Alexandria and Fairfax County may lose almost $1 million in federal matching grants to buy and improve parkland as a result of the Reagan administration freeze on federal park funds.

Two weeks ago, Reagan announced the freeze as well as plans to eliminate all future funds for historic preservation or the purchase of additional parkland.

Among the grants affected are $450,000 to develop Chinquapin Park in Alexandria and $500,000 to help Fairfax County create an 11-acre park beside the future Huntington Metro station.

"People are very upset," said James Chasnovitz, Alexandria's landscape architect. "But we're going ahead with the planning on the project anyway in the hope that Congress will restore the funds. After all a freeze is not a turndown."

The grant for the Huntington park, known as Mount Eagle, "is the only one we have that's in jeopardy," said Joseph Downs, director of the Fairfax County Park Authority.

All the grants must receive federal and state approval. The Fairfax grant was approved by the state last week, barely 10 days after Reagan announced the freeze in his State of the Union message Feb. 18.

Both grants come from the U.S. Land and Water Conservation Fund, established by Congress in 1965. The Fund has paid out to state and federal park projects. Half the funds have been used to buy or develop parkland in more than 28,000 state and local parks, with Virginia receiving $52.8 million.

The 11-acre Mount Eagle site is being leased from Metro by Fairfax County for use as a park. A Metro contractor has dumped construction dirt "and done $150,000 worth of grading free to develop the site as a park," Downs said this week.

Downs said the need for Mount Eagle as a park is crucial because it is in an area that soon will be highly developed with a population projected to be 100,000.

The end-of-the-line Huntington subway station is actually "Metro's largest single construction project," said Metro spokesman Cody Pfanstiehl. "What we have is a 75-foot building lying on its side on the side of a hill, with a 2,500-car parking lot in it and Metro's only 'inclinator,' an escalator for the handicapped."

The grant for Chinquapin Park, which is beside T. C. Williams High School, was to provide tennis and handball courts, improve the community garden and provide playgrounds and ball fields.