2.1 Climatic and Geologic Settings

Mining occurs in a wide range of geologic and topographic settings, across a full range of climatic conditions. There are large mines in production in desert environments such as the Atacama Desert in northern Chile, the Gobi Desert in Mongolia or the Namib Desert in Namibia, in tropical rainforests in equatorial countries such as Papua New Guinea and the Democratic Republic of Congo, at elevations above 4000 m in the Andes Mountains of Peru and Chile, and in polar environments in Canada or Russia where permafrost significantly modifies the character of the groundwater flow regime from that which occurs in more temperate settings. Each setting presents unique challenges in water management.

Water issues differ between areas where the climatic regime is one of water surplus (annual precipitation exceeds evapotranspiration) versus mines located in a climatic regime of water deficit (annual potential evapotranspiration exceeds annual precipitation). At mines located in a region of water surplus, excess water can accumulate in a tailings storage facility if management controls are not in place. Operational flexibility decreases and the geotechnical risk profile increases when excess water accumulates in a tailings storage facility. Similarly, surface water accumulating in an uncontrolled fashion can result in increased risks of water inflows to an open pit, or to underground workings.

At mines located in climatic regimes with a water deficit there are often no available sources of surface water and the water required for ore processing is obtained from nearby aquifers, if present. In an extreme case, mine operations in a desert setting might involve not only mineral resource extraction but also “mining” of a groundwater resource that could take many centuries to recover to the pre-development state. The climatic regime is also a key determinant of natural groundwater recharge rates and seasonal variations in those rates, both factors in determining the rates and time scale at which mobile contaminants in mine waste are released to the surrounding environment. The magnitude of a solute load released from a mine waste facility, and the time scale for release, can vary dramatically when comparing mine sites in arid and wet climates.

Mines are located in many different geologic settings. The geologic units hosting an ore deposit might be igneous, sedimentary and/or metamorphic bedrock in a low-relief ancient landscape or rugged mountainous terrain. An intrusive ore body associated with mineralized ore fluids could have penetrated a sedimentary sequence of shale, limestone, or sandstone or an igneous/metamorphic sequence of granites and gneisses. In glaciated terrain, a hydrogeologic assessment when selecting a site suitable for a mineral waste facility often requires investigation of groundwater flow in glacial deposits such as tills or outwash sands that overlie bedrock. In the humid tropics, site assessments typically require hydrogeological characterization of a thick saprolite soil and highly weathered rock that commonly forms as surficial layers across the landscape above more competent bedrock.

A full understanding of the geologic setting and how it might influence groundwater conditions at a mine site is fundamental to sound operating practice and environmental protection. A common theme in mine operations that do not perform according to geotechnical design expectations is an inadequate appreciation of the influence of the site geology on hydrogeologic conditions. Sites that present particular challenges in defining the role of groundwater flow include mines located in karstic limestone terrain, sites located in steep mountainous terrain when the bedrock groundwater flow system is controlled by the fracture network permeability, or where fault zones or dykes act to compartmentalize the groundwater flow system into discrete sub-basins, with groundwater leakage between compartments.

Mine developments occur in a diverse range of hydrologic settings. A mine site could be located in the headwater region of a small upland catchment, in a small contributing watershed of several square kilometers, or in a downslope area within a much larger surface water catchment of say, several hundred square kilometers. Wetlands may be a dominant feature in a basin and have a close relationship to the groundwater flow system. Groundwater divides may not coincide with surface water divides established by the surface topography. There are a number of instances where an open pit or underground mine is located in one watershed and the tailings storage facility is located in an adjacent catchment, contributing flow to different stream systems. There are even examples of mines located at the continental divide in South America where, depending upon how the mine facilities are positioned, some of the mine seepage discharges to local streams that report ultimately to the Atlantic Ocean, with other waters reaching the Pacific Ocean.