The root of the waste problems in land lies in the leachates and mounting amount of wastes. Such leachates which ooze out of the garbage heap are known to move slowly through the layers of the soil beneath and contaminate the water resources deep down the land. However, the problem of soil pollution differs from air and water pollution in the respect that the pollutants remain in direct contact with the soil for relatively long periods. The widespread industrialization and increasing consumption have changed the very complexion of soil. Thus, the soil is getting heavily polluted day by day by toxic materials and dangerous micro-organisms which enter the air, water and the food chain.
Soil pollution is defined as the addition of substances to the soil, which adversely affects the physical, chemical and biological properties of soil and reduces its productivity.
Types of Soil Pollution:
Negative Pollution- Deterioration in the productivity of soil due to a reduction in quality or quantity of topsoil is called negative soil pollution. It is caused by over-use and erosion.
Positive Pollution- It is a reduction in soil productivity and deterioration in the quality of plants due to the addition of pollution from the air, faulty sanitation, industrial effluents and supra-optimum fertilizers and pesticides.
Third Pollution- It is landscape pollution in which the land is so severely misused that it becomes filthy and odorous because of the dumping of garbage, rubbish, sludge, ash, industrial wastes, etc. over it.
Third Poison- It is groundwater pollution caused by seepage and leaching of sewage, toxic chemicals and extra minerals from the surface.
Soil pollution is direct if the pollutants are passed over it directly. Example- industrial effluents, fertilizers. It is indirect if the pollutants reach soil from other resources like air and water. Example- acid rain.
Sources of Soil Pollution:
Domestic wastes include garbage, rubbish material like glass, plastics, metallic cans, paper, fibres, cloth rags, varnishes, containers, paints etc. Leachates from dumping sites and sewage tanks are harmful and toxic, which pollute the soil.
Industrial wastes are the effluents discharged from chemical industries, paper and pulp mills, tanneries, textile mills, steel industries, distilleries, refineries, pesticides and fertilizer industries, pharmaceutical industries, food processing industries, cement industries, thermal, atomic and electrical power plants, mining industries etc. Thermal power plants generate a large quantity of ‘Fly ash‘. Huge quantities of these wastes are dumped on soils, thus contaminating them.
Industrial wastes also contain some organic and inorganic compounds that are refractory and non-biodegradable. Industrial sludges are even more dangerous than industrial solid wastes to dispose of tidily. The composition of industrial sludges vary enormously; the common boiler scale, for example– consists of calcium carbonate and flue gas sludge. This desulfurisation sludge is generated when calcium hydroxide or limestone are used to trap sulfur dioxide from escaping gases in coal-fired power plants. These wastes also consist of calcium salts and several toxic, volatile elements such as arsenic, selenium, mercury, lead and cadmium, which pose detrimental effects on the environment.
Pesticides are chemicals used in killing pathogens, pests and unwanted growth in agriculture, horticulture, forestry and water. Most pesticides are broad-spectrum, killing most of the organisms. They are, therefore, called biocides. Many of them are also persistent. Some of them even adversely affect the useful organisms in later stages. The phenomena are called ecological boomerang or ecological backlash. For example- weedicides added in the Aswan dam of Egypt for controlling aquatic weeds damaged crops as well as fish production in the sea (wherever dam water was discharged). Pesticides that are persistent in nature are chlorinated hydrocarbon insecticides. Example- DDT, HCH, BHC, Endrin, Aldrin, Lindane, Heptachlor, Endosulfan, Dieldrin etc. Residues of these pesticides in the soils have long term effects, especially under temperate conditions.
Soil also receives excreta from animals and humans. The sewage sludge contains many pathogenic organisms, bacteria, viruses and intestinal worms which cause pollution in the soil.
The sources of radioactive substances in soil are an explosion of radioactive devices, radioactive wastes discharged from industries and laboratories, aerial fall out etc. Isotopes of radium, uranium, thorium, strontium, iodine, caesium and of many other elements reach the soil and persist there for a long time and keep on emitting radiations.
Effects of Soil Pollutants:
A direct public health hazard.
External groundwater pollution.
Accumulation of hazardous substances, in the soil or water that can enter into the food chain.
An account of pollutants, such as odours, into the atmosphere.
Aesthetic losses within limits.
Control of Soil Pollution:
Sewage Treatment- Municipal sewage wastes and toxic gases containing a large amount of flammable and poisonous hydrogen sulfide. It is the same gas that also bubbles from gutters with a highly offensive odour.
Incineration- Waste is burnt aerobically at 900-1000°C. The hot gases and smoke are further passed into a chamber where the temperature is 1300°C. It burns the smoke particles. The gases released from the second chamber are taken to a wet scrubber for removing suspended particles and soluble gases. Ash formed in an incinerator is collected and disposed of in landfills.
Pyrolysis- The waste is heated anaerobically at a temperature of 1650°C. It yields industrial gas, alcohols, and a number of other chemicals. The bulk of the waste is reduced. The residue is disposed of in a landfill.
Biogas Plants- Cow dung, human excreta and putrescible garbage can be mixed and used for the generation of biogas. Manure is produced as a by-product.
Composting- Putrescible waste is shredded, mixed with sewage sludge and spread in open to form compost. The mixture is moistened and turned at intervals. Compost becomes ready within 3-6 months.
Fungi to increase Soil Fertility- Fungi will enable tropical trees and plants to grow in the poorest soils.
Microbial degradation of biodegradable substances is also one of the scientific approaches for reducing soil pollution.