29 Soil Pollution

J.S. Laura

Objectives:

• To study different Types of Soil Pollution.
• To study the Causes of soil pollution.
• To study the Effects of soil pollution.

Soil, a nature’s wonderful gift shaped the lives of all the living beings, directly or indirectly. Soil is the upper layer of earth that c is made of mineral and organic constituents. It is produced due to complex processes of modifications of the solid crust. Soil is a living medium in which plants grow and it provides habitat to various organisms. Soil supports cultivation of vegetation to fulfill human and animal food requirements. Linked to primary food chain of ecosystem polluted soil can result into virtual destruction of the lower life forms on chemical effects. Lower food chain pyramid level foreign organic chemicals become more telescoped for each consumers of the food chain. The contaminated soil affects the ecological balance of any system. Most plants are incapable to adaption when the chemistry of the soil changes so radically in a short period of time. Soil erosion becomes another problem when fungi and bacteria begin to decline that bind the soil together. The fertility of the soil slowly recedes, making the land incompatible for agriculture and other local vegetation to sustain. Soil pollution is caused by human activities, especially unhygienic habits, wrong agricultural practices and solid and liquid waste disposal.

Past land use, current activities on the site and nearness to pollution sources greatly impinge the soil properties. Substances such as pesticides, fertilizers and other amendments have knowingly added to soil by humans. Accidental spills and leakage of chemicals which are used for commercial or industrial purposes have also been an additive source of contamination.

Some pollutants are moved through the air and deposited as dust or by precipitation. Penetration of synthetic pesticides and insecticides deteriorate the soil quality. Atmospheric pollution is also responsible for soil pollution to a great extent. Entry of xenobiotic components and changes in the natural environment can lead to soil pollution (Fig. 30.1). These harmful chemicals settle on the soil due to the effect of gravity and pollute the soil. Hydrocarbons, polyaromatic carbons, petroleum, lead and other heavy metals are the most common type of pollutants. Few examples of soil pollutants with their hazardous effects are discussed below:

Agent Orange was one of the weapons used during the Vietnam War. It had adverse impact on human and environment which included skin cancer in soldiers, and even infertility. Also due to its impact as a defoliant, it was later banned. DDT (dichlorodiphenyltrichloroethane) and Gammaxene were first widely used as insecticides at the end of World War II but insects soon developed resistance to these chemicals and owing to their synthetic origin, these chemicals persisted in the environment for long. They entered the food chain and disrupted calcium metabolism in birds leading to thinning of eggshells. Many higher organisms became endangered due to accumulation of these chemicals in their bodies. Falcons, eagles and brown pelicans were a few of them. Though DDT has been banned in many countries but still countries like USA have been exporting this hazardous chemical to other developing countries. The global land deterioration rate has been increased perilously in recent years and prone to hike more in further decades.

Fig 30.1 Sources of soil pollution.  Image:https://media.treehugger.com/assets/images/2011/10/environmental-heavy-metal-contamination.jpg

30.2 Types of Soil Pollution

There are many different ways which may cause soil pollution:

• Landfill seepage
• Release of industrial waste into the soil
• Leakage of contaminated water into the soil
• Parting of underground storage tanks
• Excess use of pesticides, herbicides or fertilizer
• Discharge of Solid waste
• Heavy metals
• Pesticides
• Solvents
• Petroleum hydrocarbons

• (i) Rainfall, such as acid rain, rain falling onto a sanitary landfill, oil or chemical waste spilled onto the ground
• (ii) Human activities

• (iii) Physico- chemical alterations, which allow polluting substances to move within or between soil layers

Soil pollution may be caused by any substance that poses harm to living organisms. Such substances deteriorate soil quality and even disturb the natural composition of soil leading to soil erosion. The Sources and effects of contaminants are helpful in distinguishing various types of soil pollution. Based on this, three types of pollution have been identified – Agricultural, Industrial and Urban pollution.

1. Agricultural Sources

Agricultural practices devote to soil pollution profoundly. Although fertilizers enhanced crop yield but they greatly cause pollution and effects soil quality. The massive application of nitrogenous and phosphate fertilizers induced acidifying effect to the soil. There are various  processes which decide the fate of fertilizers in the soil are : plant and animal uptake, adsorption, exchange in the soil, leaching or infiltration of ground water, gaseous losses to the atmosphere and surface loss in the solid form by runoff and erosion.

