Groundwater in Hawaii is present as basal water, as dike-impounded water, and as perched water. Basal water, the fresher part of it forming a lens-shaped body floating on saline groundwater, is in dike-free volcanic rocks and in sedimentary rocks. Dike-impounded water is confined to volcanic rocks in eruptive zones. Perched water is present in all rocks and at all altitudes.

Most recharge to groundwater is in the wet interior mountains. Therefore, the main areas of recharge are upslope from developed areas where most wastes are disposed of in the subsurface. This natural deterrent, by position of the recharge area, so far has kept much of the groundwater in its pristine state. Deterioration of the groundwater will increase as land development encroaches toward the recharge areas--the degree will depend greatly on the waste-disposal practices.

Most natural discharge of groundwater is along the shore, downslope from disposal wells. Contamination of the beaches, rather than the water supply, is the main concern regarding subsurface disposal of wastes in the low areas under present conditions.

The success of injection operations depends primarily on injection capacity and fate of the injected waste. To evaluate these factors properly, an understanding must be obtained of local hydrogeologic conditions, hydrodynamics of injection under Ghyben-Herzberg lens conditions, and possible chemical and biologic effects. The hydrogeology of Hawaii is understood fairly well and, where adequate information is not available, it usually is possible to collect these data by careful field investigation. Considerable information is available from other parts of the world on the hydrodynamics of wastewater injection. However, much of this information is not directly applicable to injection in the Hawaiian environment. Particularly troublesome are the complications caused by the extreme hetero eneity of receiving formations and Ghyben-Herzberg lens effects. Likewise, because chemical and geologic reactions depend on the nature of the injected waste, the receiving waters, and the receiving formations, many of the data collected elsewhere are not applicable in Hawaii.