Soil Pollution
Unfavourable alteration of soil by addition or removal of substances and factors which decrease soil productivity, quality of plants and ground water is called soil pollution.
Sources of Soil Pollution
Soil pollution mainly results from the following sources
1. Industrial Wastes
2. Urban Wastes
3. Radioactive Pollutants
4. Agricultural Practices
5. Chemical and Metallic Pollutants
1. Industrial Wastes
Disposal of industrial waste is the major problem responsible for soil pollution. These industrial pollutants are mainly discharged from pulp and paper mills, chemical industries, oil refineries, sugar factories, tanneries, textiles, steel, distilleries, fertilizers, pesticide industries, coal and mineral mining industries, metal processing industries, drugs, glass, cement , petroleum and engineering industries etc.
Hazardous wastes generated from chemical industries which find their way into the soil environment.
Industry Wastes
i. Fertilizers Arsenic, ammonia, fluoride,heavy metals, acids, tar, phenolics, cyanide, polyaromatic hydrocarbons (PAH)
ii. Caustic Soda Mercury, chlorinated hydrocarbons, lead carbonate, asbestos.
iii. Pesticides Chlorinated aromatics, chlorobenzene, organophosphorus derivatives, solid residues of pesticides, chlorophenols, dioxins.
iv. Organic chemicals Chloro, nitro and phenolic substances
v. Organic solvents Organic liquid and semi solid residues
vi. Fibre Zinc, polysulphides ,lead salts, manganese
2. Soil Pollution by Urban Wastes
Urban wastes comprises both commercial and domestic wastes consisting of dried sludge of sewage. All the urban solid wastes are Commonly referred to as ‘refuse’: Solid wastes and refuse, particularly in urban areas contribute to soil pollution. This refuse contains garbage and rubbish materials like plastics, glasses, metallic cans, fibres, paper, rubbles, street sweepings, fuel residues, leaves, containers, abandoned vehicles and other discarded manufactured products.
3. Radioactive Pollutants
Radioactive substances resulting from explosions of nuclear de- vices, atmospheric fall out from nuclear dust and radioactive wastes (produced by nuclear testing laboratories and industries) penetrate the ‘soil and accumulate there creating land pollution. Radio nuclides of radium, thorium, uranium, isotopes of potassium (K-40) and carbon (C-14) are very common in soil, rock, water and air. Explosion of hydrogen weapons and cosmic radiations induce neutron-proton reactions by which nitrogen (N-IS) produces C-14. This C14 participates in the carbon metabolism of plants which is then introduced into animals and man.
4. Agricultural Practices
Modern agricultural practices pollute the soil to a large extent. Today with the advancing agro-technology, huge quantities of fertilizers, pesticides, herbicides, weedicides and soil conditioning agents are employed to increase the crop yield. Many agricultural lands have now excessive amounts of plants and animals wastes which are posing soil pollution problems. Apart from these farm wastes, manure slurry, debris, soil erosion containing mostly inorganic chemicals are reported to cause soil pollution. USA
alone produces about 18 million tonnes of agricultural wastes every year. Some of the agents responsible for this pollution are as follows.
(a) Fertilizers
Now-a-days agricultural practices rely heavily on artificial fertilizers, which generally contain one or more of the plant nutrients i.e. nitrogen, phosphorous and potassium. Critical pollution problems arise mainly from their excessive application rates. Although the fertilizers are used to fortify the soil, yet they also contaminate the soil with their impurities, when the fertilizers are contaminated with other synthetic organic pollutants, the water present in the soil may also get polluted.
(b) Pesticides
Among pesticides the most important are the chlorinated hydrocarbons e.g. D.D.T., B.H.C., aldrin, endrin, dieldrin, lindane, chlordane, heptachlor and endosulphan. Organo-phosphates include-malathion, parathion, ethion, fenthion, trithioin, dursban, dimethoate, phosdrin and metasystox etc. The remnants of these pesticides may get absorbed by soil particles which may contaminate root crops grown in soils.
