Introduction

Soil is a necessary component of terrestrial ecosystems since the expansion of plants and biogeochemical cycling of nutrients based upon it. Pollution, along through other kinds of degradation, these as erosion and the continuing swell of urbanisation, poses a threat to the sustainability of soil resources. Soil pollution can as well be a danger to human physical condition when potentially toxic materials shift through the food chain or reach groundwater that provides as drinking water.

In contrast through air and water, the soil is more variable and complex in composition and it purposes as a sink for pollutants, a filter that delays the passage of chemicals to the groundwater and a bioreactor in which many organic pollutants can be decayed. As a consequence of its occurrence at the interface between the land and the atmosphere, soil is the recipient of a diverse range of polluting chemicals transported in air. Further contributions of pollutants to the soil take place as an effect of agricultural and waste disposal practices, but in common, the most severe pollution generally effects from industrial and urban land utilize

Soil Pollution and Contamination: A Global View

Certain contaminants/pollutants these as heavy metals are part of the earth obviously. Their natural background levels in soils are altered to diverse levels of pollution generally as a consequence of anthropogenic activities.

It is generally recognized that most of the soil in technologically advanced regions of the world is polluted or contaminated, at least, to several extent. Realistic estimate of areas influenced through soil pollution is hard owing to unreliable official figures and paucity of data for many parts of the world. Technologically contaminated land tends to enclose higher concentrations and a greater possible range of pollutants than other sources of pollution. There are between 50,000 and 100,000 contaminated sites in the United Kingdom (UK) that occupy up to 100 000 ha. In the United States of America (USA), 25,000 contaminated sites have been recognized, 6000 sites are being cleaned-up in the Netherlands, there are recognized to be at least 3,115 sites in Denmark and 40,000 suspect areas have been recognized on 5,000 - 6,000 sites in the former western part of Germany.

Soil Pollutants and Their Sources 

Heavy metals

Heavy metals contain a density of≥ 6gcm -3 and take place logically in rocks. They belong to the group of constituents geochemically explained as 'trace elements' since they together comprise less than 1percent of the rocks in the earth's crust. All trace metals are toxic to living organisms at extreme concentrations, but several are necessary for normal healthy development and reproduction through plants and/or animals at certain critical concentrations for example Co  (for  bacteria  and  animals),  Cr (animals), Cu (plants and animals), Mn (plants and animals), Mo and Ni (plants), Se (animals) and Zn (plants and animals). Elements as Ag, As, Ba, Cd, Hg, Pb, Sb and Tl have no known necessary function, but cause toxicity above an assured tolerance level. The most significant heavy metals through regard to potential hazards and incidence in contaminated soils are As, Cd, Cr, Hg, Pb and Zn.

Sources of heavy metals that contaminate or pollute a agreed soil site are:

Metalliferous mining for example As, Cd, Cu, Ni, Pb and Zn

Metal smelting e.g. As, Cd, Pb and Tl

Metallurgical industries

Other metal-using industries for example the electronics industry (using Cd, Ni, Pb, Hg, Se and Sb); pigments and paints (Pb, Cr, As, Sb, Se, Mo, Cd, Co, Ba and Zn); the plastics industry (polymer  stabilizers e.g. Cd, Zn, Sn and Pb) and chemical industry (using catalysts and  electrodes e.g. Hg, Pt, Ru, Mo, Ni, Sm, Sb, Pd and Os);

Waste disposal

Corrosion of metals in use

Agriculture  - As, Cu and Zn (from pig and poultry feeds), Cd and U impurities  (in  some  phosphatic fertilizers), As, Cu, Pb and Zn (from metal-depend pesticides); Forestry-wood preservatives containing As, Cr and Cu; Fossil fuel combustion - Cd, Zn, As, Sb, Se, Ba, Cu, Mn and V (from coals and oils) and  Se, Te, Pb, Mo, Mn and Li (added to fuels and lubricants to improve their properties);  Sports  and  leisure  activities - game  and  clay  pigeon  shooting involving the utilize of pellets enclosing Pb, Sb, Mo and Bi.

