| OCR Text |
Show HAWAII 713 rots rather readily. The weight of the later overlying lava flows tends to reduce greatly the porosity and permeability of the weathered materials, with the result that the older alluvium is characteristically less permeable than the over- lying lava rock. In some areas, however, the alluvium is not everywhere rotted, water being allowed to percolate through the unfilled interstices. Other supplies of ground water are perched upon ash and soil beds inter- stratified with lava flows. High-level artesian water. -Artesian water, underlain by dense intrusive rock and capped with impermeable sediments, exists on Oahu. On Maui, a perched aquifer containing water under an artesian head was discovered in 1941, the water being confined under pressure in permeable basalt lying between dense lava flows. The Coastal Artesian Areas of Oahu It has been the view of knowledgeable ground water hydrologists that the main body of fresh basal water of Oahu and of other comparable islands conforms generally to the shape of a double-convex lens. This lens rests upon the underlying salt water. It arches above sea level, extending to distances below sea level about 40 times greater than the elevation of the arch above sea level, and tapering at the seashore. Along some portions of the seashore of two islands, overlying structures of caprock have altered the sharply tapering edge of the lens. In some places, artesian conditions have been created. Character and functions of caprock. -Bodies of relatively impervious caprock rest upon the sloping surface of pervious rocks along a considerable portion of the Oahu seacoast. These caprock formations are far from homogen- eous; they consist mostly of layers of sediments on older lava flows. Their mud and clay constituents are the most abundant and are far more compact than the underlying water-bearing basalt. The structure as a whole tends to be im- pervious to percolation of water, and to provide an effective barrier to the flow of the basal water that saturates the highly permeable basaltic rocks upon which it is superimposed. There are continual accretions from rainfall to the water in the basal water lens. The water in the lens tends to move outward and to escape into the ocean at the tapering edge, although the sloping wall of caprock acts as a seaward barrier. The ground water at the edge of the lens is thus forced by this wall both upward and downward, the top of the blunted edge being forced above sea level about 1 foot for each 40 feet of the distance to which the lower point is forced below sea level. The effect of the caprock is to trap water that percolates into the permeable rock beneath it from the rock in the central portion of the island. Creation of artesian conditions. -Pressure is exerted by the water that saturates the contiguous permeable rock inland or "mauka" from the caprock, and that presses the confined water against the sloping wall. The water-bearing |
