Metro's tunnels have more than 1,100 serious water and silt leaks that threaten to slow train service on the regional subway system, officials said yesterday.

An additional 2,300 leaks plague the system, but they aren't as severe, Metro officials told the transit board's operations committee. Most of the leaks occur on the Red Line, which has more miles of tunnels built in rock that is more susceptible to water damage.

Officials said it would cost about $1 million to repair most of the leaks over the next 15 months, but they said more will have to be spent in the future as other leaks develop.

They said the problem, if unchecked, could cause track to erode. In extreme cases, seeping water could cause a cavity in the earth that could cause the street above to cave in, said Metro construction chief Edwin Keiser.

After viewing several slides yesterday showing some of the worst damage, Metro board member Joseph Alexander, of Fairfax, said, "To me, that's a gross situation that should never be allowed to happen."

Problems with leaks point up the broader concern of how Metro is going to handle the constant need to maintain the system when the agency also has to operate and complete construction of the planned 103-mile system.

Under a five-year plan approved this year, local governments will contribute $200 million for repairs and upkeep. Nevertheless, rehabilitation costs over the next 20 years will be more than $100 million a year. Replacing 300 rail cars by the year 2012 will cost $1.25 billion.

Leaks occur because the tunnels are built below the water table. Metro has changed to a new method of building tunnels that seals out many leaks, but the problem persists on most of the older tunnels built in the last 15 years.

In addition to being unsightly, the leaks cause the system's steel and metal parts, such as the tracks, to erode. In extreme cases, Metro has had to shut down the track and use a single track to run trains in both directions, which slows operations.

Of the 1,100 serious leaks, 717 are on the Red Line, 215 on the Orange Line, 155 on the Blue Line and 13 on the Yellow Line. Of the 2,300 less damaging leaks, 2,135 are on the Red Line.