The water dripping into the tall Plexiglass tube looks innocuous enough, but this liquid is not anything that a person would want to drink. The water comes from a mine adit at the United Keno Hill Mine and is heavily polluted with zinc.

In the real world, this contaminated water is running down Galena Hill near Elsa. At the bottom of the hill, the water crosses the Silver Highway and then trickles into Christall Creek, which drains into the South McQuesten River.

Here in the Environment Canada (EC) laboratory in Whitehorse, the water is part of an experiment being conducted by the Environmental Protection Branch. The focus is not just on the polluted water, but on the soil sample in the Plexiglass tube. EC officials want to determine just how much of the zinc is being captured -- or attenuated -- by the soil.

This attenuation is a natural process by which concentrations of contaminants are immobilized. Soil can attenuate metals by chemically binding the metal ions. This process seems to be taking place on Galena Hill because water samples taken from the top of the hill have substantially higher levels of zinc than samples taken at the bottom.

Somehow, as the polluted water flows downhill, the zinc is being absorbed -- at least for now. But Vic Enns, EC's head of pollution abatement, says that no one should start thinking that attenuation is a panacea for mine pollution.

"Attenuation is not the final solution. It can be with petroleum products but not with metals. It is just buying you time. The process may be reversible and there are limits on how much the soil can absorb," he says.

EC began monitoring this adit more closely several years ago when the amount of water flowing out of it increased, causing ice sheets to form on the Silver Highway below. And even though three to four times more water is flowing out of the shaft, the concentrations of zinc in the water are still high, so the increased flow has not diluted the contaminants.

Enns says they do not know why there is more water flowing out of the adit, but they want to make sure that the polluted water does not reach Christal Creek. The first step is to determine how water and various pollutants such as zinc are moving downslope.

Soil with a small particle size was used for the test. To ensure that it is soil -- not plant matter -- that is binding up the zinc, the samples were first put through a sieve to remove the organic matter.

Enns says that attenuation can delay the release of contaminants into water, but it is unlikely that it will have that effect indefinitely. "At some point the attenuating capacity will be exceeded," he says.

Any number of changes can also cause the soil to start releasing the zinc that it has already absorbed. "If the pH of the water changes because of snowmelt or rain, some of the zinc could be mobilized again," he says.

So far the soil packed into the Plexiglas tube in the lab is showing an ability to stop large quantities of zinc in its tracks. The test began in February, and the soil sample in the tube is still binding up zinc from the water sample.

The test will be continued until the concentration of zinc in the water going into the tube is the same as what comes out of the other end. The filtered water samples are sent to a Vancouver laboratory for analysis.

In some places engineered wetlands have been built that rely on natural attenuation by soil and vegetation to control pollution. Microorganisms can actually transform some organic contaminants into less toxic compounds.

But Enns says that it would be premature to hope that an engineered wetland could solve the problems found at Galena Hill. For one, zinc is more resistant to attenuation than most metals. Also managed wetlands are not effective in winter when the ground is frozen.

Enns says that once they better understand what mechanisms are at work on Galena Hill, there may be a way to the zinc pollution there by mimicking those natural processes in a water treatment system.

But he says that their main concern right now is to understand the risk to the environment. "By understanding what is happening on the hillsides, we will have a better idea of when the risk to the creek could become significant, assuming the natural attenuation capacity of the soil will slowly become used up," he says.

For more information on mining pollution, contact Environment Canada at 667-3400.