Waiehu deep monitor well

Last updated February 15, 2013
See USGS site for original article

Waiehu deep monitor well chloride data collected by U. S. Geological Survey in cooperation with the County of Maui Department of Water Supply. U. S. Geological Survey data after September 30, 2012 are provisional and subject to revision.

Introduction (summarized from US Geological Survey Water-Resources Investigations Report 00-4223)

The Waiehu deep monitor well, drilled to an altitude of -1,020 ft, penetrates through most of the transition zone. Because the amount of water that can be developed from a freshwater lens for potable use is constrained by the salinity of the water, the altitude of the top and the midpoint of the transition zone (where chloride concentration is 2 percent and 50 percent that of seawater) and the thickness of the transition zone are important. Since 1985, water samples for chloride-concentration analysis have been collected from discrete depths in the Waiehu deep monitor well to track the vertical movement of the transition zone. Chloride-concentration data from the well would be expected to most reflect changes in the aquifer as a result of pumping from the Waihee and Waiehu Heights well fields because the deep monitor well is located closest to these well fields. The movement of the transition zone is in response to water-level changes caused by variations in pumping and rainfall. Because the well is uncased below sea level, it is possible that the chloride concentration of the water collected at any depth in the well is affected by borehole flow. Thus, chloride concentration for a given depth may not reflect the actual chloride concentration of water in the aquifer at that depth. Nevertheless, discrete sampling with depth over time provides a general pattern of transition-zone thickness and relative movement. A detailed discussion of borehole flow in deep monitor wells including some analysis of the Waiehu Deep Monitor well can be found in Effects of groundwater withdrawal on borehole flow and salinity measured in deep monitor wells in Hawai‘i—implications for groundwater management (Rotzoll, 2010). Well-construction information is available for selected wells in the Iao and Waihee area.

Summary of recent conditions

Chloride concentrations at Waiehu deep monitor well increased at depths of -475 ft to -700 ft during October 2012 – January 2013. Adjustments to the sampling depth spacing were made to provide more resolution in the part of the transition zone where chloride concentrations change rapidly with depth.  

The profile plots indicate a rise of about 196 ft in the midpoint of the transition zone since August 1985. The expanded profile plot indicates that the midpoint of the transition zone (-626 ft altitude) has risen by about 3.2 ft since October 2012 and risen about 9.4 ft since January 2012.

A plot of the position of two salinity values (2-percent and 50-percent seawater chloride concentration) in the transition zone shows the rise of the transition zone through time. This plot indicates that the rise had been fairly steady from the early 1990’s until the present. However, the most recent data show a steeper trend line for the rise of the 2-percent seawater position. This rise appears mainly because the part of the transition zone where chloride concentrations change rapidly with depth has moved into a part of the aquifer where the monitoring well samples are collected at wider depth intervals. The State of Hawaii Commission on Water Resource Management also monitors the Waiehu deep monitor well and the Iao Deep monitor well.

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 ground-water 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

Chloride concentrations at selected depths in the Waiehu deep monitor well, 1985 to present.

Chloride-concentration profiles for the Waiehu deep monitor well, August 1985, January 2012, and January 2013.

Expanded chloride-concentration profiles for the Waiehu deep monitor well, January 2012, October 2012, and January 2013.

Altitude of the top (2-percent seawater) and midpoint (50-percent seawater) of the transition zone, Waiehu deep monitor well, 1985 to present.