Environmental Impact of Abandoned Mine Waste: a review

1.2. Problem

Rocks and ore deposits are composed of a pool of chemical constituents. The major elements (Si, Al, Ca, Mg, etc.) are invariably accompanied by minor (Fe, Mn, Ti, P, etc.) or trace amounts of other metals. Among these, heavy metals can be defined (Adriano, 2001) as those having a metallic density >5 gcm^-3 (e.g. Cu, Co, Ni, Pb, Zn, etc.). Other important constituents, particularly utilized in modern industrial activities, are antimony, arsenic, bismuth, cadmium, chromium, germanium, selenium, tallium, etc. In addition, many metals are essential for life functions. Chief concern focuses on Cu and Zn, which are essential micronutrients but may be harmful when present in large concentrations, and on Cd, Hg and Pb, which have no known beneficial metabolic role but are known toxins (Kabata-Pendias and Pendias, 2001; Ghorbel et al., 2010). The important point is that many of these metals are also potential contaminants to the environment, and constitute a potential risk to vegetation and human health, when their concentrations are above a certain threshold (Davies, 1987; Kabata-Pendias, 2004). Yet, these metals are ordinarily present in rocks, sediments and soils, but locally may become concentrated in rocks as ore bodies and generally dispersed in the environment through pollution as a consequence of mining the ores. (Davies, 1987; Alloway, 1995).

Mining is only one of the pathways by which metals enter the environment. Mining itself affects relatively small areas, and this could not pose problems. The environmental problem arises when ores are mined, milled and smelted, and a certain amount of metals is released in the surrounding areas and to waterways. Depending on the nature of the waste rock and tailings deposits, a wide dispersion of the metals both in solution and in particulate form is possible (Sivri et al., 2010). Erosion of waste rock deposits or the direct discharge of tailings in rivers results in the introduction of metals in particulate form into aquatic ecosystems (Helios-Rybicka, 1996; Cidu et al., 2009). Smelting of ore deposits results in the release of metals to the atmosphere (Mihalik et al., 2011); when metals have been released through the atmosphere, they end up as diffuse pollutants in soils and sediments. (Nriagu, 1990; Salomons, 1995).
Figure 2. Geological and hydrological hazard determined by mine dumps in the Metalliferous Hills district (Southern Tuscany, Italy). (Photo Bini).

A second environmental (geomorphologic) concern is connected to mining operations. Excavation of ore bodies brings out important landscape modifications; earth movements, dam building and impoundment construction, may create severe geological hazard (Figure 2). Erosion processes may concur to convey waste in rivers nearby; landslides may be activated in loose material dumps, with relevant risk for population living in the conterminous land; surface hydrology and hydrological processes may be strongly modified, and constitute a further concern.

Mining areas are frequently constituted of highly tectonized and fractured rocks and detrital fragments (Tanelli, 1985), easy erodible by runoff and percolating water. The causes of accelerated surface erosion are related to both geological (tectonic structure, lithology), morphologic and climatic conditions (steep slopes, rainy events distribution, temperature gradients); vegetation cover may have great influence on attenuating, or enhancing, erosion phenomena, when land cover is scarce or lacking, as it happens frequently with metal-contaminated sites. A correlation between tectonic structure, minerogenesis and surface processes has been recorded (Lattanzi et al., 1994; Benvenuti et al, 1999; Mascaro et al., 2000) at nearly every mine site.

Access to exploitation areas is allowed through earth movements such as opening new tracks and new excavations, and this may enhance earth surface processes, landslide formation, hydrological regime alteration, contaminant dispersion (Helios-Rybicka, 1996). The hydraulic characteristics of mineral bodies (e.g. coarse grain size, permeability, hydraulic conductivity), in turn, are responsible for water percolation and circulation in the subsoil, where contaminants are convoyed to groundwater (Cidu et al., 2009). Moreover, apart from geomechanical processes (subsidence, slumps, landslides, erosion) which may lead to the complete destruction of soils and irreversible changes in the landscape, the drainage of open pits influences hydrological systems (Helios-Rybicka, 1996).

A third set of problems, of sanitary type, may occur with workers involved in mining operations. Today, heavy metals are considered as hazardous substances which can induce environmental threats and a risk to human health (Ghorbel et al., 2010). Health risk assessment depends upon complex interactions of several parameters such as waste mineralogy, exposure, climate conditions, contamination transfer, contact, ingestion or inhalation of metals, time elapsed since mine closure.

Yet, if absorbed in sufficiently high amounts, heavy metals can be toxic and even lethal. The toxicity of heavy metals to humans is well documented by several outbreaks of massive poisoning and epidemiological studies (Steinnes, 2009). In particular Ag, As, Be, Cd, Ce, Ge, Hg, Pb, Tl are examples of potentially harmful elements (PHEs) that have no proven essential functions, and are known to have adverse physiological effects at relatively low concentrations (Abrahams, 2002).

Examples of toxicity by heavy metals are known since the Antiquity (Nriagu, 1983). For instance, one of the supposed causes for the Roman Empire drop is the increasing lead toxicity from Pb-bearing potteries and wine containers, as it was found in Roman findings and bones (Stiles et al., 1995). Lead (plumbism) and Hg (mercurialism) poisoning cases were frequently recorded in workers employed in mining industry and even in hat factories in Tuscany (Dall’Aglio et al., 1966). At present, diseases and toxicity related to microelement contamination (Cr, Cu, Ni, Pb, Tl, Zn,) of air, water and soil from human activities are well established (Thornton, 1993; Abrahams, 2002). For example, the most notable cause of Tl poisoning occurred adjacent to a cement works in Germany (Abrahams, 2002).

The history of lead and its use by man dates back to almost 9.000 years. The toxic nature of lead compounds was well understood, and there was a variety of local names for lead poisoning: plumbism, saturnism, potter’s rot, painters’colic, lead palsy (Davies, 1987). The main target for health hazard by lead are the hematopoietic, the nervous and the cardio-vascular systems (Bernard, 1995). Early scientific investigation of river pollution demonstrated that dissolved lead caused the formation of mucus on the gills of fish; stickleback (Gasterosteus aculeatus L.) was the species most sensitive to lead pollution (Davies, 1987).

Cadmium, in contrast, is typically a heavy metal of the 20^th century, since over 60% of the world production has taken place during the last 50 years. It is likely, however, that cadmium constituted a potentially harmful element to exposed humans, occurring in nature together with zinc, lead and copper. The major human health hazard of Cd is a decline in the renal function even at moderate intake (Steinnes, 2009)

Chromium is well known as a toxic metal in the Cr^VI form, provoking severe metabolic disorders, membrane damage, cancer and contact dermatitis (Bini et al., 2000).

Mercury is a heavy metal ubiquitous in the environment, whose global emissions have been estimated around 4.000t/year, constituting a health risk for the human population; indeed, excessive Hg exposure resulting from burning the Au/Hg amalgam, vapours inhalation, contaminated fish consumption lead to adverse health effects (Thornton, 1996; Steinnes, 2009), causing episodes of poisoning (neurotoxicity, mercurial tremor, psychomotor retardation).

Arsenic is well known as a highly poisonous metalloid, particularly in the water-dissolved form; As concentrations up to 600 µg/L in artesian well waters in Taiwan, India and Bangladesh, for example, have been related to increased risk of skin and internal cancers affecting more than 40 millions people (Steinnes, 2009); a reduction to 10 ppb of As concentration as a tolerable level in drinkable waters, therefore, has been recently proposed by the World Health Organization (2004).