Mining Operations and Acid Mine Drainage

The Mining Project Cycle

Figure 1.1 depicts the mine project cycle consisting of key sequential phases, each entailing specific activities. A detailed discussion of the key phases of a mining project cycle is presented in literature (Robertson et al., 2017; Gorman and Dzombak, 2018). In summary, a typical mining project cycle consists of the following steps.

1.2.1.1 Prefeasibility Stage

This step entails project ideation and scoping, including identification of prospective mining sites. Preliminary geological investigations during this stage are limited to analysis of data based on desktop studies. Preliminary environmental scoping may be conducted during this phase. However, compared to subsequent steps, activities in this stage have limited environmental impacts and contribution to the formation of AMD.

1.2.1.2 Detailed Feasibility/Exploration Phase

This stage involves conducting environmental and social impact assessments (ESIAs) covering exploration, mining, metallurgical processing, and mine closure (Joyce et al., 2018; European Community, 2019). However, by nature, such ESIAs are overly broad and limited to scoping on potential social and environmental health impacts, hence have limited capacity to adequately address the potential generation and impacts of AMD. Field geological exploration and sampling involving excavation of test pits, drill holes, and subsequent sampling and assaying are also conducted during this phase. Activities conducted in this stage and, subsequent ones, have potential significant effects on the biophysical environment and AMD formation (Figure 1.1).

1.2.1.3 Design and Engineering

This stage involves the design of the mine layout and key infrastructure, including surveying of mining pits, roads, processing plants, and waste

FIGURE 1.1

Mining activities contributing to the generation of acid mine drainage.

and wastewater disposal facilities including waste rock dumps and tailings dams (Gorman and Dzombak, 2018). This stage may include excavations and drilling during geotechnical site investigations, and land clearing to create access roads and set up site camps.

1.2.1.4 Construction

This stage entails significant land and vegetation clearing, excavation, drilling and blasting, and traffic movements during the construction of infrastructure. Dewatering via groundwater pumping may occur to facilitate

FIGURE 1.2

Conceptual depiction of the pre-mining water balance (a) compared to the hydrological impacts of mining activities (b). The size of the arrow is qualitatively indicative of the magnitude of the water balance components relative to the pre-mining state.

construction of infrastructure. Typical infrastructure constructed during this stage includes groundwater dewatering systems, mine access points, roads, metallurgical processing plants, and tailings dams (Gorman and Dzombak,

  • 2018). Compared to the pre-mining state, this phase is also characterized by an increase in build-up areas and impervious surfaces, which in turn influences hydrological response (Figure 1.2).
  • 1.2.1.5 Mine Operation and Management

This is the main phase in a mining project cycle, during which significant changes in the biophysical environment occur. Key activities during this stage include large-scale excavations, drilling and blasting during the extraction of ore (Hartlieb et al., 2017; Winn, 2020). In this phase of the mining project cycle, the removal of overburden rock generates large quantities of waste rock and run-of-mine. In cases where the ore body occurs below the groundwater table, mine dewatering via pumping may also occur. Frequent movement of vehicular mining equipment such as excavators, bulldozers and loaders occurs, causing soil compaction, soil detachment and dust generation. It must also be noted that subsequent mineral processing may require large quantities of water from surface or groundwater sources. In addition, mineral processing itself generates large quantities of solid waste including waste rock, mine tailings, and wastewater (Giblett and Morrell, 2016; Habib et al„ 2020).

1.2.1.6 Mine Decommissioning and Closure

This is the final stage of a mining project entailing the decommissioning of mining pits and infrastructure. During this stage, the mine dewatering/ groundwater pumping ceases, and groundwater upwelling/rising may occur. In some cases, rehabilitation of waste dumps and mine tailings are undertaken during this stage (Sanders et al., 2019).

In summary, mining activities entailing land/vegetation clearing, excavations, drilling and blasting have significant effects on the surface and groundwater hydrological systems. Mineral processing and waste disposal may release sulphidic wastes and wastewaters which may undergo oxidation to release AMD and associated contaminants. In subsequent sections, the impacts of the mining activities on hydrology and AMD formation are discussed.

 
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