Many different chemicals are regarded as pollutants, ranging from simple inorganic ions to complex organic molecules.
The water pollutants are all divided up into various classes. Every class of pollutants has its own specific ways of entering the environment and its own specific dangers. All classes have major pollutants in it that are known to many people, because of the various health effects.
Organic compounds are compounds that consist of long bonds, usually made up of carbon. Many organic compounds are basic fabrics of living organisms. Molecules built of carbon and of carbon and hydrogen are non-polair and have little to no water solubility. They have little to no electrical charge.
The behaviour of organic compounds is dependent upon their molecular structure, size and shape and the presence of functional groups that are important determinants of toxicity.
It is important to know the structure of organic compounds, in order to predict their fate in living organisms and the environment. The organic compounds that are dangerous to the environment are all man-made and have only existed during the last century.
Organic pollutant: oil
(environmental disaster in Spain)
There are many different types of organic pollutants, examples are:
- Hydrocarbons. These are carbon-hydrogen bonds. They can be divided up into two classes, the first being single-bonded alkanes, double bonded alkenes and triple bonded alkynes (gasses or liquids) and the second being aromatic hydrocarbons, which contain ring structures (liquids or solids). Aromatic hydrocarbons such as PAH's are much more reactive than any of the first class kinds of hydrocarbons.
- PCB's are stable and unreactive fluids that are used as hydraulic fluids, coolant/ insulation fluids in transformers and plasticizers in paints. There are many different PCB's. None of them are water-soluble. In many countries PCB's are restricted.
- Insecticides such as DDT's are very dangerous because they accumulate in fat tissues of lower animals and then enter the food chain. They have been restricted for decades.
- Detergents. These can be both polair and non-polair.
Take also a look at detergents in freshwater and organic pollution in freshwater
Some inorganic pollutants are not particularly toxic, but are still a danger to the environment because they are used so extensively. These include fertilizers, such as nitrates and phosphates. Nitrates and phosphates cause algal blooms in surface water, which causes the oxygen level of the water to decline. This causes oxygen starvation because of the uptake of oxygen by microrganisms that brake down algae. This is called eutrophication.
The first class we will refer to here is metals. Metals are good conductors of electricity and generally enter chemical reactions as positive ions, known as cations. Metals are natural substances that have consisted through weathering of ore bodies, where they were deposited during volcanic action. They can be relocated into situations where they can cause serious environmental damage. Examples of metals are: lead, zinc, manganese, calcium and potassium. They can be found in surface waters in their stable ionic forms. Unnatural metals can be very dangerous, because they often come from man-made nuclear reactions and can be strongly radioactive.
Metals can react to dangerous products with other ions. They are often involved in electron transfer reactions involving oxygen. This can lead to the formation of toxic oxyradicals.
Metals can form metalloids and then bond to organic compounds to form lipophilic substances that are often highly toxic and can be stored in the fat-supply of animals and humans. Metals can also bond to cellular macromolecules in the human body.
Heavy metals are the most dangerous metals. They have a density greater than 5 and are therefore called heavy.
Metals cannot be broken down into less harmful components, as they are non-biodegradable. The only chance organisms have against metals is to store them in body tissues where they cannot do any harm.
Organisms need metals, as they are essential for their health and are usually essential components of enzymes.
The half-lives and the ways of decay of radioactive isotopes determine how dangerous they are to humans. Humans create all radioactive isotopes in the nuclear industry. There are still debates going on about whether the benefits of nuclear power exceed the dangers of radioactive radiation. When an atom of a radioactive substance decays, it can produce four kinds of particles: alpha, beta, gamma and neutrons.
Alpha particles can only travel a short distance through air and human tissues, but they can be very damaging if they collide with cells because of their large mass. They are positively charged.
Beta particles are more penetrating, but they do much less damage than alpha particles. They are negatively charged.
Gamma rays are highly penetrating. Their damage is similar to that of beta rays.
Neutrons are liberated through radiation and react with other elements through collision. They are the basis for nuclear fission in a reactor.
