Otherwise well-informed people around Washington keep telling Bruce K. Chapman what a "nice, quiet time" he's going to have as Reagan's new director of the U.S. Census Bureau. After all, they say, the 1980 census is finished.

Such ignorance brings out his missionary zeal, the same zeal that a younger Chapman once trained acidly on conservative Republicans such as Ronald Reagan and Sen. Barry Goldwater.

"The Census Bureau is a treasure trove of information" that businesses have learned to mine pretty well, Chapman says.

Aside from the big headcount every 10 years, "most people . . . don't have a clue what goes on. Among other things, we have a monthly population and housing survey in the neighborhood of 60,000 [households], which is an enormous sample compared to other polling organizations. This is the biggest polling organization in the country."

A longtime afficionado of statistics, which he has used in writing books and as secretary of state for the state of Washington, Chapman's appointment has been praised in and out of the bureau. He already seems to understand the sentiment expressed with off-handed pride among the doctorate-rich data specialists who populate the aging yellow brick compound at Suitland, Md. "After all, the census doesn't relate to anything much -- except money and political power."

Chapman, 40, brings to his new job a reputation as a political "Mr. Clean," a moderate Republican who ran unsuccessfully for governor of Washington last year. He smiles and shrugs at the mention of his younger days as a member of the Ripon Society, a maverick group of liberal Republicans, and his 1966 book The Party That Lost Its Head, which he co-authored with George Gilder, now the Reagan administration's economic guru, on Goldwater's rise.

Now, Chapman says, he classifies himself as a "strong Republican" and "a supporter of the president and the administration."

Chapman said the White House selected a Census Bureau director who was far from its own philosophy because it wants to keep the bureau above politics. He said he has been told by people such as pollster Richard Wirthlin that "they have a strong desire to protect the integrity of the bureau and to maintain its reputation for reliability."

Chapman inherits an agency somewhat demoralized by budget cuts, a spate of retirements in key positions and allegations that the 1980 census undercounted the population. But he noted the courts have tended to favor the bureau, so far, in rulings on the undercount. And he indicated the problems related to budget cuts can be handled by "unpleasant but not tragic" measures such as charging for some services and delaying some reports.

Chapman said he hopes to expand the bureau's already ambitious agenda of data analysis, with more detailed profiles on the shift of people and money to the Sun Belt, and legal and illegal aliens, among others.