern Oahu generally has low to moderate permeability and does not form significant aquifers. Saprolite that develops from weathering of Waianae Volcanics, Koolau Basalt, and Honolulu Volcanics has very low permeability.
The principal occurrences of ground water on Oahu are shown in figure 55. Areas of dike-impounded water coincide with the main rift zones in the Koolau Basalt and the Waianae Volcanics. In the Schofield area, water is impounded to high levels probably by either dikes or buried ridges. Freshwater lenses exist in the Kahuku, north-central, southern, and southeastern Oahu areas (fig. 55). Most of these areas have a coastal caprock.
Areas of thick valley fill are partial barriers to ground-water movement because the weathered valley fill and underlying weathered volcanic rocks have low permeability. Thus, water levels might be at different altitudes in adjacent areas separated by thick valley fill. In the western parts of the north-central and the southern Oahu ground-water areas, Koolau Basalt may overlie older rocks of the Waianae Volcanics. The surface of the Waianae Volcanics consists of soil, saprolite, and pyroclastic material of low permeability that separates the two volcanic-rock aquifers and causes a discontinuity in ground-water levels between the aquifers.
Water-level data (fig. 56) emphasize the differences among the ground-water areas of Oahu. Ground water generally moves from inland areas toward the ocean. Water levels in rift zones are as much as 1,600 feet above sea level in the Waianae rift zone and 1,000 feet above sea level in the Koolau rift zone. Ground water in the rift zones is impounded in compartments between dikes where the volcanic rocks are saturated with thick columns of freshwater. The exact thickness of the freshwater in the rift zones is not known, but the freshwater is thought to extend far below sea level. Water levels in the Schofield ground-water area are about 275 feet above sea level. The Schofield area receives recharge from the adjacent Koolau and Waianae rift zones; the water then flows to the north or south across boundary zones with slightly lower water levels and recharges downgradient areas that have freshwater lenses.
Water levels in the freshwater lens of the southeastern Oahu area (fig. 56) generally are less than 10 feet above sea level near the western boundary and the levels decrease eastward.
Water levels in the southern Oahu ground-water area (fig. 56) generally range from about 25 to 30 feet above sea level inland to about 15 to 20 feet above sea level near the shore where the water is under artesian pressure because it is confined by caprock. The caprock impedes the seaward movement of fresh ground water. In the eastern part of the area, thick valley fill and underlying weathered rocks form partial barriers to ground-water flow. In the western part of the area, the weathered zone near the unconformity separating Koolau Basalt from underlying Waianae Volcanics impedes the flow of water between the two volcanic-rock aquifers.
In the north-central Oahu ground-water area (fig. 56), water levels in the freshwater lens range from more than 20 feet above sea level in the southwestern part where the caprock is thick, to less than 3 feet above sea level nearshore in the northern part where the caprock is thin. The weathered zone separating Koolau Basalt from underlying Waianae Volcanics and a thick sequence of valley fill form barriers to ground-water flow in the western and eastern parts of the area, respectively.
In the Kahuku ground-water area (fig. 56), water levels in the freshwater lens range from about 20 feet above sea level inland to less than 10 feet above sea level near the shore. The nearshore artesian pressure is the result of confinement by caprock.
Fresh ground water in the volcanic-rock aquifer moves seaward and discharges upward through the caprock (fig. 57). Such discharge can form springs near the inland margin of the caprock, as in the Pearl Harbor area. Springs also can form where valleys are eroded into dike-impounded water bodies or where perched water bodies emerge at cliff faces (fig. 57).
Barriers separate some of the ground-water areas or subareas, and accordingly, water levels are significantly different across such barriers. In southeastern Oahu (fig. 58), dikes in the Kaau rift zone separate the southern Oahu ground-water area from the southeastern Oahu ground-water area. Water levels in the southern Oahu area are about 10 to 20 feet higher than those in the southeastern Oahu area because of the barrier effect of the dikes. Likewise, northeast-trending dikes within the southeastern Oahu area create a barrier that causes water levels to be about 7 to 8 feet higher in the western part of the area than in the eastern part (fig. 58). In the eastern parts of the southern and north-central Oahu ground-water areas, low-permeability valley-fill deposits and underlying weathered volcanic rocks impede ground-water movement and also create differences in water levels in adjacent subareas.
