Many of these chemicals and pesticides are known to persist for long periods in the environment and their concentration builds up geometrically as they are transferred to different stages of the food-web. Serious cases of fish mortality have occurred following the leaching of poisonous biocides from agricultural fields to nearby rivers or streams after rainfall. One such well-documented case that attracted considerable public attention in America, was a case of large-scale fish kill in the lower Mississippi river in which five million fish died. Careful investigations indicated that the fish had died as a consequence of dumping of Endrin-rich agricultural wastes and runoff into a tributary of the Mississippi river at Memphis. The widespread use of DDT as an insecticide has also aroused considerable concern in recent years and some countries have already legally banned its use.
Agroecosystems are specially vulnerable to the wide range of pesticides applied to maximise growth of crops. Residues of various weedicides and insecticides often accumulate in agricultural soils more rapidly than they are degraded. Insecticides are designed to kill insects and in many cases, they may not be toxic to plants. In contrast, some herbicides differ from insecticides in killing both desirable species as well as the intended target.
Their impact may not be limited directly to the plant community or agricultural crop but they may also act indirectly through effects on such soil microbes as nitrogen fixing blue-green algae and bacteria. This in turn may impair the growth and production of higher plants. The impact of such chemical wastes as phenols, acids, metals, etc.
, applied to soil can also be fairly high and may range from selection towards tolerant species to complete destruction of vegetation and also soil sterilization. Extensive researches in the USA have established widespread distribution of DDT residues through food chains in several lakes. Residues were detected in shallow and deep water mud samples, crustaceans, whitefish, duck, ring-billed and herring gulls and other fauna. Both DDT and Dieldrin have seen shown to pass from mother to offspring through the placenta in mice and certain other animals, possibly including man. Residues of Dieldrin, Endrin, DDT and DDE have been detected in all major river basins in the United States.
It is also believed that the declining percentage of immature biro in the American bald eagle population is due to DDT and other hard biocides. The following are some general properties of pesticides or their residues: (1) They often strike not only the intended pests but also several nontar- get organisms. (2) Many of them persist and cannot be disposed off.
(3) They may cause such unintended effects as resistance, faunal displacement and other population changes. (4) They may be carried to places far removed from the points of application or origin, and water is a very effective medium for their dispersal. (5) Their concentration and magnification in biological systems may- lead to certain unexpected or untoward results. Most of our pests are basically created in four ways (Rudd, 1971): (a) certain species of plants or animals may be selectively propagated; (b) many species are moved from one place on earth to another either intentionally or unintentionally; (c) a reduction in biological diversity often takes place either intentionally or accidentally; and (d) living organisms may adapt to changed environment and, in the case of pest, ‘biological weeds’ can be evolved.
Quite often a pest in one land is not a pest in its native land since its translocation to a remote place (mostly by man) means that its natural predators and parasites are left behind, thereby permitting its numbers to increase enormously in the new habitat. Most pesticides are employed to check or control the disturbances caused by a reduced or unstable biological community but unknowingly, we tend to accentuate the problems of both production and pollution instead of containing them. The consequences of pesticides are almost invariably adverse and harmful as is evident from the second peak. In the nineteenth century, the ladybird beetle was brought from Australia to California to control a scale insect pest of oranges. The beetle successfully kept the pest under check for more than five decades until about 1946 when DDT began to be used in the citrus orchards.
The beetle was susceptible to DDT and hence its population declined. This led to a marked increase in the population densities of the pest. A subsequent withdrawal of DDT again restored the natural balance of biological control within a few years. In fact, DDT is one of the most effective pesticides known. It was banned in the USA in 1972. Its remarkable insecticidal properties were first discovered in 1939. It was used during the Second World War in combating typhus, and after the War it began to be used for various commercial applications. This usage eventually reached a point where it became a ubiquitous contaminant of fish, penguins, birds and humans.
