The mud flats and rocky stream beds of the area just north of the San Gabriel Reservoir provide challenging weekends of high-speed splashing, slipping and sliding for thousands of off-road vehicle enthusiasts.
But federal and county officials say that the thrills they thrive on may be creating a long-term risk to the water supplied from the reservoir to an estimated 2.5 million people from the San Gabriel Valley to Long Beach.
They contend that the off-road vehicle enthusiasts, who flock to a half-mile-wide, 2 1/2-mile-long strip creeping down the northwest corner of the reservoir, pose a strong threat of contaminating the water with oil, gasoline and cleaning solvents.
The county's Main San Gabriel Basin watermaster's office, which determines how much water can be pumped out of the basin each year, has suggested that the vehicles be moved out of the reservoir area and restricted to specific areas patrolled and monitored by the U.S. Forest Service.
About 200,000 off-road vehicles will be used on the disputed area this year, most of them on weekends when riders have to pay a $2 fee to enter the area, said Donald Stikkers, district ranger for the Forest Services's Mt. Baldy Ranger District.
The Forest Service, which oversees vehicle activity in the area, agrees that some action is necessary and has begun a study to determine if moving the activity is feasible. Stikkers said he expects to present possible solutions to the watermaster's office in about a month.
However, he said, it will take at least a year to formulate and implement any new policy, so off-road vehicle activity will be allowed to continue in the area at least for the time being.
Authorities stress that the contamination poses no immediate threat to the water supply. But the problem has grown worse as the level of the reservoir has dropped and riders in search of mud drive deeper into the reservoir, dripping and spilling contaminants as they go.
'Only Source of Pure Water'
"This source of water (the reservoir) is the only source of pure water we have in the basin," said Linn Magoffin, chairman of the watermaster's office.
Magoffin said that if something is not done soon, the contamination level could rise above established safety standards and the water supply, which travels through a reservoir and river system from the mountains to the sea, could be imperiled.
The reservoir-river system is the main source of replenishment of ground water in the Upper San Gabriel Valley Basin, which starts at the mountains and extends south to Whittier Narrows.
Water travels from the reservoir to a spreading ground in Irwindale, then seeps into wells that serve most of the population from Azusa south to Whittier and from San Dimas west to Monterey Park.
As it moves south, the water goes into another spreading ground at Whittier Narrows, and that water supplies wells that serve about half the population of the Central Basin, which runs from Commerce and Montebello to the Orange County line and from La Puente west to Long Beach.
"We don't want to wipe out ORV activity in the mountains, but we've taken many years developing these dams, and you just don't let a handful of ORV people in and jeopardize it," Magoffin said.
Municipal water officials and suppliers who rely on the reservoir agree that something should be done to try to halt the contamination.
"We just want to stop the problem before the reservoir has to be monitored and the Health Department comes in and closes the entire area down. It's called preventive maintenance," said Tom Shollenberger, water superintendent for the city of Alhambra and a member of the nine-man watermaster's office board.
"We're really concerned about contamination from that area because we serve over 14,000 customers from there," said Edward Heck, general manager of the Azusa Valley Water Co., one of two San Gabriel Valley suppliers that get water directly from the reservoir.
"It could be a catastrophe if significant contamination occurs," he said.
Bill Temple, general manager of Covina Irrigating Co., a wholesaler who also gets his water directly from the reservoir and sells to 14 cities, water companies and agricultural interests, agrees.
"We're all concerned because the possibility of reservoir contamination definitely exists," Temple said.
"We get all our water from the wells, and we feel we have an obligation to maintain the purity of the water up there," said Bob Berlien, water manager for the city of Arcadia.
After touring the area with members of the watermaster's office board, Magoffin met with Stikkers in early November to discuss the problem and sent a letter to the U.S. Forest Service, which has jurisdiction over law enforcement in the area, suggesting that the off-road vehicles be moved out of the reservoir area.