Water levels in the Pearl Harbor area and other places are directly affected by large ground-water withdrawals (fig. 59). As ground-water withdrawals increased in the Pearl Harbor area from 1901 through 1980, water levels in observation wells decreased. During years when withdrawals decreased, water levels rose. The long-term trend, however, has been one of water-level decline.
Recharge to Oahu's aquifers is not evenly distributed. Annually, some areas receive a significant amount of recharge (greater than 150 inches), and in other areas, recharge is low (less than 10 inches). Water-conveyance structures are used to transport water from areas of excess to areas of deficiency. The amount of mean annual predevelopment recharge available to Oahu's aquifers was estimated for each of the seven major ground-water areas (fig. 60). Estimates of recharge for the areas range from less than 4 to about 369 million gallons per day and are greater for areas at higher altitudes and for larger areas in the eastern and central parts of the island.
Reported ground-water withdrawals by decade from 1901 to 1980 from wells and shafts were computed for each of the seven major ground-water areas on Oahu (fig. 61). Discharge from shafts in dike complexes was not included. The Waianae rift zone and the southeastern Oahu ground-water areas each had less than 5 million gallons per day of withdrawals. Between 1921 and 1980, withdrawals in the Koolau rift zone area increased to a maximum of about 13 million gallons per day during 1971-80. Withdrawals in the Schofield ground-water area increased over the decades to a maximum of about 15 million gallons per day during 1971-80. In the Kahuku ground-water area, maximum withdrawals of about 31 million gallons per day was during 1941-50, when sugarcane was cultivated in the area. Since 1940, withdrawals in the north-central Oahu ground-water area have ranged from about 40 to 60 million gallons per day. The southern Oahu ground-water area withdrawals were greatest, ranging from about 81 million gallons per day during 1901-10 to about 265 million gallons per day during 1971-80. Withdrawals in the north-central and southern Oahu ground-water areas have decreased in the 1990's because sugarcane is no longer cultivated on Oahu. In addition to the withdrawals, an unknown quantity of ground water is discharged naturally to the ocean by submarine springs and seeps.
A synopsis of the ground-water areas of Oahu follows:
· The Kahuku ground-water area consists of dike-free volcanic rocks in the northern end of the Koolau Range and overlying unconsolidated and consolidated sedimentary deposits. Aquifers were heavily developed for agriculture before 1971 when sugarcane cultivation ceased; consequently, estimated water use declined after 1971. Most fresh ground water is found as a lens in the Koolau Basalt. Regional ground-water movement is northeastward from highlands of the Koolau Range towards the ocean. Mean annual predevelopment ground-water recharge from direct infiltration of rainfall was about 3.8 million gallons per day, but ground-water flow through the area is much greater than this because of ground-water inflow from the adjacent dike complex. Discharge is predominantly from the volcanic-rock aquifers into the overlying sedimentary deposits and from there to the ocean.
· The Koolau rift zone ground-water area in eastern Oahu consists mostly of dike-intruded Koolau Basalt but also includes extensive areas of unconsolidated sedimentary deposits and local areas of the Honolulu Volcanics and consolidated sedimentary deposits. Most of the area is mountainous and has been deeply dissected by erosion. Land in the coastal areas is used for agricultural, military, and urban purposes. The area is wet, exceptionally so in the mountains. The Koolau Basalt is the principal aquifer; sedimentary deposits are poorly permeable and yield little water. These deposits form a caprock that confines water in the Koolau Basalt inland and in the coastal plains. Regional ground-water movement is from the highlands to adjacent ground-water areas and directly to the ocean. Dike-impounded water is most important in this ground-water area, and some water levels are as much as 1,000 feet above sea level. Mean annual predevelopment recharge was about 368 million gallons per day and was entirely from rainfall. Discharge is to streams and by ground-water outflow to adjacent ground-water areas; withdrawals from wells, shafts, and springs; evapotranspiration; and outflow to the ocean.
