Mining operations lead to considerable land disturbance and the accumulation of large amounts of by-products such as waste rock and tailings. Open storage of waste rock on the ground surface is associated with a number of negative environmental impacts because waste rock often contains contaminants such as radioactive material, heavy metals, etc.
The main impacts are physical and chemical weathering of contaminants from waste rock by wind and water, which eventually may lead to atmospheric, surface water, and groundwater contamination. One of the methods of mitigating these impacts is to cap waste rock with a soil cover composed of uncontaminated material. The main functions of such a cover are to serve as a shield to wind and water erosion of waste rock, reduce water flux into waste rock to negligible amounts, and to accumulate water in the soil cover itself for plant growth. To limit deep percolation, some water is removed from such a cover by evapotranspiration. Unlike resistive covers such as clay liners and geosynthetic materials, soil covers have better long-term stability and do not require an extensive maintenance. A soil cover is also usually more affordable than other types of covers.
One of the main advantages of such a cover is that it allows the use of soil natural to the site and, thus, is conducive to plants native to the area to grow. This is especially important because the main purpose of any reclamation is to return the mining site and the waste rock storage area to a predevelopment state with a similar biological productivity and biodiversity of the land. However, soil covers are not devoid of limitations. These systems do not perform well under humid conditions with frequent and intense rainfalls. Therefore, their application is limited to mainly arid and semi-arid regions.
For a soil cover to accumulate a sufficient amount of water for plant growth and to limit deep infiltration of precipitating water into waste rock, it must have sufficient water storage capacity or the maximum amount of water a soil can hold after gravity drainage. Depending on soil texture and organic matter content, soils can have higher (like clay) or lower (like sand) water storage capacity. Sand is a coarse-textured soil with poor water retention properties. However, quite often the area of disturbance is composed of coarse-textured soil. Despite its low water retention, sandy soil is able to support such forests as the jack pine, spruce and aspen forests. These field soils most likely developed layers during soil formation that increased water storage and reduced flux through soil layers. Similar to natural layered soil systems, reclamation soil cover with artificially created layers can have increased water storage allowing it to support vegetation.
The optimal number of layers and soil cover design are inherently site-specific. The choice of optimal soil cover design depends on many factors, including climatic conditions, type of vegetation to grow on a soil cover, budget, purposes of a cover, etc. The following recommendations are given on a basis of laboratory and some field experiments, where the original natural sand from the site has been tested as is without any fractionation or separated into coarser and finer fraction and layered as finer-over-coarser. The depth of tested covers was 1 m.
In terms of climatic conditions, in arid regions, for example, with sparse and low-intensity rainfalls, two-layered (finer-over-coarser) or even a cover made of non-layered native to the site sandy soil could be sufficient. In regions with frequent and intense rainfalls, four-layered covers with more flow barriers may be a better option. The design of the cover also depends on the type of vegetation. For example, non-layered and two-layered covers may be optimal for tap-rooted species, whereas four-layered covers may be better for deep-rooted plants, since this cover stores more water at 50-75-cm depth than other covers.
In cold semi-arid regions, where the frost-free period is only about three months a year on average, a single-layered cover may have higher freezing depth as compared to the two-layered covers and the four-layered. The layered covers have the finer layer as the first layer, which freezes to lower depth as compared to the coarser soil, since larger mineral surface and the extensive network of fine pores in finer soil would interrupt ice formation.
Dobrovolskaya Y.V. (2013) Improving water storage of reclamation soil covers by fractionation of coarse-textured soil. M.Env.Sc. thesis. Univ. of Saskatchewan, Saskatoon, Canada.