When the issue was water conservation, he brought a portable toilet into the board room. When it was the Lorton prison, he hopped over a low fence to talk to some guards. When it was a question of school safety, he demonstrated how to use the crossbows that some students had made in a shop class.

Those may not have been the high points of Fairfax County Board Chairman John F. Herrity's biography, but it was enough yesterday to win him the "1983 Grandstander of the Year Award."

Herrity, a Republican, good naturedly, if somewhat unhappily, accepted the award, presented by Democratic rival Sandra L. Duckworth, the board's Mount Vernon District supervisor.

"Oh my," sighed Herrity, as he made his way to a podium to accept a plaque and a minature grandstand from Duckworth. "Just what I needed. Christmas comes early."

Some of other supervisors laughed uncomfortably. "Sandy, you're going to be getting runner-up, the way you're going here," said Supervisor Thomas M. Davis, a Republican from the Mason District.

Some supervisors, including Duckworth, have complained that Herrity often tries to horn in on their issues. Herrity replies that, because he is elected at-large and lacks his own district, he is bound to step on someone's toes whenever he gets involved in a controversy.

Yesterday, however, the supervisors, who are up for election this fall, put aside any ill feelings toward Herrity. "It was," said Duckworth after the presentation, "all in good fun."