The Carnegie Library, an ornate Washington landmark at Mount Vernon Square, was seriously damaged by a cascade of water and mud that filled its basement and utility area during a severe thunderstorm two weeks ago, officials of the University of the District of Columbia said yesterday.

The 84-year-old building, which houses UDC administrative offices and a graduate library, is closed until mid-September while extensive repairs are made to its heating, air-conditioning and electrical systems, officials said.

University trustees have budgeted $250,000 for the cleanup and repair work, but the total cost of the loss, including damage to library files and archives, is uncertain, according to Edward Holland, UDC vice president for administrative services.

"Unfortunately, we needed an ark and we had a library," Holland said, alluding to the torrential rain on July 11. "It's shame to see something like this happen. We put a lot of money into that building."

In the late 1970s, the District government spent $4.2 million to renovate the Italian-style marble structure, which had served as the city's main public library for 69 years.

Where the damaging mud came from is a matter of dispute.

Holland and other UDC officials say it came from a massive mound of dirt -- excavated during Metro subway construction -- that occupies more than half a city block just north of the library. The university has asked the District government to seek reimbursement from Metro's contractor, Holland said. But the construction company that piled up the dirt between Seventh and Eighth streets NW, north of Mount Vernon Square, said dirt from its pile did not damage the library.

"We didn't see any indication that any material went off the pile and into the {storm} sewer or the building," said George E. Howard, project manager for the Perini Corp. of Framingham, Mass., which is constructing a $47 million section of Metro's Green Line on Seventh Street.

The rain "picks up some residue and mud from a lot of places," Howard said. He said the mound -- about 20 feet high in some places -- has occupied the city-owned block without incident since mid-1984. He said he expects that it will be leveled in about a month when the dirt is used to cover the Seventh Street subway tunnel between M and Q streets NW, which is almost complete.

During the July 11 storm, which dumped nearly 1 3/4 inches of rain in an hour, storm sewers near the library became clogged with mud, causing water to pour into the building through an entrance under its main outside stairway, Holland said.

He said the water rose to a depth of 12 feet in a sub-basement room containing the boiler, electrical panel and air-conditioning equipment, then covered a basement library area and community meeting room to a depth of about four inches.

He said the D.C. Fire Department's pumping equipment broke down twice, prompting the university to hire a company to pump out the water.

Since the flood, the building has been without electricity. Among those displaced are the staff of UDC's board of trustees and incoming president Rafael L. Cortada.