Volcanic Domes—Hawaiian Islands
In contrast to the very large area of similar geology of the Columbia Plateau Basalts, lava flows associated with some volcanic eruptions can have very heterogeneous aquifer systems. The Hawaiian Islands provide the classic example (Peterson 1972; Cox 1954; Visher & Mink 1964; Takasaki 1978).
Each of the Hawaiian Islands consists of shield volcanoes forming from one to five volcanic domes. Each dome consists of thousands of individual basaltic lava flows coming from either craters or fissures. Lava flows cooling above sea level are thin bedded, highly fractured, or composed of vesicular and very permeable basalt. Those cooled under water are more massive and less permeable. However, owing to lowered sea levels during the Pleistocene and isostatic sinking of the islands, highly permeable, air-cooled basalt is found below present-day sea level. Interbedded with the lava flows are ash beds, which have a lower permeability. In the zone in which lava flows originated, igneous dikes with low porosity and permeability cut across the lava beds. The original dome structure may be partially eroded. Sediments have accumulated along some of the coastal areas. These coastal plain sediments consist of both terrestrial and marine deposits. The cross section of Figure 29 is through an idealized Hawaiian volcanic dome, with both the original and eroded states illustrated.
The Hawaiian Islands are typical examples of oceanic islands where fresh ground water is underlain by salty ground water. Because fresh water is less dense than salty water, the fresh ground water beneath an oceanic island can be thought of as “floating” as a thin lens in the salty ground water. The fresh ground water grades into salty ground water in a zone of mixing.
Ground water in the Hawaiian Islands is contained in the highly permeable basalt flows. It is recharged by rainfall, which can average as much as 20.8 ft (635 cm) per year on Oahu. In the interior of the islands, the basalt flows are isolated by cross-cutting igneous dikes, which are ground-water dams. The ground water is trapped at a high elevation behind these dams. High-level ground water is also found as perched water in lava beds overlying low-permeability ash beds. If the infiltration is not trapped on an ash bed or behind a dike dam, it moves downward to the basal ground-water body, which is a fresh-water lens in dynamic balance with salty ground water. The basal ground water may be unconfined or, in areas of coastal plain sediments, may be confined by the lowpermeability sediments locally termed caprock. A cross section of the occurrence of ground water is shown in Figure 30. A bird’s-eye view of the island of Oahu indicates areas of high-level water bodies impounded by dikes (Figure 31).
Springs issue from ash-bed perched aquifers and also from dike-dammed water bodies. Some of these are 300 ft (100 m) or more above sea level. High-level water is developed by tunnels into the tops of ash beds or penetrating dike dams. Most ground water is developed from the basal ground-water body. Unconfined basal water is collected in horizontal skimming tunnels, called Maui tunnels, which are at sea level and slightly below. These skimming tunnels can develop water where the fresh-water lens is very thin and conventional wells would pump brackish or salt water. Maui tunnels are capable of producing up to 3.1
104
gal/min (2
103
L/s), although in most cases the yield is much less. Where the basal water is confined by coastal plain sediments, conventional wells are used for ground-water development.