Hydrocarbon pollutants

Hydrocarbon pollutants from petroleum mostly comprise a range of saturated alkanes from CH₄   to C₇₆H₁₅₄. Aromatic hydrocarbons and organic components enclosing nitrogen and sulphur can as well be significant constituents of several petroleum deposits. The hydrocarbons derived from coal and petroleum tends to form the main group of organic macropollutants in soils. Organic solvents can be significant soil pollutants at industrial sites.

Sources of hydrocarbon pollutants comprise fuel storage and distribution, disposal of utilized lubricating oils, leakage of solvents from industrial sites and coal stores.

Toxic Organic Micropollutants (TOMPs)

The most common toxic organic micropollutants in the soil environments  comprise: polycyclic aromatic hydrocarbon (PAHs), polyheterocyclic hydrocarbons (PHHs), polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and pesticide residues and their metabolites for example organochlorines (DDT,  BHC,  Endrin), organophosphates (Malathion, Parathion) and Carbamates (Aldicarb). The gravest problems connected through TOMPs are the subsequent contamination of surface and groundwaters and entry into the food chain through crops, particularly.

Other industrial chemicals

It has been approximated that between 60,000 and 90,000 dissimilar chemicals are in current commercial use, worldwide. Even though not all of such constitute potential toxicity hazards, many will cause pollution of soils as a consequence of leakage during storage, from utilize in the environment or from their disposal either directly, or of wastes containing them. Apart from industrial utilizes, a huge number of chemicals are utilized in domestic products and so their employ and disposal is less controlled than that of industrial chemicals that are subjected to strict regulations. The total world production of hazardous and special wastes was 338 x 106 tonnes in the year 1990.

Nutrient-rich wastes

The major nutrient-rich wastes are sewage sludges and livestock manures. Sewage sludge, though a valuable source of plant nutrients (N and P especially) and of organic matter which has beneficial effects on soil aggregate stability, contains potentially harmful substances such as heavy metals (Cd, Cu, Ni, Pb and Zn) and toxic organic micropollutants (polychlorinated  biphenyls, PCBs;polychlorinated terphynyls, PCTs; polychlorinated naphthalenes, PCNs;polychlorinated benzenes; polychlorinated dibenzodioxins, PCDDs; halogenated aliphatics; polycyclic aromatic hydrocarbons, PAHs; aromatic amines; nitrosamines; phthalate esters and pesticides). As well,livestock manures contain huge amount of N, P and K, but the manures may as well enclose residues of feed additives that might include As, Cu and Zn and antibiotics such as sulfonamides fed to pigs and poultry.

Radionuclides

Nuclear accidents as those at Windscale (UK) in the 1957 and Chernobyl (Ukraine) in the 1986 resulted in many different radioactive materials being dispersed into the atmosphere. The greatest long-term pollution difficulty through radionuclides in soils is caused via 137Cs which has a half- life of 30 years and acts in a manner similar to K in soils and ecosystems.  Atmospheric testing of nuclear weapons dispersed huge amounts of 90Sr which has a half-life of 29 years and behaves similarly to Ca in biological systems and poses a particularly dangerous hazard to humans since it is stored in the skeleton.

Pathogenic organisms

Soils can be contaminated through pathogenic organisms these as bacteria, viruses and parasitic worm eggs from diverse sources: the burial of dead animals and humans, manures and sewage and germ warfare experiments. The soil's own microbial biomass will demolish many of such pathogens but some appear to survive for relatively long times.

Transport Mechanisms Conveying Pollutants to Soils

Pollutants reach soils by four main pathways: atmospheric deposition of particulates; sorption of gases; flUV ials transport and deposition; and placement.

