CARBONDALE, Colo. (AP) _ Toxic wastes seeping from the abandoned mines of the Rocky Mountains are destroying Western streams and rivers, and it may cost billions of dollars to repair the damage, experts say.

In Colorado alone, there are an estimated 10,000 abandoned mines leaking a variety of chemicals - a legacy from the 19th century mining boom, says state geologist John Rold. More than 450 miles of streams can't support fish because of the pollution.

But scientists are hopeful that a new method of purifying tainted water by using peat bogs and limestone may soon provide a cheap, simple way of cleaning up some of the waste.

The problem of mine waste seepage is a persistent one because mines don't stop draining, nor is it limited to the Rockies, Rold said.

''In the East it's worse. Nobody really knows how many old mines there are in West Virginia, Pennsylvania and some other states back there,'' he said.

In Charleston, W.Va., Perry Bryant of the Citizens Action Group said it will cost an estimated $50 million to clean up just two mines in the Cheat River drainage near Morgantown.

''There's no one solution to mine waste,'' said Rob Walline, a chemist with the federal Environmental Protection Agency. ''The main thrust now is expensive chemical treatment, but this (the peat bog-limestone method) is an attractive alternative.''

At the Thompson Creek coal mine near Carbondale, researchers have installed a peat bog and a limestone bed to take out the dangerous minerals that leak out of the mine.

Water is taken from the mine adit and sprinkled on the peat, which removes heavy metals through a chemical reaction known as an ion exchange, said Al Howard, a professor at the Colorado School of Mines.

To restore the water's balance of alkalinity and acidity, it is run down over the limestone after leaving the peat, and it enters the creek with good quality, researchers said.

Dave Holm, supervisor of the state inactive mines reclamation program, began tinkering with peat bogs nearly two years ago after he saw how beaver ponds naturally cleaned mine waste at a coal mine near Oak Creek.

''There was a tremendous amount of bright, red water coming out of that mine but the beaver ponds seemed to have a big impact on water quality,'' Holm said. ''Peat is an excellent absorber of heavy metals and the whole thing came together when I had time to think.''

Holm said researchers in West Virginia experimented with limestone but found that once water left heavy metals on the rock, it was rendered useless.

By filtering the water through peat first, he said, the limestone can balance the water's acidity or alkalinity without interference from heavy metals.

Holm isn't ready yet to badger the Legislature for money to expand the ''living treatment system'' to more mines. ''I still think we need more research before we go after the money,'' he said.

Howard said researchers still don't know how long a particular bog can filter heavy metals. Nor do they know yet what should be done with the peat once it is saturated with metals.

Holm said he installed two experimental peat bogs for about $17,000 each. A chemical treatment plant, in addition to requiring power and roads, would cost at least $250,000 to build and $20,000 per year to operate, he said.

''A lot of the old mines in Colorado don't have roads to them,'' Howard said. ''Around Silverton, there are mines above 13,000 feet that miners hiked into and took their ore out with mules. There's no way you could build a treatment plant in places like that.''

Holm believes peat bogs could work ''even better in the East because the climate is so much better at lower altitude. Peat bogs can grow and regenerate back there with very little maintenance.''

Ironically, he said, he expects opposition from environmental groups because of ''their protective stance toward wetlands.'' Using peat bogs for mine waste purification would mean the bogs could not be used as nesting grounds because of the poisonous metals left behind.

''We'd probably have to fence and cover the peat bogs to prevent use by waterfowl, but the dent would be negligible,'' he said. ''It wouldn't be a big withdrawal of wetlands.''

''It's a simple but not a perfect solution,'' he said. ''Mine waste is a very difficult problem and technologies aren't that elegant. We're trying to use natural mechanisms.''