Similarly, Pesticides spray (Fig 30.2) also cause soil pollution. Not only this, these harmful chemicals penetrate deep inside the soil and even may contaminate the ground water aquifers causing diseases life blue baby syndrome, etc.

Surface runoff due to rain and irrigation also contaminate the nearby water streams and aquatic life.

2. Industrial Sources

Improper waste disposal contaminates the soil due to addition of harmful and unwanted chemicals thus affecting plant and animal species.

These chemicals are also found to contaminate local water streams. About 90% of oil pollution is caused by industrial waste products.

Toxic fumes that arise from the regulated landfills contain chemicals that return back to earth as acid rain and damage the soil profile.

Fig 30.3 Industrial pollution Image: http://cf.Itkcdn.net/greenliving/images/std/139838

3. Urban Activities as a source of soil pollution

Human activities have direct as well as indirect impacts on soil quality.

Improper drainage and increase run-off contaminates the nearby land areas or streams.

Improper solid and liquid waste disposal deposits a number of chemicals and pollutants onto the soil which further seep into groundwater and contaminate the local water system.(Fig 30.4)

Due to excessive deposition of waste, there is decrease in bacterial population in the soil that deems the soil unfit.

Decomposition by bacteria generates methane gas contributing to global warming which is a greenhouse gas.

30.3 Causes of soil pollution

1. Industrial Activity: From the last century industrial activity has been the biggest grantor to the problem, principally since the extent of mining and manufacturing has expanded. Most of the industries are dependent on extracting minerals from the soil. The contaminated i.e. coal, iron ore or its by products are not disposed off in a manner that can be considered safe. As a result, the industrial waste dawdled in the soil surface for a long time and makes it inadmissible for use.
2. Agricultural Activities: Due to modern use of pesticides and fertilizers, the chemical utilization has been gone up staggeringly. These fertilizers are made up of chemicals that are not synthesized in nature and so cannot be broken down by it. Due to this the seepage occurs and gets mixed with the ground water and slowly dwindled the fertility of the soil. Also the composition of the soil gets damaged by the chemicals and erosion becomes easier by air and water. Plants also absorb these chemicals and when they disintegrate becomes part of land and hence causes soil pollution.

Besides human sources, trace metals can also be found in the parent material from which the soil is developed. However, the amount of these elements may be increased due to several human activities i.e. applications of chemicals, sewage sludge, farm slurries, etc.

Long-term and extensive use of land for agriculture with frequent application of agrichemicals is one of the major causes of trace metal, such as copper, nickel, zinc and cadmium, accumulation in soil.

Accumulation of Cu in agricultural soil is a consequence of the century – old practice of using copper  sulphate (Bordeaux mixture) and other copper containing fungicides to control vine downy mildew. Widespread distribution of Cd and its high mobility makes it a potential contaminant in a wide range of  natural  environments.   Phosphatic   fertilizers   are   one  of  the   most   ubiquitous  sources   of  Cd contamination in agricultural soils throughout the world. Soils may get contaminated due to addition of heavy metals in soil by application of increased doses of the fertilizers, pesticides or agricultural chemicals. A number of factors required to decide whether the said inputs will be toxic or not and to what extent.

These factors are:

Specific chemical and physical trace metal characteristics, Soil type, Land use, Geo-morphological characteristics within the soil type, and Exposure to emission sources.

The processes which affect the mobile transformation and toxicity of metals in soil are of great importance in the rhizosphere which is actually the zone where soil root development takes place. Due to this we need to understand the behavior of trace elements in the soil, how they are built up in the soil and their uptake by the plants. Metals are chemically very susceptible in the environment which may result in the mobility and bioavailability to living organisms. Trace metals should be in soluble state or bound to mobile particles in order to be translocated to the soil profile by water.

Complexes are formed due to the combination of metals and particles of organic matter present in the topsoil which can be translocated angularly along the profile depth. So many health problems like vomiting, abnormalities, nausea and gastrointestinal occurs due to long term deposition of heavy metals. For e.g. Arsenic from smelter emissions and pesticides when used in the soil remains for hundreds of years in it. Pesticides(Fig30.5) in agriculture clings chemicals in the environment for a long time which can lead to the soil poor quality and also affects the beneficial organisms.