Atmospheric deposition: Light particulates can become airborne and obtain transported from one location to the other. For instance, there have been cases where pollution is straight traceable in soil up to 40kilometres downwind of smelters. The critical factors computing the distance transported and the area of land influenced are: the height of the emission above the ground, the wind speed, constancy of the atmosphere and the size, shape and density of the particles and amounts of the pollutants emitted.

Sorption of gases: Volatile organic solvents become dispersed in the atmosphere  and  later  get  sorbed  from  the  atmosphere  onto  soils  or plants. FlUV ial transport and deposition: This mechanism is only important in land subjected to flooding. This was an important pollution pathway in areas of metalliferous mining in the nineteenth century when Pb-Zn mines in the UK discharged waters from ore dressing operations directly into streams and rivers. This led to the allUV ial soils in most flood plains of rivers draining mining areas being severely contaminated with Pb, Zn and other metals. Soils on the flood plains of many major rivers in  the  world  which  drain  industrial  and  urbanized  areas  have  been significantly contaminated with a diverse range of substances from flood waters.

Placement: This can occur in many ways: dumping injection, surface spreading, etc; the most obvious being the spreading of wastes such as sewage sludge or metal-rich manures from pigs or poultry. Phosphatic fertilizers can contain significant concentrations of Cd and U and have been partially responsible for the significantly elevated concentrations of these elements in many parts of the world.

The Consequences of Soil Pollution

Soil pollution can restrict the uses to which land is put because of the ability of pollutants to be hazardous to human health, harmful to living resources or to damage buildings and services.

Types of Hazards Associated with Contaminated Soil

Direct ingestion of contaminated soil: As, Cd, Pb, CN-, Cr⁶⁺, Hg, coal tars (PAHs), PCBs, dioxins, phenols, pathogenic bacteria, viruses, eggs of parasites.

Inhalation of dusts and volatiles from contaminated soil: organic solvents, radon, Hg, asbestos. Plant uptake from soil: As, Cd, 137

Cs, Hg, Pb, 90Sr, Tl, PAHs, various pesticides. Phytotoxicity: SO₂, Cu, Ni, Zn, CH₄, Cr, B. Toxicity to soil microbial biomass: Cd, Cu, Ni, Zn.

• Deterioration of building materials and services: SO₂, SO₂, Cl, tar, phenols, mineral oils, organic solvents. Fires and explosions: CH₄, S, coal and coke dust, petroleum oil, tar, rubber, plastics, high calorific value wastes.
• Contact with contaminants during demolition and site development: tars, phenols, asbestos, radionuclides, PCBs, TCDDs, bacteria, viruses.
• Water contamination: CN-, SO₂, metal salts, hydrocarbons, solvents, surfactants, sewage, farm wastes, and pesticides.

Soil-Plant Transfer of Pollutants

The transfer of pollutants from soil into plants through the roots varies considerably depending on the properties of the plants (genotypic factors) and the extent to which the pollution is absorbed in the soil in an unavailable shape. The common orders of magnitude of the relocate coefficients for several heavy metals are 0.01 - 0.1 for As, Co, Cr, Hg, Pb and Sn; 0.1 - 1.0 for Cu and Ni; and 1.0 - 10 for Cd, Tl and Zn (which are the most readily accumulated by plants). Organic pollutants tend to have lower coefficients; a value of 0.033 has been suggested for dioxins and PCBs. The lower chlorinated congeners of PCBs tend to be more bioavailable and biodegradable than those through higher numbers of Cl atoms in their structure.

Ecotoxicological Implications of Soil Pollution

Zinc concentrations close to the maximum allowable level of 300 mg/g for European soils treated through sewage sludge, can cause a mark in the  activity of the nitrogen-fixing bacteria  of  the species  Rhizobium leguminous via trifolii which take place in nodules in the roots of white clover. Although it appears that the order of toxicity of heavy metals to these bacteria is Cu > Cd > Ni > Zn, the greatest risk is posed by Zn. This is because many soils which have received several applications of sewage sludge might be approaching the maximum permissible concentration of 300 ug/g for Zn, but remains well below the maxima for these other metals.

