The Fairfax County Park Authority has received a $500,000 federal grant to help it acquire 12 acres for a park at the new Huntington Metro station, just south of Alexandria. The station, scheduled to open next fall, will be the first subway stop in Fairfax County.

The park authority is negotiating with Metro, which owns the land, and although a purchase price has not been set, it could exceed $1 million, according to a park authority official.

To complete the purchase, the park authority must come up with $500,000 to match the federal grant, which last winter was blocked by the Reagan administration as part of its overall plan to freeze park grants around the country.

The grant, from the U.S. Department of Interior, was freed this summer when Congress required the administration to distribute most of the 1981 park money in Interior's Land and Water Conservation Fund. The 15-year-old fund, established by Congress, funnels some of the revenue from Interior's off-shore oil and gas leases into federal and state-approved park projects.

In approving the grant, Interior noted that it will provide park land "in a high-density urban setting" that "will buffer the Huntington Metro" development from nearby residential neighborhoods.

The proposed park site includes the remains of an earthwork fort, one of several built by Union troops on hills around Washington during the Civil War.

The park authority, which already is using the undeveloped land as a community park under a $13,000-a-year lease with Metro, will seek help from the Fairfax County Board of Supervisors in raising the $500,000 in matching money.

Even after the land is bought, the park authority will not have funds to develop the park for several years, according to Louis Cable, assistant director of the park authority.

"Fortunately, we've had some help from the Metro contractor (the George Hyman Construction Co. of Bethesda), which donated about $80,000" worth of construction work, Cable said.

In exchange for using part of the site during Metro construction, Hyman built an entrance road and parking lot and graded land for a future ball field, two tennis courts and a basketball court.

Neighborhood children already are using the site, said Cable, and it has been sown with grass and equipped with a few picnic tables.