Mining Operations and Acid Mine Drainage Formation

Mining operations may contribute to AMD through the following: (1) directly by altering and exposing the previously buried/submerged sulphidic geological materials to weathering agents, and (2) indirectly by changing surface and groundwater hydrological flows and regimes. Table 1.1 presents a summary of mechanisms and processes related to mining operations.

TABLE 1.1

Contribution of Various Mining Activities to AMD Formation and Dissemination

Mining Activity

Impact

Contribution to AMD

Vegetation clearing during construction

Increases soil erosion and runoff

Disseminates sulphidic materials and AMD

Traffic movement during haulage

Sediment detachment and compaction

Increases erosion and transport of sulphidic sediments

Mine dewatering via groundwater pumping

Lowers groundwater table and alters flow directions

Promotes AMD formation by exposing previously submerged geological materials to weathering

Drilling and blasting of rock masses

Causes hydraulic fracturing or fracking of aquifers and changes water flow directions

Fractures promote AMD transport and expose previously submerged geological materials, while increasing surface area

Excavation of mining pits

Breaks down and exposes previously buried geological materials

Promotes AMD formation by breaking down rock materials and exposing them to oxidizing agents

Milling/crushing of mineral ores

Reduces particle size and increases surface area

Promotes AMD formation due to increased surface area

Metallurgical

processing,

including

hydrometallurgy

Size reduction and increases in surface area, and use of strong extracting solutions

Promotes AMD formation in mine tailings and colloids in mine effluents. Strong acids/oxidizing agents accelerate weathering

Mine tailings / wastewater disposal

Environmental pollution

Promotes AMD formation from mine tailings and colloids in wastewater

Post-mine closure

Environmental pollution

AMD formation and dissemination via groundwater upwelling, erosion and runoff from waste dumps and mine tailings

1.2.3.1 Vegetation Clearing and Impervious Surfaces

Increased surface water runoff and soil erosion caused by land clearing and impervious (Asabonga et al., 2017; Jarsjo et al., 2017; Awotwi et al., 2019) promote the mobilisation and transport of sulphidic sediments and fine- textured mine wastes (Table 1.1). Traffic movements may also detach sulphidic mine wastes and sediments, thereby increasing their susceptibility to water and wind erosion (Patra et al., 2016). These processes mobilise and disseminate sulphidic geological materials and sediments, resulting in offsite impacts. Reduced evapotranspiration caused by vegetation clearing also increases the risk of deep drainage into buried sulphidic mine wastes and thus the formation of AMD.

1.2.3.2 Excavations, Drilling and Blasting

Excavations, drilling, and blasting expose previously buried/submerged sulphidic rock materials to weathering agents (Scheiber et al., 2018). Fractures, cracks and drill holes promote oxygen ingress into the sub-surface, causing in situ oxidation and AMD formation (Figure 1.2). Fractures and cracks also increase the permeability of rock formations to both water and oxygen (Bao and Eaton, 2016; Fie et al., 2016). This in turn increases connectivity and preferential flow pathways for the transport of AMD and associated contaminants.

Large quantities of waste rock or run-of-mine materials are generated during excavation, blasting and drilling to remove overburden geological material (Ruiseco et al., 2016; Pearce et al., 2019). Excavation, drilling, and blasting also cause rock fragmentation into particles relatively smaller than the original geological materials (Singh et al., 2016; Iravani et al., 2018). The reduced particle size and resulting increase in surface area create ideal conditions for subsequent weathering processes and AMD formation. Excavation, drilling and blasting, transport, and stockpiling of ores and waste rock also disperse sulphidic materials over large areas.

1.2.3.3 Mine Dewatering

The decline in groundwater levels caused by mine dewatering may expose previously submerged sulphidic rock formations to weathering conditions including oxidation and AMD formation. Groundwater upwelling may mobilise and transport AMD and contaminants from oxidized underground sources to the surface and aquatic systems as reported in a number of studies (Gomo, 2018; Migaszewskiet al., 2019). The impact on groundwater upwelling on AMD is most pronounced in water-limited environments such as the tropics. This is because, cyclic phases of groundwater upwelling in the wet summer season, followed by a decline in groundwater levels in the dry winter season, may subject sulphidic geological materials to wetting and drying cycles. In fact, the repeated cycles of drying and wetting of excavation pits and drill holes followed by groundwater rise or upwelling promote AMD generation. This phenomenon is well-pronounced in old coal mine workings in Hwange, Zimbabwe, for example, where coal mining dates back to the 1960s (Gwenzi et al., 2018b).

1.2.3.4 Mineral Processing and Waste/Wastewater Generation

Mineral processing entails the use of comminution circuits for mechanical size reduction using grinding and milling to increase surface area for subsequent mineral extraction in some cases (e.g., gold extraction) and/or to meet specifications for certain applications in other cases (e.g., coal) (Giblett and Morrell, 2016; Habib et al., 2020; Martinez et al., 2020). Three processes are critical in the context of AMD: (1) size reduction may generate sulphidic solid wastes in the form of waste rock or dust, (2) the use of strong extracting solutions in metallurgical processing generates sulphidic mine tailings and wastewaters laden with fine particles and colloids, and (3) strong extracting solutions such as peroxides and acids may promote weathering and AMD formation. Fine particles and colloids are also prone to both wind and water erosion.

1.2.3.5 Mine Decommissioning and Closure

In most developed countries using the mine bonding system such as Australia and Canada, among others, the environmental regulatory authority in the ministry responsible for environment often takes over the responsibility for post-mine closure monitoring and management (Cheng and Skousen, 2017; Sanders et al, 2019). In contrast, for most developing countries (e.g., Zimbabwe, Zambia, etc.), this phase marks the period of the most significant environmental impacts. This is partly because there is lack of clarity on the responsible authority between the ministry of mines on one hand, and the ministry of environment on the other hand. Moreover, due to the lack of a mine bond system, resources for such monitoring and management of mines in the postclosure phase are often unavailable after mine closure. Environmental regulatory agencies in most developing countries also often lack expertise and resources to mitigate AMD and its associated impacts, because no such bond systems exist. The mine or environmental bond system consists of an upfront, gradual set-aside or the allocation of financial resources to cater for mine closure and associated environmental health risks, rehabilitation and cleanup (Pepper et al., 2014; Cheng and Skousen, 2017). At the end of the mining operation, such bonds are either: (1) relinquished or forfeited to the state or its delegated regulatory authority that then takes over the responsibility of environmental management, or (2) repaid to the mining company in cases where mine rehabilitation and closure are done to the satisfaction of the regulatory authority. Due to the challenges associated with restoration of mined sites, and the potential for long-term adverse impacts (e.g., AMD) in the post-mine closure period, the former option is more common than the latter.

In summary, compared to the operational phase, the post-mine closure stage is characterized by relatively minimal environmental monitoring and control, thus, leading to significant AMD-related environmental impacts. Specifically, mine excavation pits, drill holes and cavities, which are typical relicts of mining operations, act as potential hotspots for the formation and transport of AMD and the associated contaminants. In addition, the termination of mine dewatering accompanied by frequent groundwater upwelling and flooding of mine pits promote AMD formation and dissemination. Indeed, several legacy cases of post-mine closure AMD occur globally, pointing to the lack of adequate control systems to manage AMD in several countries (Favas et al., 2016; Gwenzi et al., 2017, 2018b; Kim and Choi, 2018; Tabelin et al., 2019).

 
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