HAWAII VOLCANIC-ROCK AQUIFER STUDY

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HAWAII VOLCANIC-ROCK AQUIFER STUDY

HYDROLOGY OF OCEAN ISLANDS

The ultimate source of freshwater in oceanic islands is precipitation, which includes rain, fog, and snow. Water from precipitation can run off the land surface to the ocean through streams, infiltrate the subsurface and recharge groundwater, or return to the atmosphere through evapotranspiration (evaporation and transpiration by plants). Humans sometimes take water from streams or pump water from the aquifer to irrigate crops; some of the irrigation water may also seep into the ground and recharge the aquifer.

In oceanic islands, freshwater in the saturated part of the aquifer forms a lens-shaped body underlain by saltwater from the ocean. The freshwater lens is buoyed by the underlying saltwater because freshwater is less dense than saltwater. Between the freshwater lens and the underlying saltwater is a brackish-water mixing zone. The freshwater lens is thicker where recharge rates are high or aquifer permeability is low, and thinner where recharge rates are low or permeability is high.

Freshwater in the lens flows from inland areas, where most recharge occurs, to coastal areas, where groundwater discharges naturally to springs, streams, and the ocean. Depressions in the land surface can expose the groundwater and form wetlands, lakes, and ponds.

Wells that pump water from the lens cause a reduction in the natural groundwater discharge to streams, springs, and the ocean. Pumping wells also cause the freshwater lens to become thinner by lowering the water table and causing the brackish mixing zone to rise.