The `Iao aquifer is the second largest in the state – but to Maui County residents, no other aquifer matches it in importance. Located deep within the mountains of West Maui, it is the source of three quarters of all the fresh water carried in the pipes of the Maui Department of Water Supply and nearly all the water the department delivers to homes and businesses along the island’s isthmus, all the way down to the posh resorts and trophy houses of Wailea and Makena.
In August 1997, a series of events led the state Commission on Water Resource Management to limit the county’s withdrawals from the aquifer to no more than 20 million gallons a day. This amount was thought to be the aquifer’s sustainable yield – the amount of water that can be withdrawn, in an optimum configuration of wells, without causing salt water levels to rise. In Hawai`i, underground aquifers are formed when a lens of buoyant fresh water floats atop denser ocean water. Between the two is a transition zone of brackish water. Pump out the water too quickly and the transition zone rises while the available supply of fresh water diminishes.
At the time of the Water Commission’s order to the Maui Department of Water Supply, the transition zone had been rising. “Twenty mgd [million gallons a day] clearly cannot be sustained based on the current infrastructure,” commission staff reported. “If additional wells are drilled at some distance from the existing pumping center, and pumpages are readjusted, `Iao aquifer may sustain 20 mgd.”
The county pledged to bring on line new wells that would even out pumping within the aquifer and increase the safety margin. The wells were drilled and withdrawals have been reduced by 10 percent – down to 18 mgd from more than 20.
And the transition zone?
Last November, the Maui Board of Water Supply received a report from Ellen Kraftsow of the Water Department’s staff. “The gist of it,” she told the board in summarizing the report, “is that the transition zone is continuing to rise.”
The only member of the Board of Water Supply this seemed to worry was Jonathan Starr: “I would like to read into the minutes and make sure that all the board members are aware that although we have reduced pumping in the `Iao aquiferÉ we’re still seeing the transition zone, where the salt meets the freshwater, is rising and the water levels are still dropping, which means it looks like `Iao has not reached equilibrium yet.”
By February and March 2001, water levels in the Waiehu test hole — a monitoring well in the `Iao aquifer — reached the lowest level recorded since 1982 for those months.
Running on Empty?
As it turns out, the 20 mgd estimate of the aquifer’s sustainable yield may be too high. A recent study by the U.S. Geological Survey concludes that the 1980-era model used to calculate sustainable yield is not as accurate as more sophisticated models developed since.
Gordon Tribble, chief USGS hydrologist for Hawai`i, says the “jury is still out” as to whether reductions in withdrawals from the `Iao aquifer will be sufficient to avoid damaging it. “The technique used to establish sustainable yield is not as technically rigorous as modern methods allow us to be. The model doesn’t simulate the flow of water in the aquifer.”
The new wells Maui has drilled are in the Waihe`e aquifer, adjacent to the `Iao aquifer. More wells are being planned there. Tribble expressed concerns with this approach: “One thing we didn’t address explicitly in the report but [which] is well known to most hydrologists is that individual aquifer sectors are treated independently,” he continued. “What that means is you can pump sustainable yield from one aquifer … and are presumed not to have an effect on the adjacent aquifer. This is a hydrologic oversimplification, since pumpage in one aquifer sector will affect adjacent aquifer sectors, provided they’re at the same elevation and there are no extreme geologic barriers between them.”
Sustainable yield for the `Iao aquifer – and many other aquifers throughout the state – was arrived at using a model called the Robust Analytical Model, or RAM, developed by John Mink, an independent hydrologist. Mink chafes at the notion that the estimates of sustainable yield resulting from the application of RAM are substantially off base. “The sustainable yield of 20 mgd is holding up – it may be a little less or a little more, but not very different,” he told Environment Hawai`i. “The USGS is trying to sell the idea of doing numerical models. We could do it, too, but they don’t tell us any more than what we empirically have derived.” The use of numerical models involves “a lot of unproved assumptions,” Mink continued. “The sophistication of the numerical procedure doesn’t convert to a greater accuracy.”