Then started a popular public movement in the USA that asked the Government to protect the public from the general toxification of the environment by DDT and other persistent poisons. When most of these protests and appeals failed, law courts were approached; they intervened and forced the Government to ban further use of DDT. All these historical and other aspects of DDT are well reviewed by Dunlop, (1981). DDT and other pesticides have in recent years been widely used to control mosquitoes with a view to eradicating malaria. Such use succeeded in the early stages and in India, the number of malaria cases showed a steep decline within a decade of DDT applications (started in 1952). From about 100 million cases annually in 1950, the number fell to 50,000 in 1961. However, by 1970 once again the number of cases started rising and more recently there has been a marked resurgence of malaria in India, Sri Lanka and Central America.
This resurgence seems to have been paralleled by intensified agriculture and the associated increased use of pesticides. In fact, many people believe now that the most serious threat to public health is the uncontrolled use of pesticides for agronomic practice (Chapin and Wasserstrom, 1981). In India, widespread resistance to pesticides has developed in Anopheles spp. particularly in areas which have recently shifted to high-yielding forms of agricultural production. Resistance seems to have emerged with the introduction of the Green Revolution technology, particularly of high-yielding varieties of rice and possibly the switch-over from organic manures to chemical fertilizers may also have contributed to this sorry state.
In America, entomologists have developed a kind of integrated pest management system involving a judicious and proper timing of pesticide applications so that the crops are protected from predators only at the most vulnerable stages of growth. Current estimates of pesticide use in the USA vary between 5 X 108 to 1 X 109 pounds per annum (Berry, 1979: Pimental et al. 1980). Human beings are exposed to pesticides mainly through the intake of food and water but also by inhaling contaminated air. Several pesticides are teratogenic, mutagenic, or carcinogenic. Bees vitally aid the pollination of several plants.
Pesticides have significantly affected some honey bees and other useful insects whose populations have declined. According to Pimental et al. (1980), annual agricultural losses due to poor pollination from pesticides can be as high as $ 4000 million in the USA. Sometime crops are damaged (instead of being protected) by pesticide applications, e.g., application of improper dosage under unfavourable conditions Herbicides that drift from a treated crop to a nearby crop also cause serious environmental problems.
Presistent herbicides also can injure crops planted in rotation. Drifting or leaching pesticides drain into nearby water in rotation. Drifting or leaching pesticides drain into nearby water bodies, causing fishery and wildlife losses. Wild birds and mammals exposed to pesticides suffer by death from direct exposure to high doses; reduced survival, growth and reproduction from exposure to sublethal dosages; and habitat reduction through elimination of food resources. No less important are the effects of pesticides on insects, earthworms, invertebrates, protozoa, and microbes found in soils, especially the decomposers. According to Pimental et al.
, human pesticide poisonings, reduction in natural enemies because of pesiticide usage, increased resistance to pesticides in insects and mites, and honey bee poisonings account for about 70 per cent of the calculated socio- environmental costs for pesticides in the USA. The estimates of economic benefits of pesticide usage in the same country equal some $ 10900 million per year (including tentative social and environmental guesses) amounting to $ 839000000 (Pimental et al., 1980). Chemical pesticides are by no means the only way to control pests. Some other general methods of controlling pest numbers include such means as (a) bioenvironmental, e.g., host resistance, biological control and sexual sterility attractants; (b) biophysical, e.g.
, barriers; (c) chemical, e.g., repellents and (d) quarantine.
The main criticisms of ecologists to present-day pesticide applications are that such applications lack selectivity and controllability. When a broad-spectrum or persistent biocide is applied, a large fraction of its spray goes to the unintended zone; a significant fraction drifts to unintended surface; some part goes to the atmosphere; surviving residues tend to leach off into water or enter the organic food-web and become biologically magnified or concentrated there. Transport of pesticide residues by air from the point of application to a remote area and their subsequent biological magnification in an ultimate consumer can often lead to toxic levels. In aquatic habitats, gradual and slow releases of persistent biocides and their residues enable their biological capturing’, storage, and magnification in aquatic food-webs.
