Effects of pollutants on the aquatic environment
Effects of pollutants on the aquatic environment

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5 Toxic pollutants

The term ‘toxic’ is a rather misused one. It is misleading to refer to one material as a toxic substance and to another as non-toxic without qualification. The toxicity of all materials depends on their concentration. Further complications may be introduced by the fact that some materials (e.g. selenium) are essential components of an animal’s diet, yet in anything other than very low concentrations they may have a toxic effect.

Other environmental factors to be taken into account include:

  • the extent of biodegradation
  • the rate of accumulation of a substance in the biota (and so within the food chain)
  • the retention time of a substance within an organism.

An important mechanism for toxicity in the body is the poisoning of enzymes, which are the catalysts of all the bodily functions.

The terms used when explaining the effects of a toxic substance on an organism are:

  • lethal – causing death by direct poisoning
  • sublethal – not sufficient to cause death, but leading to a reduction in the number of species and/or individuals by, for example, causing a change in behaviour, growth or reproductive success
  • acute – causing an effect (possibly death) within a short period of time
  • chronic – causing an effect (lethal or sublethal) over a prolonged period of time
  • accumulative – having an effect that is increased by successive doses.

To test the effects of toxicity, the LD50 (lethal dose) test is commonly used. The LD50 is the dose that is large enough to kill 50% of the sample of animals under test. Some examples of LD50 values for different chemicals are given in Table 1.

Table 1  Oral toxicities of some chemicals in small mammals

LD50

(mg per kg body weight)

Examples

Classification

1–10

arsenic

highly toxic

10–100

cadmium

copper

lead

mercury

moderately toxic

100–1000

aluminium

molybdenum

zinc

slightly toxic

>1000

sodium

iodine

calcium

potassium

relatively harmless

Certain inorganic substances, such as cyanides, fluorides, sulfides, sulfites and nitrates, may be classified as toxic.

  • Compounds of cyanide and sulfide interfere with the use of oxygen in respiratory reactions in cells.
  • Excess fluoride can lead to mottling of teeth and bones in humans.
  • Nitrates can cause ‘blue baby syndrome’.

However, the final effects of a toxic substance in water depend on environmental factors such as hardness, temperature and pH. Salts of heavy metals such as copper, silver, lead, gold, nickel, chromium, zinc, cadmium and mercury are toxic and will generally kill most aquatic organisms at very low concentrations, but they are generally less toxic in calcium-rich water (Wilson, 1988); a nickel–cyanide complex is 500 times more toxic to fish at pH 7 than at 8, because the complex dissociates into cyanide and nickel ions and a proportion of the cyanide forms the highly toxic undissociated hydrogen cyanide (HCN); and ammonia is 10 times more toxic at pH 8 than at 7 (EIFAC, 1968). Also an additive effect, synergism, may occur – for instance, evidence has been found for synergism between mercury and uranium (Sánchez et al., 2001).

Two particularly significant groups of toxic pollutants are the heavy metals, and synthetic organic substances such as some of the pesticides.

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