Mining and the environment

Abstract

Metal ores and coal are mined in large quantities: for nonferrous metals alone, total global annual production amounts to approximately 50×10 t (50 Mt, Lottermoser 2010). Mining products are vital and indispensable to our modern society and contribute significantly to our wealth. Currently, the monetary value of global copper and gold production amounts to about €100,000 million/year for each of these metals. On the other hand, mining and the mineral processing steps associated with them generate huge amounts of wastes, some of which are chemically reactive. Whereas the annual production rates of metals or coal can be determined fairly well, appraising the amount of mining waste deposited in a country or continent is a very challenging task, and any estimates are necessarily rough. A country-by-country inventory for the European Union in 1999 assessed the quantity of stored waste rock—mainly overburden material—as 4,700 Mt and that of the processing wastes or tailings as 1,200 Mt (BRGM 2001). In the opinion of the authors of this study, the quantities reported represented lower limits to the actual quantities involved. The authors also noted that differences in the national regulations of the 13 EU member countries resulted in a degree of uncertainty in the data collected. Globally, material flows of mine waste by far exceed those of waste from non-mining industries and of municipal waste. Heaps of waste rock covering land surfaces is estimated to require about 1 to 3 ha/Mt of solid waste. These piles may generate dust and discharge fine particles into surface waters, and they also deface the local landscape: open-pit mining operations, for instance, leave large cavities in the scenery. Leachates from overburden and gangue materials that do not contain a substantial amount of sulphidic minerals create seldom a chemical water quality problem. By contrast, deposits of processing wastes (tailings) often have a severe impact on all types of natural waters. The extraction of non-ferrous metals from sulphidic ores has been going on for a long time. Because such ores contain pyrite in notable concentrations, the tailings also contain pyrite, often in a finely milled form. Pyrite, and to a smaller extent additional other pyritic minerals, are oxidised when they come into contact with air. Together with percolating water, this produces a leachate containing sulphuric acid, known as acid mine drainage (AMD) or acid rock drainage. The acid produced by the oxidation of sulphidic minerals dissolves various other minerals that may contain heavy metals, arsenic and other hazardous elements. AMD, its acidity and the resulting elevated concentrations of noxious substances represent the key issue when assessing the risk to natural waters and soils posed by mine tailings. Coal deposits also contain pyritic minerals. Hence, the outflows from underground mines and open-pit lakes often turn acidic. Physically unstable tailing dams are at risk of bursting during heavy rainfall, potentially causing a disastrous mudflow. Such events attract widespread attention in the media, whereas the chronic impacts of AMD are hardly ever reported on. The formation of AMD is a slow process that continues even after mining operations have ceased. Therefore, the outflows from abandoned mines and tailings—several thousands of orphaned sites exist in the EU—also need to be investigated. The monitoring data gained need to be assessed in relation to the risk that contamination poses to ecological and human health. Such results and monitoring data represent one starting point for remedial measures. Often, such measures are very costly, and a natural attenuation approach is sometimes the only feasible solution. The most important key point for remediation is understanding the biogeochemical behaviour of the constituents of the tailings both in general and under the specific climatic and hydrological conditions prevailing at a particular site. Severe failures in tailing handling have resulted in environmental disasters such as the Ajka red mud spill in Hungary in 2010 (Mayes et al. 2011) and the Baia Mare and Baia Borse Responsible editor: Philippe Garrigues