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The way freshwater ecosystems deal with an excess of metals
| Three countries—the United States, Germany, and Russia—with only 8% of the world’s population consume about 75% of the world’s most widely used metals. The United States, with 4.5% of the world’s population, uses about 20% of the worlds metal population and 25% of the fossil fuels produced each year.|
How metals get into freshwater
Metals are introduced in aquatic systems as a result of the weathering of soils and rocks, from volcanic eruptions, and from a variety of human activities involving the mining, processing, or use of metals and/or substances that contain metal pollutants. The most common heavy metal pollutants are arsenic, cadmium, chromium, copper, nickel, lead and mercury. There are different types of sources of pollutants: point sources (localized pollution), where pollutants come from single, identifiable sources. The second type of pollutant sources are nonpoint sources, where pollutants come from dispersed (and often difficult to identify) sources. There are only a few examples of localized metal pollution, like the natural weathering of ore bodies and the little metal particles coming from coal-burning power plants via smokestacks in air, water and soils around the factory.
The most common metal pollution in freshwater comes from mining companies. They usually use an acid mine drainage system to release heavy metals from ores, because metals are very soluble in an acid solution. After the drainage process, they disperse the acid solution in the groundwater, containing high levels of metals. See also acids & alkalis.
The term ‘heavy metal’ is somewhat imprecise, but includes most metals with an atomic number greater than 20, and excludes alkali metals, alkaline earths, lanthanides and actinides.
What happens when an excess of metals enters freshwater ecosystems?
When the pH in water falls, metal solubility increases and the metal particles become more mobile. That is why metals are more toxic in soft waters. Metals can become ‘locked up’ in bottom sediments, where they remain for many years. Streams coming from draining mining areas are often very acidic and contain high concentrations of dissolved metals with little aquatic life. Both localized and dispersed metal pollution cause environmental damage because metals are non-biodegradable. Unlike some organic pesticides, metals cannot be broken down into less harmful components in the environment.
Campbell and Stokes (1985) described two contrasting responses of an organism to a metal toxicity with declining pH:
- If there is little change in speciation and the metal binding is weak at the biological surface, a decrease in pH will decrease owning to competition for binding sites from hydrogen ions.
- Where there is a marked effect on speciation and strong binding of the metal at the biological surface, the dominant effect of a decrease in pH will be to increase the metal availability.
Generally the ionic form of a metal is more toxic, because it can form toxic compounds with other ions. Electron transfer reactions that are connected with oxygen can lead to the production of toxic oxyradicals, a toxicity mechanism now known to be of considerable importance in both animals and plants. Some oxyradicals, such as superoxide anion (O2-) and the hydroxyl radical (OH-), can cause serious cellular damage.
Some inorganic pollutants are assimilated by organisms to a greater extent than others. This is reflected in the Bioconcentration Factor (BCF), which can be expressed as follows:
BCF = concentration of the chemical in the organism / concentration of the chemical in the ambient environment.
The ambient environment for aquatic organisms is usually the water or sediments. With inorganic chemicals, the extent of long-term bioaccumulation depends on the rate of excretion. Toxic chemicals can be stored into tissues of species, especially fat tissues. Bioaccumulation of cadmium in animals is high compared to most of the other metals, as it is assimilated rapidly and excreted slowly. Also the sensitivity of individuals of a particular species to a pollutant may be influenced by factors such as sex, age, or size. In general the concentrations of metals in invertebrates is inversely related to their body mass. In fish, the embryonic and larval stages are usually the most sensitive to pollutants.
Benthic organisms are likely to be the most directly affected by metal concentrations in the sediments, because the benthos is the ultimate repository of the particulate materials that are washed into aquatic systems.
Some metals, such as manganese, iron, copper, and zinc are essential micronutrients. They are essential to life in the right concentrations, but in excess, these chemicals can be poisonous. At the same time, chronic low exposures to heavy metals can have serious health effects in the long run.
Tolerance to metals has also been recorded in invertebrates and in fish. After exposure for 24 hours to a copper concentration of 0.55 mg/l, rainbow trout showed a 55 per cent inhibition of sodium uptake and a 4 per cent reduction in affinity for sodium, which resulted in an overall decrease in total sodium concentration of sulphydryl-rich protein (Lauren and McDonald 1987a,b). The protein was considered to be a metallothionein. These low molecular weight proteins contain many sulphur-rich amino acids which bind and detoxify some metals. The pretreatment of an organism with low doses of a metal may stimulate metallothionein synthesis and provide tolerance during a subsequent exposure (Pascoe and Beattie, 1979).
Many rivers are polluted with heavy metals from old mine workings and some species of algae become very tolerant to polluted conditions. A survey of 47 sites with different concentration of zinc found the filamentous green alga 'Hormidium rivulare' to be abundant everywhere, tolerating zinc concentrations as high as 30.2 mg Zn/l.
Toxicity of metals
For the protection of human health, the maximum permissible concentrations for metals in natural waters that are recommended by the Environmental Protection Agency (EPA), are listed below:
Maximum Permissible Concentrations (MPC) of Various Metals in Natural Waters For the Protection of Human Health
Source: EPA (1987); Federal Register 56 (110): 26460-26564 (1991).
|Metal ||Chemical Symbol ||mg m-3|
This table gives an idea of the relative toxicity of various metals. Mercury, lead and cadmium are not required even in small amounts by any organism.
Because metals are rather insoluble in neutral or basic pH, pHs of 7 or above give a highly misleading picture of the degree of metal pollution. So in some cases it may underestimate significantly the total of metal concentrations in natural waters.
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