The Hawaiian Islands are made up of one or more shield volcanoes that are composed primarily of extremely permeable, thin basaltic lava flows (within the flows are a few ash beds.) Ordinarily, basalts are among the most permeable rocks on earth.
When rain falls on the Islands, it does one of three things: (1) wets the land surface, shallow infiltration saturates the uppermost soil layer and replaces soil moisture used by plants and then is absorbed by the vegetation and/or evaporates (evapotranspiration;) (2) runs off, eroding the land, forming valleys and gouges in the mountain slopes (and also creates some spectacular periodic waterfalls;) or (3) percolates into the ground (slowly sinks into the ground and becomes groundwater.)
The latter contributes to the groundwater recharge of the area (in the Koʻolau, it takes about 9-months for the rain, now groundwater, to seep down through cracks and permeable materials in the mountain.) Other recharge components include cloud drip (moisture condenses on the trees and leaves as clouds/fog drift through) and irrigation of an area helps add to the recharge.
Rainfall percolating through the ground may accumulate in three principal types of groundwater bodies: (1) high-level bodies perched on relatively impervious soil, ash or lava layers; (2) high-level bodies impounded within compartments formed by impermeable dikes that have intruded the lava flows; and (3) basal water bodies floating on and displacing salt water.
The principal source of fresh ground water in the Hawaiian Islands is the roughly lens-shaped basal water body floating on and displacing denser sea water. (It varies by area, sometimes there is high-level confined water.) Recharge of the basal water body results directly from percolating rain water or by underground leakage from perched-water bodies and bodies impounded by dikes.
The Ghyben-Herzberg principle applies to this basal water that suggests the top of fresh water above sea level should be balanced by a thickness of fresh water below sea level about 40 times as great. That generally means, for every foot of fresh water above sea level, there is 40-feet of fresh water below it.)
Water resources were becoming a challenge in the growing Honolulu community. “From the outset it was the Board’s major problem to supply the City of Honolulu with water from sources within its own boundaries as long as that remained possible.”
“The Board’s long range plan, however, contemplated the eventual necessity of going outside the boundaries of the District of Honolulu for additional artesian water but it had been thought that the time when this would have to be done was far in the future. However, when the emergency arose it was possible to advance this phase of the program by deferring the infiltration projects.”
“Bond moneys that would have been applied to infiltration were transferred to a new project through which additional artesian water will be brought into the city from a 284-foot inclined shaft and electrically-operated underground pumping station in North Hālawa Valley.”
“War has delayed the completion of the North Hālawa project and has greatly increased its cost. Army and Navy authorities have given us splendid cooperation on this project, and, although we cannot be certain how soon all the materials and equipment required for completion of the installation will reach us, progress on its construction has been satisfactory, and it should be completed within the year 1943.” (Report of the Board of Water Supply, Ohrt’s Report, January 28, 1943)
North Hālawa Valley overlies the Pearl Harbor aquifer, an important source of potable water for the island of Oʻahu. Freshwater in the Pearl Harbor aquifer is part of a large, lens-shaped body of ground water that is thickest in the central part of Oahu and thins toward the coastline.
This lens of freshwater, known as the ‘basal lens,’ floats on saltwater that penetrates from the ocean into the basalt flows of which the island is composed. (Izuka, USGS)
In some early installations, vertical wells were drilled in the tunnels to develop additional water. Hālawa was different; it is referred to as a skimming tunnel. It’s commonly called the Hālawa Shaft.
Skimming tunnels consist essentially of a vertical or inclined shaft constructed from the ground surface down to about the water table and one or more horizontal, or nearly horizontal, tunnels constructed laterally at, or just below, the water level to collect water. (Peterson)
The fundamental advantage of the skimming tunnels over conventional wells is their capability to produce large quantities of fresh water from lenses so thin that drilled wells would recover only brackish water.
For this reason, skimming tunnels are especially useful in some of the dry leeward coastal areas of Hawaiʻi, as well as on many small oceanic islands with extremely thin fresh-water lenses.
Owing primarily to economic considerations and also to the greater flexibility of modern deep-well pumping stations, no new major skimming tunnels have been constructed in Hawaiʻi since the early 1950s. (Peterson)
The Hālawa Shaft facility is at an elevation of 165 feet above sea level; it’s one of five main shafts operated by the Honolulu Board of Water Supply. (The other main shafts include Wai’alae, Kalihi, Makaha and Pearl City.)
Approximately 15-million gallons of pure water is pumped every day from the Hālawa shaft by three pumping units which have a capacity of 18 to 20-million gallons per day.
The water pool is a ‘hole’ at the top of a 919-foot long water development tunnel below. The Hālawa Shaft was put in operation on August 22, 1944. (Papacostas)
The completion of the Hālawa Shaft made possible the importation of water from the Pearl Harbor area to Honolulu permitting a reduction in draft from the Honolulu aquifer.
This change in draft has raised the water levels in Honolulu to the extent that this aquifer now appears to be functioning well within the limits of its safe yield. (Ground Water Development, 1958)
While I was at DLNR, I had the opportunity to have a private tour of the Hālawa Shaft. The lack of a key to unlock a gate on the stairs leading down the shaft caused quite an embarrassment to the Water staff.
Rather than turn back, we climbed over the gate and were able to view the shaft and water pool.