Fig 30.5 Pesticide use pattern in India Image: https://image.slidesharecdn.com/soilpollution2-160715162454/95/soil-pollution-19-638.jpg?cb=1468599920

3.    Waste Disposal: Finally, a growing cause for concern is how to dispose of our waste. While industrial waste is sure to cause contamination, there is another way in which we are adding to the pollution. Every human produces a certain amount of personal waste products by way of urine and feces. Though most of it founds its fate in the sewers but a large amount is dumped on the landfills in the form of diapers. Even the sewage ends up stacking up into the landfills where the biological waste pollutes the soil and water.

4. Accidental Oil Spills: Oil spills can happen during storage and transportation of chemicals. This has been observed at most of the fuel stations. The chemicals present in the fuel deteriorate the quality of soil and make them unsuitable for cultivation. These chemicals can enter into the groundwater through soil and also make the water undrinkable.

5. Acid Rain: Sulphur dioxide (SO2) and oxides of nitrogen and ozone to some extent are the primary causes of acid rain. These pollutants originate from human activities such as combustion of burnable waste, fossil fuels in thermal power plants and automobiles. These constituents interact with reactants present in the atmosphere and result into acid deposition. When air pollutants combine with the rainwater, it forms acid rain. Along with the wet deposition there are also dry depositions of acids, which can be transformed into salts in the soil and cause the same environmental damage, as do the wet deposits. These acids change the structure of soil and make it acidic thus deeming it unfit for cultivation.

5. 6. Radioactive pollutants – Explosions and wastes from nuclear testing laboratories and industries generate radioactive dust which penetrates into the soil thus contaminating it. On the basis of toxicity radioactive waste is classified as high, intermediate, or low level toxic. A waste can remain radioactive from seconds to minutes, or even for millions of years depending on the radionuclide contained in it. The radionuclide distribution in the soil includes the source term and the release conditions, transport and dispersion mechanisms, and the properties of the ecosystem (Fig30.6). In the soil the nature of radionuclide retention and the affinity to certain soil constituents depends upon its chemical form and the reactivity.

The charges on the surface of the soil particles, and the chemical properties help to establish covalent bonds the interactions between radionuclide and the soil. Most of the sand and silt fraction of the soil mainly quarts and feldspar are derived from the parent rock. Due to the relatively low specific surface area, their role in contaminant interaction is the smallest, and the attachment occurs through reversible sorption. Minerals such as clay result from weathering processes including physical, chemical, and biological processes. With small particle size and large specific surface area clay are the carriers of permanent surface charge which make them important matrices for contaminant retention. Consequently, clay minerals, Fe, Al-oxides are abundant in amorphous form, and organic matter, containing functional groups, such as OH– and COOH– undergo a variety of interactions with contaminants.Uranium, Radium, Thorium are radio nucleoids , Potassium (K-40) and Carbon (C-14) isotopes are normally found in soil, rock, water and air.

Explosion of hydrogen weapons and cosmic radiations include neutron, proton reactions by which Nitrogen (N-15) produces C-14 which participates in Carbon metabolism of plants and then into animals and human beings.

Radio nuclides such as Strontium90, Iodine-129, Cesium-137 and isotopes of Iron which are highly toxic are present in radioactive waste. In bones and tissues of humans and animals Strontium is found its way to get deposited.

The harmful effect of radioactive material depends upon their half life for e.g. nuclear reactors produce waste containing Ruthenium-106, Iodine-131, Barium-140, Cesium-144 and Lanthanum-140 along with primary nuclides Sr-90 with a half life 28 years and Cs-137 with a half life 30 years hence more the half life more will be the toxic effect.

Sr-90 and Cs-137 deposited on the soil through rain water where they are tightly held with the soil particles by electrostatic forces. Once deposited on the soil all the radio nuclides emit gamma radiations.

7. Climate change: It is another important factor which likely to affect all the degradation threats, and hence soil properties. Soils on the edge of stability are likely to be effected by the changes in the seasons like to drier summers, low rainfall soil suffer irreversible shrinkage upon drying on the other hand in rainy season increase the risk of flooding and salinization in low-lying coastal soils, one major effect of flooding is reduction of total microbial biomass. Climate change will affect important soil functions while effecting soil properties.

