Applications of Hydrology Principles in Acid Mine Drainage Prevention
The fact that hydrology controls the formation, mobilisation and dissemination of AMD forms the underlying principle for the design and operation of wet and dry engineered covers for AMD prevention. A detailed discussion of wet and dry covers is presented in Chapter 6 of this book. Therefore, only an overview of the application of hydrology in the design and operation principles of wet and dry covers is highlighted here.
220.127.116.11 Wet or Water Covers
Wet or water covers prevent AMD generation by restricting oxygen ingress by maintaining permanent saturation (Aubertin et al., 2016; Kama and Hettiarachchi, 2018). The use of water to restrict oxygen ingress is based on the fact that oxygen has low solubility and diffusivity in water. According to Equations 1.1—1.4, the lack of oxygen prevents oxidation of sulphidic materials and formation of AMD. Wet covers are commonly used in temperate and permafrost environments, where precipitation far exceeds potential evapo- transpiration (Aubertin et al., 2016). Such environments lack strong seasonality patterns, hence it is possible to maintain permanent saturated conditions over the sulphidic wastes. However, wet covers are not ideal in water-limited environments such as the tropics, due to strong seasonality associated with distinct wet and dry seasons, which induce strong wetting and drying cycles.
18.104.22.168 Evapotranspirative 'Store-Release' Covers
Evapotranspirative covers are also known as dry, water balance or store- release covers (Gwenzi, 2010). The design objective of the covers is to reduce drainage, and to some extent oxygen ingress into buried sulphidic wastes (Knidiri et al., 2017). This is achieved by: (1) storing water in a storage layer consisting of soil or benign geological material during or soon after rainfall events, and (2) then releasing it back into the atmosphere as water vapour via evapotranspiration (Gwenzi, 2010). Accordingly, vegetation plays a key role in the hydrology and performance of store-release covers. Uptake of soil water by plant roots and the subsequent loss of such water via transpiration account for the soil dewatering process and reduction of drainage. In principle, evapotranspirative or store release covers are designed to enhance soil moisture storage and the evapotranspiration components of the water balance. Evapotranspirative covers are ideal in water-limited environments such as the tropics, where potential evapotranspiration far exceeds precipitation. These climatic conditions are ideal for maximum soil water loss via evapotranspiration. A detailed review of global literature on the design and performance of store-release covers is presented in earlier studies such as by Gwenzi (2010).