Deep freshwater found on Big Island

WAILUKU — The deepest hole ever drilled on an oceanic island turned up lots and lots of unexpected freshwater on the Big Island.

The surprised scientists now think similar conditions might exist on Maui, and they would like to drill a hole to find out.

Professor Donald Thomas of the University of Hawaii Center for the Study of Active Volcanoes gave a briefing on the decade-long research project last week at a meeting of the state Commission on Water Resource Management.

The 11,551-foot hole is near Hilo International Airport, and the drilling has been going on for a decade, at a cost of more than $10 million.

The researchers from the U.H., Cal Tech and Cal-Berkeley weren’t looking for freshwater, although they expected to find saltwater. Their first goal was to try to understand how the Hawaiian Islands formed.

As it turned out, the standard model was too simple. Experts from 30 institutions had their ideas about what a really deep look inside Hawaii would reveal.

“In some ways, every one of them was wrong,” Thomas said.

His particular interest is in what geochemistry and isotope chemistry of water can reveal about hydrogeology.

Hawaii is a rainy place, and the old view was that water soaked into the ground, where the freshwater floated on the heavier saltwater that infiltrates every island from the ocean.

The “lens” of freshwater weighed down on the saltwater, so that if you drilled into the islands, eventually you would hit fresh (or, near the coast, brackish) water. For every foot that the head of water rose above sea level, there would be about 40 feet more below sea level.

Most of Maui’s population gets most of its water from this lens under Iao Valley. It is more than 600 feet deep.

The drillers were trying to reconstruct the history of Mauna Kea. They were astonished, after they passed through the saltwater they expected to find, to find a layer of freshwater.

And then further layers of salt and freshwater interleaved down through the rock.

Some of the water is artesian — when the well pipe was uncapped, it was a gusher.

This was not of much practical value in Hilo, which is wet already. But it is conceivable that similar gushers exist on other islands or even on the dry Kona side of the Big Island.

Using geochemical tracers, Thomas believes the water he found at a depth of around 1,000 feet fell 2,000 years ago on the 7,000-foot level of Mauna Kea.

At the airport, newer Mauna Loa flows have covered older Mauna Kea flows. But during a period estimated at 150,000 years, the Mauna Kea lava was on the surface, where it weathered into sand and gravel.

It has since subsided as much as 3,800 feet.

The unstable gravel layer is about 1,500 feet thick — “not a very comfortable thought,” said Thomas.

As water from high on Mauna Kea worked its way down, it hit this tilted layer of porous material and the pressure of water above forced it to move through the sand and gravel, rather than continuing down.

Research submarines have now detected springs of freshwater entering the ocean a thousand feet down. (There are fresh springs — big ones — entering the ocean on the Kona side, but these are shallow and originate in a different geological process.)

Thomas says now that scientists realize how little they know about the islands, they cannot say confidently that similar fresh layers exist elsewhere. But he thinks there’s a good chance on Maui, where Haleakala lava overlies West Maui lava.

The alluvial Central Valley does not easily reveal where the best place might be to drill, but Thomas would like to do an assessment to determine a couple of likely sites.

He suspects that older islands might have an even better chance of forming the gravel layers — because they had more time to weather — so conceivably Molokai has deep entrained water.

On the other hand, the northern half of Molokai fell into the ocean long ago, with unknown effects on the deep water layers.

Lanai, which is very short of water, is a less likely candidate since it is a single volcano, with less opportunity of having a new volcano cap the weathered flank of a “sister” volcano with lava.

This new understanding, Thomas says, suggests that ordinary drillers might have missed some exploitable groundwater because they avoided areas that looked dry.

It may be that nearshore areas, depths below brackish aquifers and high saddles of mountains would hit freshwater.

Roughly, he thinks there is 10 times as much freshwater underground as was previously thought.

“I don’t want you to go out saying, aha! we have got all this water stored underground,” he told the commission.

Council Member Michelle Anderson, chairwoman of the County Council Water Policy Committee, was at the commission meeting. She asked Thomas if Hawaii County had considered making use of the water he found.

“That would be like bringing coals to Newcastle,” said Thomas, since Hilo already has billions and billions of gallons of water closer to the surface.

Maui, on the other hand, is pumping its most valuable surface aquifer, Iao, as hard as it can and still is looking for more water.

• Harry Eagar can be reached at heagar@mauinews.com.