Hydrological Impacts of Mining

Mining operations have significant impacts on local hydrological processes and water balances. Figure 1.2 depicts the hydrological impacts of mining operations relative to the pre-mining state. Specifically, mining activities reduce water infiltration, while increasing surface water runoff and soil erosion. Mining operations also remove vegetation cover, thereby reducing evapotranspiration. Mine dewatering increases groundwater pumping/ abstraction, resulting in significant declines in groundwater levels and potential ground subsidence. Impervious surfaces associated with build-up areas reduce groundwater recharge. The mechanisms and processes responsible for the changes in surface water and groundwater hydrology are summarized in the subsequent sections.

1.2.2.1 Land Clearing and Impervious Surfaces

Vegetation cover protects soils against wind and water erosion, while plant roots increase soil aggregation by binding soil particles together. Vegetation cover also attenuates raindrop impacts and surface water runoff velocity, thereby increasing infiltration during rainfall events. Hence, land clearing, including the removal of vegetation, exposes soil to wind and water, leading to soil detachment and subsequent erosion (Asabonga et al., 2017; Jarsjo et al., 2017). The replacement of natural surfaces and vegetation cover by build-up areas consisting of mine buildings and impervious surfaces such as roads and roofs alters the water balance and affect hydrological processes (Figure 1.2). First, the removal of vegetation reduces interception of rainfall by vegetation covers, root uptake of soil moisture, and transpiration (Awotwi et al., 2019). Impervious surfaces associated with roads and build-up areas reduce infiltration and promote rapid generation of runoff water, resulting in peak water flows or flash floods (Figure 1.2). The removal of vegetation reduces root uptake of soil moisture, hence reduces evapotranspiration. In turn, reduced root uptake of soil moisture may potentially increase soil moisture storage and increase the risk of drainage (Ketcheson et al., 2016).

1.2.2.2 Vehicular Traffic Movements

Frequent movement of both heavy and light vehicular equipment including excavators, bulldozers, loaders and passenger vehicles has potential impacts on soil physical properties and hydrological behaviour (Weyer et al., 2019). The effects of traffic movement on soils and hydrology depend on soil type and moisture conditions. For example, movement of vehicular traffic on moist soils results in soil compaction via increased soil bulk density and reduced porosity (Strahm et al., 2017). Soil compaction is more pronounced on fine-textured soils such as clays and loams compared to sandy soils. Reduced porosity increases surface water runoff volumes and velocity during rainfall events. On dry soils, movement of vehicular traffic detaches soil particles and sediments. This increases soil erodibility or susceptibility to wind erosion and dust generation (Patra et al., 2016).

1.2.2.3 Excavation, Drilling and Blasting

Excavation, drilling and blasting alter the natural landforms and surface topography, which may in turn change the surface and groundwater hydrology (Hartliebet al., 2017; Winn, 2020). Furthermore, excavations conducted to remove overburden materials to expose the ore body may create surface and underground cavities and pits, which act as artificial depressions. These cavities and pits alter the flow directions of surface water runoff and may even change river flow regimes and flow directions (Newman et al., 2017; Zhang et al., 2017).

Drilling and blasting cause fractures or cracks in geological rock formations (Zhang et al., 2018). Recent concerns in the mining industry have also focused on the impacts of hydraulic fracturing/fracking (Taherdangkoo et al., 2017; Wu et al., 2019). Flydraulic fracturing or fracking involves injecting pressurized water, sand and chemicals into a rock formation through a well, in order to induce rock fragmentation and increase permeability of rock formations (Bao and Eaton, 2016; He et al., 2016). In cases where the ore body is submerged under groundwater, drilling and blasting may cause fractures or cracks in groundwater-bearing rock formations or aquifers. Fractures and cracks alter groundwater flow directions and result in groundwater upwell- ing, where groundwater rises to the surface under hydraulic pressure (Toner et al., 2017; Morrison et al., 2019). Groundwater upwelling is particularly common in confined or semi-confined aquifers, where groundwater occurs under hydraulic pressure.

1.2.2.4 Mine Dewatering via Groundwater Pumping

Mine dewatering entails the pumping of water from mine working areas such as mine pits to facilitate mining operations (Szczepiriski, 2019). Such water may originate from two sources: (1) surface water runoff from surrounding areas, and/or (2) groundwater in cases where mining occurs below the groundwater table. Mine dewatering systems include wells, well galleries, boreholes and pumping units. Subsequently, groundwater pumping lowers the groundwater table and alters groundwater flow directions. Excessive dewatering or groundwater pumping may significantly alter the groundwater balance, leading to aquifer drying or groundwater recession. Excessive groundwater pumping also creates a large void space in the subsurface. Therefore, as the geological material settles to fill up the void space, this causes ground subsidence (Morrison et al., 2019).

1.2.2.5 Mineral Processing and Waste/Wastewater Generation

A wide range of mineral processing technologies exist, including pyromet- allurgy, and hydrometallurgy (Giblett and Morrell, 2016; Habib et al., 2020). Mineral processing requires large quantities of water (e.g., washing, cooling) and may increase surface water and groundwater abstractions. In turn, mineral processing, particularly hydrometallurgy, generates large quantities of waste- waters, which may contribute to surface water and groundwater pollution.

1.2.2.6 Mine Decommissioning and Closure

The termination of mine dewatering/groundwater pumping may be accompanied by groundwater upwelling and flooding of mine pits. Excessive surface water runoff, and water and wind erosion may also occur from waste rock dumps, and mine tailings, thereby generating dust and sediments.

 
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