Subaqueous disposal means placement of tailings into or beneath a water cover. Deposition of tailings into a lake is the most common subaqueous method. In many instances the embankment dam is constructed as a water retaining structure and the impoundment is filled with water into which the tailings are discharged.
Mildred Lake at the Syncrude oil sands mine in Alberta is one of the largest subaqueous tailings facilities in the world. A topographic depression has been turned into a large lake by construction of embankments. The tailings are discharged into the water which forms a “thick” water cover. The closure plan for this facility includes placement of coke over the tailings surface, and retention of a water cover. The idea is that the coke that settles over the submerged tailings will prevent future resuspension of the tailings.
In Ontario and Quebec, acid generating tailings are often placed beneath a water cover both during operation and at closure—see this link for details. The advantage of a permanent water cover is that the potentially acid generating tailings are kept saturated and thus do not generate acid. Two issues arise:
- How deep a water cover is required to prevent suspension of tailings?
- What is the probability that in a dry period the water cover evaporates and the tailings become unsaturated and hence acid-generating?
To answer the first, consider that waves generated by wind over the surface of the tailings may cause turbulent water to resuspend settled tailings. In theory the water cover should be deep enough to preclude wave-induced turbulence from disturbing settled tailings. At one mine in Quebec, the operating water cover is one meter thick although the regulators are now calling for an increase of this thickness to 1.3 m.
To answer the second question, you will have to set as a design criterion some drought recurrence interval and calculate the return period for which the water may evaporate, the lake turn dry, and the tailings generate acid. The deeper the water pool, the less likely it is to all evaporate in a dry spell.
During operation and post-closure, waves may also lead to erosion of the upstream face of the embankment. Sound erosion-resistant layers must be in place to prevent wave-induced erosion.
Another issue with a water cover is this: how do you get the tailings into the pool so that there is a uniform thickness of water over the tailings? I have seen floating pipelines out into the pool with discharge from the end of the pipe. You need to keep moving the pipe to get uniform coverage and retain uniform water depths. This is impossible in winter if the water freezes. In this case, the tailings build up and may create islands of tailings with a surface above the water that are revealed in the spring.
Waves soon wash down the islands. Or the tailings consolidate and settle and the tailings surface submerges back beneath the water surface. The regulators comment that the required water cover depth is not achieved, but that is the way it is. For it is expensive and unsafe to go in to mechanically redistribute the tailings to get them at least one meter below the water surface.
Many now say that a post-closure water cover is not acceptable. The primary reason is that a water retention structure as is required for closure, is still a dam that requires perpetual active maintenance. Many believe this is simply not possible. Thus there is a call to remove the water cover at closure and replace it with another, more permanent cover solution.
One way to create a permanent closure cover at a tailings facility that, during operation, involves a water cover, is as follows: as you approach closure, deposit tailings from the embankment in order to create a tailings beach that slopes away from the embankment into the pool. Keep the general level of the beach at or just below the average level of water in the pool. Doing that keeps the tailings saturated most of the time, either because the tailings are below water or kept saturated by capillary rise.
If the impoundment is large, it may not be practical to place tailings to fully displace all the pool. In such a case, push the pool as far from the embankment as possible and retain the pool as a polishing pond—an area where water flowing into the pool from the upgradient watershed may dilute constituents emanating from the tailings.
A classic case of planning a closure water cover for tailings is when the tailings are placed into a used open pit. I am involved with one such. The plan is to fill the pit with tailings, but keep a thirty-meter thick water cover over the tailings at closure. No science went into determination of this number. It sounds goo though.
I am concerned that in the goodness of infinite time, the walls of the open pit will fail, or at least ravel back to a flatter inclination than currently. The slope failure material will slough into the water and tailings and possible disturb the tailings, befouling the thirty meters of anticipated clean water.
I am convinced the better way is to fill the pit entirely with tailings, and for some years after filling let the tailings settle, as they will. Then fill in the resulting depression with rockfill and create a mound big enough to deal with long-term tailings settlement. Divert surface flow around or away from the pit, now filled and covered, and you have a geomorphically stable landform.