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Hanawi Stream near Nahiku, Maui, Hawaii.


Recent hydrologic conditions, Iao and Waihee aquifer areas, Maui, Hawaii


Iao and Waihee aquifer areas on Maui, HawaiiThe Iao and Waihee aquifer areas, on the eastern side of West Maui Mountain, are the principal sources of domestic water supply for the island of Maui. In collaboration with the County of Maui Department of Water Supply (DWS) and the State of Hawaii Commission on Water Resource Management, water-level and deep monitor well salinity data for the Iao and Waihee aquifer areas are displayed here (click on the above links or the highlighted text to see each section). These web pages are prepared to provide water-resource managers, scientists, and the public with timely information on the status and trends of hydrologic conditions in central Maui.

Background information about the Iao and Waihee aquifer areas

Since the introduction of pumping in 1948, groundwater levels have declined and the chloride concentrations of pumped water have risen above predevelopment levels at all of the well fields. In addition, the transition zone between freshwater and saltwater, which has been monitored at the Waiehu deep monitor well since 1985, has been rising during the period of record.

The Iao aquifer lies on the flank of West Maui Mountain and encompasses about 24.7 square miles. The boundaries of the Iao aquifer, as defined by the Commission on Water Resource Management (1990) and Mink and Lau (1990), are: the ridge south of Waihee River and north of Kalepa Gulch extending from the coast to the summit of West Maui Mountain; the crest of the West Maui Mountain; the ridge north of Waikapu Stream extending from the crest to the isthmus; and the southern divide of Iao Stream to Kahului Bay (a detailed map is available from US Geological Survey Water-Resources Investigations Report 00-4223). The area is characterized by a steep and mountainous region to the west, an area of sloping alluvial and colluvial plains extending east from the mountains, and an area of lithified sand dunes and coastal plains near the ocean. The adjacent aquifer area north of the Iao aquifer is the Waihee aquifer which extends north to Kahakuloa Stream.

The fresh groundwater system in the Iao aquifer contains: (1) dike-impounded water, (2) a freshwater lens floating on saltwater, and (3) perched water. The dike-impounded water body is found in the mountainous interior part of the aquifer. A freshwater-lens system is found within the dike-free volcanic rocks and also in the coastal sedimentary deposits. Perched water in the sedimentary deposits overlying volcanic rocks is vertically separated from the freshwater lens by a zone of unsaturated rock. The general movement of fresh groundwater in the Iao aquifer area is from the dike-impounded water body into the freshwater-lens system and then to the ocean (US Geological Survey Water-Resources Investigations Report 00-4223). Similar occurrences of groundwater are expected in the Waihee aquifer area.

Additional information about the Iao and Waihee aquifer areas can be found in:

"Ground-water availability in the Wailuku Area, Maui, Hawai'i" by Stephen B. Gingerich, 2008, US Geological Survey Scientific Investigations Report 2008-5236

"The response of the Iao aquifer to groundwater development, rainfall, and land-use practices between 1940 and 1998, Island of Maui, Hawaii" by William Meyer and Todd K. Presley, 2001, US Geological Survey Water-Resources Investigations Report 00-4223

"Analytical versus numerical estimates of water-level declines caused by pumping, and a case study of the Iao aquifer, Maui, Hawaii" by Delwyn S. Oki and William Meyer, 2001, US Geological Survey Water-Resources Investigations Report 00-4244.


Commission on Water Resource Management, 1990, Water Resources Protection Plan: prepared by George A.L. Yuen and Associates, Inc., for Department of Land and Natural Resources, State of Hawaii, 262 p.

Mink, J.F., and Lau, L.S., 1990, Aquifer identification and classification for Maui: groundwater protection strategy for Hawaii: Honolulu, Hawaii, University of Hawaii Water Resources Research Center, Technical Report no. 185, 47 p.

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