30.4 Effects of Soil Pollution

Soil system is very dynamic, complex and important ecological factors. Every plant depends on it for their nutrient and water supply. Past land use practices and current human activities are responsible for soil pollution. To have maximum from land pesticides, fertilizers and other amendments to soils have been added by farmers. Accidental spills and leaks of chemicals used for commercial or industrial purposes are other sources of soil contamination. Another factor like acid rain increases the exchange between hydrogen ion and nutrient cations like potassium (K+), magnesium (Mg2+) and calcium (Ca2+) in the soil leading to acidification of soil. These cations are rapidly leached out in soil solution along  with sulphate from acid input which results in nutrient deficiency and altogether disturbing the nutrient cycle in the affected soils. The loss of soil fertility results into decrease in the growth of plants. The number of hydrogen and aluminum ions in solution increases and the available nutrients decrease due to acidity. This will disturb food chain and microorganism along with the whole ecosystem .If we take the example of oil then In assessing the environmental situation relating to oil contamination, it is important not only to determine the spread of pollution, but also the depth of its penetration (Rogozina, 2006; Pikovskiy, 1988). There have been cases of large-scale hydrocarbon accumulation in soils in the territories of oil-gas production enterprises, refineries, transport pipelines and accidental spills. Oil products are characterized by high migration capacity and enter into deep horizons of soil. Most oils have certain content of metals. Concentrated in asphaltenes and tars heavy metals can cause deleterious effect on the soil. Among metals like Pb, Cd, Hg, Zn and Cu can be very noxious to the ecosystem and human beings. Enzyme system function get disturb as majority of heavy metals have strong affinity for sulfur forming bonds with sulfur groups in enzyme. They can easily enter a food chain and reach toxic levels due to their high persistence. These may ultimately kill many species of fish, birds and mammals (Qiu, 2011).

1. Effect on Human health: Since we are dependent on the soil system for food and other reasons, soil health is of utmost importance. People can get soil contaminants from the animals raised for food from contaminated soil, and people by consuming animal products such as meat, eggs and milk may be exposed to these contaminants. Various diseases are also transfer to us by the crops and plants grown on polluted soil for e.g. salmonella being a food borne disease, one of the common route of this bacteria in vegetable is through soil .Plants retain those toxins and when they die, it decompose the toxic material back into the soil and in due course the soil will become unusable. Many bacterial infestations spread through contact with polluted soil like, Leptospirosis, Actinomycosis, Anthrax, Botulism etc. It is through contaminated soil that many helminthes like Ascaris lumbricoides, Necator americanus, Ancylostoma duodenale etc. enters human beings and some other examples like Tetanus bacteria and some Hanta virus are transmitted to human through infected rodent excreta which may be in soil. . Long term exposure to such soil can affect the genetic make-up of the body, causing congenital illnesses and chronic health problems that cannot be cured easily.

2. Effect on Growth of Plants: Elevated levels of soil contaminants can negatively affect plant vigor, animal health, microbial processes, and overall soil health and hence the ecological balance of any system gets affected. Most plants are unable to adapt when the chemistry of the soil changes so drastically in a short period of time. For example, legume plants are able to fix nitrogen in the soil through a symbiotic relationship with Rhizobium bacteria in their root nodules of crops (including beans, lentils, peas, and peanuts) and are often used to replenish nitrogen levels in depleted soils. However zinc contamination can disrupt the nitrogen fixation process as bacteria are sensitive to it and hence key nutrient nitrogen for plant growth may then no longer be available to the plant or to the rest of the system. Moreover fungi and bacteria found to help in the soil aggregation begin to decline, which leads to an additional problem of soil erosion. Soil loses its fertility gradually making it unfit for cultivation not able to support most types of vegetation just like deserts. Some contaminants may change plants’ metabolic processes and reduce yields or cause visible damage to crops like oil pollution .Oiled shoots of crops may wilt and die off stomatal blockage thereby inhibiting respiration  photosynthesis and transpiration. Germination, growth performance and yield of these crops are hindered by oil spillage (Aghalino, 2000). Oil and oil products are characterized by high migration ability and penetrate into deep horizons of soil. There have been cases of large-scale hydrocarbon accumulation in soils in the territories of oil-gas production enterprises, refineries, transport pipelines and accidental spills.

