Pacific Islands Water Science Center
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Recent hydrologic conditions, Iao and Waihee aquifer areas, Maui, HawaiiLast updated September 19, 2010
The Iao and Waihee aquifer areas, on the eastern side of West Maui Mountain, are the principal source 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, pumpage, water-level, chloride-concentration, and rainfall data for the Iao and Waihee aquifer areas through the second quarter 2010 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. Pumpage and chloride-concentration data are current through the end of June 2010. Rainfall data are current through the end of May 2010. Water-level data are current through August 2010. Waiehu deep monitor well was last sampled on July 8, 2010. Iao deep monitor well was last logged in February, 2010. Streamflow information is updated through June 2010. Well-construction information for wells described in this web page is available here.
Summary of recent conditions
The 12-month moving mean of Iao aquifer area total pumpage was 15.54 Mgal/d as of June 2010 or about 0.75 Mgal/d more than in June 2009. The 12-month moving mean of Waihee aquifer area total pumpage was 5.90 Mgal/d as of June 2010 or about 0.44 Mgal/d more than in June 2009. Most water levels have increased from April to August 2010 but are lower than this time last year. Chloride concentration of the pumped water from well fields is higher in most well fields relative to the same time last year. Chloride-concentration data in July from the Waiehu deep monitor well indicate the middle of the transition zone has risen by about 3.0 ft since January 2010 and about 6.8 ft since July 2009. The 12-month moving means of rainfall are below the long-term means of rainfall at the rain gages used for these quarterly updates. Streamflow was below the median during April-June 2010 at the gaging stations on Waihee River and Iao Stream.
Background information about the Iao and Waihee aquifer areas
Since the introduction of pumping, 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 moving upward 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 (click here to see a detailed map from US Geological Survey Water-Resources Investigations Report 00-4223 in .pdf format). 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 (click here for illustration in .pdf format). 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 aquifer area 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.
A general description of the USGS cooperative program with the County of Maui Department of Water Supply can be viewed here.
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.