Testing by the Navy on one of its massive underground fuel tanks at Red Hill suggests that tank corrosion at the facility is more extensive than it predicted, elevating concerns among Honolulu Board of Water Supply officials and environmentalists about the aging facility’s potential for leaks and the risk that poses to Oahu’s drinking water.

As part of an agreement with federal and state regulators, the Navy sampled 10 steel plates from one of its 20 tanks at the Red Hill Underground Fuel Storage Facility near Pearl Harbor. At least five of the plates, which measured 1-foot by 1-foot in size, showed considerable corrosion, according to the Board of Water Supply, which was briefed by the Navy on the findings. More troubling, parts of the tank’s original, quarter-inch-thick steel liner have become extremely thin and it appears the Navy overestimated the remaining thickness of some of the plates.

For instance, the Navy had anticipated the thickness of one steel plate to be between 0.135 to 0.187 inches. However, testing in June indicated that the remaining thickness was about half of that. The steel liner is the only thing separating the fuel from a release into the environment.

“The Navy’s predictions are so far off, and the corrosion is so far gone, that the risk of another significant leak, potentially catastrophic, is far more severe than previously thought,” said Marti Townsend, director for the Hawaii Sierra Club, in a press release. “Once the fuel escapes there is no way to get it back. It is irresponsible to continue the current course of action on these tanks.”

Townsend said the Navy needed to retire the tanks and relocate the fuel away from drinking water supplies.

Department of Health officials have said that fuel leaks, including a 27,000 gallon release at the facility in January 2014, are nearly impossible to clean up.

Officials have worried that past and future fuel leaks could migrate to an aquifer that supplies drinking water to residents from Moanalua to Hawaii Kai. The aquifer sits just 100 feet below the tanks. Fuel leaks also pose a risk to a nearby Navy well that supplies drinking water to about 65,000 people at Joint Base Pearl Harbor-Hickam.

If the aquifer is polluted by a major failure at one of the Red Hill tanks, the Navy’s own studies indicate the cleanup of the critical water supply could take decades or be cost prohibitive.

The corrosion testing is required as part of an agreement that the Navy entered into with the U.S. Environmental Protection Agency and the Hawaii Department of Health after the 2014 fuel release.

The Navy and regulators are also in the process of assessing six options for improving the tanks, including building new tanks, constructing a tank within a tank and double lining the tanks. The Navy informed regulators in August that it would likely be recommending the option that its own report described as involving “minimal changes to the status quo.” The corrosion testing is likely to increase pressure on the Navy to choose more aggressive tank protection options.

A plan for tank improvements will have to be approved by regulators who are expecting the Navy to take into account its corrosion study, as well as studies on leak- detection methods, in making its formal recommendation, which is due at the end of the year.

Ernie Lau, manager and chief engineer of the Board of Water Supply, said that the findings, which have not been released in a final report, raise concerns about the Navy’s current methods of inspection and repair of the tanks. The Board of Water Supply is urging the Navy to expand its corrosion sampling to more tanks.

The Navy didn’t respond directly to an emailed question from the Honolulu Star-Advertiser asking whether it would be willing to test for corrosion in additional tanks. Instead the Navy issued a news release, stating it would brief elected officials and regulators on the latest information during an annual state Senate task force meeting on Oct. 3.

The Navy stressed that the corrosion study is “ongoing and has not been completed,” with final results expected in late October and warned against making conclusions based on incomplete information.

“Ensuring tank integrity is the top priority and key to the exhaustive review approach we’ve agreed to,” said Lt. Cmdr. Blake Whittle, fuels director at Fleet Logistics Center Pearl Harbor, in the news release.

In recent years the Navy and the Defense Logistics Agency have spent $45.3 million to help identify the best alternative to improve the facility, while conducting groundwater studies, the Navy said. An additional $260 million has been spent since 2006 on maintaining and modernizing the facility and conducting environmental testing.

“We have not and will not rush to judgment or conclusions, and we will continue to keep the drinking water safe, no matter what,” said Rear Adm. Brian Fort, commander, Navy Region Hawaii and Naval Surface Group Middle Pacific. “There is much more work to do in studying, analyzing and then implementing all the right initiatives at Red Hill.”