Heavy metal is a general collective term which applies to the group of metals and metalloids with an atomic density greater than 4 g/cm³. Although it is a loosely defined term (Duffus, 2002), it is widely recognised and usually applies to the widespread contaminants of terrestrial and freshwater ecosystems. The heavy metals which are included in APIS are cadmium, chromium, copper, mercury, lead, zinc, arsenic, boron and the platinum group metals, which comprises platinum, palladium, rhodium, ruthenium, osmium, and iridium. Unlike almost all organic pollutants, such as organochlorines, heavy metals are elements which occur naturally in the Earth’s crust. They are therefore found naturally in soils and rocks with a subsequent range of natural background concentrations in soils, sediments, waters and organisms. Anthropogenic releases can give rise to higher concentrations of the metals relative to the normal background values.
Sources of atmospheric emissions
Although heavy metals differ in their chemical properties, they are used widely in electronic components, machinery and materials. Consequently, they are emitted to the environment from a variety of anthropogenic sources to supplement natural background geochemical sources. Some of the oldest cases of environmental pollution in the world were caused by heavy metal extraction and use, for example, copper, mercury and lead mining, smelting and utilisation by the Romans.
The amounts of most heavy metals deposited to the surface of the Earth from anthropogenic activities are many times greater than depositions from natural background sources. Combustion processes are the most important sources of heavy metals, particularly, power generation, smelting, incineration and the internal combustion engine (Battarbee et al 1988; Duce et al. 1991; Galloway et al. 1982; Hutton & Symon 1986; Nriagu 1989; Nriagu & Pacyna 1988).
Concerns about Heavy Metals
The Dangerous Substances Directive of the European Union (76/464/EEC) defines dangerous chemicals as those which are toxic, persistent and / or bioaccumulative. As they are elements, heavy metals cannot be broken down and will therefore persist in the environment. Unlike many organic pollutants, which eventually degrade to carbon dioxide and water, heavy metals will tend to accumulate in the environment, especially in lake, estuarine or marine sediments. Metals can be transported from one environment compartment to another. The information in APIS is primarily focused on metals which are transported via the atmosphere. Many of the heavy metals are toxic to organisms at low concentrations. However, some heavy metals, such as copper and zinc are also essential elements. Concentrations of essential elements in organisms are normally homeostatically-controlled, with uptake from the environment regulated according to nutritional demand. Effects on the organisms are manifest when this regulation mechanism breaks down as a result of either insufficient (deficiency) or excess (toxicity) metal. Whether the source of heavy metals is natural or anthropogenic, the concentrations in terrestrial and aquatic organisms is determined by the size of the source and adsorption/precipitation in soils and sediments. The extent of adsorption depends on the metal, the absorbent, the physio-chemical characteristics of the environment (e.g. pH, water hardness and redox potential) and the concentrations of other metals and complex chemicals present in the soil water, river or lake. The concentration of metal in bioavailable form is not necessarily proportional to the total concentration of the metal.
Heavy metals accumulate in organisms as a result of direct uptake from the surroundings across the body wall, from respiration and from food. Uptake via food is most important in terrestrial organisms and it may also be important in the aquatic environment. Dietary uptake can include heavy metals adsorbed on particulates present on the surface of leaves etc, which have not been absorbed by the plant.
The free ion is generally the most bioavailable form of a metal, and the free ion concentration is often the best indicator of toxicity. However, there are exceptions, such as the well known case of mercury, where the organic form, (methylmercury) is more toxic than the inorganic ion. Metals exert toxic effects if they enter into biochemical reactions in the organism and typical responses are inhibition of growth, suppression of oxygen consumption and impairment of reproduction and tissue repair.
Critical levels and loads
Most critical levels for heavy metals have been developed to protect humans, although some have been suggested to protect aquatic organisms and agricultural soils which receive sewage sludge.
Critical loads for protection of ecosystems have recently been calculated for a number of heavy metals (cadmium, lead, copper, nickel and zinc) for six UK habitats (ROTAP, 2012). However, for many metals there is a large discrepancy between emission estimates and measured deposition. There is also too little quantitative data to accurately identify the sources of the metals currently recorded in UK air. Until these issues can be resolved by further work, it is not practical for APIS to provide site relevant heavy metal critical loads. However, this does not preclude consideration of heavy metals critical loads in the assessment of relevant1 new developments on a case by case basis provided deposition at the site can be validated using information sources on heavy metals that already exist. This information might include, for example: existing soil guideline values in H1 in H1 in England and equivalent horizontal guidance in Scotland and Northern Ireland and existing information on the designated site. Further development of heavy metal critical loads may allow their future inclusion in APIS (e.g. through the APIS site relevant critical load tool).
Most critical levels derive from the European Union's Dangerous Substances Directive (76/464/EEC), US EPA water quality criteria (US EPA 1998), the new Air Quality Directive 2008/50/EC and World Health Organization air quality guidelines for air (World Health Organization 1987). The Air Quality Directive 2008/50/EC merges most of existing legislation into a single directive (except for the fourth daughter directive) with no change to existing air quality objectives. Lead for example has an annual average upper assessment threshold of 70 % of limit value (0,35 µg/m3), with a lower assessment threshold of 50 % of limit value (0,25 µg/m3). Finally, the United Nations Economic Commission for Europe adopted a Protocol on Heavy Metals in 1998. The ultimate objective of this is to eliminate any discharges, emissions and losses of cadmium, lead and cadmium to air.
The European legislation is transposed in England by the Air Quality (Standards) Regulations 2010, while the Air Quality Strategy is the means by which it is implemented in the United Kingdom. The EU sludge to land directive (86/278/EEC) sets critical levels for heavy metals in agricultural soils that receive sewage sludge.
The UK is obliged under the Dangerous Substances Directive (76/464/EEC) to set Environmental Quality Standards (EQSs) for selected (List I and II) chemicals in water which are of national concern and the Environmental Protection Act 1990 requires EQSs for further chemicals (the Red List) in order to protect the aquatic environment. EQS values for mercury and cadmium are available.
Article 16 of the Water Framework Directive (2000/60/EC) (WFD) sets out "Strategies against pollution of water", outlining the steps to be taken. The first step was to establish a First list of priority substances to become Annex X of the WFD. This first list was replaced by Annex II of the Directive on Environmental Quality Standards (Directive 2008/105/EC) (EQSD), also known as the Priority Substances Directive , which set environmental quality standards (EQS) for the substances in surface waters. The EQSD established in Annex I, limits on concentrations of the priority substances in surface waters of 33 priority substances and 8 other pollutants. Heavy metals included in the list were cadmium, mercury, lead and nickel.
At the Bad Harzburg Workshop in 1997 (UN ECE Convention on Long-range Transboundary Air Pollution, Task Force on Mapping 1998), representatives from many Member States of the European Union agreed approaches to set critical limits/loads and effects-based standards for heavy metals in freshwaters and soils. For both soils and freshwater, a critical limits approach is recommended for cadmium, mercury and lead for key receptors. The relevant receptors in freshwaters are fish for mercury and water for cadmium and lead and invertebrates, food crops and groundwaters (and possibly soil micro-organisms) in soils.
1Relevant installations are any plant where metals are likely to be limited by permit conditions or where metals are identified as a potential emission. At present these are likely to be (but not limited by) IED Chapter IV (waste incineration and co-incineration plant), ferrous and non-ferrous metals installations. It may also include chromium (and other) plating works.