With her carefully coiffed honey blonde hair, pink-lipsticked smile and electric blue eyes, Liz Hartwell may not look like an iron-willed champion of the environment.

Although 53-year-old Hartwell has retained the Southern belle charms she acquired in her youth in Danville, Va., when it comes to what she lovingly refers to as "the cause," the lady is a tiger.

For 13 years Hartwell has led battles to preserve the natural splendors of Mason Neck, a sparsely-populated wilderness at the southernmost reach of Fairfax County. The Neck's most celebrated resident, the southern bald eagle, is one of 223 species of birds, 26 kinds of mammals and 15 species of amphibians and reptiles who make their home in the area's unspoiled marshes and forests.

Since the roar of bulldozers first threatened the scenic peninsula in 1965, Hartwell has fought to save her beloved Neck more than a dozen times - from such monuments of progress as a "satellite" city, a trash landfill, an airport, a deep-sea port, an experimental chemical spraying tower and a natural gas pipeline.

"You would not believe the absolutely atrocious things people want to do with Mason Neck," said Hartwell, leaning over her battle-scarred walnut work table piled high with manila folders, card files, a large map of Mason Neck, a stack of form letters and a thick roll of postage stamps. The paraphernalia is from her most recent fight against a sewer line proposed to run across Mason Neck.

"There has been a major threat to Mason Neck about once every six months for 10 years. The point is, you have to convince this very pro-development area that the Neck should be saved for forests and creeks and marshes."

With her eyes sparkling behind mascare-rimmed lashes, Hartwell described her love affair with nature that began as a child - fishing in lazy streams, visitin "Granny" at Colonial Beach and watching the sunset on the Potomac River.

She left Danville to study English at Mary Washington College in Fredricksburg and moved to Washington in 1943 "to help win the war." While working as a typist for the Army Security Agency she met Major Stephen Hartwell, an editorial and research specialist, and they were married in 1946.

Five years later the Hartwells moved into a home in Mount Vernon because it was just two blocks from the Potomac and "last in the line" of developed suburbs. In 1959 they moved to a home right on the river in Mason Neck.

With her two young sons she hiked through the quiet forests, steered her tiny boat through the scenic inlets of the great marsh and became enamored of the bird whose freedom she would later champion - the bald eagle.

"I'd never really watched birds or thought about the bald eagle, but the first time I ever saw that tremendous, magnificent creature I felt absolute total pure joy," she recalled. "I had one fly about 20 feet over my head, and I could see the eight-foot wingspan, the golden talons, the white head and white tail."

So in 1965 when Hartwell discovered that a developer planned to build a satellite city for 20,000 people on Mason Neck that included construction of a sewage treatment plant in the great marsh area, site of a communal eagle roost, she prepared for her first battle.

"I just thought, this can't be," said Hartwell, who organized the Conservation Committee fro Mason Neck and wrote hundreds of letters enlisting the aid of citizens, conservation organizations and federal, state and local officials.

She gave speeches and made a movie of the historic, scenic and recreational values of the area to show to civic organizations and garden clubs. And she attended over 40 county hearings and workshops on the proposed satellite city rezoning application and persuaded others to attend and testify.

"Developers would flap their arms like wings at me in the halls and make eagle noises," she grinned, throwing back her head and exploding with a husky, Rosalind Russell laugh. "But it never did get me down because I knew I was going to win."

Hartwell won the battle in 1967 when the Fairfax County Board of Supervisors approved a plan for a state park, two regional parks and wildlife refuge on Mason Neck. But she's still fighting the war.

Although now divorced and living in her first suburban home in Mount Vernon, she's still Mason Neck's chief crusader. Her current campaign protests a proposal for a connector sewer pipeline to be built from Harbor View Estates on the western edge of the Neck to the newly upgraded lower Potomac treatment plant on the eastern side.

"Here I am, back in the sewer again," chuckled Hartwell, who has belonged to 20 conservation organizations and estimates she's attended 10 meetings a month for 10 years - all without pay. "But I thrive on controversy. And researching the facts is the important thing. If you've got your facts to prove your suppositions, you've got it made.

"But I didn't do it myself," she's quick to add. "I'm the catalyst who got hundreds of other people involved."

Although Hartwell downplays her own importance, both her fans and her opponents say they respect the formidable accomplishments of the woman who won the 1976 Conservationist of the Year Award from the Virginia and National Wildlife Federations and Sears & Roebuck, and was the first woman appointed to the Northern Virginia Regional Park Authority in 1972.

"She's probably one of the most effective persons I've ever had the opportunity to work with in conservation," said Park Authority executive director Darrell Winslow. "She's a very dedicated, hard worker who does her homework and tries to do everything that's proper and right"

"I think the world of Liz," said developer John Ringle who ran head on into Hartwell when his plans to build a resort in Virginia threatened Mason Neck. "The nicest thing she had to say was it was abominable, and I dropped my plan. She's an attractive, intelligent woman who's done good work."

"No question she's a fighter," said Jack Herrity, chairman of the Fairfax County Board of Supervisors. "She's not an unreasonable type person, and she realizes there has to be a balance between economic need and the environment."

Although Hartwell said she's "not the type of person to march in protest on the Pentagon or burn her bra," she's proud of her track record of beating the system from within.

"One individual can do something," she nodded. "But my only importance has been in being a leader for the cause."