ASPEN, Colo. (AP) _ At scores of lakes perched in the harsh alpine tundra in the shadow of the Continental Divide, Forest Service technicians are racing to beat winter as they search for evidence of a silent lake-killer, acid rain.

The first comprehensive study of acid rain in the West began this month under a congressional mandate, but because the Forest Service wouldn't let the Environmental Protection Agency fly helicopters into wilderness areas, foresters have to go after the samples the hard way - saddling up.

Although it is not yet officially autumn, snow, sleet, and high winds already have brought misery to 14 Forest Service crews who pack 200 pounds of equipment on horseback miles up to wilderness lakes.

The $5.2 million study will measure the chemical composition of 888 lakes. Each sample is worth $5,000 to $7,000, figuring the cost of obtaining and analyzing it, said Larry Svoboda, the EPA's regional acid rain program chief.

''If we find acidic lakes, there's no way we can say whether it is natural acidity or man-caused acidity,'' Svoboda said. ''What we will get is baseline data that will give us the ability to monitor lakes so we can tell in the future when there is a change.''

For example, he said, high acidity in a lake might be caused by acid rain, but could be caused by drainage from an abandoned mine.

Forest Service employees Tim Snowden, Carrington Brown and Steve Rieser had a smooth trip Monday to Independence Lake, a few hundred yards from the spine of the Divide in the Hunter-Fryingpan Wilderness Area east of Aspen.

There wasn't a tree in sight at 10,000 feet, where the brown and still- slightly-green tundra gives way to boulders. Lichen takes several hundred years to grow, and scientists say it takes 1,000 years to replace a single inch of topsoil.

The sky was blue and the sun shining, but once the foresters got out on the lake in a small raft, the wind began gusting. Rieser tethered the raft to a boulder to keep it from scooting across the lake in the stiff wind.

In rubberized nylon suits designed to protect them should they fall into the nearly freezing water, Brown and Snowden spent more than an hour carefully drawing samples and measuring the depth and temperature of the lake.

The wind whipped the fresh snow on Blue Mountain, the 13,700-foot peak that guards the lake, but the weather didn't deteriorate to the snow and sleet that plagued researchers on Sunday.

Acid rain potentially is a far greater threat in the West than in the East because of western lakes' vulnerability, said Dennis Haddow, air quality specialist for the Forest Service regional office in Denver.

A lake's susceptibility to acid rain depends on its bedrock geology, soils development, and forest canopy, Haddow said. ''Up here we have only one out of three of those. There is no forest above timberline and there is so little soil that you might as well not have any.''

The lack of a forest canopy or soil replacement hurts a lake's buffering capability, said Ranger Jack Troyer. ''It's like Rolaids. In the East they've even tried things like putting lime into lakes to buffer them.''

The results of the study are to go to Congress next year.