Not that the RAM is perfect: “Of course, RAM can be improved upon,” Mink says. “It’s a first model. It treats the entire aquifer as a single cell, so it doesn’t take into account pumping centers and what have you. It’s a first approach, and should be regarded as such.” Still, he says, when it was developed, “it was the first time that we had a tool in which we could say É how the volume [of water] in the aquifer was behaving.”
Mink rejects the notion that the sustainable yield figures for Hawai`i aquifers need to be recalculated. “I don’t think there is any need now for the Commission on Water Resource Management to revise the sustainable yield figures. There should be a continuing review of them. These aquifers, like the Pearl Harbor aquifer, are very large and they respond very slowly to whatever situations are imposed upon them. As I see it right now, there’s no need for it. But as I’ve said in my writings É there has to be a review every so often.”
Sustainable yields are “a management tool,” Mink says. “At the best of our knowledge in 1977 or 1978, they were a template for management, and I think they’ve worked very well. And surely É they are not absolute and over time they can be changed, but there was nothing like that that existed before. Everything was kind of vaporous. No one knew what anyone else was referring to.”
Case-by-Case Review
The Commission on Water Resource Management seems to be in no hurry to conduct a wholesale revision of its sustainable yield estimates. In a written response to questions from Environment Hawai`i, its director, Linnel Nishioka, said: “The more important issue for the CWRM is that from a comprehensive statewide perspective is what should the next step be? We strongly believe that the major limitation on many of the current numeric models, including some by USGS, is the lack of adequate verified baseline data on water levels, transition zones, climate, recharge, soil moisture, rainfall and fog-drip. Without this quality baseline data, another model would be subjected to the ‘garbage in, garbage out’ problem.
“Based on our assessment, the priority for the commission at this point is not to fund yet another numeric model but to concentrate on gathering and verifying the necessary validation data through increased deep monitoring wells, water use records, climate, recharge and rainfall and other verified data that could provide the important baseline information for future SY studies. Prior to any new SY studies, we believe that an updated statewide recharge analysis should be done.”
The daily maximum and minimum data show how recent conditions compare to the range of daily water levels measured in the historical record. The entire period of record is used to estimate the maximum, minimum, and average water levels that historically occurred in each day and recent water levels are plotted to show how they compare to the historical range. (Source: USGS)
The commission plans to look at sustainable yields across the state on a “case-by-case” basis, Nishioka said. Last year, the commission revised the Pearl Harbor aquifer sustainable yield using “the best information available,” she went on to say, including results from a RAM and two separate numerical models. “To the CWRM, the comparison of all the models was more valuable than any single one on its own,” she stated.”
The revision resulted in a drop of nearly 20 percent in the quantity of water thought to be available for daily pumping. From nearly 200 mgd, the aquifer’s sustainable yield was reset to approximately 165 mgd. Part of the decrease was attributable to irrigation in the central plain having been reduced with the demise of sugar. Still, should the `Iao aquifer’s sustainable yield be adjusted in similar fashion, the result would be a yield of 16 mgd instead of the current 20 mgd.
For Further Reading
For recent information on water levels in aquifers and streams across Hawai`i, see: [url]http://hi.water.usgs.gov/recent/index.html[/url]
The Commission on Water Resource Management’s website is: [url=http://www.state.hi.us/dlnr/cwrm/download/planning/hwpfinal.pdf]http://www.state.hi.us/dlnr/cwrm/download/planning/hwpfinal.pdf[/url]
In August 1990, Environment Hawai`i reported on the first Hawai`i Water Plan. Cover articles on Maui water appeared in the August 1997, September 1997, and July 2000 editions.
The Maui Board of Water Supply maintains a website, including links to transcripts of many of its meetings. See: [url=http://www.mauiwater.org]http://www.mauiwater.org[/url]
— Patricia Tummons
Volume 11, Number 11 May 2001
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