By Jim Higgins
When a tailings pond dam at the Mount Polley copper and gold mine in British Columbia failed recently, over 10 million cubic metres of alkaline water and 4.5 million cubic metres of liquefied tailings were released into local watercourses.
Although the toxicity of the released material has proven to be much less than many feared, the event may have serious ramifications for the mining industry. It may further call into question the use of water-covered tailings management areas (TMAs) retained by dams.
An evapotranspiration cover/engineered bioreactor system provides an economic alternative to water-covered TMAs. It involves:
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- Draining tailings ponds after mine closure.
- Covering the resulting dry surface with local vegetation such as trees.
- Converting the tailings dams into permeable dikes holding back only relatively dry materials.
- Installing a stormwater management system.
- Treating released leachates with an advanced engineered bioreactor located down-gradient of the new lowered dikes.
Tailings Dam BackgroundVideo of the Mount Polley tailings dam failure.
Most mines impound tailings behind large dams in TMAs to contain and manage residues. These areas allow solids in the tailings slurry pumped from the mine’s mill to settle out. Decanted water is recycled back to the mill. After a mine is closed, these areas are often used for the long-term storage of tailings.
The tailings ponds then become water covers. These are especially appropriate where the ore that was mined contained sulphidic material. A one-to-two-metre deep layer of water maintained over the tailings will greatly limit the influx of oxygen. This mitigates the generation of acid rock drainage, which adversely impacts the health of aquatic animals, insects, and plants.
Tailings dam failures are rare. However, there have been a number of highly publicized ones such as those at Stava in Italy in 1985, the Sullivan Mine in Canada in 1991, the OMAI mine in Guyana in 1994 and the Los Frailes mine in Spain in 1998. Rates of tailings dam failure have, however, increased in recent years.
It has been concluded that of the roughly 3,500 tailings dams worldwide, there are between two to five “major” failures each year. Furthermore, these statistics are for physical failures only and do not include “environmental” failures where the structures of dams were maintained, but leakages of polluted mine drainage water occurred.
Most jurisdictions now require mining firms to have a closure plan before opening new mines. It must detail how the maintenance of tailings dams will be carried out after closure.
One alternative to storing tailings long term in water-covered, dammed tailings areas is to place them under water in lakes, rivers or seas. However, submerged disposal of tailings is controversial. Other options such as placing the tailings in mined-out open pits or dewatering them and placing them back in mineshafts are only possible in some cases.
Dry cover systems
There are two main kinds of dry covers. Barrier covers seek to totally and permanently isolate the underlying tailings under covers of impermeable plastic, clay, or earthen layers. Evapotranspiration (ET) covers involve phytoremediation technology.
The kinds of vegetation that are planted on barrier-type dry covers usually have roots that will not penetrate cover material. In effect, this limits cover vegetation to various grasses and necessitates long-term maintenance programs to prevent deeper rooting vegetation such as trees and bushes from growing. This is a difficult problem for mines in forested areas, where trees are the natural vegetation.
With ET covers, the penetration of roots through the cover material or into the underlying tailings is not a problem. Any infiltrating water will be treated in down-gradient wastewater treatment systems, such as constructed wetlands. With ET covers, woody plants native to the area of a mine can be used.
Constructed wetlands are passive natural wastewater treatment systems, consisting of multiple, in-ground cells arranged in one or more parallel flow paths. There are various types, including free water surface wetlands and sub-surface flow wetlands. Here, the water being treated flows beneath the surfaces of permeable beds. Wastewater may flow either horizontally or vertically in a sub-surface wetland.
Wetlands are already used at many mine sites for treating mining and sanitary wastewater. These treatment systems are attractive because they are economic to build and operate and require relatively little attention long after mine decommissioning. However, they have limitations, including:
- Requiring relatively large surface areas.
- Limited overall contaminants removal, only up to 40 to 60 per cent for many pollutants.
- Sometimes-erratic treatment capabilities.
- Inability to handle some mining wastewater contaminants such as dissolved metals
- Poor or non-operability during winter.
Engineered bioreactors, also called engineered wetlands, are types of in-ground wastewater treatment systems that evolved from sub-surface constructed wetlands. With engineered bioreactors, design, morphology, operatingmethods, substrates, flows, and/or other process conditions in wetlands, are manipulated and controlled to perform whatever the ambient conditions.
There are several types of engineered bioreactors, including aerated and non-aerated aerobic and anaerobic.
The aerated bioreactor engineered wetland (BREW Bioreactor) injects air from a nearby blower via aeration tubing under its aggregate substrate.
Many types of anaerobic bioreactors can be used as engineered bioreactor cells for treating wastewaters. These include denitrification bioreactors; successive alkalinity-producing systems for neutralizing acid rock drainage without generating ochre; and biochemical reactors for removing many dissolved metals and metalloids.
The substrates of most anaerobic bioreactors contain high molecular weight active media that can be degraded microbially in the bioreactors. These generate lower molecular weight breakdown products that are metabolised by characterizing bacteria. These are different for the various anaerobic bioreactors.
Aerobic and anaerobic in-ground engineered bioreactors have much higher treatment efficiencies and can handle higher wastewater flow rates than constructed wetlands. In addition, they are modular and can operate successfully at higher contaminant loading rates. The capital expenditure for engineered bioreactors is often roughly half of conventional mechanical wastewater treatment plants. Operating expenditures and life cycle costs are also much lower, and they can operate in frigid conditions.
Integrated dry cover systems for closed mines
ET covers and engineered bioreactors can be combined with other technologies to create an integrated system that is a viable and economic long-term alternative to water covers and tailings dams.
During construction and operational phases of a mine, stripped overburden and cleared brush are stored for use at closure. Prior to closure, an appropriate engineered bioreactor system is constructed down gradient of the tailings management area. It is relatively small in area and designed to treat any leachate, drainage and meteoric water under worse case possible contaminant concentrations and extreme environmental conditions.
Lined stormwater ditching is constructed around the tailings management area to isolate it, diverting away water that might otherwise enter from the surrounding watershed. These stormwater ditches will direct un-contaminated water into a down-gradient stormwater wetland, which will remove any suspended solids before discharging it to local watercourses. Effluent from the new engineered bioreactor system will also be directed into the stormwater wetland.
On closure, a section of the TMA’s dam will either be breached or replaced with a permeable section, depending on design and local morphology. This allows the tailings pond to be drained into the engineered bioreactor system, where its water is treated. Any future percolation through the tailings is also treated. The resulting dry tailings surface is then contoured to improve surface drainage. Following this, it is covered with previously stored or acquired organic material, to allow vegetative growth.
The former tailings dam will become a dike, holding back largely dry materials. As appropriate, this dike can then be lowered, contoured and vegetated in the same manner as the area above it. The consequences of any future break in the dike would be minimal, compared to a breach in a dam holding back water and tailings.
Once the surface of the now hydraulically-isolated tailings is vegetated, the amount of water that can infiltrated into it and percolate through it, will be limited. The amount of water that can infiltrate will be further reduced by evapotranspiration from the cover’s vegetation, and surface contouring.
Regulators, the public and other stakeholders can be expected to be even more skeptical or resistant to water covered tailings management areas. Engineered bioreactors and evapotranspiration systems can provide viable alternatives for mine closure.
Jim Higgins, Ph.D., P.Eng. is with Environmental Technologies Development Corporation. This article appeared in ES&E’s September/October 2014 issue.