Plans to open the new $11.5 million library in Sterling and a new library in Leesburg by the end of the year are threatened by the county's fiscal plight and the likelihood that there won't be funds to pay for 75 new workers needed to staff the facilities.

The Loudoun County Board of Supervisors is expected to seek cuts of up to $12 million in county programs and services in fiscal 1992, raising the possibility that some county staff members will be laid off and all but dismissing prospects for staff expansion.

Sally Hunt, director of Loudoun County Library Services, said that although she expects both projects to be built on schedule by December -- money to build them has already been secured and construction is underway -- chances that they will open their doors to the public on time are slim.

"We've got equipment, books and buildings, everything except staff," Hunt said. "There just isn't any way we can open the libraries without any additional staff."

No final decisions about expanding the library staff will be made for about three months, but the library service is already feeling the pinch: The 65 workers currently employed are being asked to forgo any raises or cost-of-living adjustments this year, Hunt said.

County Administrator Philip A. Bolen said that "libraries are an important element in the quality of life and an integral part of our total education system," but he acknowledged that the libraries were in a difficult position at a time when the high-priority fire-and-rescue and school services also are under the gun.

Loudoun now has five libraries, which had a total budget of $2.8 million last year. The two new libraries, which together with the expansion of the Purcellville library are projected to cost more than $10 million to build, would more than quadruple the floor space of Loudoun's libraries, Hunt said.

County library officials say they are seeking ways to lessen the impact of the possible staff shortages, while keeping their vision of a modern, fully equipped library system alive.

Two weeks ago, the library service sent a letter outlining possible solutions to advisory boards across the county.

Diverting all of Loudoun's library resources to the Eastern Loudoun Regional Library in Sterling is the only way the new library could open on time and stay open 64 hours a week as originally planned, the letter said. But that would mean the openings in Leesburg and Purcellville would have to be postponed for a year, it said, and that the existing libraries in Lovettsville, Middleburg, Purcellville, Sterling, the Thomas Balch library in Leesburg, the audio-visual service and the Bookmobile would all have to be shut down.

Other solutions included delaying the opening of the new Leesburg library for a year and cutting back opening hours by almost two-thirds at the regional library, or delaying the opening of the regional library for a year, opening the Leesburg library for only 20 hours a week and opening the expanded Purcellville library in 1992.

While Loudoun's ability to provide adequate library service to its citizens is coming under growing pressure, demand for better libraries is increasing, Hunt said.

Library circulation, the number of items checked out, reached 500,000 in 1990, a 40 percent increase since 1985, she said.

Thirty-five percent of those checkouts were from the Sterling library, which is issuing library cards at the rate of 300 a month, many in anticipation of the new regional library, Hunt said.

But the Sterling library is little more than a corner in the community center with few books and no seats, and like many other libraries around the county, it cannot hope to keep up with the ever-increasing demands of a growing population.

Many involved with the county's libraries are fatalistic about the prospects. "The supervisors and the taxpayers are in a difficult position and something has got to give," said George V. Hart, president of the Loudoun County Public Library Foundation, a private group that raises funds for the library service. "We're all vulnerable at this point."

"The continued growth of the county has put pressure on the spaces and staff," Hunt said. "We're falling further and further behind the rest of the region in sophistication and quality of service."

Joan Alsop, chairwoman of the Library Board of Trustees, said, "At this point, I think the chances the {regional library} will open are slim to none . . . unless there's a miracle."