The radioactivity of a substance is measured in bequerels, but this does not express the amount of tissue damage the radiation causes. That is why the amount of radiation causing 1 kg of tissue to absorb 1 joule of energy is now expressed in grays. Different kinds of radiation can do different kinds of damage, because the energy is imparted into tissues in different ways. This is expressed in sieverts. An amount of alpha radiation can do twenty times the damage of the same amount of beta radiation. Radioactive matter has to be held in storage for different periods of time, in order to erase the danger. How long it has to be stored depends upon the half-life of the isotopes; the time taken for half of the atoms of a radioactive isotope to decay.
Discharge of sewage water represents a mayor global source of pollution. Domestic and industrial wastes are discharged unto surface water through sewage systems. In some cases industrial waste is released directly unto surface water. The quality of sewage water that enters the surface water depends upon the pollutants that are present in the sewage water and the extend to which it is treated before it is brought in contact with surface water.
Domestic sewage water mainly consists of paper, soap, urine, faeces and detergents. Industrial wastes are varied and depend upon the specific processes of the plants that they origin from.
Heavy metals are associated with mining and smelting operations, chlorophenols and fungicides with pulp mills, insecticides with mothproofing factories, several different organic chemicals with the chemical industry and radioactive substances with nuclear power plants.
On land the releases of industrial waste are closely controlled, but offshore oil and manganese extraction lead to direct discharge of pollutants into the seas. Radioactive waste is dumped into the sea in large concrete barrels to decay, but often the barrels will start to have defects after a while. Representatives of factories often ship waste onto sea to dump it illegally, because it is very expensive to have their water purified.
Oil is released into the sea through oil tankers and shipwrecks and pesticides are applied to water to control aquatic pests. Paints on boats will decay during long trips on the ocean and will eventually end up in the water.
During the growth period of crops nitrates and phosphates are absorbed by plants, but when the plants die they are released from dead plant material into the soil and will often end up in surface water.
Except for the deliberate causes of surface water pollution, pollutants can also enter the water environment accidentally, for instance through atmospheric deposition. Pesticides can enter surface water easily this way, because they are applied as droplets or sprays. Pollutants present on land can enter surface water through heavy rainfall or infiltrate into the soil and enter surface waters through groundwater.
The effects of pollutants are noticed mostly in small inland seas and lakes. This is because the oceans have a natural dilution system for incoming pollutants, whereas lakes have no effective outlet. Due to this, much depends upon the rate of degradation and precipitation that will remove the pollutants from water.
Pollutants can exist in water in different states. They can be dissolved or they can be in suspension, which means that they exist in the form of droplets or particles. Pollutants can also be dissolved in droplets or absorbed by particles. All states of pollutants can travel great distances through water in many different ways.
Particulate matter may fall to the bottom of streams and lakes or rise to the surface, depending on its density. This means that it mostly remains on the same location when the water does not flow very fast. In rivers, pollutants usually travel great distances. The distance they travel depends upon the stability and physical state of the pollutant and the speed of flow of the river. Pollutants can travel farthest when they are in solution in a river that is fast flowing. The concentrations on one site are then generally low, but the pollutant can be detected on many more sites than when it would not have been so easily transported.
In lakes and oceans pollutants are transported through currents. There are many currents in the oceans, which are wind-driven. This enables a pollutant to travel from one continent to another.
We usually count on the ability of the oceans to reduce pollutants in concentration, the so-called 'self-cleaning ability' of oceans. But this does not always work, because the movement of the currents in the oceans is not uniform. This causes inshore waters to often have substantially higher levels of pollution than the open sea.
When persistent pollutants accumulate in fish or sea birds they cannot only become a toxic danger to aquatic food chains, they can also travel great distances within these animals and end up in the food chains of non-polluted areas.
Physical processes determine the movement of chemicals within water; movement depends upon properties of the chemicals themselves and properties of the water. These processes will be overviewed here.
Water is a polar liquid. This means that the oxygen atom in a water molecule attracts the electrons of the hydrogen atoms, so that these develop partial positive charges. The oxygen atom gets a partial negative charge, through which it can attract atoms of other water molecules to form hydrogen bonds. In non-polar compounds, such as hydrocarbons, there is hardly any charge separation and consequently they do not dissolve in water.