Cases of Exposure to Pollution

The seveso chemical plant explosion

On 10th July, 1967, a massive release of 2, 3, 7- tetrachlorodibenzo-p- dioxin occurred from a chemical plant in Seveso near Milan in Northern Italy. The chemical plant was being utilized for manufacturing 2, 4, 5- trichlorophenol (2, 4, 5 - TCP). A security disc in a reaction vessel ruptured and a plume of chemicals containing 2, 4, 5 - TCP below 30 to 50 m above the factory. As it cooled, the material in it was deposited over a cone shaped area downwind from the factory about 2 kilometres long and 700 metres wide.

Dioxin is both extremely toxic and stable, and at sufficient dose levels, it is known to affect foetal development and to have porphyrinogenic effects (often manifesting as digestive system and skin disorders). It is not used commercially, but found as a contaminant when 2, 4, 5 - TCP is synthesized by the hydrolysis of tetrachlorobenzene at high temperatures, and it is used to make 2, 4, 5 - T and 2, 4 - D (herbicides); dioxin is often present in trace amounts in these compounds. Dioxin is also produced as a by-product of waste incineration and it continues to be of major concern as a general environmental pollutant.

A few months after the accident, 176 individuals, mostly children, were found to have the skin condition chloracne, 50 of whom came from the most contaminated area. Some neurological abnormalities including polyneuropathy were as well noted. There was too proof of liver enlargement initially, and afterward, reported cases of enhanced hepatobiliary (liver) cancer, raised levels of leukaemias and other haematologic neoplasms in men and enhanced myeloma and myeloid leukaemia (cone marrow cancers) in women. Lastly, many farm animals died and the site became a wasteland of dying plants and deserted homes.

The Bhopal Catastrophic Release of Methyl Isocyanate

The accident at Bhopal, India occurred on 3rd December, 1984 at the Union Carbide factory which had been producing the insecticide carbaryl for about eighteen years. Methyl isocyanate (MIC) was one of the main ingredients of carbaryl. MIC was produced from monomethylamine (MMA) and phosgene, phosgene being generated via reacting chlorine and carbon monoxide;   chloroform served as the solvent. On the night of the accident, several water almost certainly inadvertently got into a tank where Al tonnes of MIC were being stored, causing a runaway chemical reaction. The heat caused through the reaction led to a devastating explosion. About 100,000 people living in the vicinity were influenced through at least 2,000 deaths reported.

Toxic Oil Syndrome in Spain 

Between May and August 1981, in Madrid (Spain), there was an endemic of cooking oil poisoning that affected about 20000 persons with nearly 400 deaths. The illness began in each patient through a fever followed via severe (acute) respiratory symptoms and a variety of skin rashes similar to measles. Many of the patients expanded signs of cerebral oedema (swelling of the brain) and cardiological abnormalities.

The cause of the sickness was traced to adulterated cooking oil that was fraudulently sold to the public as pure olive oil. On analysis, the oil was discovered to contain up to 90percent rapeseed oil, fluctuating amounts of soya oil, castor-oil, olive oil and animal fats. The oil as well enclosed between one and 50 ppm aniline, 1500 and 2000 ppm of acetanilide and a number of other anilides. Such compounds might have been accountable for the symptoms generated through the toxic oil.

River Oil Contamination by the Toxic Oil

In the year 1968, in Japan and Taiwan, a number of families expanded chloracne, the skin condition which influenced the victims of Seveso. Chloracne is more severe than the kind of acne that happens in adolescents and it has a rather dissimilar distribution on the body. Epidemiological studies revealed that those affected had been exposed to a particular batch of rice oil that was contaminated with PCBs.

