A constant flow of water ripples from beneath a heavy metal door into a garish orange pool outside the entrance to the old Argo Tunnel here. The orange is streaked with bright-green algae that thrive in the acidic outflow, and minerals that the water has leached out of the mountain collect in lumpy crystals on the rock at the pool's edge.

"Almost all the mines we'll see today drain here," says Donald Macalady, staring down at the shallows on a clear, cold morning. An hour earlier, he had spread open a U.S. Geological Survey topographical map on a table in his office at the Colorado School of Mines, where he is a professor in the department of chemistry and geochemistry. On the map, tiny half-black squares littered contour lines all along the route of the introduction-to-mine-drainage field trip he had planned. "Every one of these," he said of the black squares, "is a mine."

A water-chemistry specialist with a keen interest in environmental issues, Mr. Macalady is one of several faculty members at Mines who study mines' acid drainage and how to treat it. He is especially interested in improving passive-treatment techniques, such as filtering metal-laden water through easy-to-maintain systems that rely on microbes to neutralize acid and cause the metals to precipitate out.

Now, after filling a travel mug with coffee and making the winding, beautiful drive up Clear Creek's canyon, he is standing by the Argo Tunnel's portal and the orange-and-green pool. Gold was discovered in the mountains here in 1859 and silver soon after, setting off a mining boom that continued for decades. The plain inscription above the metal door says "1893" -- the year work began on the tunnel, which was intended both for shipping ore and for draining the many mines that angle into the mountains rising above Clear Creek.

By the time it was finished, in 1910, the tunnel stretched 4.6 miles and was collecting water from an unknown number of mines that groundwater would otherwise have filled. Indeed, Mr. Macalady says the Argo Tunnel was shut down in 1943 after four miners broke into an underground aquifer and drowned in the resulting flood.

The mines themselves closed one by one, but the drainage water continued to flow. The problem, Mr. Macalady says, is that the rock though which the mines were blasted is laced with troublesome metals, many of them bound up in pyrites. (The best known is iron pyrite, or fool's gold.) As long as the rock is undisturbed, the metals stay put. But once tunnels begin admitting air, the metals oxidize, and after that any groundwater seeping through the rock can dissolve them and carry them away. Aluminum, arsenic, cadmium, copper, iron, manganese, zinc -- it's a whole alphabet of problems.
And plenty of water seeps into the tunnel. The average outflow is 250 to 300 gallons per minute, and it's highly acidic, with a pH of 2.5 to 3. Until a few years ago, it poured right down into Clear Creek, playing havoc with fish and other aquatic life. In 1983, the tunnel and its network of mines were included on the federal government's list of hazardous-waste sites to be covered by the Environmental Protection Agency's Superfund Program.

In 1998, Mr. Macalady says, the EPA and the Colorado Department of Public Health and Environment opened a $4.7-million treatment plant that sits a few feet down the hillside from the portal. Mary Scott, the project manager, agrees to lead a quick tour of the facility, which has two "treatment trains" and costs $1-million a year to operate. It is anything but passive.

The plant treats the water by mixing in sodium hydroxide to increase its pH drastically -- to 9.9 -- and to force the metals to precipitate out. Polymers are added to give the tiny, freed-up metal particles something to latch onto, and they settle to the bottom of a tank as sludge. Clear water from the top of the tank is filtered and then treated with carbon dioxide to bring its pH level down to about 8, or nearly neutral. Then the water -- no longer orange and virtually metal-free -- is discharged into Clear Creek.

The sludge, meanwhile, is compacted into a crumbly, cakelike substance. Ms. Scott says the plant ships about 20,000 pounds of it to a landfill every day, enough to fill a good-size truck. It is not hazardous, she says, and the metals won't leach back out.

"You can see why there's so much interest in passive treatment," Mr. Macalady says a few minutes later, climbing back into his pickup. The federal government pays 90 percent of the plant's operating expenses, but after 2009 Colorado will bear all the costs itself. He heads for the other end of town, where a Mines colleague, Thomas Wildeman, built a pilot project back in 1993 to show that a constructed-wetland approach could work here.

