The mining industry plays a key role in the global economy through the provision of industrial raw materials, contribution to gross domestic product, and employment creation (Golev et al., 2016). Moreover, according to a World Bank report, minerals such as gold are estimated to have a multiplier factor of about

1.7 to 1.8, indicating that for each mining job, the sector creates an additional 1.7 to 1.8 jobs through expenditure effects and backward linkages (Chuhan-Pole et al., 2017). It must be noted, however, that the mine project cycle consists of several steps and activities, including: (1) prefeasibility studies, (2) exploration, (3) design and engineering, (4) construction, (5) extraction and processing, and (6) mine decommissioning and closure (Durucan et al., 2006). Without proper planning, these mining activities can cause significant adverse impacts on the biophysical environment. Specifically, mining activities may cause land degradation via: (1) disruption of ecosystems and loss of biodiversity, (2) changes in surface and groundwater hydrology, and (3) environmental pollution, including acid mine drainage (AMD) (Ochieng et al., 2010).

AMD is formed through the oxidation of sulphidic rock materials including waste rock, mine tailings, and even in situ rocks exposed to water and oxygen during excavation and drilling operations (Pope et al., 2018; Wright et al., 2018). Two pre-conditions are necessary for the formation of AMD: (1) sulphidic rock materials with acid-generating capacity exceeding acidneutralizing capacity, and (2) a combination of water and oxygen, which promotes the oxidation process (Skousen et al., 2019; Campbell et al., 2020).

Unfortunately, AMD is a global environmental problem, which has been widely reported in several countries in nearly all continents. To date, AMD has been reported in Africa (Fosso-Kankeu et al., 2017; Gwenzi et al., 2017; Ochieng et al., 2017; Mungazi and Gwenzi, 2019), Europe (Grande et al., 2018), North America (Campbell et al., 2020), South America (Galhardi and Bonotto,

  • 2016), Asia (Hao et al., 2017), and the Pacific/Oceania (Wright et al., 2018). At the moment, several cases of AMD problems exist in South Africa (Ochieng et al.,
  • 2010), Zimbabwe (Gwenzi et al., 2017; Mungazi and Gwenzi, 2019), Australia (Wright et al., 2018), China (Hao et al., 2017), the United States (Campbell et al., 2020), and Canada (Genty et al., 2016). In these case studies, AMD has been reported in both underground and surface mining operations, waste rock dumps, and mine tailings (Gwenzi et al., 2017; Ochieng et al., 2010; Mungazi and Gwenzi, 2019). Moreover, AMD has been detected in a wide range of mining operations, including coal, gold, and uranium mining, among others (Choudhury et al., 2017; Humphries et al., 2017; Casagrande et al., 2020).

The environmental, human and ecological health risks of AMD are well- documented (Liao et al., 2016;) and discussed in detail in Chapter 5. Some of the impacts of AMD include: (1) soil pollution, including that of agricultural soils used for food production (Liao et al., 2016; Fernandez-Caliani et al., 2019),

  • (2) surface and groundwater pollution (Ochieng et al., 2010; Wright et al., 2018), and (3) disruption of aquatic ecosystems and functions (Leppanen et al., 2017). For example, significant pollution of soils, surface water, and groundwater by AMD has been reported at Iron Duke Mine in Zimbabwe (Gwenzi et al., 2017; Mungazi and Gwenzi, 2019). In Zimbabwe, anecdotal evidence also indicates significant human health effects due to downstream contamination of drinking water sources by AMD from underground coal mine workings dating back to the 1960s (Gwenzi et al., 2018b). This and several other studies show that AMD may have a long latent or lag time, between exposure to oxidative conditions and the manifestation of AMD (Kanda et al., 2017; Wright et al.,
  • 2018). This is because AMD only manifests when the acid-generating capacity exceeds the acid-neutralizing capacity derived from calcium and magnesium oxides and carbonates (Kanda et al., 2017).

The significant impacts of AMD and the challenges associated with its remediation once it occurs justify the need for AMD prevention during the whole mining project cycle. However, existing literature, including reviews on AMD, is dominated by studies on the formation, hydrochemistry, environmental health impacts, and remediation and control using either passive or active methods (Buxton, 2018; Park et al., 2018; Skousen et al., 2019; Tabelin et al., 2019). By comparison, limited attention has been paid to the role of mining activities including excavations, drilling, blasting, and metallurgical processes (e.g., comminution, pyrometallurgy, and hydrometallurgy) on the AMD generation. Similarly, a clear hydrological perspective on the generation, dissemination and control of AMD is largely missing.

Therefore, the main objective of this chapter is to discuss how mining activities and hydrology contribute to AMD formation, dissemination and prevention. The specific objectives are: (1) to discuss the role of mining activities, including excavations, drilling and metallurgical processes on hydrology, and subsequent AMD formation, (2) to discuss how hydrology controls AMD formation, mobilisation and dissemination, (3) to summarize how a fundamental understanding of hydrology is used as a basis to design engineered cover systems for preventing AMD formation, and (4) to highlight constraints and knowledge gaps on AMD.

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