Do you see any patches of oil on the water?

Oil may be leaking from an abandoned oil drum or from a factory or motorboats.

Pesticides

Pesticides are a diverse group of poisons of widely-varying chemical affinities, ranging from simple inorganic substances to complex organic molecules. Of the latter, some are natural metabolites, particularly of plants, while others are synthetic derivatives of natural products or completely synthetic substances produced in chemical factories under conditions which do not exist in the natural world. They live in common only that each pesticide is highly toxic to some forms of life and of intermediate or negligible toxicity to others, and that they have been widely introduced into the natural environment. Pesticides are introduced into aquatic systems by various means: incidentally in the course of their manufacture, and through discharge consequent upon their use. Surface water runoff from agricultural land and the side-effects of aerial spraying are especially important, and many serious pollution incidents arises through the accidental or negligent discharge of concentrated pesticide solutions which have been used for agricultural purposes such as sheepdipping. Additionally, many pesticides are deliberately introduced into bodies to kill undesirable organisms such as insect or molluscan vectors of human diseases, weeds, fish and algae.

Pesticides ire also used in many industrial processes, for example in the manufacture of textiles and in the production and processing of perishable materials such as paper and timber products. They are therefore an important component of many industrial effluents. In many countries, the special significance of pesticides as pollutants (and as widely used toxic chemicals in the working environment) has led to the development of strict controls on their use.

Insofar is it is possible to generalise about the polluting effects of such a diverse (group of substances, the following points are perhaps of greatest significance. First, effective pesticides are more or less selective in their effects, that is they are extremely toxic to some forms of life and relatively harmless to others. Second, their modes of application vary according to the circumstances. In some cases, pesticides are applied in relatively high concentrations for relatively short periods. This pattern of application typically occurs where pesticides are applied to water to kill weeds, disease vectors or other undesirable organisms, or as an incidental effect of aerial spraying of crops. Here, the principal concern may be to determine their short-term toxicity to non-target organisms, and it may be advantageous to devise specific toxicity testing protocols in order to estimate the impact of the pesticides on the receiving-water biota. However, in lowland rivers draining agricultural areas, pesticides arc more likely to be present at low but fairly consistent levels, and in this case the major areas of toxicological interest will be their potential sublethal effects, their capacity to accumulate in individual organisms and via the food chain, and the development of resistance through acclimation and/or genetic adaptation. Many pesticides arc known to be refractory to chemical and biological degradation and their persistence in the environment has for many years been a cause for concern.

Detergents

Synthetic detergents are in interesting group of pollutants because they were virtually unknown before 1945, yet within a few years became responsible some spectacular water pollution which, usually, came rapidly to the attention of the general public. The alkylbenzene sulphonate detergents rapidly replaced soap is domestic and industrial cleaning agents because of their cheapness and greater efficiency, and particularly because they did not cause precipitation of calcium salts in areas supplied with hard water. unfortunately they were not readily broken down by sewage treatment processes, giving rise to problems of toxicity to the receiving-water biota, and of foaming in watercourses and treatment works.

In areas where industrial usage of detergents was pronounced (for example, in textile-processing Industries) whole towns were frequently covered in detergent foam; in waste treatment works, a number of serious accidents occurred through, for example, operatives falling into sedimentation tanks which were concealed under a thick layer of foam. Consultation between regulatory authorities and the detergent manufacturers led to research which showed that modifications to the manufacturing process could produce linear alkylbenzene sulphonate (LAS) detergents, which were rapidly degraded in conventional waste treatment plants. From 1965 onwards, 'soft' or biodegradable detergents were introduced for domestic use, and although these ire generally more toxic to aquatic organisms, their unbranched Hydrocarbon chain is more readily broken down in treatment processes and in practice, toxicity and foaming problems had largely disappeared by the early 1970s. Synthetic detergents remain significant causes of pollution in some circumstances however. 'Soft' detergents are not suitable for use in certain industrial processes. Detergents are widely used as components of oil-dispersants, particularly in coastal and marine habitats, and are often more toxic to aquatic organisms than the oil itself. Finally, some components of detergent formulations exert adverse effects of their own. The best known example is the high level of phosphate found in many formulations. Less widely appreciated are the adverse effects of boron, from perborate additives to detergent formulations, which can cause adverse effects on crops if contaminated surface waters are used for irrigation.

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