2. Decreased Soil Fertility: The toxic chemicals present in the soil can decrease soil fertility and therefore decreases land production. The addition of toxic chemicals which impact chemical properties of soil that regulates life processes is known as Chemical degradation process. These toxic chemicals also known to affect other processes of soil also like nutrient availability, nutrient uptake and nutrient element mobility. Contaminated soil is then used for horticulture purpose. Fruits and vegetables thus produced deficient in nutrients and may have poisonous substances that definitely going to cause serious health problems in people consuming them. Agricultural inputs or contaminants also affect many soil chemical properties functions, mainly pH. From the atmosphere or the use of fertilizers, agrochemicals, manures or waste materials on land the deposition and accumulation of metals and metalloids in topsoil takes places which, under acidifying conditions if released in toxic concentrations, can severely impair plant growth and food quality. Pesticides reaching the soil and causing degradation of some aspect of soil properties cannot be underestimated and such possibilities become high when pesticides are applied at high rates over many years and most of them remain persistent in soil varying degrees. Pesticides enter soils from spray drift during foliage treatment, wash off from treated foliage, release from granulates or from treated seeds in soils (PAN, 2010). Pesticides in soils have been identified to have major effect on soil microbes. Significant irreversible changes in soil microbial populations are being caused by Pesticides by affecting soil micro flora and micro fauna and harm soil fertility. Petroleum hydrocarbon also affects soil fertility, thus affecting the physico-chemical properties of the soil such as temperature, structure, nutrient status and pH. The oil obstruct proper soil aeration as oil film on the soil surface acts as a physical barrier between air and the soil ,it ‘sterilize’ the soil and prevent crop growth and yield for a long period of time (Onwurah, 1999). The diversity and activity of soil microbes play a vital role in recycling of plant nutrients, maintenance of soil structure and detoxification of noxious chemicals. Crude oil spillage is one of the factors that  affect soil micro flora activities in the soil. The fungi population abundance may decrease with time since, by the depletion in nitrogen due to biodegradation as it is being used up during biodegradation of petroleum compounds (Haris, 1962). This reduces the amount of nitrogen available to the flora hence their population would decline. In the oil contaminated soils hydrocarbon-oxidizing microorganisms are absent and biological self-purification system of soil gets hampered. This results into increase of accumulation of exchangeable bases in the oil polluted soils in comparison to Exchangeable cations (Ca2+, K+, Mg2+, Na+) which further affects the ionic stability of the soil . The absence of nitrogen, phosphorus and other biogenic elements and diffusion of atmospheric oxygen all that are necessary for microbial activity are blocked due to hard saturation of soils with heavy oil fractions–asphaltenes and tars leads to complete suppression of microbiological degradation in contaminated soil . To curtail this problem, liming, fertilization, enhanced ploughing and harrowing are recommended. Further the farmers are advised to carry out remediation processes on oil polluted soils and not to cultivate untill.

4. Changes in Soil Structure: Soil structure usually gets alter after death of many soil organisms (e.g. earthworms) in the soil apart from this; it could also force other predators to move to other places in search of food. Degradation of soil physical properties, particularly the decrease in porosity from compaction and the destabilization of structure from loss of SOM, affects a range of soil functions. Water storage and flood regulation functions are degraded with a loss in porosity. Poor aeration of the soil which results in root stress might also be a factor. If we take the e.g. of petroleum products which are very toxic to living organisms in soils which indirectly control the chemical and biochemical activities in soils for plant. As a result of crude oil pollution which is denser than water may reduce and restrict permeability, soil physical properties such as pore spaces might be clogged which reduces soil aeration, infiltration of water into the soil, increased bulk density, depletion of soil nutrients and inhibition of enzymatic and soil microbial activities of the soil which may affect plant growth.

Another factor i.e. climate change also have influence on soil structure (type, spatial arrangement and stability of soil aggregates) is a more complex process. The most important direct impact especially during heavy rains, thunderstorms and even ‘rain bombs’ is the aggregate-destructing role of raindrops, surface runoff and filtrating water, the increasing hazard, frequency and intensity of whichare characteristic features of climate change. The indirect influences are caused by changes in the vegetation pattern and land use practices. The activities of microorganisms, especially beneficial microorganisms are so important to nutrient release to crops/vegetation grown on a particular soil hence the need for avoidance of pollution and contamination of soil is vital.

