Methods such as subaqueous disposal are available to control and reduce acidic drainage from tailings and can be applied to many, but not all tailings management facilities. These available methods have various drawbacks and in particular high cost. There is a need for alternate methods which can reduce the cost of closure and perhaps eliminate some of the drawbacks associated with the available methods. This report reviews one such alternate method that is increasingly being considered for the closure of sulphide tailings impoundments – the application of elevated water table concepts.
The concept of using an elevated water table within tailings is of great interest as available data suggests that the rate of oxidation of sulphide tailings, in conditions where the water table approaches the surface of the tailings, is very low and similar to the rate of oxidation of sulphide tailings maintained under water cover. There are three basic approaches that can be applied to raise the elevation of the water table and the associated capillary zone within tailings:
- Modifying the water balance of the tailings;
- Enhancing the water retention ability of the tailings; and
- Constructing groundwater flow barriers within the tailings.
Modification of the water balance involves increasing water input into, or decreasing water losses from, the tailings. Water retention can be improved by enhancing the physical characteristics of the tailings prior to placement – thickened tailings which provide a very high level of saturation and considerable capillary zone height represent one means of achieving this objective. The concept of a groundwater flow barrier involves the installation of a barrier within tailings to reduce the horizontal and often preferential downgradient flow of pore water within the tailings. The suitability of these approaches is dependent upon site specific conditions.
Not all tailings management facilities are well suited for the application of elevated water tables. Tailings stacks were historically designed to minimize the water level within the tailings and maintain as small a surface pond as possible. The application of elevated water table concepts to raised stacks presents a considerable challenge. The application of the concept to other tailings impoundments (e.g. valley dam impoundments) is possible and may be economically beneficial in comparison to other closure options.
Elevated water tables have been an intrinsic component of closure strategies for tailings management facilities but have only recently become proposed or applied as a principal basis of closure plans. As such, experience in the application and performance assessment of elevated water tables is recent and being accumulated. The developing state of knowledge is evidenced by the relatively limited data available regarding the application of elevated water tables to sulphide tailings impoundments. Nine sites, however, provide relevant but preliminary data:
* Five sites (the Elura mine in Australia, the Greens Creek mine in Alaska, the Cluff Lake mine in Saskatchewan, Les Mines Selbaie in Québec, and the Kidd Creek metallurgical site in Ontario) involve the ongoing or discontinued disposal of thickened tailings. These sites represent a broad range of site conditions, and tailings disposal and closure strategies.
* Three sites (the Falconbridge New Tailings site, the Dona Lake mine site, and the Stanrock mine site – all in Ontario) involve the modification of the water balance of the tailings. At the Falconbridge New Tailings site, research has been carried out to investigate the important relationships between oxygen consumption rates within tailings, the degree of saturation, and depth to the tailings water table. At the Dona Lake site, a closure plan is being implemented which relies in part on the elevation of the water table within the coarse tailings beach area. The proposed approach for the historic Stanrock tailings stack is based on replacing spigotted tailings dams with engineered water retaining structures – this would ultimately allow the water table within the tailings to rise and inhibit acid production from much of the tailings. Chemical treatment of acidic drainage from the Stanrock tailings would continue for an interim period following the construction of the new containment structures.
* The Sturgeon Lake site in Ontario involves a combination of a modified water balance and a groundwater flow barrier.
Chemical treatment of acidic drainage may be required, in some instances, after the application of an elevated water table concept. Key sources of acid production could include sulphide solids in the near surface and not sufficiently saturated zone, and fluctuations in the water table elevation. The effect of water table fluctuation could be mitigated in some cases by the use of a cover to reduce evaporative losses, increase infiltration, or to elevate the water table to the cover. The additional and interim use of chemical treatment may be cost beneficial when compared to other options.
Numerical modelling techniques are being used to predict the performance of elevated water table concepts in controlling or reducing acidic drainage. These techniques draw upon soil science and civil engineering methods of predicting water movement in soil, and can be complemented by numerical models for the prediction of acidic production in tailings. These models are important as they provide a means of preliminarily assessing the performance of a closure option, and of addressing the effect of changes in ambient conditions (e.g. drought conditions).
Preliminary and conceptual cost comparisons were carried out based on the closure of three types of tailings impoundments: a tailings stack; a valley impoundment underlain by a pervious zone; and a valley impoundment underlain by an impervious zone. Closure options included the perpetual collection and treatment of acidic drainage, the use of an engineered cover, and applications of elevated water table concepts. The first order estimates of closure costs indicate that elevated water table concepts, when suitable, can provide significant closure cost saving in comparison to collection and treatment, and the use of an engineered cap.
