The most productive and most widespread aquifers consist of thick sequences of numerous thin lava flows. On Oahu, the vertical hydraulic conductivity has been estimated to be hundreds of times less than the horizontal hydraulic conductivity. Lava tubes are extremely permeable features that can vary from less than 1 foot to more than 50 feet in diameter and can be several miles long. They form as molten lava drains out from under a solidified crust. The hydraulic conductivity of clinker zones ranges from several hundred to several thousand feet per day, which is similar to that of coarse, well-sorted gravel.
Dikes tend to channel ground-water flow parallel to the general trend of the dikes.
In some areas, the level of dike-impounded water is much higher than that of the regional, freshwater lens. Pyroclastic rocks include ash, cinder, spatter, and larger blocks. These deposits have hydraulic conductivity between 1 and 1,000 feet per day. Exposed weathered profiles include inches to several tens of feet of saprolite. In areas of high precipitation, saprolite may be from 100 feet to as much as 300 feet thick.