NORTH COUNTY - An old dairy at the planned Gregory Canyon landfill will become an anti-terror training ground next month for 40 to 60 Marines from Camp Pendleton, a military official said Wednesday.

Local environmentalists said they are worried the exercise could damage the fragile environment near the San Luis Rey River or ignite a wildfire in a tinder-dry area at one of the driest times of the year. Marine officials said care will be taken to avoid starting fires and damaging the environment.

County officials said they do not believe there will be damage at the site, which after a decade of controversy and legal challenges has been approved for a landfill.

"We reviewed it and determined that there was no environmental impact from this activity," said Jack Miller, assistant director for the San Diego County Department of Environmental Health.

Opponents of the training aren't sure about that, and they say the site is the wrong location for a military exercise.

"I'm kind of at a loss to understand why this would happen," said Everett DeLano, an Escondido attorney with the North County environmental group RiverWatch. "I thought there was this thing called Camp Pendleton up there that had some ground for training."

With the goal of gauging how well the Marines would perform in unfamiliar territory, such as they might encounter in Iraq or some other foreign country, the exercise was moved off the nearby 125,000-acre base, said Master Sgt. Wayne Jones, chief of training and experimentation for the I Marine Expeditionary Force. Jones said he is also a military liaison for the FBI, which is involved in the exercise.

"They've been training together for six months already, and they have been doing that on base. They have become very familiar with the training area," Jones said in a telephone interview Wednesday. "This way we will know that they really are ready to do a real-world operation in an unfamiliar area."

Jones said the practice operation will begin July 28, with actors and instructors posing as terrorists taking over the Verboom dairy house and reconnaissance teams watching them from a distance. On those days, fewer than 10 people will be involved in the exercise.

Then on the evening of July 30, Jones said, a team of Marines from the 11th Marine Expeditionary Force and a single FBI agent will conduct a raid lasting 60 to 90 minutes. Two to four helicopters will place dozens of team members on the ground. After using a small explosive or a mechanical instrument to bust through a door, rifle-toting team members will round up the fake terrorists, he said.

Jones said the noise may travel a half-mile, but no farther. He said the small explosive is about as loud as "a large firecracker," and the practice rifle fire will be kept inside the building.

There will be no dramatic explosions or fires visible, Jones said.

"It's nothing as exciting as you would see on television," he said.

Still, the idea of a military exercise on the landfill is disturbing to Ruth Harber, secretary-treasurer of RiverWatch and a nearby resident.

"I am very upset," Harber said. "You know how dangerous it is to shoot in these dry conditions. The fire could run up the canyon and destroy the Medicine Rock of the Pala Band (of Mission Indians), burn up the sacred mountain and burn up all the properties on the south side of the mountain where I live."

Medicine Rock is a boulder at the base of Gregory Mountain, which is held to be sacred by local American Indians.

Richard Chase, chief executive officer for Gregory Canyon Ltd., the business partnership that owns the 1,770-acre landfill site, said the partnership was contacted two or three months ago by the Marines and FBI.

"We're just trying to be accommodating to a polite request," he said. "We assumed that it would be a good, useful thing."

Chase said the partnership is not being paid. Jones said the same thing.

"He's not being compensated in any way," Jones said of Chase.

The Marine master sergeant said measures will be taken to prevent a fire. Besides limiting rifle fire to inside the building, the explosion would entail using a charge mixed with a water-gel formula that puts out flames, he said. Fire extinguishers will be available if things get out of hand.

There is cleared vegetation next to the house and the nearest thicket of brush is 150 feet away, Jones said.

The fire issue aside, RiverWatch is concerned about the potential for damaging habitat that has been deemed suitable for the endangered arroyo toad.

"Troops trample all over the place," Harber said.

Jones said organizers will limit activities to 10 or 15 acres, and steer the team away from sensitive habitat.

Jones also said the Marines will avoid damaging the old farm structure by framing a temporary structure with a door, to avoid busting through the door of the house itself.

- Contact staff writer Dave Downey at (760) 740-5442 or ddowney@nctimes.com.