Unions that have bitterly opposed plans to equalize federal paychecks with those of state and county workers may want to rethink things, and demand immediate pay parity with Fairfax County.

The issue of federal-local government pay equity has been raised by the Ford, Carter and Reagan administrations. All three felt annual pay proposals for feds are based on wage surveys -- made by government workers -- of unrepresentative high-wage private firms. They felt including local government pay in the survey would produce a more accurate picture of what feds should earn.

Obviously the federal vs. county pay partisans never heard of Fairfax County. Fairfax, it turns out, really knows how to treat a public servant.

When the January federal pay raise goes into effect top civil servants (at Grade 18) will get $68,700 a year. The most senior members of the Senior Executive Service will be paid $72,300. That kind of money would shame Fairfax's executive headhunters.

Fairfax recently lost its bid to keep its 55-year-old school superintendent from retiring. It offered to give him combined salary and retirement compensation of $157,000 a year, for four years, so he would stay on. The proposal raised such a flap that the superintendent decided to resign anyhow.

Now poor old Fairfax must find a new superintendent. Although the national school superintendents' job grapevine is one of the best, the county is taking no chances. It has hired two talent scouts, at $15,000, to help spread the word that the county has a job opening.

Just the other day Fairfax approved a 10 percent pay raise -- to $87,439 per year -- for its county executive. The Washington Post yesterday quoted county officials as saying County Executive J. Hamilton Lambert, who is 43, gets offered a job "every week" so the raise was needed to keep him.

For that kind of money, Fairfax could hire almost anybody in the federal government. It could get (and give them a healthy raise in the process) half the nation's governors. It could hire Rep. Geraldine Ferraro, or Sen. Barry Goldwater. They now get only $72,600. If Ferraro becomes vice president she will make only about $7,000 a year more than the county executive of Fairfax.

For $87,439 the county could get proven executives such as Secretary of State George P. Shultz ($83,300) or Defense Secretary Caspar Weinberger who, were he Fairfax County executive, would not be troubled getting up nights with red alerts.

Half a dozen federal workers -- several of them Fairfax County residents -- called this columnist yesterday about the pay story. Some said the county pay structure is out of whack -- feds pay taxes too! Others said the federal pay situation is a mess.

One woman suggested that the Grace Commission -- which recently took the federal government to task for paying and pensioning its people too generously -- be reactivated to look at counties. Starting you know-where.