Passive treatment wetlands aren’t a cure all for mining contamination

Passive wetland treatment systems have been discussed for the proposed Casino Mine and the closure plan for the currently operating Minto Mine.

Submitted by Lewis Rifkind,

Yukon Conservation Society

Passive wetland treatment systems have been discussed for the proposed Casino Mine and the closure plan for the currently operating Minto Mine.

Passive treatment is a catch-all term for a range of naturally-derived technologies used to treat and purify water. It can be used to treat mine waste water to remove contaminants such as metals that can harm the environment.

The process relies on bacteria and plants to remove the contaminants. The passive treatment process is sometimes referred to as a constructed wetland.

The easiest way to visualize this is to think of a wetland, such as a weedy marsh along the banks of the Yukon River. Anyone who has ever fished for northern pike in these areas will have noted the presence of dense weeds and muddy sediment. It is the interaction between these weeds and muddy sediment that acts to filter the slow moving water that passes through it. This is the reason that wetlands, in general, are often referred to as “the kidneys of the world.”

Passive treatment has less to do with the soil and the plants and more to do with the bacteria that live within and around them. The soil and the plants are only important to the process because they provide the right conditions for different types of bacteria to grow. The bacteria are the real workhorses: the chemical reactions that they use to feed themselves remove contaminants as a result of the other chemicals they create.

The many different types of bacteria within the soil give passive treatment systems their innate ability to purify water so effectively. The more types of bacteria the soil has within it, the more substances — including heavy metals such as zinc, iron, lead, copper, and cadmium — it can treat.

Another example of a passive treatment system, but one that functions without soil, is called in-situ treatment. It can be used in large open pit mines or in underground tunnels that have been flooded with contaminated water.

This water is treated with a bacterial food source (usually a carbon-rich substance like molasses or ethanol) that promotes bacterial growth within the water column. As the bacteria feed and grow, they turn the currently soluble contaminants — heavy metals such as zinc, lead, cadmium, and copper — into insoluble forms through processes such as anaerobic respiration and fermentation.

With the bacteria doing the majority of the work, it is no surprise that maintenance and upkeep costs are greatly reduced, particularly for remote mine sites, where current active treatment systems (similar to municipal water treatment systems) require around the clock maintenance and supervision from on-site personnel.

As such, these types of wetlands have been proposed for large projects, such as the Casino Mine Project, near Pelly Crossing. The water contained behind the massive tailings dam will gradually be released to percolate through a series of passive treatment wetlands.

In theory, the idea is good: contaminated water slowly moves through the wetland, bacterial transformations occur, and the final result is clean water. The bacteria do most of the work and people just have to come in and check up on it from time to time.

However, in practice, it is rarely this simple. Passive treatment can be fickle, just like the bacteria that do the majority of the work. As a natural system, there is no guarantee that the bacteria will do exactly what the mine operators need them to do and when. It may take a year or more for bacterial populations to be built up in sufficient numbers to even begin purifying contaminated water effectively. Even then, nature is often chaotic, not orderly, and much more research is required into the application of passive treatment systems in order to discern the best way to gain some level of control over the processes.

There has been much research into many natural wetlands that exist and that have been shown to sequester heavy metals over hundreds, if not thousands, of years. Researchers cannot be sure that this removal has been efficient or consistent over this time period. Before miners apply this technology to large-scale mining practices, Yukoners must ensure that it can work over a long period of time.

Furthermore, passive treatment systems are far from “walk away” systems. They still require periodic maintenance to ensure that they are working at their optimum capacity. Like anyone who keeps a garden will know, a significant portion of time throughout the short growing season is spent removing unwanted weeds. The same is true for a passive treatment wetland. Unwanted plant growth must be controlled to ensure that the constructed wetland functions as efficiently as possible. Just like an unkempt garden won’t produce the greatest yield, an overgrown passive treatment wetland won’t yield high contaminant removal efficiencies.

It may seem like a treatment wetland is a perfect place to see ducks, moose and beavers, but the water, soils, and plants could be highly contaminated, depending on the site-specific situation. There could be circumstances when wildlife should be prevented from accessing certain wetland treatment areas.

It must be emphasized that this technology is still in its relative infancy. There is also concern that the idea of passive treatment systems may breed complacency. It is not a system that can be implemented and left to its own devices to purify mine waste water while the mine operators stand back.

The harsh truth is that passive treatment systems will require significant human intervention, now and long into the future, over the hundreds of years that will be required to decontaminate some of our proposed massive mining developments.

Mine operators, mine regulators, and mining watchdogs should remember that passive treatment, though full of promise, is certainly not as passive as they might like.

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