The Waldo-Hancock Bridge will be closed to vehicles and pedestrians as soon as the new Penobscot Narrows Bridge opens to traffic, and eventually it will be taken down.

The Maine Department of Transportation will monitor the bridge to ensure that it does not pose a danger, according to spokeswoman Carol Morris, but that project will have to wait until funding is available. The project will compete with other long-range projects in the state.

Initial cost estimates put the tab at about $12 million, but Morris said the department will have to conduct more specific cost estimates before starting the project.

“There are a lot of variables,” Morris said.

The department hopes to leave the concrete piers that support the old bridge in the river. The Coast Guard would have to approve that idea, but it could save the state a significant amount of money, she said.

There has been some discussion of constructing a footbridge from the western shore to the western pier as part of developing educational displays near the area around the western tower on the new bridge.

“Ultimately it would be nice to enhance that lower pylon area around the observatory as an educational and historical area that would celebrate the engineering achievement of the new bridge and the history of the old bridge,” Morris said.

The concrete piers could be a part of that overall historical display.

No one in the department is working now on developing plans for dismantling the old bridge. However, in response to questions on the DOT Web site, engineers drew up a likely scenario that would have the bridge taken down in the reverse order from its construction.

First, all elements not needed structurally would be removed and hauled off the western end of the bridge.

Second, the concrete deck would be removed.

Once the deck is removed, the superstructure steel would be dismantled and lowered to barges on the river, leaving only the towers and the cables. Using explosives, crews would drop the cables into the middle of the river. The cables would have to be pulled out of the river. Then the towers would be cut into pieces and dismantled.

The department has not developed estimates on how long the dismantling process would take.