9.3 Tailings Storage Facilities and Waste Rock Stockpiles
A common step during the closure of tailings facilities, waste rock stockpiles, and spent heap leaches is to place a soil cover on top of the mine wastes. Two types of soil covers are considered based on the design intent; isolation covers and infiltration covers. The main intent of an isolation cover is to prevent contact with the mine wastes by humans and wildlife, to minimize long-term mobilization of mine wastes by either wind-blown erosion or surface runoff, and to provide a substrate for re-vegetation of the disturbed landscape. Figure 22 shows an example of an isolation cover in a semi-arid environment, placed on a re-contoured waste rock stockpile. The surface topography created includes consideration of surface drainage patterns for large run-off events.
Infiltration covers provide isolation but they also have as their design basis the intent to reduce the amount of infiltration that enters the underlying tailings or waste rock. Soil covers can be as thin as 30 cm, or up to 2 m in thickness, depending on design intent and availability of suitable soils. Cover design draws heavily on concepts of unsaturated flow through soils and quantifying the exchange of water and energy at the soil – atmosphere boundary.
Figure 22 – Example of an isolation cover placed on a re-contoured waste rock pile.
A number of factors are considered when evaluating the potential benefits of cover placement on mine wastes. A soil cover can be designed to enhance storm runoff in comparison to that occurring on the tailings surface or waste rock stockpile if it were left uncovered. If surface runoff does not come in contact with underlying mine wastes, it is likely to be of suitable quality to allow direct release to the environment, without need for water treatment. Tradeoff studies are sometimes undertaken to assess the benefits of a higher level of cover performance relative to an anticipated reduction in requirements for water collection and costs of water treatment after mine closure.
If mine wastes yield solute loads to groundwater that present a concern for water quality, or the anticipation is that at some point in the future solute loads will increase as mineral weathering progresses, an infiltration cover design might be adopted that decreases solute loads reporting to the base of the facility. The timing for placement of a soil cover during closure activities is often based on geochemical assessment of the length of time before the neutralization capacity of the tailings or matrix materials in waste rock is consumed and acidic conditions begin to develop. A second design consideration for reactive mineral wastes is the extent to which a soil cover might reduce the rate of oxygen resupply into interior regions of a stockpile, thus reducing the rate of sulfide mineral oxidation. In this latter case, the porosity of the soils used in the cover and the water content within the cover that is retained over the climate cycle determines the oxygen flux into the mine waste. Soil covers also suppress preferential flow through waste rock stockpiles due to the reduction in net infiltration and their more uniform surface characteristics when compared to uncovered waste rock. The pore water flux entering the underlying waste rock is more likely to be transmitted through granular matrix materials due to capillary pressure effects, contributing to more uniform flushing of the stockpile with a consequent influence of the spatial and temporal variability in solute loads.
An excellent review of the function, design and monitoring of single layer or multi-layer soil cover systems is provided in a guidance document developed by the International Network for Acid Prevention (2017). Several of the principal design concepts for infiltration covers are noted here. From the perspective of modifying the amount of infiltration passing through mine waste, soil covers are designed as water storage and release systems, barrier systems, or they may incorporate both elements. The design intent of a store and release cover is to have sufficient capacity to store wet-season precipitation within the root zone of the soil cover and then to rely on evapotranspiration to remove that water during the dry season. Over the annual cycle, net infiltration through the cover is reduced. A barrier system is designed to promote surface runoff and interflow within the cover system as the means of reducing downward percolation into mine wastes. In barrier systems, use is made of low-permeability compacted soils such as clays or geosynthetic (geomembrane) liners. Unlike a liner system place beneath a tailings facility, which has hydraulic head acting across the liner, a geosynthetic liner incorporated within a soil cover is less prone to leakage through installation defects due to a much-reduced water head. Experience suggests that for an effective store and release cover system, a net infiltration in the range of 5 – 10% of the annual precipitation would be considered an excellent outcome in more humid climates, with a net infiltration of 20 – 30% of annual precipitation being a reasonable estimate of performance for many sites. Net infiltration through a cover would be lower in an arid climate. If a geomembrane is incorporated in the liner system, net infiltration is usually expected to be on the order of 1 – 2% of annual precipitation.
In projecting cover performance into the future, the influence of multi-year climate cycles needs to be considered, rather than the average annual precipitation and temperature condition. Because potential environmental risks can exist long into the future following mine closure, it is becoming more common that cover performance is also evaluated (modeled) for various climate change scenarios.