Metro's Medical Center Station is acting like a sink, drawing groundwater that would normally flow into Rock Creek but instead is streaming toward the Red Line stop.
That's one of the observations made by experts studying the way water is seeping into the Metro, a longtime problem that has become a costly headache for the transit system.
Scientists at the U.S. Geological Survey have been investigating the leaks for more than a year under a $100,000 contract with Metro. Their findings, which are expected in early January, should help the transit agency to begin to figure out a way to stop or deflect the leaks, Metro officials said.
The Metro subway is one of the deepest in the world, with all tunnels and underground stations at or below the water table. The water problem is most serious and getting worse along a nine-mile stretch of the Red Line from Farragut North to Medical Center in Montgomery County.
Water is eating away at the track bed, the power system, electrical components and the steel girders that support fire pipes, communications cables and power lines throughout the subway tunnels. Rail fasteners are rusting, and communications cables are corroding.
Mineral deposits left by dripping water have built up along the track bed, turning insulators into conductors and interfering with the proper flow of electricity from the 750-volt third rail to the trains.
Of Metro's approximately 40 miles of mined subway tunnels, nine are severely wet, 18 are moderately wet and 12 are dry, Metro officials said.
The water problem, flagged by Metro as a concern as early as 1985, has accelerated at an alarming rate, according to Metro General Manager Richard A. White. The number of leaks plugged by Metro workers has ballooned from about 500 a year in the 1980s to a projected 4,600 this year.
Every month, Metro pumps about 1.25 billion gallons of water from the subways, the equivalent of 1,786 Olympic swimming pools.
Water and its mineral deposits are damaging electronic and mechanical equipment, requiring replacement well before the end of its normal life cycle.
In the current fiscal year, Metro expects to spend $1.83 million to grout 4,600 leaks and $3.9 million to replace 28,000 feet of rail, 300 insulators and 8,050 track fasteners.
Metro engineers say the water is not yet threatening the structural integrity of the stations or subway tunnels.
The water pouring into the stretch between Farragut North and Medical Center has as much to do with geology as it does with the engineering techniques available when that section of the Red Line was built in the 1970s.
The troubled section was mined through rock beneath Rock Creek. The rock has joints, fractures and tiny channels that allow groundwater to flow down, hydrologists say.
The section from Farragut North to Medical Center is the only portion of Metro that was built in rock without a protective waterproof liner around the concrete tunnel. Tunnels built later were wrapped in a waterproof lining of polyvinyl chloride, which catches water and diverts it around the tunnel into a drainage system.
Engineers who designed the subway from Farragut North to Medical Center wrongly believed that if the tunnel concrete was dense enough, it would deflect water and no protective liner would be needed, Metro officials said.
The engineers also didn't realize that the Medical Center Station was carved through an igneous, or volcanic, rock known as saprolite, while the rest of the nine-mile stretch was carved through bedrock.
Earl A. Greene, groundwater specialist at the U.S. Geological Survey, said the fact that Medical Center was carved through saprolite may explain the heavy flooding at that station. He and other scientists are studying whether the saprolite is more permeable than the bedrock, allowing water to move through it more easily.
"Our hypothesis is that geology is influencing a lot of this," Greene said.
Greene said it is clear that the water leaking into Medical Center comes from the ground and not from nearby Rock Creek. Greene and a team of hydrologists and geologists have drilled holes around the station to measure the water and examine the way it behaves. They are also mapping fractures in the saprolite and trying to determine whether they are connected and whether water is moving from fracture to fracture.
"We're like detectives," Greene said. "We're gathering lines of evidence that we can put together to tell a story."