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Hardrock Mining: Environmental
By its very nature, mining can cause large disturbances to the land. Not only can there be a direct impact as a result of mining, but the withdrawal and discharge of water, manipulation of topography, and releases of particulates and chemicals can all have indirect impacts on various habitats.
Beneficiation processes can use large quantities of water. Large withdrawals of groundwater or surface water can affect aquatic habitats and wetlands by lowering the water table or decreasing recharge.
Surface Water and Groundwater
Hydrology relates to the flow characteristics and patterns of surface water and groundwater, including springs, wetlands, brooks, streams, rivers, ponds, lakes, and aquifers.
Mining can impact hydrological systems by it nature. The massive quantities of water necessary for many beneficiation processes can disrupt surface water and groundwater flows. The removal of water from mine workings can result in drawdown. Drawdown can reduce the amount of water available for recharging wetlands and surface water, thereby affecting any organisms that depend on those waters.
Water that percolates through waste piles and tailings can become contaminated with heavy metals. This water can contaminate both groundwater and surface water. Additionally, the hydraulic head pressure of tailings ponds can cause hydraulic mounding underneath and adjacent to the pond, potentially altering the local groundwater flow pattern.
Control and Remediation
One of the most effective control measures for minimizing surface water and groundwater impacts is to reduce the amount of water needed during mining operations as much as possible. Recycling water can dramatically reduce overall water withdrawals. Tailing impoundments can be operated to recycle water back to the mill and can be underlain with a liner to prevent the release of contaminated water.
The effect of pit design, specifically the depth of the mining operation, on groundwater and surface water should be considered. Pits can fill with water after mining is completed and this water can become contaminated with metals.
Mining subsidence is the surface impact of collapsing overlying strata into mined-out voids. Subsidence may manifest itself in the form of sinkholes or troughs. Sinkholes are usually associated with the collapse of a portion of a mine. Sinkholes or depressions interrupt surface water drainage patterns, affecting ponds, streams, and wetlands. Reducing the withdrawal of groundwater through specific practices such as recycling mine water can reduce the potential for subsidence.
The threat and extent of subsidence is related to the method of mining employed. In many instances, traditional room and pillar methods leave enough material in place to avoid subsidence. However, high volume extraction techniques, such as pillar retreat and longwall mining, result in a strong likelihood of subsidence. Preventing subsidence involves leaving support mechanisms (e.g., pillars) in place after completing the mining operation or backfilling with waste rock.
Aesthetics involve the general visual environment, including the overall scenery and unique topographical characteristics. Since most mining operations result in large land disturbances, aesthetic impacts can be significant. Recontouring the land to reduce unnatural anomalies, backfilling holes, revegetating, and promoting wildlife habitats can all improve the aesthetics of a mining operations.