The Ups and Downs of the ‘Iao Aquifer

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Ever since the designation of the ‘Iao aquifer as a ground water management area, questions have lingered over the actual amount of water that’s available for withdrawal from that source.

The aquifer was designated in July 2003, when the 12-month moving average of water withdrawals exceeded 18 million gallons a day, a figure equal to 90 percent of the aquifer’s sustainable yield of 20 mgd. (The sustainable yield is the estimate of the amount of water that can be taken daily without diminishing the aquifer’s yield over the long term.)

But new information suggests the withdrawals may have been overstated. Last year, the Maui Department of Water Supply noticed a large gap between the amount of water it pumped from the aquifer and the amount actually consumed by end users. Until 2002, the volume of unaccounted-for water was trending down, to the point that, by January 2002, it was practically non-existent. In the months following, the unaccounted-for water shot up dramatically; from July 2002 through January 2004, the amount ranged between 3 mgd and 4 mgd, or roughly a fifth of the county’s total withdrawals from the aquifer.

To investigate the reason for the large discrepancy, the Department of Water Supply hired a consultant, Carl Freedman. Freedman reviewed meter readings, electrical consumption at pumps, records from the Honolulu Board of Water Supply, which does billing for the county, and other information. His conclusion: “The exceptional increase in unaccounted-for-water … in Spring of 2002 is primarily the result of measurement and reporting methods. Best current information does not support a conclusion that actual unmetered water flows have increased recently on the central district system.”

Freedman found problems in a number of areas. But the largest single discrepancy he traced to the quantity of water attributed to Wailuku Shaft 33. An error in entering data on a spreadsheet and poorly calibrated flow meters were among the problems he found in analyzing production records from this source. He wrote: “It is likely that the DWS production reports overestimate production from Shaft 33 for the period of April 2002 at least through the end of calendar year 2003 by approximately 0.860 mgd.”

The period of overestimating the draw from Wailuku Shaft coincides with the period of use that led to the designation of the ‘Iao aquifer as a ground water management area. Had the more likely actual use figures been used in calculating the county’s draw, the trip-wire of 18 mgd averaged over a 12 -month period would not have been pulled.

And instead of Wailuku Shaft 33 provid ing 5.77 mgd to Maui’s central district system, it probably yielded about 4.9 mgd.

So should the aquifer have been designated? Maui deputy corporation counsel Jane Lovell says, “If the commission ever took a hard look at the numbers, they’d have to un-designate.” That’s not something she or the head of the Department of Water Supply, George Tengan, intends to pursue, though. “We see many benefits to designation,” Lovell told Environment Hawai‘i. Separately, Tengan told The Maui News: “I personally was opposed to state designation of ‘Iao aquifer. Over time, I have taken a second look and have come to consider designation is not that bad, especially in ‘Iao.”

Work is continuing on studies to refine the sustainable yield estimate for ‘Iao. Meanwhile, other indicators suggest that whether the 20 mgd figure is accurate or not, current pumping rates and well configurations are not protecting the aquifer in the long term. At Wailuku Shaft 33, the water table has sunk from a high of about 16 feet to about 10 feet above sea level as of November 2004, accord ing to the U.S. Geological Survey. Other wells in the aquifer have shown similar trends over the last decade or more.

— Patricia Tummons

Volume 15, Number 7 January 2005

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