The least expensive method of double-walling the Red Hill fuel tanks could cost between $500 million and $2 billion, the Navy said in a newly released estimate.

The most expensive tank-within-a-tank option, with carbon steel and full interior and exterior coating, is estimated at between $2 billion and $5 billion.

Replacing the World War II-built tank farm with similarly fortified underground or concrete-encased fuel tanks would run as much as $10 billion.

The costs are part of an “administrative order on consent” entered into by the Navy, Defense Logistics Agency, U.S. Environmental Protection Agency and state Department of Health to upgrade Red Hill after 27,000 gallons of fuel spilled from Tank 5 in 2014.

The Red Hill Bulk Fuel Storage Facility, with 20 giant underground tanks that cumulatively hold 250 million gallons of fuel, is deemed by the military to be an essential facility for operations across the Indo-­Asia-Pacific.

At a Red Hill public information “workshop” last week, Waianae resident Kapua Keliikoa-Kamai said, “If we can spend billions of dollars in a war in somebody else’s country, we need to spend billions of dollars for national defense here in my home.”

Keliikoa-Kamai favors secondary containment — in this case double-walling the tanks — in five years. The Sierra Club of Hawaii has a similar stance. The consent agreement requires that whatever upgrade is selected needs to be completed by September 2037.

In December the Navy submitted a tank upgrade study that examines three single-wall options and includes improved repair and restoration as the most basic option, as well as two double-wall and one tank-within-a-tank designs.

The cheapest single-wall option is projected by the Navy to cost between $180 million and $450 million.

Costs were made available at the workshop at Moanalua Middle School and are broad in range so as to not affect bidding for the eventual projects, the Navy said.

The Navy last month also completed a Red Hill “alternative location study” that looks at 12 potential relocation sites for what it said would be an “extraordinary project” to duplicate in 40 tanks the same 250 million­-gallon capacity that exists now.

The replacement facility would need to be hardened to the same level as Red Hill — which is buried 100 feet underground — to withstand missile or electronic attack, the Navy report said.

That would mean similarly burying the new tanks 100 feet below ground or installing them at a shallower depth and covering them with the equivalent thickness of concrete.

The 12 sites include the favored location — Kapukaki — which is near the Red Hill tanks and adjacent to pipelines to Joint Base Pearl Harbor-Hickam. The Sierra Club said the site is immediately mauka of the fuel farm.

The Navy report said the Kapukaki option, which would not be part of the consent agreement 2037 deadline, would “more likely” be completed by late 2051.

Other locations — including some the Navy admits are clearly not viable — are Hickam field, the Navy-Marine Golf Course, Makalapa Crater military housing area, Salt Lake District Park, Aliamanu Military/Coast Guard Reservation, the commercial quarry north of Red Hill, adjacent to Tripler Army Medical Center, adjacent to Fort Shafter, Campbell Industrial Park, Lualualei Naval Magazine and a Navy facility between Marshall and Namur roads.

Salt Lake District Park sits on the edge of a caldera that would provide some natural cover, but “the fact that it is a local public park is sufficient reason enough to reject it as a viable location,” the Navy report said.

The use of Lualualei on the Waianae Coast would require a new, 25-mile fortified pipeline tunnel, meanwhile.

Marti Townsend, director of the Sierra Club of Hawaii, said the proposal the Navy is favoring for relocation “is just putting 40 tanks uphill from the existing 20 tanks — and that doesn’t seem to be a significant improvement on our situation.”

The Sierra Club said more than 200,000 gallons of fuel have leaked from Red Hill since 1943. In an opinion piece, Townsend said the Health Department has admitted that storing millions of gallons of fuel 100 feet above the Southern Oahu Basal Aquifer, the principal source of drinking water for 763,000 residents, is “inherently dangerous.”

Townsend said she wants to see “secondary containment at Red Hill as soon as possible,” adding, “We’re asking for five years.” But even 10 years would be better than what’s currently proposed, she said.

“The Navy needs to put the funding in place that represents how significant they think these tanks are to their mission,” she said. “If it’s that mission-critical, then put the money up.”

A circuit judge’s ruling last month that Red Hill is not exempt from state fuel tank upgrade laws gives the Health Department a chance to “take a stronger position with the Navy” on repairs, Townsend said. A new state Senate bill, meanwhile, would give “new urgency to bringing the Red Hill fuel tanks into compliance,” she said.

Much of the concern over Red Hill centers on the tanks’ location 100 feet above the water supply aquifer, which is in saturated volcanic rock. The Navy thinks deep claylike barriers to any leaked fuel could extend through the aquifer in valleys separating the Board of Water Supply’s Halawa and Moanalua water shafts from Red Hill.

Beneath Red Hill’s 20 feet of concrete is lava rock that in some cases is solid and elsewhere is like cinder blocks. Leaked fuel may have become bound up in concrete and rock and/or attenuated by microbial action, the Navy said.

But the Health Department and EPA recently warned the Navy that it may be “drawing conclusions prematurely” about groundwater flow, and to make sure the data is “scientifically rigorous.”

The Navy’s own well for Joint Base Pearl Harbor-­Hickam is closest to the Red Hill fuel farm. “Very, very low” levels of fuel constituents have been found in a groundwater monitoring well, but no hydrocarbons have been found in the drinking water source itself, the Navy said.

“The water from the Navy’s well has always been safe to drink — all the way back through the requirement to provide water testing when the Safe Drinking Water Act came into existence. The water has always tested safe to drink,” said Capt. Rich Hayes, commander of Naval Facilities Engineering Command Hawaii.

The Navy said the Red Hill tanks are not leaking and that is has spent more than $250 million improving the facility since 2005.

The Board of Water Supply said in February that all three single-wall upgrade options are “not sufficiently reliable given the history of leaks at the site.” Other than tank relocation, only the tank-within-a-tank option “could reliably protect the aquifer.” That option would cost between $2 billion and $5 billion.

The Health Department and EPA are reviewing the tank upgrade alternatives. Approval of a final plan could come by the fall, the Navy said.

Then “we’ve got to go to Congress and say, ‘All right, this is what everybody has agreed to. Give us the money to make it happen,’” said Mark Manfredi, the Red Hill program director. “Then we’ve got to go through the detailed engineering planning. So we probably would not actually start construction until maybe the 2020-2022 time frame, unless that could be accelerated.”

With double-lining, the first two or three tanks could possibly be done in about three years, he said.

“Some tanks would be completed in a few years; then a few years later there would be another batch of tanks done,” Manfredi said. Any tanks not done by 2037 would be drained and, because they have no fuel, could be worked on past 2037, he said.