At the end, the individuals observed were considered to have met the diagnostic criteria for what has come to be recognized as Yusho disease. In addition to chloracne, the patients of Yusho illness had a number of systemic complaints counting loss of appetite, lassitude, and nausea and vomiting, weak point and loss of sensation, and hyper pigmentation of the face and nails, in some cases. A number of non-specific symptoms that persisted in the patients include feeling of fatigue, headache, abdominal pain, cough with sputum, numbness and pain, transforms in menstruation in females, sensory neuropathy, retarded expansion in children, abnormal teeth  growth and anaemia in several patients. The most striking observation was a marked enhance in serum triglyceride levels. To what extent the non precise symptoms are seen in the patients through Yusho disease were entirely due to PCBs is difficult to say because it is recognized that they were as well description to polychlorinated dibenzofurans(PCDFs) and polychlorinated quarterphenyls (PCQs) - compounds shaped when PCBs are heated. In animal models, PCDFs and PCQs are more toxic than PCBs.

Polybrominated Biphenyls (pbbs) in Cattle Feed

Polybrominated biphenyl is used mainly in plastics as a fire retardant. At Michigan  in  May  and  June of 1973, several 10 to 20  bags  of  PBB (labelled 'Firemaster') were sent in error, instead of magnesium oxide (a livestock food additive, "Nutrimaster"), to a grain elevator. Therefore PBB became incorporated into the feed and dispensed throughout the state to be fed to the unsuspecting cows.

Soon, reports of sick cows began to surface. Dairy products and meat from the influenced cows were contaminated. Over 1000 farmers were unfavorably concerned. Several of the adverse effects comprised acne, dry skin, hyper pigmentation and discolouration of the nails, headaches, nausea, depression and a number of non-specific symptoms. Serum levels of hepatic enzymes were higher in the Michigan farmers than their neighbouring controls.

Mercury Poisoning in Minamata and Niigata

The Minamata Bay disease (Kibyo or "mystery illness") was first noted at the end of 1953 when an unusual neurological disorder began to affect the villagers who lived on Minamata Bay, Japan.  Those who were affected presented with a mixture of signs relating to the peripheral and central nervous systems. The prognosis of the condition was poor; many patients became disabled and bedridden and about 40percent died.

The disorder was connected through the consumption of fish and shellfish caught from the bay. The fish and shellfish were contaminated through methyl mercury. The source of the mercury was effluent liberated into the bay from a factory that was manufacturing vinyl chloride using mercury depend catalyst prior to the time when there were regulations forbidding its utilize in Japan.

The Minamata Bay sickness concerned about 700 people and there was one more outbreak of methyl mercury poisoning in Japan (Niigata) in 1965 influencing about 500 individuals. This consequence from pollution of the Agano River through industrial effluent containing methyl mercury and following consumption of contaminated fish via the local population.

Methyl Mercury Poisoning in Iraq

Iraqi government imported a huge consignment of seed grain treated through an alkyl mercury fungicide and distributed them to the mostly illiterate rural population for sowing. The sacks were marked through cautioning labels 'for sowing and not for eating' in English and Spanish.

The seed had been treated by a red dye to distinguish it from edible grain, but the farmers originate that they could eliminate the dye via cleaning and equated this through the elimination of the poison. The grain was 1^st utilized to build bread in the year November, 1971 and the 1^st cases of poisoning appeared in December. Through the end of March 1972, there had been over 6,530 admissions to hospital and 459 of such had died. On the whole, this episode became a major outbreak in that the incidence of disease was estimated to be as high as 73 per 10,000.

Love canal chemical odours

Love Canal was a waste disposal site enclosing together municipal and chemical misuse disposal of over a 30 year period up to 1953. Homes were then built on the site during the year 1960s and leachates began to be detected in the late 1960s. The reporting of chemical odours in the basements of homes in the Love Canal district, USA, led to a toxicological investigation which made this area famous in the history of waste disposal and resulted in energetic regulatory movement in the USA. Dibenzofurans and dioxins were among the chemicals detected in the organic phase of the leachates. Animal studies specified possible dangers of immunotoxic, carcinogenic and teratogenic results. The episode resulted in significant fears of ill-health and much psychological stress. Limited summarize of residents identified low birth weights in the offspring of Love Canal residents.

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