Sweat beaded above Christopher Engel's upper lip as the 17-year-old Michigan native set down a hay bale and trudged back to the truck for more. He was hot, sticky and sunburned.

Engel, nine other teenagers and two adults had been up since 6:30 a.m., working to prevent erosion at the Manassas National Battlefield Park, and they still had several more hours of work to go.

Is this any way to spend your summer vacation?

Absolutely, said 20 high school students from across the country who volunteered to spend three weeks at Manassas trying to repair the damage done by Northern Virginia developer John T. "Til" Hazel before Congress added his land to the national park at an estimated cost of more than $100 million.

"I wanted to do something constructive," Engel said. "If there's anything I can do to help our park system, it's good."

Two teams of students participated in the program sponsored by the Student Conservation Association, which sponsors environmental projects in national parks from Wyoming to New Hampshire.

Participants pay only their transportation costs -- the association provides the gear -- and they get to go camping for a week as their reward.

Engel's group, which was the second team to arrive, lived in tents on the battlefield and worked seven hours a day reseeding, building dams and clearing a path on the Stuart's Hill tract, as Hazel's land has come to be known.

The National Park Service paid to remove three model homes -- built before Congress voted to take the land -- but much of the 558-acre parcel remains a dust bowl, torn up by bulldozers and denuded of vegetation.

"These days, conservation is cleaning up after other people," said Ryan Hollander, 17, of Wethersfield, Conn.

Living and working on the battlefield has been a new experience for most of the 11th- and 12th-graders.

The teenagers have had to fend largely for themselves. They shop for and cook their own meals and do their own laundry, some of them for the first time.

"Out here, you really appreciate things you take for granted -- like water," said Samantha Loucks, 17, of Charleston, Ill., taking a large swig from her plastic jug.

"These are all kids who are really motivated," said group leader Michael Andberg, who teaches art at McLean Langley High School. "If I had kids like this to teach, I'd have a sensational job."

The participants are learning as they go along, sometimes from their own mistakes.

The first team put hay bales across streams that were carving channels through the park's open fields, but heavy rains swept the bales away.

Next time, the students drove spikes into the earth to hold the hay in place.

The reseeding work has been more successful.

"This whole segment is no longer an erosion problem," said group leader Lanara Roell, of Charlottesville, pointing to a hillside where green spikes of grass were already sprouting through the jute netting laid down by the previous team.

Team members said they signed up for the program -- rather than staying home to work -- because they wanted to make a substantive contribution to improving the environment.

"It's more important to actually help something than working for money," said Wes Galyardt, 16, of Ralston, Neb.

Said Hollander, "This is the green decade, when everything is either going up or coming down, and I'd like to help."

The Student Conservation Association spent about $50,000 on the battlefield program -- $35,000 from the National Park Service, the rest from private donors, said Executive Vice President T. Destry Jarvis.

Next year, the organization hopes to expand the program to 30 or 40 participants, Jarvis said.

"There's certainly no end of work to do," Jarvis said.