Water tends to form aggregates in which four other molecules surround each water molecule. Cations and anions have an affinity for the parts of water that carry an opposite charge, so that the water aggregates are disrupted and the ions dissolve. Many organic salts and polar organic compounds are water-soluble, but non-polar organic liquids are not.
From this we can conclude, that molecules that can perform charge separation can easily dissolve in water, whereas molecules that do not have charges are not very water-soluble.
A consequence of polarity is the hydrophobic effect. In the process of forming aggregates with charged molecules water actively excludes non-polar substances. This leads to the formation of phospholipid bilayers, which contribute to the movement of hydrophobic pollutants though membranes.
The level of hydrophobicity is determined by the water/ octanol partition coefficient. The concentration of a compound in octanol is divided through the concentration in water. The higher the number that results from this calculation, the more hydrophobic the compound in question is.
Whether a compound remains in the water is also determined by its vapour pressure. Vapour pressure means the tendency of a liquid or solid to volatilise. Vapour pressure increases when temperatures rise, as surface molecules increase in kinetic energy. Then more molecules in a watery solution have the tendency to vaporize, which means they are no longer in solution.
The partition of the chemical between different environmental compartments air, water and soil is another important factor. The escaping tendency or 'fugacity' of a substance determines the movement from one compartment to another.
Molecular stability is a factor that determines the time a chemical remains in the environment and the distances it can travel. In the environment chemical and biochemical processes, such as hydrolyses and oxidation, break down chemicals. The break down is not only determined by the stability of the chemical, but also by the environmental factors temperature, level of solar radiation, pH and nature of absorbing surface. For instance, the pH of water determines the water-solubility of metals. Sometimes biotransformation of a compound in the environment during break down is not very positive, because it can lead to increased toxicity of a chemical.
When pollution enters the body of an organism it causes a variety of changes. These changes can either serve to protect the organism against harmful effects or not.
The first response of an organism to pollutants is to bring a protective mechanism into action. In most cases these mechanisms maintain the detoxification of pollutants, but in some cases they produce active substances that can cause more damage to the cell than the original pollutant.
Another response is to reduce the availability of pollutants by binding them to another molecule, to excrete or store them.
Next to protective mechanisms an organism can also bring a mechanism into action that repairs damage caused by pollutants.
Responds to toxicity and the uptake of pollutants not only depends upon the pollutant that enters the organisms body, but also upon the kind of organism in question.
Water pollutants can have many different effects on organisms, always depending on the pollutant and the organism in question. Here the general effects a pollutant can have are discussed.
Many compounds that enter the body of an organism are known to cause damage to DNA. These compounds are called genotoxins, due to their genotoxic effect.
Usually when pollutants damage DNA a natural repair system in an organism will return it to its usual state, but when this goes wrong for any particular reason cells with damaged DNA can divide. Mutant cells are than produced and the defect can spread, causing the offspring of the organism in question to have serious defects that are often very damaging to their health.
Examples of genotoxins are PAH's, aflatoxin and vinyl chloride.
In all of these genotoxins it is not the original compound that reacts with DNA, for this is relatively stable. Highly reactive short-lived products produced from the original compound by enzymes usually cause the reactions.
Several pollutants are carcinogenic, which means they can induce cancer in the body of humans and animals. Carcinogenic pollutants are pollutants that play a role in one or more of the stages of cancer development in an organism.
Pollutants can be inductors; this means that they introduce cancer-forming properties in the cells of an organism. They can also be promoters, which means that they promote the growth of cells that have cancer-forming properties. Finally, they can be progressors, which means that they stimulate unrestrained division and spreading of cancer cells. When one of these substances is absent cancer cannot be induced.
When cancer cells are malignant, they can spread through the human body rapidly, causing defects to healthy cells and immunity mechanisms. They will destroy normal body cells and cause cancer in organs and systems.
The nervous system of organisms is very sensitive to toxic effects of chemicals, both naturally occurring and man-made. Chemicals that cause neurological effects are called neurotoxins. Examples of dangerous neurotoxins are insecticides.
