Correction: An earlier version of this story misidentified the planned intersection of the Purple and Red lines. The two will meet at Bethesda Metro Station. This version has been corrected.

It won’t happen for a long time — not until 2016 — but Metro riders in Montgomery County might as well start preparing for a major disruption. The transit agency said Thursday that it plans to close a four-station stretch of the Red Line for 14 weekends to fix a problem that routinely causes train delays in that part of the rail system.

“Water infiltration,” engineers calls it.

Meaning a leaky tunnel.

Groundwater seeping onto the track bed near the Medical Center station regularly causes malfunctions in the electrified third rails. For years, the resulting service disruptions have been a leading cause of Metro commuting headaches in Montgomery.

Once the tunnel is better sealed, Metro General Manager Richard Sarles said, riding the subway there should become a much more predictable experience.

WMATA released this video on Thursday outlining the process of making improvements to the Medical Center station on the Red Line in Montgomery County. (WMATA)

But first comes the work, which Metro engineers outlined Thursday for members of the transit authority’s board of directors.

The Medical Center and Bethesda stations, which are adjacent to each other, will be closed on the 14 weekends. At the same time, the Grosvenor-Strathmore station, north of Medical Center, will be open only for riders traveling toward Shady Grove. And the Friendship Heights station, south of Bethesda, will be open only for passengers headed in the direction of downtown Washington.

Shuttle buses will be used to ferry riders between Grosvenor and Friendship Heights.

The first seven weekends of disruption will be nonconsecutive, followed by seven weekends in a row of annoyance. Metro said it hasn’t decided when in 2016 the work will be done but that summer and fall are the most likely periods.

The uncertainty has to do with Maryland’s planned Purple Line, a 16-mile light-rail transit route in Montgomery and Prince George’s counties. The Purple Line, scheduled to be built from 2015 to 2020, will intersect with the Red Line at Bethesda, requiring the station to have to be significantly renovated.

While the station is shut down for leak repairs, Sarles said, Metro wants to take advantage of the closing by doing the Purple Line work at the same time.

Before deciding when that time will be, he said, Metro officials need to coordinate with their counterparts at the Maryland Transit Administration.

The water-logged tunnel between the Red Line’s Medical Center and Friendship Heights stations has forced Metro’s leak-pluggers and water mitigators into a resource-sucking game of Whac-A-Mole. Here’s why the tunnel is so wet:

“The Red Line in that area, as our customers know, has been problematic,” Sarles told reporters after the meeting. “We’ve come up with an engineering solution to relieve all the bad effects of having water leak into the track area.”

The repair project, estimated to cost $13 million, will involve installing large, precast panels in the tunnel to keep water out.

Why is the tunnel leaky? As the U.S. Geological Survey put it, “The potential or driving force of water intrusion into the Medical Center station . . . is due to the difference between the hydraulic head in the fractures in the surrounding rock and the tunnel, which is atmospheric pressure.” In other words, it’s complicated.

Sarles, an engineer, said, “I have worked in and around a lot of tunnels in my life” and almost all of them were leaky. The difference: “They all have pumps in them, because water flows into tunnels.” But Red Line tunnels don’t have pumps. For reasons not immediately clear, the builders didn’t see fit to install any.

So when water appears, pumps and other equipment have to be hauled in.

“Since Metro opened the Bethesda and Medical Center stations in 1984,” Sarles’s staff said in a report, “water infiltration in this section of the Red Line has caused operation disruptions and has required extensive maintenance over the years to control it. The tunnel requires ongoing pumping, dredging and cleaning to keep switches in service” and prevent electrical problems. The cost? About $4 million a year.

Before the repair project can begin in 2016, Metro needs to complete additional engineering work and solicit bids from contractors, Sarles said. When the work is finished, the problem should be fixed “for decades,” he said.

“What this basically is, we have a leaky roof,” Sarles said. “It’s analogous to your house. And we’re putting a new roof in. After that, it’s just a matter of maintenance.”

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