Local 2: Water supply on Oahu (v1.0)

The Board of Water Supply (BWS) is responsible for Managing Oahu’s municipal water resources and distribution system. It provides residents with a safe and dependable drinking water supply. The BWS is the largest municipal water utility in the State, serving approximately 150 million gallons of water a day to roughly one million customers on Oahu. The BWS is a financially self-sufficient, semi-autonomous city agency. Its operations and projects are financed with revenue generated by water transmission and distribution fees. The BWS also pursue federal and state grants to help subsidize BWS projects. The BWS responsibilities include repairing and replacing ageing infrastructure and maintaining and updating critical components in the water distribution system. There are other state of Hawaii government entities in the ecosystem too like Safe Drinking Water, and Clean Water Division.

The supply of water to residents on Oahu is somewhat privatized. Even the BWS is somewhat semi-private. There are a number of companies that provide drinking bottled water and are water suppliers to residents and businesses like Aloha Water Company, Kalaeloa Water Company, Island Ice and Water Company, Hawaiian Isles Water Company, Menehune Water Company, Aloha Kangen Water, Kangen Water Hawaii, Waterworks Hawaii, and American water. Water purification for residents and businesses is also done by a number of companies like Optimal Water, Pure Water Technology, and Hawaiian Cool Water LLC. 

Oahu’s municipal drinking water is provided by groundwater resources, which are dependent on recharge from rainfall. On average, an estimated 1.8 billion gallons of rain per day fall on Oahu. Some rain runs off: historically, about a third. Some rain is absorbed by plants and then evaporates, about a third. Historically, about a third of that rain soaks in and recharges the groundwater. It takes between one year and 25 years to reach the aquifer. Groundwater sources are less susceptible to drought and climate variability. The BWS continuously monitors rainfall and groundwater levels to ensure the island's drinking water supply is preserved. The BWS taps some of this water with horizontal dike tunnels. The island’s geological water system isn’t a one-way arrangement. Groundwater is always flowing somewhere, and eventually it will all exit, either through human-created means or through the island’s system of freshwater submarine springs.

The occurrence of ground water on Oahu is determined by the type and character of the rocks and by the presence of geohydrologic barriers. The primary modes of freshwater occurrence on Oahu are firstly as a basal lens of fresh ground water floating on saltwater (Basal ground water is characterized as a lens-shaped water body floating on saltwater). Secondly as dike-impounded ground water (Ground-water reservoirs impounded by volcanic dikes receive a substantial part of the total recharge to ground water on the island of Oahu because they generally underlie the rainiest areas. They accumulate the infiltration from rainfall, store it temporarily, and steadily leak it to abutting basal reservoirs or to streams cutting into them). Thirdly as perched ground water (Perched water is found in areas where low-permeability rocks impede the downward movement of ground water sufficiently to allow a perched water body to develop within otherwise unsaturated rocks). Saltwater occurs at depth throughout much of the island. 

The origin, mode of emplacement, texture, and composition of the rocks of Oahu affect their ability to store and transmit water. The volcanic rocks are divided into four groups: (1) lava flows, (2) dikes, (3) pyroclastic deposits, and (4) saprolite and weathered basalt. Stratified sequences of thin-bedded lava flows form the most productive aquifers in Hawaii. Dikes are near-vertical sheets of massive intrusive rock that typically contain only fracture permeability. Pyroclastic deposits include ash, cinder, and spatter; they are essentially granular, with porosity and permeability similar to those of granular sediments. Weathering of basaltic rocks in the humid, subtropical climate of Oahu alters igneous minerals to clays and oxides, reducing the permeability of the parent rock. Saprolite is this highly weathered basalt material. 

A regional aquifer system composed of the Waianae aquifer in the Waianae Volcanics and the Koolau aquifer in the Koolau Basalt is subdivided into well-defined areas by geohydrologic barriers. The aquifers are separated by the Waianae confining unit formed by weathering along the Waianae-Koolau unconformity. In some coastal areas, a caprock of sedimentary deposits overlies and confines the aquifers. 

The island of Oahu has been divided into seven major ground-water areas delineated by deep-seated structural geohydrologic barriers; these areas are further subdivided by shallower internal barriers to ground-water flow. The Koolau rift zone along the eastern (windward) side of the island and the Waianae rift zone to the west (Waianae area) constitute two of the major ground-water areas. North-central Oahu is divided into three smaller ground-water areas, Mokuleia, Waialua, and Kawailoa. The Schofield ground-water area encompasses much of the Schofield Plateau of central Oahu. Southern Oahu is divided into six areas, Ewa, Pearl Harbor, Moanalua, Kalihi, Beretania, and Kaimuki. Southeastern Oahu is divided into the Waialae and Wailupe-Hawaii Kai areas. Along the northeast coast of windward Oahu is the Kahuku ground-water area.

This map shows the groundwater collection sites: groundwater areas of oahu - Search Images (bing.com)