A U.S. judge in Alexandria said yesterday that Fairfax County has discriminated against black and female job applicants in some categories in the past, but praised its "impressive and persuasvie" affirmative action program adopted in 1978.

U.S. District Court Judge Albert V. Bryan Jr., ruling in a suit brought by the Justice Department, rejected charges that Fairfax had engaged in systematic discrimination against women and minorities in all categories.

Bryan said that prior to the affirmative action program-approved by the Board of Supervisor in a 5-to-4 vote-there was "purposeful discrimination" against blacks in the hiring of police officers, firefighters and service and maintenance workers.

Bryan also found that Fairfax-from 1974 through 1978-had discriminated against women in hiring truck drivers, laborers, and other workers who generally perform manual labor.

Bryan said that because the county had made "convincing and satisfactory progress" in hiring blacks in the categories listed, he did not find it necessary to set and enforce numerical employment goals.

Because of what he called "persistent" disparity of women hired as truck drivers, laborers and other service and maintenance workers, Bryan prohibited the county "from engaging in any act, pattern or practice (3f) discrimination" against women in that category.

County Attorney F. Lee Ruck, whose office, along with the personnel department, was tied up by the suit for two months, said:

"I think it's a vindication of the county's affirmative action program . . . Obviously the county is not without a blemish in one category."

The county's new acting personnel director, Cornelius J. Kane, said "it's going to take a little bit of plainning" to get more women in the service and maintenance area. "We'll go out to women groups, but not just any women's group."

Kane said the service and maintenance jobs generally require "very heavy lifting. This is not a job category that traditionally attracts women's," he said.

While the county did hire 11 women in this category during 1978 exceeding its goal by one, the overwhelming number of worker-577 out of 599, or 96 percent-were male.

John F. Herrity, chairman of the Board of Supervisors, bitterly attacked the Justice Department for suing the county, but vice chairman Martha V. Pennino (D-Centreville) took another view.

"I don't agree with you completely," she said after Herrity spoke. "There is much to be done yet if everyone is going to get an equal break from the county concerning equal employment.

As Judge Bryan emphasized, the county saved itself from a more stringent action by adopting its affirmative action program, which took effect Jan. 1978. A key amendment, ceiling for specific goals for hiring blacks, other racial minorities and women, was approved by the supervisors by only a 5-to-4 vote.

During 1978-the first full year of the program-these were the minority and female hiring goals and actual hires:

Officers and administrators-The goal for minorities was four and one hired (25 percent). The goal for females was two and was met 100 percent.

Professionals-The goal for minorities was 13 and 17 were hired (131 percent). The goal for females was 16 and 43 were hired (275 percent).

Technicians-The goal for minorities was 10 and six were hired (60 percent). The goal for females was eight and 19 were hired (237 percent).

Protective services (including police and fire)-The goal for minorities was 15 and 26 were hired (173 percent). The goal for females was 12 and 11 were hired (92 percent).

Paraprofessionals-The goal for minorities was 13 and four were hired (31 percent). The goal for females was 13 and six were hired (46 percent).

Office and clerical-The goal for minorities was 44 and 27 were hired (61 percent). The goal for females was 63 and 168 were hired (267 percent).

Skilled crafts-The goal for minorities was 14 and six were hired (43 percent). The goal for females was nine and three were hired (33 percent).

One of the turning points in the case was Bryan's decision not to use the Justice Department's definition of Fairfax's labor pool as the metropolitan area, including the District of Columbia.

Instead, Bryan used Fairfax's definition, which says Northern Virginia, based on applicants, is the major source of labor. Under the Fairfax definition, there was be far fewer blacks in its labor pool because Northern Virginia has fewer blacks than D.C.