Three studies are underway on Oahu to find sites to house 2,700 Marines from Okinawa, Japan, but it may take until 2026 to implement what is estimated to be a $2.5 billion move.

The Defense Department said it is conducting a Marine Corps Base Hawaii Optimization Study examining existing Kaneohe Bay land; an “Oahu Land Use Study” looking at defense property on the island; and a third study analyzing former defense property in the Kalaeloa area.

The Marine base at Kaneohe Bay is getting crowded — particularly with the ongoing addition of 900 aviation Marines as part of helicopter and tilt-rotor Osprey units.

All three studies are expected to be completed by December but are only the beginning of complex planning that will accompany what the U.S. Government Accountability Office now says will be the exodus of more than 10,000 Marines from Okinawa.

The GAO said in a report this month that 4,700 Okinawa Marines would be shifted to Guam, 2,700 to Hawaii, 2,500 to Australia and 800 to the mainland, with 15,000 expected to remain in Japan.

As of Tuesday there were 7,525 Marines at Kaneohe Bay, 1,734 at Camp Smith and 97 elsewhere in Hawaii, Marine Corps Base Hawaii said.

Navy Adm. Samuel Locklear III, head of U.S. Pacific Command at Camp Smith, said in testimony in March to the House Armed Services Committee that the Marine move to Guam would be completed by 2020, and the move to Hawaii by 2026.

“We continue to work with the government of Japan towards the relocation of U.S. Marine Corps forces from Okinawa in a way that is geographically distributed, operationally resilient and politically sustainable,” Pentagon spokes­woman Army Lt. Col. Cathy Wilkinson said Tuesday in an email.

The relocation of U.S. Marine Corps units from Okinawa will occur “when appropriate facilities in Guam, Hawaii and elsewhere are available to receive them,” Wilkinson said.

The plan is dependent on an ongoing supplemental environmental impact statement for Guam, as well as future construction of the facilities needed to support Marine relocation there, she said.

Environmental planning for Guam is expected to wrap up in 2015, and sometime after that a Hawaii environmental impact statement will be started, Kathleen Hicks, principal deputy undersecretary of defense for policy, said in a May letter.

In its report, the GAO, which is the investigative arm of Congress, took the Pentagon to task this month for developing a $12.1 billion “rough order of magnitude” cost estimate to realign Marines from Okinawa to Hawaii, Guam and elsewhere — excluding the cost of sending rotational forces to Australia.

Of that, the Defense Department indicated the U.S. share would be $9 billion and Japan’s share $3.1 billion, GAO said.

But the estimate “is not reliable because it is missing costs and is based on limited data,” the report said.

“We found DOD’s estimate only partially meets best practices for being comprehensive,” the GAO concluded.

Indeed, the report is titled “More Reliable Cost Estimates and Further Planning Needed to Inform the Marine Corps Realignment Initiatives in the Pacific.”

Hicks took issue with that.

“We have expressed concern with the title of this report, which suggests the department currently has the ability to provide comprehensive cost estimates and complete planning for all realignment initiatives but has failed to do so,” she said in correspondence.

She noted that the environmental studies have to be completed before the Defense Department can develop reliable cost estimates.

The flap follows 2011 cost projections for a previous plan — now abandoned — to move 8,600 Marines from Okinawa to Guam that had risen to as much as $29.1 billion for the shared U.S. and Japa­nese costs, according to the GAO.

Congress froze funding and demanded that the military re-examine the move.

Efforts to realign U.S. forces in Japan date from 1995 and discontent among Okinawans over the U.S. military presence there.

The United States also is realigning Marines to meet requirements for the “re-balance” of forces to the Pacific. The Pentagon in 2012 decided to send some of the Okinawa Marines to Hawaii, Guam, Australia and the mainland, the GAO said.

Officials said in 2012 that the old Naval Air Station Barbers Point and Pearl City Peninsula, where SEAL Delivery Vehicle Team 1 has a compound, might be looked at to house some of the extra Marines.

“These (Oahu) studies are in their very early stages; as more information becomes available, we’ll be better able to make informed decisions,” Chuck Little, a spokes­man for Marine Forces Pacific at Camp Smith, said in an email.

Part of the realignment plan includes establishing Marine Air-Ground Task Forces in Hawaii, Guam and Australia, the GAO said.

The Marines said such a task force is made up of a ground combat element (infantry, heavy weapons, artillery), a logistics element and an aviation element, all centered around a command element.

Wilkinson, the Pentagon spokes­woman, said the three studies underway in Hawaii will help develop a “master plan” for Marine Corps facilities in Hawaii, including a Marine Air-Ground Task Force.

THE LAY OF THE LAND

Three studies are underway to examine basing possibilities for 2,700 Marines from Okinawa, Japan:

>> Marine Corps Base Hawaii Optimization Study, examining existing Kaneohe Bay land
>> Oahu Land Study examining available defense property on the island
>> Analysis of former defense property in Kalaeloa