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Show 712 GROUND WATER RIGHTS IN SELECTED STATES lens-shaped section, but without filling the voids in the rock of this upper zone through which it descends. In fact, the "lens" of water exists, but it is not symmetrical in configuration, and the occurrences of ground water throughout the island necessarily are not uniform. The utlimate source of fresh ground water and of surface water is precipita- tion upon the island. The exposed portions of the original volcanic formation, particularly in the high mountain elevations, act as the intake area of the main ground water system of the island. Basal Water Basal water is characteristic of the large islands in the Hawaiian archipelago. On Oahu it is divided into (1) shallow water, usually without confining beds, and (2) water occurring in important basalts. This latter includes the greatest underground reservoir on Oahu, which is of outstanding importance in the economy of the Island. While the basal water table slopes toward the seashore, the gradient is relatively flat, indicating that the rocks are exceedingly perme- able. In portions of the coastal plains of Oahu and Kauai, water in the basalt under the relatively impermeable "caprock" is under artesian pressure, being confined between the caprock and the underlying salt water. These coastal areas are supplied from water in the much more extensive portions of the basalt inland from them. The artesian and nonartesian waters are thus in direct contact-a physical relationship that is important in its effect upon rights to the use of the waters. High-level Water The occurrences of high-level water in Hawaii consist chiefly of water (1) confined by intrusive rocks, mainly dike complexes, (2) perched on ash or tuff beds, (3) perched on soil beds, and (4) perched on alluvium. The term "perched" is used with referecne to water resting upon a relatively impervious body, which in turn rests upon an aerated zone. Water confined by dikes. -The largest known bodies of high-level water on Oahu are those confined in the dike complexes associated with the rift zones. Water from precipitation on the high elevations enters and saturates the perme- able rock within the dike complexes and is held up by relatively impermeable barriers, chiefly dikes. They act as natural reservoirs, part of the impounded water being held in storage and part being allowed to escape. Water confined by dikes and not floating on salt water has been found on several islands in addition to Oahu. Perched-water supplies.-The second largest group of high-level water sup- plies of Oahu appears to be in perennial stream valleys where older alluvium has been covered by later lava flows. The Hawaiian alluvium is relatively im- pervious, the constituent grains being of basaltic debris which weathers and |