The water pollution is brought about by effluents from factories (the organic matter in such contaminated water is broken down by bacteria which consume oxygen and the oxygen demand is usually tested by ‘Biochemical Oxygen Demand’ or BOD test’). Some of which may have toxic substances like metallic zinc and copper, by crude oil mostly in seas and beaches, and pesticides. The pollution of soil is by all agents that pollute the water. Pollution of environment by pesticides is the result of extensive and intensive cultivation necessitating continuous and more frequent use of pesticides. The pollution was not as serious a problem with the earlier phytogenous and inorganic pesticides as with the present organic pesticides, some of which are highly persistent.
Pesticide residues in soil
Pesticides reach soil as a result of direct application to it or by drift during dusting and spraying to foliage or washed down by rains after application. From soil the persistent chemicals enter into soil invertebrates or water, or are broken down by micro organisms and physical factors. Such chemicals are mostly DDT and Dieldrin (Aldrin also breaks down in soil to Dieldrin). Both the DDT and dieldrin are reported to be more prominent soil contaminants in Western Countries, because of a large scale and indiscriminate use. The persistence of chemicals in soil is influenced by various factors like. The granular formulations persist longer than emulsions which again persist longer than wettable powders. Heavy clayey soil retains pesticides longer than light sandy ones and both organochlorines and organophosphates persist longer in acid than in alkaline soils. Higher the organic matter and clay mineral content in a soil longer is the persistence of a chemical. Higher the temperature and soil moisture less is the persistence. The break down of insecticides is more in soils having microorganisms than in sterile soils.
Pesticide residues in water
The pesticides reach water in lakes, ponds, tanks, rivers and streams from application of the same in mosquito control, deposits from aerial spraying of cultivated crops, as a surface runoff from soil treated with the chemical, from disposals and wash off of insecticide containers, etc. An of the absorption of the chemicals by bottom sand mud, plankton, aquatic plants and invertebrates and of the break down due to hydrolysis, toxic concentrations in water seldom build up. However, the two main hazards of residues in water are the deposition of chemicals in the bodies of fishes. which are effective filters of suspended particulate matter and in the bodies of aquatic organisms that form the food for the fish and as a result of both of these more chemical gets deposited in the fishes than is found free in water. The accumulation of pesticides in fishes is of great importance because they form food for the man and birds.
Pesticides in the biota
The plants retain pesticides on their surface or in tissues as a result of chemical control of pests. The organophosphorus compounds and systemic poisons are soon broken down but organochlorines like dieldrin are retained for relatively long periods. It is important that crops used for human or animal food should not contain residues of pesticides and hence tolerance levels that can be permitted in crops used for food have been fixed for most chemicals. The residues of organochlorines occur in the tissues and eggs of many species of birds and these residues might have accumulated from their food, mainly from fish and also from grains, leaves, etc. The birds accumulating lethal doses of the poison may get killed, followed by any predatory and scavenger birds feeding upon them.
The DDT is the most common pesticide than all other organochlorines present in human tissue and at present, an average amount of DDT present in human body is 5 to 10 ppm. The other common residues are from aldrin and dieldrin. The organochlorines can be transferred from the mother to foetus so that babies may be born with some pesticide chemical in their bodies, and milk from such mothers will also contain the chemical that together will accumulate in the babies’ bodies. However, these residues do not cause any immediate harm to the man but the possible long-term effects needs further studies. Fortunately in tropical country like India the chances of break-down of chemicals into harmless compounds are more. The possible pollution of human and cattle food can be minimized by several ways as.
Using less persistent chemicals like organophosphorus compounds and carbamates, which are equally or more effective than the persistent organochlorines against the pests they are usually used. However, they are more toxic to mammals than organochlorines and so adequate precautions have to be taken during and immediately after application. A better use of pesticides like applying them just around plants instead of by broadcast allover the planted area, seed dressing, seedling dips, using granular formulations on or around the plants or in furrows, etc. The particular microorganisms in soil degrade particular pesticides; for example a strain of the bacillus Nocardia alba, degrades parathion to the extent of 61% in 8 days under optimum conditions. Further a break-down of particular chemical is faster in soils treated with the same pesticide in the previous season because of a presence of the appropriate microorganisms 1n the soil. The possibility of introducing such microbial cultures to soil to hasten the break-down of pesticide will be an interesting field of exploration. Resorting to control of pests through ways other than chemical, like biological method.