One of the plums that can come with serving on the Fairfax County Board of Supervisors is a seat on the board of the Metropolitan Washington Council of Governments, the premier forum for attention-getting regional issues such as water supply, sewage treatment, air quality and land use.

In what she acknowledges as a power play - "I flexed my muscles" - Supervisor Martha V.Pennino (D-Centreville) says she and Supervisor John P. Shacochis (R-Dranesville) have lined up the votes to put the two of them on the COG board in place of John F. Herrity (R), chairman of the Fairfax board, and Supervisor Audrey Moore (D-Annandale). The Fairfax board is entitled to two of the 24 seats on the COG board; the posts are decided by a majority vote of the Fairfax supervisors.

Pennino said she has received pledges of support from four Democratic colleagues - Joseph Alexander (Lee), Alan Magazine (Mason), James M.Scott (Providence) and Warren I. Cikins (Mount Vernon) enough for a majority. Although most of the supervisors could not be reached, Moore conceded that Pennino and Shacochis, who is now an alternate to the COG board, have the votes, and Herrity acknowledged that "there has been some wheeling and dealing going on."

Pennino said she used direct action to marshal her apparent victory.

"I decided I wanted it," said Pennino, who previously served on the board as amembr and chairman. "I looked them both (Herrity and Moore) in the face and told them . . .They were a little taken aback.

"It certainly perplexes me," said Moore, who would like to know what Pennino, a frequent ally of liberal Democrat on the board, and Shacochis, a conservative, have in common.

"I want to stay on COG," said Moore, who often has been a minority of one in the tough stands she has taken to clean up sewage pollution and slow down growth in Fairfax. "I don't think there's any reason to take me off the board."

Herrity said he hasn't made up his mind whether he will fight to hold on to his seat. "It's been readily apparent that I differ violently with the board (of supervisors) on a whole a group of issues."

He added, philosophically, "You don't have to be on COG to be effective regionally . . . I'm not sure it's worth a big fuss, unless you consider publicity a necessary ingredient for your career, which I don't."

Pennino, however, said she wanted Herrity off the COG board so he couldn't use it as a political platform if he decides to run against Rep. Herbert E. Harris II (D-Va) next year for Harris's 8th District congressional seat. "When it comes down to partisan politics," she said, "I'm a Democrat."

Furthermore, she said, Herrity "is not interested in regional issues, apart from water." She described herself as a dedicated regionalist "who made a real contribution when I served on the COG board previously."

Pennino acknowledged that the COG post "puts me in a better position" if she decides to run for chairman of the board of supervisors in 1979.

In addition to her apparent capture of the COG post, she was elected last week as president of the general membership, which includes all officials of governments participating in COG.

Pennino has been a member of the board of supervisors since 1968. Durong the term of the previous board (1972-1976), she often was eclipsed by her more liberal colleagues who many times dominated the board. But now, she says, "the old coalition is dead. It's a new ballgame."

In the new ballgame, which is played a little more conservatively, Pennino is a respected power, as evidenced by her election as board vice chairman.

Assumong Pennino and Shacochis do have the votes, they would be appointed to the COG board when the supervisors hold their annual reorganization meeting Jan.9.

In other appointments, according to Pennino, Alexander will retain his seat on Metro's board - a plum nearly as juicy as a Metro board alternate - and Shacochis will continue as a Metro board alternate. She said she, Alexander, Shccochis and Marie B.Travesky (R-Springfield) will serve on the Northern Virginia Transportation Commission, viewed as a sort of secondary bonus.