Amid ongoing fuel leaks between Red Hill and Pearl Harbor, the Navy said it is investing over $750 million in improvements to its aging fuel storage tanks that lie 100 feet above the Moanalua-Waimalu groundwater aquifer — while also pursuing a double-wall tank experiment.

At a state fuel tank advisory committee hearing held Thursday over Zoom, the Navy said that on July 16 a release of about 150 gallons of marine diesel into Pearl Harbor was detected at Kilo Pier due to corrosion that created a small hole in a pipe.

The state Department of Health was immediately notified, 110 gallons was recovered and the pipeline was emptied, said Capt. Bert Hornyak, commander of the Fleet Logistics Center at Pearl Harbor.

While a small leak, it is the latest in a series of fuel releases from the Red Hill Bulk Fuel Storage Facility on down to Pearl Harbor that has the Sierra Club of Hawaii clamoring for relocation and the Navy struggling to keep the World War II-built fuel farm operating where it is as “critical to the national security of our nation.”

“Operator error” caused the release of 1,618 gallons of jet fuel at Red Hill on May 6, the Navy said. Between March 2020 and May 2021, 7,100 gallons of fuel was recovered from the Hotel Pier area. In 2014, meanwhile, 27,000 gallons of fuel spilled from Tank 5.

In 2019 the Navy recommended a restoration plan for the Red Hill tanks and added leak prevention, detection and mitigation, which the Sierra Club said was “the least protective” of six upgrade options that also included interior coatings, composite tanks and full tank-within-a-tank.

On Thursday, Capt. Gordie Meyer, commander of Naval Facilities Engineering Systems Command Hawaii, continued to push that restoration plan as “best available practicable technology” while also highlighting the promise of a double-wall test.

Meyer said that “dual path” includes an agreement with Gaztransport &Technigaz North America, known as GTT NA, to explore secondary containment at Red Hill with a system used for oceangoing tankers transporting liquefied natural gas.

“The initial proposal uses the existing (Red Hill) tanks only as support and anchoring points,” Meyer said. “The system would provide a new outer stainless steel membrane, new inner membrane and a monitored interstitial (space) between the two.”

The technology “continues to prove promising” with a pilot project in Tank 1 expected to be upgraded to a secondary containment system starting in the fall of 2023, he said.

The two-year project could lead to “full-scale construction of the remaining tanks” using the same approach, according to Meyer. The proposal by GTT NA would require no excavation atop the tanks, which are buried under 100 feet of volcanic rock, the Navy said.

GTT NA’s “solution for access to the Red Hill tanks will be through the existing eight-foot upper hatch in each tank,” according to Navy Region Hawaii.

The 20 tanks are 250 feet tall. Each can store 12.5 million gallons of fuel. Meyer said 18 tanks are operational or going through the clean, inspect and repair process.

He said all tanks in serv­ice continue to pass twice-yearly “tightness” testing.

“It is critical to continue to have the tanks inspected with current approved standards while several research and development efforts are in progress — pursuing both paths simultaneously,” Meyer said during the virtual meeting.

“We do not want to sacrifice the current upgrades with available technology today while we await new technology. We need to do both,” he said.

Fuel tank advisory committee member Melanie Lau asked for an update on possible alternative sites for Red Hill, noting that old underground fuel tanks are being relocated above ground at Naval Base Kitsap in Washington state and Naval Base Point Loma in California.

“There is practicable technology available,” she said.

Meyer said those tanks “are very different than what we have here at Red Hill.” The mainland tanks were built “just below the ground and covered with earth.”

With Red Hill, which feeds Joint Base Pearl Harbor-­Hickam by gravity, “It’s not just the tanks that need to be relocated — it’s the entire fuel distribution system. That also is a consideration.”

There also is the security of having the Red Hill tanks buried so deep, he said. “Again, these are strategic resources. Moving them above ground causes that issue and concern,” Meyer said.

But Lau said, “I disagree with you in terms of it being a site that is protected.” The whole reason to have Red Hill underground during World War II “was to protect it from being bombed,” she said, adding that “warfare has improved since then, and everybody knows where the tanks are now.”

Construction on the tanks was finished in 1943. The Honolulu Board of Water Supply has taken the position that tank-within-a-tank storage is needed or the fuel farm needs to be moved to protect the aquifer.

In 2018 the Navy estimated that the least expensive method of double- walling the Red Hill fuel tanks would cost between $500 million and $2 billion.

The most expensive tank-within-a-tank option was pegged at $2 billion to $5 billion. Replacing the tank farm with concrete-encased tanks elsewhere would run as much as $10 billion.