Incumbent Nancy K. Falck defeated conservative Lloyd L. Thoburn in yesterday's Republican primary for the Dranesville District seat on the Fairfax County Board of Supervisors in a close race marked by big spending and harsh rhetoric.

Falck will face Lilla D. Richards, a civic activist who demolished her opponent, Franklin S. Holland, the assistant manager of a Vienna vacuum cleaner and sewing machine repair store, in the Democratic primary.

Falck won 53 percent of the vote to Thoburn's 47 percent, while Richards garnered 93 percent of the vote to Holland's 7 percent.

In the Springfield District, former School Board member Toni M. Carney won a landslide victory over civic activist Beatriz (B) Garcia in the Democratic primary for the county board, capturing 62 percent of the vote with all ballots counted. Carney will face incumbent Supervisor Elaine McConnell, a Republican, in the Nov. 3 elections.

In addition, three persons filed before yesterday's 7 p.m. deadline to run as independents in the November election.

Thomas E. Giska filed to run in the Springfield District, and Daniel Edward Belsole filed to run in the Mason District against Supervisor Thomas M. Davis, a Republican. Davis has no Democratic challenger. Former state delegate Robert L. Thoburn, father of Lloyd Thoburn, filed to run in the Dranesville District.

Before the primary, Republican officials had said that if Falck, an eight-year incumbent, won less than 65 percent of the primary vote, it might be a sign that she would be vulnerable in the November election. Falck had said that she hoped to receive "in the high 50s."

Last night, Falck said that Lloyd Thoburn's campaign had engaged in "smears," adding, "Despite all the tricks, I still pulled off a landslide."

Democratic Party sources said that Lloyd Thoburn may have benefited from a crossover vote by Democrats who cast ballots in the Republican primary to boost the chances of Thoburn, who they thought would be easier for Richards to beat in November.

Voters in Virginia do not register by party and can vote in either party's primary. Technically, they are supposed to support the winner of the primary in which they voted, but they are not obligated to do so.

Voter turnout was low in the Springfield primary, where about 1,650 people voted, or 3 percent of those eligible to cast ballots. About 15 percent of those eligible in the Dranesville District voted yesterday.

Candidates in the three Fairfax County supervisor primaries focused on transportation and development issues. Though the campaigns were generally low key, harsh rhetoric was often traded, with some candidates attacking their opponents' records and questioning their political motives.

The campaigns of Lloyd Thoburn, 25, a religious conservative and administrator of his family's Fairfax Christian School, and Falck, 57, took on unusually personal tones.

Thoburn spent more than $28,000 -- $19,000 for printing and postage alone in the last month -- and Falck spent more than $13,000, on their campaigns.

Thoburn accused Falck of "pure spite" in voting against rezoning requests that his family brought before the county board, saying she opposed them because she does not like the family's conservative politics.

Falck, who at the time said she opposed the rezonings for environmental and land use reasons, dismissed the family during the campaign as "rich developers" and said Thoburn was challenging her in the primary to get even.

In contrast, the Richards-Holland race was mild mannered. Each candidate refrained from criticizing the other and focused instead on what they claimed was Falck's failure to plan adequately for the tremendous growth in the Dranesville District.

Democratic Party regulars viewed Holland, 31, as a bit of a curiosity.

A self-described conservative Democrat, Holland said he was essentially a single-issue candidate who was running for office because of his anger over local transportation problems.

Richards has been a fixture in local land use debates for nearly 20 years, earning praise for her knowledge of local affairs and county land use policies.

In the Springfield District, Carney, 45, had the backing of most party regulars in the Democratic primary, although Garcia supporters questioned Carney's party loyalty.

They noted that from 1978 to 1984, Carney was the School Board appointee of Republican Supervisor Marie B. Travesky, who was unseated four years ago.

Carney, in response, said she had always considered herself an independent. She joined the Democratic Party last summer.

Local party officials, who noted that switching parties is common in Virginia politics, voiced concern that a debate about Carney's past political affiliations could disrupt the party's unity and damage the Democrats' chances of defeating McConnell in November.