According to the European Environmental Agency, heavy metal contamination (Figs. 1-2) is the most prevalent form of soil and groundwater contamination in Europe . Contamination by heavy metals and metalloids, including but not limited to arsenic, cadmium, chromium, lead, mercury, nickel and uranium, occurs as a result of activities such as mining, smelting, manufacturing (especially metallurgy, battery production, metal plating, tanneries, chlorine production and wood treatment), use of munitions, and disposal of heavy metal-bearing wastes in landfills . Unlike organic contaminants, heavy metals cannot be broken down into less toxic byproducts and are therefore very persistent. Due to this persistence, remediation of heavy-metal contamination requires either physical removal from the contaminated media or conversion of the metal into a less toxic or insoluble form. Health effects of chronic exposure to heavy metals include carcinogenesis and neurological damage.
Figure 1: Summary of soil contaminants in surveyed areas of Europe 
Figure 2: Summary of groundwater contaminants in surveyed areas of Europe 
Table 1 shows the prevalence of each form of contaminant in the surveyed countries . Heavy metal contamination is particularly prevalent in countries where mining is common, post-Communist countries that had poor environmental controls in the past , and countries recently affected by violent conflict . The Balkan countries (Croatia, Montenegro and FYROM) are particularly affected, as are Austria, Cyprus, France and Italy. Worldwide, nations with mining-dependent economies are hotbeds of heavy metals contamination, and rapidly-industrializing developing countries are affected as well.
Table 1: Relative contributions of selected contaminant types to a survey of contaminated sites in Europe 
Abbreviations used- AT: Austria, BE: Belgium, HR: Croatia, CY: Cyprus, FI: Finland, FR: France, HU: Hungary, IT: Italy, LT: Lithuania, MK: Former Yugoslav Republic of Macedonia, ME: Montenegro, NL: Netherlands, NO: Norway, SK: Slovakia, ES: Spain, CH: Switzerland
Arsenic (As) known to be toxic since antiquity, exists in the environment at high concentrations both naturally and due to human activity. Acute exposure to arsenic can be fatal at relatively low concentrations, and chronic exposure can cause cancer and a skin condition known as hyperkeratosis . Weathering of sulfide minerals in the Himalayan Mountains results in concentrations of arsenic along the Bengal River in Bangladesh and India that are high enough to accumulate to dangerous concentrations in locally-grown food. Elsewhere, arsenic is commonly associated with acid mine drainage, pesticides and wood treatment. Arsenic contamination is relatively uncommon in Europe, but is widespread enough in the United States that it was listed number one on the Agency for Toxic Substances and Disease Registry’s (ATSDR) Substance Priority List . Other regions significantly impacted by mine wastes, particularly in Mexico and South America, also feature high levels of arsenic contamination. High Groundwater concentrations of arsenic are usually remediated by immobilization, as it easily binds to iron oxides including LDHs such as green rust.
Cadmium (Cd) is present at small concentrations in the environment but is often associated with zinc ores, causing much higher concentrations is some areas. In addition to being a byproduct of zinc mining, Cd is also used in metal plating and rechargeable batteries . Exposure to Cd can occur occupationally or through release into the atmosphere by fossil fuel combustion or waste incineration, but the most common source of exposure to Cd is cigarette smoking. Cd is a potent carcinogen and is currently listed at number seven of the ATSDR’s Substance Priority List  and also listed on the European Commission (EC)’s priority list .
Chromium (Cr), an element commonly used in tanning, wood treatment and metal plating, exists in two forms: the relatively harmless Cr (III) and the carcinogenic Cr (VI). In addition to the industries mentioned above, Cr (VI) can also be released into the environment through the mining of ferrochromite (FeCrO4) . The most effective way to remediate Cr (VI) contamination is by reduction to Cr (III) because the trivalent form is less toxic and less soluble in water . This makes Cr (VI) an excellent candidate for reductive transformation by green rust, as it is rapidly converted into highly stable, Cr (III)-bearing iron oxides .
Lead (Pb): Due to its natural abundance and useful properties, lead (Pb) has been used in its metallic form for millennia. Its detrimental effects on human health were discovered in the 19th century, but use of lead in paint and gasoline continued for decades in some countries . Although lead emissions have declined in many parts of the world, exposure continues due to lead in mine wastes, paint, and aging water pipes, to the extent that it is ranked second on the ATSDR’s priority list  and listed on a similar list published by the EC . Continued use of leaded gasoline in some developing countries is also a concern.
