On the surface, water seems scarce in the remote saddle region between the Mauna Loa and Mauna Kea volcanoes on the island of Hawaii. The area gets between 5 and 15 inches of rain a year, according to Peter Peshut, a biologist who manages the Natural Resources Office of the Pohakuloa Training Area (PTA), a U.S. Army facility in the region.
But below the surface, tests reported at the 2014 AGU Fall Meeting reveal groundwater pockets at high elevations. The groundwater, which could be pumped to the surface to supply much-needed water or developed into a source of geothermal electricity, could help solve the region’s water and energy woes.
An Unexpected Resource
The U.S. Army maintains a primary interest in water and energy resources to serve the overall needs of the 35- by 25-kilometer PTA. The area provides training for units of all U.S. and allied militaries and can accommodate up to 2300 people.
“Currently, the Army spends nearly $2 million annually for tanker truck operations to supply water at the remote PTA,” Peshut told Eos. However, the apparent extent of the water resource at relatively shallow depth bodes well for the development of a water production system, he said.
Not only is the water close to the surface, but its deeper reaches get progressively hotter. Temperatures at depth may exceed 250°C, enough heat to create steam from extracted water. This steam can drive turbines and generate electricity.
“Energy providers are eager to explore alternative sustainable power resources,” Peshut said. He noted that Hawaiians pay the highest price per unit of energy in the country, about $0.34 per kilowatt-hour.
However, the discovery of a geothermal resource does not guarantee a drop in prices, which would depend on private industry’s development costs and business practices as well as market forces. And even if electricity were to become more affordable, some may not welcome development of this resource.
Near Pāhoa, a half-hour drive and 1700-meter descent from the PTA, the Puna Geothermal Venture provides electricity to a quarter of the island’s population. Yet “there was a great deal of opposition to the geothermal work we had done down in Puna,” Donald Thomas, a geochemist at the University of Hawaii at Mānoa in Honolulu, Hawaii, told Eos. “The community just simply doesn’t want industrial activity in their region.”
Deepening Understanding of Island Geology
The previously accepted groundwater model held that most of the island above sea level consists of porous rocks, Thomas explained. The only stored groundwater was believed to be within less permeable, relatively limited dike systems found on the rift zones and at the centers of volcanoes.
However, a 3500-meter borehole drilled near the island’s shoreline in 2007 offered new clues to hidden water reserves. Researchers expected to find a thin layer of freshwater underlain by saltwater—indicative of ocean waters penetrating the island’s permeable rocks. Instead, they found multiple layers of freshwater down to 3000 meters below sea level.
“The very deep water near the shoreline implied that we had to have high-elevation freshwater in the interior to displace sea water to those depths,” Thomas said.
To find out, Thomas and his colleagues cooperated with U.S. Geological Survey researchers to create a two-dimensional image of a cross section of the island, showing among other factors, remotely measured electrical and magnetic resistivity from the surface to several kilometers below ground.
The image indicated groundwater at elevations as high as 1000 meters above sea level, so they dug a test well. Below the ground, “we found freshwater standing at about 1500 meters above sea level,” Thomas said.
New measures of resistivity indicated hydrothermal activity at 2-3 kilometers depth, plus evidence of magma storage at around 5 kilometers.
More Freshwater Elsewhere?
The team expects that their method could find more freshwater reserves throughout the island and on other islands in the Hawaiian archipelago. In particular, they’re testing to the north of Mauna Kea, in the nearby Waimea region, for aquifer reserves and geothermal potential. Depending on what they find, industries, government agencies, and residents could soon face choices about whether or how to develop water and energy resources.
For now, though, Thomas and his team will analyze the water they found. They will measure the total dissolved solids and sodium, potassium, calcium, and magnesium. Finding an easily accessible resource is just the beginning. “Water quality,” said Thomas, “that’s the other half of the picture.”
—Leslie Willoughby, Freelance Writer
Citation: Willoughby, L. (2015), Underground water reserves found in Hawaii’s high country, Eos, 96, doi:10.1029/2015EO022539.