Northern Virginia voters returned Democratic incumbents to local government seats in Arlington and Loudoun counties and approved nine Arlington and Fairfax county bond issues yesterday, according to unofficial election returns.

Ellen M. Bozman, a Democratic-backed Arlington County Board member, easily defeated Republican challenger Robert E. Harrington. Democrat Betty W. Tatum won narrowly over Republican Bob Scheetz in a special election for a Loudoun Board of Supervisors seat.

The bond proposals, five in Arlington and four in Fairfax, were accepted by generally wide margins.

In Arlington, Bozman, 56, won a third term on the County Board, now controlled by a 3-to-2 Republican majority. Harrington, 63, a retired Air Force colonel and school business manager, was making his first bid for elective office.

In her campaign, Bozman attacked the board's GOP majority, accusing it of using "backroom" tactics in firing former county manager Vernon Ford last August. News of his dismissal, Bozman charged, was withheld for four days, threatening Arlington's open-government tradition.

Harrington campaigned for tight tax and spending policies and sought to point up Bozman's ties to the liberal Arlingtonians for a Better County (ABC). "Are we going to hold the line on taxes . . . or do we go back to the days of the ABC Democrats with excessive spending and skyrocketing taxes?" he asked.

In Loudoun, Tatum, 40, held off a challenge for the board seat to which she was appointed last July. She had been named to fill a vacancy left by the resignation of former supervisor Gerry Gardner, Loudoun's first woman member. Scheetz, 46, a former supervisor, recently switched to the Republican Party.

The contest in the Guilford District at the county's more developed eastern edge took place amid GOP efforts to make further inroads in traditionally Democratic Loudoun. Democrats have dominated the Board of Supervisors by a 6-to-2 margin.

Scheetz, a real estate broker, focused his campaign on a controversial proposal for a Sheraton hotel and conference center on Rte. 7 in Loudoun. Tatum, a former planning commission member, voted against the proposal when the commission considered it but now says she has an "open mind" about a modified version of the plan. Scheetz favored the hotel complex, saying it would provide tax revenue and jobs. He has accused Tatum of being "not sufficiently aware of what economic development means."

A $16.2 million bond proposal to finance jail expansion was approved in Fairfax, where voters rejected a similar plan last year. The bond issue is intended to finance a three-story addition to the jail, which county officials say is severely overcrowded.

Another $7.2 million bond proposal accepted by Fairfax voters will pay for building a medium-security correctional camp for persons convicted of nonviolent crimes. It had stirred strong opposition and the Board of Supervisors was split on the issue. County officials have predicted a bitter fight over the correctional camp's location.

Fairfax voters ratified two other bond proposals -- $57.2 million for school construction and renovation and $30 million for improvements in secondary roads.

In Arlington, voters approved a $3 million bond issue to finance improvements in neighborhood parks and pay the county's share of regional park projects. Arlington voters turned down a $4 million park bond proposal in 1979.

Another $2.1 million Arlington bond issue ratified yesterday will finance expansion of the county's overcrowded jail, with 48 additional cells.

Arlington voters accepted three other bond proposals -- $3 million for street and highway improvements, $1.8 million for storm drainage improvements and $1 million for community conservation projects.

The conservative Arlington Taxpayers Alliance backed the jail bond proposal but opposed the others. All five bond proposals were supported by the County Board and the Arlington Civic Federation, a coalition of county associations.