Mercury (Hg), a well-known toxin, was previously used in dental amalgam, thermometers, and the chlorine manufacturing process, and anthropogenic release into the environment continues through coal combustion, gold production, smelting of metal ores, and disposal of Hg-bearing wastes . Hg is particularly dangerous when it is methylated by bacteria in anaerobic environments, dramatically increasing its bioavailability and allowing it to accumulate at the top of the aquatic food chain, making consumption of fish such as sharks and swordfish a major cause of chronic Hg exposure . Hg contamination is a serious enough problem in the US that it is listed third on the ATSDR’s priority list of contaminants , and is also a serious problem in Europe and Japan.
Uranium (U): Since the early twentieth century, uranium has been widely used in nuclear weapons, nuclear power plants, and ammunition (in its depleted form) . In addition to the carcinogenic effects of uranium’s radioactivity, the metal itself also has toxic effects. Therefore, uranium poses a potential threat to human health in areas of significant contamination such as sites associated with nuclear meltdown, uranium mining and smelting, disposal of radioactive waste, and testing and use of nuclear weapons . Uranium (VI) is much more soluble and toxic than uranium (IV), so reduction by green rust is a promising method for remediation of uranium-contaminated sites .
Karlsruhe Institute of Technology
 Lauwagie, G.V.E. (2014). Progress in management of contaminated sites. Copenhagen, DK: European Environmental Agency.
 Bradl, H. (Ed.). (2005). Heavy metals in the environment: Origin, interaction and remediation. London, UK: Elsevier B.V.
 Carter, F.H. & Turnock, D. (Eds.). (2002). Environmental issues in east-central Europe. New York, NY: Rutledge.
 Richardson, M. (1995). The effects of war on the environment: Croatia. London, UK: E & FN Spon.
 Ahsan, H., Chen, Y., Parvez, F., Argos, M., Hussain, A.I., Momotaj, H., Levy, D., van Geen, A., Howe, G., & Graziano, J. (2006). Health Effects of Arsenic Longitudinal Study (HEALS): Description of a multidisciplinary epidemiological investigation. Journal of Exposure Science and Environmental Epidemiology, 16, 191-205.
 Agency for Toxic Substance and Disease Registry. (2014). Priority List of Hazardous Substances. Retrieved from https://www.atsdr.cdc.gov/spl/
 Pinot, F., Kreps, S.E., Bachelet, M., Hainaut, P., Bakonyi, M., & Polla, B.C. (2000). Cadmium in the environment: Sources, mechanisms of biotoxicity, and biomarkers. Reviews on Environmental Health, 15 (3), 299-324.
 European Commission. (2011). Priority substances under the Water Framework Directive. Retrieved from http://ec.europa.eu/environment/water/water-dangersub/pri_substances.htm
 Kimbrough, D.E., Cohen, Y., Winer, A.M., Creelman, L., & Mabuni, C. (1999). A critical assessment of chromium in the environment. Critical reviews in environmental science and technology, 29 (1), 1-46.
 Skovbjerg, L.L., Stipp, S.L.S., Utsunomiya, S., & Ewing, R.C. (2006). The mechanisms of reduction of hexavalent chromium by green rust sulphate: Formation of Cr-goethite. Geochimica et Cosmochimica Acta, 17, 3581-3592.
 Patte, O.H., & Pain, D.J. (2003). Lead in the Environment. In Hoffman, D.J., Ratter, B.A., Burton, G.A., & Cairns, J. (Eds.), Handbook of Ecotoxicology (pp. 373-408). Boca Raton, FL: CRC Press.
 Suzuki, T., Imura, I., & Clarkson, T.W. (Eds.). (1991). Advances in mercury toxicology. New York: Plenum Press.
 Ribera, D., Labrot, F., Tisnerat, G., & Narbonne, J. (1996). Uranium in the environment: Occurrence, transfer, and biological effects. Reviews of Environmental Contamination and Toxicology, 146, 53-89.
 O’Loughlin, E.J., Kelly, S.D., Cook, R.E., Csencsits, R., & Kemmer, K.M. (2003). Reduction of uranium (IV) by mixed iron (II)-iron (III) hydroxide (green rust): Formation of UO2 nanoparticles. Environmental Science and Technology, 37, 721-727.