Neurotoxins all somehow disturb the normal transmission of impulses along nerves or across synapses.
The consequences of neurotoxicity are varied. They can be uncoordinated muscular tremors and convulsions, malfunction of nerves and transmissions, dizziness and depression, or even total malfunction of body parts. Neurotoxicity can be so serious, that synapses are blocked. Synaptic block causes death as a result of paralysis of the diaphragm muscles and respiratory failure.
Disturbance of energy transfer
Energy transformation in organisms is done through mitochondria systems in the cells. On the mitochondrion ATP-molecules are produced, which transfer energy through the body of an organism. When ATP production is disturbed the energy transfer will cease. This will make an organism tired and lifeless and unable to function normally.
Pollutants that cause reproductive failure due to damage to the reproductive organs are called endocrine disruptors. There are several ways in which a pollutant can act as an endocrine disruptor.
The first is an oestrogenic chemical. This is a chemical that can imitate an oestrogen by binding to the oestrogen receptor. This results in the induction of oestrogenic processes, causing an organism to experience reproductive failure due to a disturbance in the reproductive system.
An oestrogenic chemical can also block the effects of endogenous oestrogens by binding to the oestrogenic receptor. This causes masculization of female organisms.
It is also possible that female reproductive chemicals are found in male organisms. This causes hermaphrodites. Imposex has been widely reported in marine organisms, for instance with dog whelks by tributyl tin.
Another series of problems is caused when chemicals block the hormone receptor sites. In this case, the normal action of the hormone is inhibited, as it cannot react with the receptor. This can cause infertility when it occurs over a longer period of time.
All behaviours are vulnerable to alteration by pollutants. Foraging levels can deplete, resulting in reduced production. Vulnerability to predators can increase, due to a depletion of vigilance. In these ways, effects of pollutants on behaviour result in lowered production and higher mortality rates.
A common result of pollution is a loss of appetite and thus less uptake of food. Searching for preys can also be affected, due to effects of pollutants on learning, search strategy and sensory systems.
These behavioural effects cause lower chances of survival of organisms, mainly animals.
One property of pollutants that should always be kept in mind is their possibility to interact with one another. Chemical reactions that cause pollutants to combine can reduce their overall chemical effect, but can also increase it, making a pollutant even more dangerous to organisms.
Toxicity of chemicals in water can be tested with aquatic animals as indicators. Toxicity tests with aquatic animals are mainly concerned with direct uptake from water. The chemicals may be in solution, in suspension or both.
To determine values for lethal concentrations organisms are exposed to different concentrations. When an effect occurs the effect-concentration of the chemical is noted. When the test-animal dies the lethal concentration is noted. This way the toxicity of a chemical is determined in a laboratory. When many of the test animals die at low concentrations of a chemical it means that the chemical in question is very toxic. When we know how toxic a chemical is, we also know the effects of this chemical when a certain concentration is present at a location.
The toxicity of a chemical for certain aquatic organisms depends upon the present concentration of the chemical and the time of exposure to the chemical. The time of exposure to a chemical during a toxicity test depends upon the test animals that are being used. Daphnia are often used for certain toxicity tests. These tests commonly take only 24 to 48 hours. By contrast, fish toxicity tests take longer, usually four days up to a week.
Data of such chemical toxicity tests not only show how toxic a certain chemical is, they also give an indication of the toxicity of a chemical in relation to other chemicals. Not all toxicity tests work to lethal end-points; sometimes a change in behaviour of an aquatic animal is the indicator of toxicity of a certain chemical.
Toxicity tests are influenced by both the properties of the chemical and the properties of the test organism. The availability of the chemical to the test organism is always an important factor, because the toxicity of a chemical declines when it is not readily available to a test organism.
Laboratories can also perform toxicity tests for chemicals present in water sediment nowadays.
A fish toxicity testing system
toxic ions hazard of irrigation water
For water terminology check out our Water Glossary or go back to water FAQ overview
For toxicity testing and toxicity responses from aquatic life check out toxicity to aquatic organisms
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