5. Effect of chemical on soil: Salinization is a process of chemical soil degradation, which greatly reduces soil productivity. Kavvadias (2014) defines salinization as the accumulation of water-soluble salts (including sodium, potassium, magnesium and calcium, sulphate, carbonate and bicarbonate) on or near the surface of the soil. Salinization involves the accumulation of different salts, but the increased content of exchangeable sodium (Na+) in a soil resulting to a completely unproductive soil is referred to as sodification. Salinization is a common problem in arid and semi-arid regions where the rate of water loss from the soil through evaporation and transpiration is higher than the amount of rainfall received. Acidification is the change in the chemical composition of the soil, which may trigger the circulation of toxic metals Acidification impacts negatively on the soil ecosystem thereby causing damage to plants. It also results in the alteration of soil water chemistry. Soil acidification results from pH decline or from acid deposition. The phenomenon of acid deposition arises from the deposition of emissions from vehicles such as SO2, power stations, other industrial processes and natural bio-geochemical cycles onto the soil surface mainly via rainfall and dry deposition Acidified soils hinder the availability of some mineral elements in the soil either by reacting to produce forms that become bound to the soil particles or form complexes or the elements are leached further down the soil structure.

Summary

Under the influences of man and land use soil is always responding to changes in environmental factors. Some changes in the soil will be of short interval and reversible, others will be a everlasting feature of soil. Due to the presence of soil pollutants soil productivity may be completely lost. Soil pollutants have an undesirable effect on the physical chemical and biological properties of the soil and reduce its efficiency. We should avoid soil that has been contaminated to grow food, because the chemicals will enter into the food chain and harm people. To avoid the contamination proper methods should be adopted for waste disposal. Industrial wastes should be treated physically, chemically and biologically until they are less hazardous to be disposed off. Soil acidity and alkalinity is being caused by acidic and alkaline wastes which should be first neutralized; Biodegradable waste should be allowed to degrade under controlled conditions before being disposed off. Time to use bio-fertilizers and manures which will reduce chemical fertilizer and pesticide use. Biological methods of pest control can also reduce the use of pesticides and thereby minimize soil pollution. It is important to note that degradation processes do not necessarily have irreversible effects on soil functions. Land management practices can be revised to reduce or even remove the degradation threat to improve soil properties and functions.

Reference:

• Haris, J. O. (1962). Microbiological Studies of Oil and Gas Pipelines. Technology Bulletin, 135-136
• Jent, B. (2007). Causes of Oil Spills. http://www.articlesbase.com/business-articles/causes-of-oil-spills-172578.html (accessed in December 2014)
• Kavvadias, V. (2014). Soil degradation. Soil science institute of AthensNational Agricultural Research Foundation. Accessed [20/11/2014]. Available at: www.prosodol.gr/sites/ L
• Nelson-Smith, A. (1973). Oil pollution and marine ecolo-gy. Plenum Press, New York. http://dx.doi.org/10.1007/978-1-4757-6063-7
• Onwurah, I. N. E. (1999). Restoring the Crop Sustaining Potential of Crude Oil Polluted Soil by Means of Azotobacter Inoculation. Plant Production Research Journal, 4, 6-16.
• Pesticides Action Network (PAN) (2010). Environmental Effect of Pesticides. An impression of recent scientific literature. Europe.
• Pikovskiy Yu.I.( 1988). Transformation of technogenic oils treams in soil ecosystems. // Remediation of oil contaminated soil ecosystems. M.: Nauka publishing, P. 7-22 (in Russian).
• Qiu H. ( 2011). Migration mechanism of organic pollutants in national water-body sediments // J. Geography and Geology.V.1. P. 239-246. [18]. USEPA. Test
• Rogozina E. A. ( 2006). Actual issues of the problems of oil contaminated soil cleaning // Oil and Gas Geology. Theory and Practice. №1. P. 1-11 (in Russian).
• Rowell, M.J. (1977). The effect of crude oil spills on soils: A review of literature. In: Toogood, J.A. Ed., The Recla-mation of Agricultural Soils after Oil Spills. Part 1: Re-search, Alberta Institute of Pedology, Canada, 1-33.
• Sparks DL. (2003). Environmental Soil Chemistry. 2nd edition. San Diego: Academic Press; 2003. 352p.
• Sposito G.( 2008). The Chemistry of Soils. 2nd edition. New York: Oxford University Press; 2008. 330 p.
• Wang, Y.P., Shi, J.Y., Wang, H., Li, Q., Chen, X.C. and Chen, Y.X. (2007). The influence of soil heavy metals pollution on soil microbial biomass, enzyme activity, and community composition near copper smelters. Ecotox-icology and Environmental Safety, 67, 75-81. http://dx.doi.org/10.1016/j.ecoenv.2006.03.007
• Yong RN, Mohamed AMO, Warkentin BP.( 1992 ). Principles of Contaminant Transport in Soils. Amsterdam, New York: Elsevier Science; 327 p.