Most commonly it is the last link in the food-web, i.e., the ultimate consumer, which exhibits toxic symptoms of concentrated residues (Rudd, 1971). In lakes, biocide dispersal involves adsorption to silt and deposition of residues and silt on Lake Bottom, from where they can be dispersed, preserved and concentrated in the lipid pool of the ecosystem by such means as bacteria and food-web transmission, leading ultimately to the final consumer. It is the relatively inert lipid fraction found in the ecosystem that acts as preservative for the residues. In oceans, airborne particles of biocide which strike water surface fail to disperse below the surface film and are absorbed rapidly by plankton.
This ensures spatial confinements of residues to the top layers of water (which are biologically active) and, hence, facilitates magnification. Since surviving biocide residues are uncontrollable, insidious and unpredictable in their actions, Rudd (1971) has advocated a complete ban on such pesticides whose toxic effects cannot be confined to the site of application. He has rightly emphasized that substantial harm has already been wrought by these biocides and even if their future use were banned now, sufficient amounts of residues will still persist in the various ecosystems to cause serious ecological effects for decades. He has further stressed diat pest control should be treated as a basically ecological rather than a chemical problem. In developing countries, poverty, malnutrition and weak health of the agricultural workers coupled with improper storage of the chemicals and the use of pesticide containers for storing food and drinks in villages contribute to accidents of pesticide poisoning. Most poor peasants live in a single-room hutment where the probability of pesticides getting into the food (often cooked in the same room) is high and the access of the chemicals to children is easy.
These hutments are frequently very close to the crop fields, and when the crops are sprayed, the spray frequently drifts into the house, exposing the children to grave risks. Pesticides pollute the environment causing health hazards and several actual episodes are known to have occurred in India and abroad. A decade ago, a mysterious disease involving pain in joints afflicted poor farmers in several villages in Karnataka State.
The crustaceans had become contaminated with parathion and endrin which were used in the paddy fields. The pesticides ultimately were ingested by the farmers who developed the disease. In Bangladesh, certain pesticides used on crops in Barisal area seeped into water-logged bog area and hundreds of cattle drinking that poisoned water died.
Asbestos is widely used as an insulating material and the chances of scrap asbestos reaching water bodies are quite appreciable. However, its recognition as a pollutant has come to be realized only since 1973, when the American Environmental Protection Agency reported that the water of Lake Superior contained hazardous amounts of asbestos from iron ore wastes being dumped in the north western portions of the lake. Asbestos is a fibrous mineral which is highly resistant to full disintegration; recent medical opinion attributes such hazards as cancer of lungs, stomach and intestines to asbestos pollution. Asbestos is now also recognized as a dangerous air pollutant since its particles can be carried into the atmosphere.
Asbestos occurs mainly in four forms: chry sotile, crocidolite, tremolite and amosite. Since the size of the asbestiform fibres in most water supplies is very small (less than 0.1 m in diameter), their presence in water cannot be detected by light microscopy. Some recent experiments have shown that asbestos fibres can be removed from water by filtration through diatomaceous earth. Much concern arises from the hazard of exposure to airborne asbestos inside buildings sprayed with asbestos-containing materials. Certain tiles, flooring and roofing materials and cement pipes used in buildings also contain asbestos. The weathering of asbestos flooring yields fairly high concentrations of indoor airborne asbestos.
The dirt and dust particles which are carried from land to water constitute a very serious pollutant.
These particulates originate from the weathering and erosion of rocks. Silt particles are mainly composed of silicates and carbonates with some phosphates and heavy metals absorbed on them. They also contain some organic compounds chiefly derived from the river biota.
Silt particles are usually 5.50 ^m in diameter. Silting can choke irrigation channels and rivers. Edible fishes, e.
g., salmon, are unable to spawn on silted gravel beds.