Serious structural problems at four Metrorail stations will threaten the safety of riders if the defects are not repaired immediately, according to transit officials.

The stations are National Airport in Northern Virginia, Archives-Navy Memorial and Rhode Island Avenue in the District, and New Carrollton in Maryland.

The Metro board, worried about passenger safety and the potential loss in revenue if train service is disrupted, is taking steps to correct the problems within two years. The board's budget committee has approved spending $520,000 this year on design work, and will be asked to spend $4.3 million in the next budget year on the repairs.

Yellow Line service at the Archives station already has been interrupted several times because of rainwater that floods the station under Seventh Street at Constitution Avenue NW.

Although that is more of a nuisance, the problems at the National Airport and Rhode Island Avenue stations are more severe because they involve cracks in the structures that hold up the bridges on which the trains run.

Metro's construction chief, Edwin C. Keiser, said yesterday that while the defects require immediate attention, none is so severe that station structures are threatening to collapse.

The structural problems highlight a broader concern facing the regional transit system, parts of which have been in use for more than 14 years. At a time when local governments that support the system are having financial troubles, the Metro system will require a huge investment in the coming years for maintenance and rehabilitation.

"Unforeseen structural problems can also be expected in other Metrorail station structures as they continue to age," warned a recent staff memorandum.

At National Airport's station opposite the North Terminal, two of 78 bearings and seven beams that support the bridges are deteriorating. The structure was finished in 1975, but train service did not begin there until 1983. National Airport is served by the Blue and Yellow lines.

"These defective bearings with cracked concrete beams could settle, causing the tracks above to settle, creating a safety hazard to train operations," Metro officials said in a memorandum. Bearings help support the weight of the bridge by allowing a bending action to reduce friction and wear.

In 1985, engineers discovered that the bridge deck supporting the tracks had settled because the contractor installed the bearings incorrectly. The bearings were replaced two years later.

At the time, engineers had no way of knowing whether other bearings also were defective, because the bearings were enclosed behind concrete.

Last year, workers installed doors allowing the bearings to be inspected and discovered the new problems.

Cracks also have been found in the concrete box girders that support the bridge carrying the outbound Orange Line over the New Carrollton Metro and Amtrak station entrances. The station is north of Route 50 in Prince George's County.

If the cracks are not patched, officials said, water will seep into them and cause the steel inside the supports to corrode.

At the Rhode Island Avenue station on the Red Line, the concrete platform, track girders and columns are visibly falling apart because of the deteriorating concrete, officials said.

Replacing the concrete at the Rhode Island station will be the most expensive of the four projects, costing $2.7 million for design and construction. New Carrollton will cost $1.3 million, National Airport $700,000, and Archives $70,000.