The Fairfax County Board of Supervisors yesterday agreed to pay an architectural firm $50,000 to design a new governmental center that the county may never build.

The board, which earlier this summer voted to postpone a referendum on whether the center should be built, approved the funds on a 6-to-2 vote with one abstention. Proponents said the county should complete plans in case voters eventually approve the $50 to $70 million complex, which would move many county offices from Fairfax City to a 183-acre site west of the city along Interstate 66.

Supervisors Thomas M. Davis III and Audrey Moore voted no and Board Chairman John F. Herrity abstained, saying the board should seek voter approval for the controversial project before spending any more funds.

"As far as I can see, the money's going to waste," protested Mason District Supervisor Davis, who said his constituents are overwhelmingly opposed to building another government facility to replace the 12-story 1968 Massey Building. "The board didn't want to put it on the ballot this year, they don't want to put it on the ballot next year. As far as I'm concerned, we shouldn't spend another penny until we're ready to go to the voters."

Fairfax already has spent more than $4 million preparing to move its government to the new location near the intersection of Rte. 50 and I-66. Most of the money was for the 1979 purchase of the vacant land, with several hundred thousand dollars more spent on surveys and on a design competition for the center.

The board yesterday selected from among four proposals an imposing design for a terraced glass-and-concrete structure built alongside an artificial lake. The winning design was submitted by a joint venture between the Canadian architectural firm of Arthur Erickson Associates and the locally influential engineering firm of Dewberry & Davis.

The board awarded the joint venture $50,000 to complete the design so the county will have a "plan that will show what it intends to build at such time that a decision is made to proceed," according to a memorandum from County Executive J. Hamilton Lambert.

Proponents of the center, including Centreville Supervisor Martha V. Pennino, say it will save the county money because the fast growing Fairfax government spends more than $2 million each year renting office space from private landlords.

The nine supervisors, in their first meeting since their August recess, also voted to create a regional "Occoquan Alliance" aimed at cleaning up the Occoquan Reservoir, which provides drinking water to 600,000 Northern Virginians. The action, which Herrity suggested, came in response to a news article on a state report that labeled the Occoquan the second most polluted lake in Virginia.

Supervisor Moore, a frequent critic of development interests, joined the unanimous vote for the "alliance" but labeled it a "red herring" aimed at distracting attention from the county's culpability in polluting the lake.

The "Occoquan Alliance" would include elected officials from all the jurisdictions that drain water into the reservoir, according to Herrity, including Loudoun, Fauquier, Prince William and Fairfax counties. Herrity said the group would study the problem, including the potential future danger from uranium mining.

Moore said regional organizations already are studying the problem. She said Fairfax, although it contains only 17 percent of the Occoquan watershed, produces significantly more of its pollution.

"I think Jack is trying to make it look like all these people in the other jurisdictions are creating the problem and Fairfax is the white knight," Moore said, adding that Fairfax should demand better pollution controls from its own developers. "I think Fairfax better put its money where its mouth is."

"I think we've done an awful lot to correct the problem ," Herrity said. "We have a small portion of the watershed, and obviously there are some problems in that watershed, but we're not the main cause."