Operational challenges in managing groundwater and mitigation of environmental impacts associated with water release vary with the type of mine waste under consideration. Assessment of the potential environmental impact associated with water infiltration through mine waste requires consideration of physical and geochemical processes, including biogeochemistry. The nature of these processes will vary with the type of mine waste. There are three broad classes of mine waste; mill tailings, waste rock, and spent heap leach.

Mill Tailings

Tailings are produced when rock containing economic grades of ore-forming minerals is crushed to facilitate extraction of those minerals in a mill. The tailings are a granular waste material that is a composite of clay, silt and sand-sized particles (Figure 2). The grain size distribution of the tailings that are produced is a function of the rock type hosting the ore and processing requirements in the mill to optimize ore recovery. Some tailings are sandy material (e.g., 80% fine sands and 20% silt and clay size particles), while other tailings can be very fine (e.g., 90% silt and clay-sized particles and only 10% sand-sized particles).

Figure 2 – A deposit of mill tailings in an active deposition area, showing a zone of fine sands in the left of the photo, and a zone of silt-sized tailings to the right. The ribbons in the deposit with a metallic luster are where sulfide minerals were concentrated during sediment transport. Scale of view is approximately 2 meters laterally.

Waste Rock

Waste rock is the rock mass that must be removed to access ore zones of economic value. It is either barren of the mineral resource being mined or contains concentrations of ore that are not cost-effective to recover and process. Explosives are used to break down the bedrock and then it is loaded onto trucks or carried by conveyor for disposal in nearby waste rock stockpiles. Waste rock produced during open pit mining can have an extreme range in particle size (Figure 3), from clay-size particles to large boulders up to a meter or more in diameter. The particle size distribution of waste rock is highly variable from site to site, and even within a single waste rock facility. A waste rock pile with 70% boulder and gravel-sized particles by weight, 20% sand-sized material, and 10% silt and clay-sized material would be characterized as a coarse stockpile, while if there were on the order of 40% boulder and gravel-sized particles, 30% sand-sized material and 30% fines, the stockpile would be characterized as a finer-grained stockpile. In addition to overall variability in particle size distribution, the process of placing waste rock in a stockpile results in additional segregation of the particle sizes during dumping and/or compaction of the waste rock creating finer textured layers as a result of truck traffic.

Waste rock produced during underground mining has a similar-sized fine fraction of particles to that from open pit mining, but is commonly absent the large boulder fraction. Soft sediments (e.g., saprolite, glacial deposits) removed to access the ore body may also be in incorporated within a waste rock stockpile. General guidance suggests that waste rock piles where more than 20% of the material by weight passes a 2 mm sieve (medium-grained sand and finer) will behave hydrologically as a “soil-like” pile, while if less than 20% of the material has a smaller particle diameter than 2 mm, the structure will respond as a “rock-like” pile (Smith and Beckie, 2003). ‘Soil-like’ in this context refers to the ability of the waste rock to exhibit capillarity effects including the retention and transmission of water under negative pore water pressures and the concomitant dominance of more “piston-like” infiltration of water. ‘Rock-like’ in this context highlights that the waste rock has limited capacity to store and transmit water under negative pore pressures and this promotes the tendency for flow of water along preferential pathways in the stockpile.

Some mining operations produce waste piles that are predominantly soil-sized materials and it can be misleading to characterize these stockpiles as waste rock. Examples include open pit mines where the ore is recovered from within a thick saprolite soil or at a coal mining operation where the fines produced by coal wash plants are stacked on the ground surface.

Figure 3 – Waste rock from an open pit operation has a wide range in particle size, and significant variability from one location to the next within the stockpile.

Spent Heap leach

In heap leach mining operations, rock is placed in constructed piles on the ground surface in multiple lifts and a chemical solution is repeatedly passed through the heap to dissolve and mobilize ore-forming metals such as copper or gold (Figure 4). The solution transports those metals to the base of the heap where the leach solution is collected for processing and re-circulation. Heap leach operations use either run-of-mine rock or crushed ore. If crushed, the typical particle size ranges from fine to coarse gravel size material, and particles are angular in shape. Depending upon the leach solution that is used and the mineralogy of the rock, there can be significant chemical degradation of the rock over time, resulting in progressive increase in finer-grained particles over time. Heap leach piles range from tens to a few hundred of meters in height, and are typically built on top of a low-permeability liner to maximize recovery of the metal-carrying leach solution at the base of the heap and to reduce the risk of leach solutions entering the underlying groundwater system.

Figure 4 – A heap leach pile nearing the end of its operational life. The pile is approximately 100 m in height.