· The southeastern Oahu ground-water area is in the dry southeastern tip of Oahu. The Koolau Basalt is the principal aquifer in the area. Unconsolidated and consolidated sedimentary deposits in valleys and the coastal plain are generally not developed for water supply; in the coastal plain, these deposits are confining units. The area is highly urbanized, and water is imported from adjacent ground-water areas to supplement the limited resource in this area. Regional ground-water movement is from the highlands southward to the ocean. Mean annual predevelopment recharge was about 17 million gallons per day from infiltration of rainfall. Discharge is by outflow to the ocean and withdrawals from wells.
· The southern Oahu ground-water area is the largest ground-water area on Oahu. The Koolau Basalt is the primary aquifer in the central and eastern parts of the area, whereas the Waianae Volcanics forms the main aquifer in the western part. Unconsolidated and consolidated sedimentary deposits form a thick confining unit near the coast. Coralline limestone within the sedimentary deposits is at shallow depths and is extremely permeable but commonly contains brackish water that is withdrawn for cooling and industrial purposes. The confining unit locally is more than 1,000 feet thick near the coast. Because of widespread development, the aquifers in the area have been studied more than those elsewhere. Regional ground-water movement is from adjacent highland rift zones and the Schofield ground-water area towards the ocean. Mean annual predevelopment recharge from direct infiltration of rainfall is estimated to have been about 229 million gallons per day. Discharge is primarily to wells and shafts and to springs in the Pearl Harbor area. Some ground water also flows out of the southern Oahu ground-water area to the adjacent southeastern Oahu ground-water area to the east.
· The Waianae rift zone ground-water area encompasses most of western Oahu. Except in the mountains, the area receives little rainfall. The Waianae Volcanics forms the principal aquifer in the area; locally, consolidated sedimentary deposits are minor aquifers, but, for the most part, these deposits have low permeability and confine water in the underlying volcanic-rock aquifer near the coast. The area is chiefly undeveloped, except for some agricultural and military development inland and residential development near the coast. Regional ground-water movement is from areas of dike-impounded water at high altitudes, as much as 1,600 feet above sea level, to downgradient ground-water areas or directly to the ocean. Mean annual predevelopment recharge to the area was about 52 million gallons per day from infiltration of rainfall. Discharge is primarily as ground-water outflow to downgradient ground-water areas and to the ocean.
·The north-central Oahu ground-water area composes the northwestern coastal area of Oahu. The principal aquifers are the Koolau Basalt in the east and the Waianae Volcanics in the west, which are overlain near the coast in most areas by a confining unit of sedimentary deposits. The area is little developed, except near Waialua. Regional ground-water movement is from adjacent areas of dike-impounded water and the Schofield ground-water area northward toward the ocean. The water in the north-central Oahu ground-water area occurs as a freshwater lens. Mean annual predevelopment recharge from direct infiltration of rainfall to the area was about 43 million gallons per day. Recharge also is by ground-water inflow from adjacent areas. Discharge is by outflow to springs and the ocean, and withdrawals from wells and shafts.
·The Schofield ground-water area is in the Schofield Plateau of central Oahu. Poorly known geologic structures of low permeability impound ground water at altitudes that range from about 130 to 275 feet above sea level. Although the Waianae Volcanics locally is present, the principal aquifer in the area is the Koolau Basalt. Regional ground-water movement is northwestward and southward from the central part of the plateau to adjacent ground-water areas. Ground-water inflow to the area from the adjacent Koolau and Waianae rift zones supplements recharge from direct infiltration of rainfall. The mean annual predevelopment recharge is estimated to have been about 79 million gallons per day. Discharge from the area is principally by ground-water outflow to downgradient ground-water areas, but some water is withdrawn by wells.