Architect Maya Lin, who as a student seven years ago designed and fought for the stark black granite Vietnam Veterans Memorial, yesterday found herself defending it once again. Before the Senate subcommittee on public lands, national parks and forests, she argued against the addition of yet another statue to the memorial grounds, this one to honor women veterans.

"It's been dedicated, and I think it's done," she said. "In allowing this addition you substantiate the assumption that our national monuments can be tampered with by private interest groups years after the monuments have undergone the proper legal and esthetic approval processes."

Still, she was not optimistic about prevailing upon the committee, which yesterday held its only public hearing on the proposal. After her testimony, wearing a look of resignation, Lin said she had "no doubt" that a bill authorizing the statue's installation would be approved. "It's going to happen," said the 28-year-old designer of the five-year-old monument. "And I just don't see where it will end."

Lin's testimony was supported by J. Carter Brown, chairman of the Commission of Fine Arts, and by Reginald W. Griffith, executive director of the National Capital Planning Commission, federal planning bodies that have ruled against the proposal recently.

Support for the statue came from 10 of the 18 witnesses, including representatives from such major veterans groups as the American Legion, Veterans of Foreign Wars, Disabled American Veterans and the Vietnam Veterans of America.

Committee staffers predict easy Senate passage, although there is no companion House bill yet. "It will be a tricky thing to be opposed to a bill that honors women," said a senior staffer. The bill was introduced last October after the Commission of Fine Arts rejected the proposal.

Yesterday was yet another of many conflicts for the memorial, which has become Washington's most popular monument, attracting more than 1 million visitors yearly. When Lin's design was chosen in 1981, it attracted a thunder of opposition. Then-interior secretary James Watt held up its construction permit until a compromise was reached and a statue and flagpole erected on the site.

The newest proposal -- a lone Army nurse -- came from the Vietnam Women's Memorial Project. Donna-Marie Boulay, who heads that group, testified that "a women's statue is appropriate ... We lived and worked amid the wounded, the dying, the dead."

Though many agree with the idea for a memorial to women, and point to a 1986 law authorizing a memorial honoring women veterans of all wars, they oppose this one because it would disturb the design.

"If this year's legislation says we stop with just one addition," said Carter Brown, voicing fears that this will open the gate for special interest groups, "what does next year's legislation say?"

Sen. David Durenberger (R-Minn.), who introduced the Senate bill along with Sen. Alan Cranston (D-Calif.), tried to calm those fears, pointing out a clause stipulating no further additions. "I want the record to show that the sponsors of S. 2402 consider this statue a fitting way to complete the Vietnam Veterans Memorial."

Opponents to this proposal also are concerned with Rep. Robert Dornan's (R-Calif.) bill, which seeks to have a tall flagpole at the memorial's apex.