Pesticides and Pollution. Kenneth Mellanby

Чтение книги онлайн.

Читать онлайн книгу Pesticides and Pollution - Kenneth Mellanby страница 9

Pesticides and Pollution - Kenneth  Mellanby

Скачать книгу

wrongly, that polluted air is short of oxygen, and they believe that they inhale more of this vital gas in the country or on the top of a mountain than when in a town. In fact there is little change in the amount of oxygen in the air even in the stuffiest room; there is certainly more in a crowded lecture room in London than in the rarer, though purer, air at the top of Ben Nevis. Industrial pollution, except for the undiluted exhaust gases from chimneys and engines, hardly reduces the amount of available oxygen. Carbon dioxide, present in pure air in very small quantities (approx. 0·03 per cent) is indeed increased by pollution, but seldom if ever to a concentration which is harmful to animals, and it may even stimulate plant growth. Man’s breathing is upset by air containing 7 per cent of CO2, and 14 per cent breathed for some minutes can be lethal; such levels of pollution have never been recorded except in such enclcsed spaces as fermentation chambers in breweries. “Stuffiness” is experienced in crowded rooms, but this is not due to the shortage of oxygen or the amount of carbon dioxide. It is due to very small amounts of organic substances given off by the other occupants of the room (“B.O.”), and to shortwave radiation from the walls and people themselves. Many Englishmen – and even more Englishwomen – think a room is stuffy and “polluted” simply because, for once, it is comfortably warm! Polluted air is usually “normal” air, in so far as its content of oxygen, nitrogen and carbon dioxide is concerned, plus the addition of small quantities of added materials. Polluted water, as will be seen in the next chapter, may provide quite different problems.

      CHAPTER THREE WATER POLLUTION

      As was seen in the last chapter, air pollution has proved difficult to study, and many conflicting results have been obtained. Water pollution seems to have provided a more satisfactory topic for investigation. This is not because the subject is simpler; in fact in some ways it is even more complicated. Air almost always contains sufficient oxygen to sustain life, and “pollution” only means adding a lesser or greater amount of some foreign substance to an otherwise wholesome atmosphere. Water, on the other hand, may be greatly depleted of oxygen so that it cannot sustain most kinds of life, or it may have various substances added, so that animals and plants are poisoned. Some waters have both reduced amounts of oxygen and appreciable amounts of poisons in solution; the inter-relations between these factors, and their effects on aquatic animals and plants, may be very complicated. The advantage of water over air, from the point of view of the research worker, is probably that it forms a definite and restricted environment, from which animals cannot easily move. It is therefore possible to study the long-term effects of pollution under fairly constant conditions, and there is no difficulty in demonstrating the serious effects which pollution can produce. It may also be suggested that water is a marketable commodity of considerable economic value, while air is, theoretically at least, “free.” So although fresh water covers under one per cent of the surface area of Britain, there are probably more scientists studying its pollution than there are investigating air which covers one hundred pet cent of the globe, land and water alike.

      Several excellent books on water pollution, and books on freshwater dealing authoritatively on aspects of the subject, have been published in recent years. These include The Biology of Polluted Water by H. B. N. Hynes, Fish and River Pollution by J. R. Erichsen Jones, the New Naturalist Life in Lakes and Rivers by T. T. Macan and E. B. Worthington, and Freshwater Ecology by T. T. Macan. The existence of this extensive and easily obtainable literature has enabled me to make this chapter much shorter than would otherwise be desirable, and to deal with the effects of pollution in a rather different way than would have otherwise been possible.

      Man’s requirements regarding water are different from those of “wild life” generally. Man demands what he describes as “pure” water; what he really means is “safe” water. This must contain only a minimum amount of salts, and must be free from those bacteria, protozoa and arthropods which might develop in his body and cause disease. Man thus deliberately interrupts the life-cycle of many other forms of life by the various methods of purification which he uses. He is less concerned with the oxygen content of the water than are the fish and insects which live in it. These efforts to produce a pure water supply for city dwellers can even be thought of as a form of “pollution,” in that water catchments alter, and sometimes sterilise, large areas of the countryside. The conflict between Manchester Corporation and many naturalists and others over the fate of much of the English Lake District illustrates this point.

      On the other hand, water which, for public health reasons, is considered to be “grossly contaminated” by sewage, may still be, from the biological point of view, a healthy and desirable environment for many animals. But by the deliberate discharge of his domestic and industrial wastes man most greatly affects streams and lakes, and so alters the whole composition of their flora and fauna.

      Natural waters may not only be “impure” from man’s point of view because of the parasites they harbour; they may contain many substances, even poisons, without any human intervention. Quite high concentrations, sufficient to poison some fish and many insects, of lead and copper are found in waters which percolate through strata rich in these metals. Streams running through forests, particularly pine forests, may be contaminated with large amounts of organic matter, and the results may be quite similar to those arising from domestic pollution. As a rule a special flora and fauna is found, consisting of plants and animals adapted to such conditions, in these impure waters. Human pollution usually happens so quickly that impoverishment occurs, often without time to allow the introduction of many of these special types of organism.

      Primitive man did not seriously harm the aquatic environment. He often lived beside rivers and lakes, and his waste products must have entered the water, but in insufficient quantities to have adverse effects on the flora and fauna. In fact excrement entering the water in this way no doubt contributed to its nutritive value, and the substances it contained entered into the normal cycles. In some of the less developed and less densely populated areas of tropical Africa we can see a similar situation to-day. The streams and ponds are full of healthy fish; the human beings have a rich internal fauna of parasite worms which pass part of their lives in the water, inside small crustaceans or fish. Man in this way contributes to the richness of wild life in his environment.

      When man came to live in towns and cities, however, his increasing numbers had a very different effect. Sewage continued to be poured into the rivers, but the quantities were so great that most unpleasant results were obtained. By the middle of the nineteenth century the Thames, and many other major British rivers, had become open sewers. There are many accounts in the literature. I myself like the account of the Reverend Benjamin Armstrong, from his diary:

      “July 10th, 1855. Took the children by boat from Vauxhall Bridge to show them the great buildings. Fortunately the Queen and Royal Princes drove by. The ride on the water was refreshing except for the stench. What a pity that this noble river should be made a common sewer.”

      Practically every other river was treated similarly. Even the Cam flowing through the Backs at Cambridge was in this way abused, as is illustrated by the (perhaps apocryphal) story of Queen Victoria’s conversation with the Master of Trinity when she looked over the bridge. “What,” she asked, “are all those pieces of paper in the water?” The Master promptly replied, “Those, Your Majesty, are notices saying that bathing is forbidden.”

      The results of all this untreated, or “raw,” sewage, vary greatly, depending on the volume of water and the amount of organic matter. As indicated above, small amounts of raw sewage may be actually beneficial to most forms of aquatic life. To-day in some rivers, including the Bedfordshire Ouse, the comparatively small number of boats present discharge the contents of their water closets straight into the water. This does not cause noticeable offence. In some parts of the Norfolk Broads it does, for there are many boats producing much more

Скачать книгу