(c) Soil Conditioners, Fumigants and Other Chemical Agents
In addition to the fertilizers, pesticides and biocides, soil conditioners and fumigants are also employed to the land system to increase and protect the soil fertility as well as to kill the hazardous insects. These chemical agents are reported to cause alterations in both agricultural and horticultural soil areas. They contain several toxic metals lead, arsenic, cadmium, mercury and cobalt etc. which when applied to a land will accumulate on the soil permanently thereby introducing these chemical components into growing crops.
(d) Farm Wastes
Increasing population of cows, cattle, pigs and poulters have resulted in considerable soil pollution. Buildings in which grazing animals are housed can be cleaned using water but the manure is also washed out and deposited as wet slurry on the land. This slurry deposited on soil may seep into ground water and pollute it. When these farm wastes are dumped into heaps, they may become a good breeding ground for insects and nuisance arise.
5. Chemical and Metallic Pollutants
A number of industries including textiles, pesticides, paints, dyes, soap and synthetic detergents, tanneries, drugs, batteries, cement, asbestos, rubber, petroleum, paper and pulp, sugar, steel, glass, electroplating and metal industries pour their hazardous effluents in soil and water creating disastrous effects on living organisms.
Effects of Soil Pollution
1. Most of the heavy metals are toxic, for example, arsenate impregnated areas can often be recognized by the absence of vegetation. The most toxic metal are nickel, chromium and cadmium. Compounds of nickel, chromium and arsenate cause lung cancer. Several nickel compounds, including the volatile nickel tetracarbonyl, are carcinogenic. Lead tetra alkyls are extremely poisomous, affecting the central nervous system. Lead itself causes lead poising characterized by painful intestinal colic, failure of kidney functions, loss of function in peripheral nerves leading to tumours and paralysis.
2. Hazards caused by biomagnification of pesticides are of increasing concern. These pesticides enter food chain, attain concentration through biomagnification and destroy members of different trophic levels, and may pass from one ecosystem to the other.
3. Persistnant pesticides become concentrated in higher links of foods. Recent declines in number of eagles, falcons hawks, gulls and brown pelicans are linked with unsuccessful reproduction. The predators accumulate DDE in their tissues from rodens and other organisms which they eat.DDE is correlated with disturbance in liver enyme action and interference in calcium metabolism. Eggs are produced with little or no hard shell, are easily broken and do not hatch.
4. There is also a strong pressure of natural selection in insect population exposed to insecticides,which leads to the development of resistance, often by metabolizing the compounds in new ways, SO in lime, heavier dosages are required to produce the same effecs or none. However the danger to the community increases with increased release of the toxic substance in the environment. The history of pesticides chemistry is, therefore of continuous struggle to keep one ‘jump ahead of the insect’. Dioxin will kill small animals but the major effect on human being are skin lesions, including a very unsightly and long-persisting acne, due to the blocking of the ducts of the sebaceous glands.
5. The PCBs, posses many of the unwelcome biological properties of organochlorine, Particularly, long persistence, lipid solubility leading to concentrated food chains and toxic effects in animals. In accidently induced contamination it has been seen o cause malfunctioning intestinal system and the brain. A widespread concern arose for the human population, because the PCBs were secreted into milk and could be shown to be concentrated in human adipose tissue. A part mild neurological effects, there has been no immediate human toxicity. They are also potent inducers of hepatic mixed function oxidases.
6. Once radioactive materials reach the soil, they are accumulated by the producers, lichens for instance, are known to concentrate Caesium-13. When the green are fed upon by herbivores and subsequently pass through the food chain, biomagnification of radionuclides occurs showing distinct effects. The nuclide which is deposited on grass, is fed by cows, and appears in milk.
7. Dumping municipal wastes, garbage and improper sanitary conditions become breeding grounds of pathogenic organisms and their vectors. The soil becomes contaminated by various bacilli those causing dysentery, cholera, typhoid, paratyphoid and other. These are carried by vectors and the human population gets infected. When introduced into cropland, spores and cysts of various helminth parasites contaminate the food crops and enter the food chain.