The project, built by the entrance to what's known as the Big Five Tunnel, was designed to handle only about three gallons a minute of its orange, 30-gallon-a-minute flow, which also drains into Clear Creek. Mr. Wildeman, who is also a professor in the chemistry-and-geochemistry department, says later that his project "was a manure bed with a green toupee."

"On top of the system, it looked like a typical wetland, with cattails and aquatic plants," he says. "But those weren't doing the treatment. Underneath was a substrate with a lot of manure, limestone, hay, and alfalfa." Microbes nurtured in that environment helped reverse the process by which the metals had oxidized in the first place. Although the project appeared as a natural wetland, the substrate materials were actually contained in low, hidden concrete tanks.

A full-scale version of the Big Five project was never approved, Mr. Wildeman says, adding that "trying to get innovative things done on a Superfund site is very difficult -- they want proven technology." But he has used the approach successfully at other sites. He is proudest of a system that handles a 1,200-gallon-a-minute outflow at the West Fork Mine, near Bunker, Mo. "We started that in 1996, and the water's always been in compliance," he says.

"The systems we put in are much less costly than the system at the Argo Tunnel," he says. "What we do needs to be looked at now and then" -- not tended around the clock. The trade-off, he says, is that passive systems take a long time to plan and require a lot of space.

Backtracking through Idaho Springs, Mr. Macalady seeks out the dirt road that leads up through Virginia Canyon toward Central City. The road twists and climbs among old mine tailings -- sloping piles of crushed waste rock.

Mr. Macalady points out that nothing grows on the tailings, where oxidized metals are concentrated at levels toxic to plants. The tailings are also troublesome because storm runoff and groundwater seepage carry the metals into gullies and streams that lead to Clear Creek. Mr. Wildeman recently worked on a study that recommended sealing some mine shafts against rain and diverting water -- especially road runoff -- away from the piles of waste rock.

What Mr. Macalady would really like to do, though, is construct small passive barriers that would treat drainage below each tailings pile. "We'd like to intercept the drainage right at the pile, before it gets into the streams," he says.

And back in his lab at Mines, two graduate students -- Miranda Logan and Jason Seyler -- are testing barrier systems that could be custom-fitted to tailings piles of any size and buried right where they would intercept the outflow. Using water from contaminated sites around Leadville, Colo., they're testing various mixtures of treatment materials, including limestone, sand, manure, walnut shavings, and pellets of compacted brewery waste. The goal is to provide the best possible environment for the microbes that will reduce the water's acidity and metal content.
After a quick lunch in Central City, a mining town that was once prosperous enough to build itself an opera house, Mr. Macalady heads down the road to another old mining town, Black Hawk. Both Central City and Black Hawk had fallen on hard times before a Colorado referendum replaced the kind of gambling done underground by men in hardhats with the kind of gambling done by people at slot machines and roulette tables. And both towns are on an eyesore of a stream called the North Fork.

Unlike Clear Creek, though, the North Fork still runs orange. "It's spring-fed, but the springs are all disturbed by mining," Mr. Macalady says. "There are so many small rivulets putting mine drainage into the river." Mr. Macalady and his students sample the water in the North Fork regularly, and he says it is "not getting better or worse." If small-scale passive barriers are proved to work, this canyon would be the perfect place for them.

In the meantime, it's a sobering ride. Orange ice collects along the stream's banks, which are dotted with long-silent earthmoving machinery. Here and there, the body of an old Denver and Rio Grande Western flatcar serves as a bridge to an abandoned mine whose tailings spill downhill. If a canyon can be said to look sad, this one does. "Every place there's one of those piles, somebody dug a hole," says Mr. Macalady, glancing at a truck rusting beside the orange creek. "I can't help being in a place like this and thinking how many people's dreams are buried here."