The Fairfax County Board of Supervisors unanimously rejected yesterday construction of a controversial sewer line proposed for the Mason Neck area, an environmentally sensitive region that area residents claim would be runied by the pipeline.

The sewer line was intended to link about 150 families in the Harbor View neighborhood of Mason Neck to the county's sewer system. The families now are served by a private treatment plant whose service costs them more than double the rate for county sewer service.

The county will hold a public hearing Jan. 8 before deciding whether to subsidize the private sewer operation, run by Utilities Inc. of North Brook, Ill., in order to lower sewer bills of Harbor View residents. A Harbor View family pays about $288 a year for sewer service compared to the typical Fairfax family bill of about $120 a year for county sewers.

The board also vetoed taking over operation of the private plant, which would have cost the county about $110,000 a year, according to county officials. Subsidizing the plant would cost the county about $42,000 a year.

In the meantime, Mason Neck environmentalists consider the rejection a victory for the area, the home of the Southern Bald Eagle and a variety of other wildlife.

"That sewer line was a great treat to the Neck," said Eliabeth Hartwell, a leader in preserving the largely undeveloped area in the southernmost point of Fairfax. "It would have opened up all kinds of development here that nobody could have stopped."

In another land issue the board approved the development plan for 198 acres in the Centreville area to build 803 single-family houses in place of 1,753 town houses and garden and highrise apartments originally proposed by the Yeonas Co.

The board approved the plan for the area south of Dulles International Airport near the intersection of I-66 and Lee Highway by a 5 to 4 vote after having rejected the same plan last week by a tie vote. The rejection of the lower-density plan was unusual for the supervisors, who traditionally favor less building than developers want.

Supervisor Joseph Alexander (D-Lee), who was absent during last week's vote, broke the tie yesterday, saying he approved of the lower density plan.

The Yeonas Co. had abandoned its higher density plan, saying it was not economically feasible since a once planned shopping center will not be built nearby.

Supporters of the earlier plan said it would have increased the availability of cheap housing.

The board also tackled commercial and industrial development yesterday, approving $206,908 for the county's Economic Development Authority to continue an advertising campaign to attract business to Fairfax.

The supervisors in February approved $559,482 for the campaign that began July 1. Authority officials told the board that 122 businesses have shown "active interest" in moving to Fairfax as a result of advertising in national magazines including Business Week and the New Yorker. They said they expect nearly 170 more businesses to announce interest by next summer in moving to Fairfax.