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1.0: INTRODUCTION AND LITERATURE
Environmental pollution as a result of man’s increasing activities has increased considerably in the past century due mainly to significant increases in economic activities and industrialization. Several studies have shown that heavy metals such as lead, cadmium, nickel, manganese and chromium amongst others are responsible for certain diseases (Wegwu et al., 2002). Metals found in waste dumpsite exist in various forms either as the pure metal or alloyed with various other metals. Heavy metals impairing the quality of the environment come from various sources that can be categorized into urban-industrial aerosols, liquid and solid wastes from animal and man, mining and industry and agricultural chemicals (Wegwu et al., 2002). The disposals of materials contaminated with heavy metals such as in waste dumpsite are of concern and pose dangers to people in contact with the contaminated soils and plants. Research made it known that toxic metals arising from human activities are accumulated in soil. The quality and quantity of solid waste generated in Nigeria vary widely from day to day, season of the year and nature of the waste disposed due to improper waste management (Wegwu et al., 2002). Aboyade, (2004) reported that concentrations of heavy metals in soil around waste dump site are influenced by types of wastes, topography, run-off and level of scavenging. The wastes at dumpsite are usually left over a long time to decompose naturally eaten by animals, picked by scavengers or washed away by floods when it rains into the larger creek and rivers thus affecting the surface water quality (Obute et al., 2001).
Recently, many studies have shown that heavy metals and metalloids with an atomic density >6 g/cm3– from these wastes can accumulate and persist in soils at environmentally hazardous levels (Wegwu et al., 2002). Most abandoned waste dumpsites in many towns in Nigeria attract people as fertile ground for cultivating varieties of crops. The cultivated plants take up the metals either as mobile ions presents in the soil solution through the root or through foliar adsorption. The uptake of the metals by crops results in the bioaccumulation of these elements in plant tissues. This is known to be influenced by the metal species, plant species and plant part (Obute et al., 2001). Indeed, it has been reported that plants grown on soils possessing enhanced metal concentration due to pollution have increased heavy metal ion content. If the consumption of these metals through plant source is not carefully regulated, it may lead to accumulation in man with attendant health hazards.
1.2: Justification of study
Urban areas known for high level of industrial activities generate more pollutants and therefore subject to the menace of resultant indiscriminate disposal of both domestic and industrial wastes. A typical example of such urban centres is Port Harcourt city located in the heart of the oil-rich Niger Delta of Nigeria. There are reports that its suburbs are loaded with toxic heavy metals and certain trace elements resulting from poor waste management programme (Wegwu et al., 2002).
The presence of toxic heavy metals in the environment continues to generate a lot of concern to environmental scientists, government agencies and health practitioners because of health implications of their presence (Awofolu, 2005). Heavy metals have been referred to as common pollutants are widely distributed in the environment with sources mainly from soils and weathering of rocks (Merian, 1991; and O’ Neil, 1993). However, levels of these metals in the environment have increased tremendously as a result of human inputs and activities (Awofolu, 2005). According to Oskarson et al. (1992), there exist transfer of heavy metals from contaminated soil to plants and from plants to animals with the subsequent transfer through the food chain up to man.
It is not uncommon to find ruminants feeding on grasses and birds feed on insects and earthworms on the dumpsite soils. High concentrations of metals in the environment may lead to accumulation, becoming toxic to plants and animals with possible danger to human health.
Solid waste disposal tends to pollute underground water at the vicinity of dumpsites which has been a serious problem for the entire world. It threatens the health and well-being of the residents, plants, and animals. All water pollution is dangerous to the health of living organisms; it has been reported that the quality of the underground water close to dumpsites is compromised resulting in serious health problems to residents. In some areas, the population has only one source of water and if this water is polluted, the population has no choice but to use it (Ince and Howard, 1999). The effect of toxic substances and a wide range of other adverse effects can occur when waste products are introduced into the water body leading to changes in physical, chemical and biological parameters such as infectious agents, temperature, turbidity, color, pH, salinity and oxygen concentrations. Changes in any of these parameters have direct environmental effects and can also produce impact by modifying other parameter.
The role of some heavy metals (Cd and Pb) is very critical in determining the quality of our atmosphere because air, soil and water are directly interacting with each other. Growing heavy metals pollution especially in air has led to increased respiratory diseases, infant mortality and also affects the functioning of the blood, liver, kidney and brain. The measurement of Pb, Cd, Cu, Zn and Ni accumulation in soil and plant appears to be a useful tool for evaluating the potential heavy metal hazards of the environment (Mudassir et al., 2005).
In Nigeria at present, little data is available on the extent of soil-vertebrates-human pollution. Clearly, there is a gap in knowledge related to dumpsite soil-water-animal-human pollution especially in Nigeria and empirical data are needed as the basis for wider modeling assessment. Lagos as an urban settlement with high industrial presence generates tremendous amount of waste which can be toxic to the environment. Most of the waste dumped in the landfills in Lagos is not treated at the point of generation before disposal and this exposes the environment especially the soil, underground water and soil organisms to direct contact with toxic substances.
Heavy metal content of soils is a critical measurement for assessing the risks of refuse dumpsites. Since these contaminants affect the environmental qualities in and around such open dumpsites, monitoring of soil qualities especially heavy metal content in dumpsite becomes necessary which can facilitate to recommend suitable remedial measures.
1.3: AIM AND OBJECTIVES OF THE STUDY
The aim of the study is to assess heavy metal concentration in soil and selected subterranean animals in dumpsites in Lagos State.
1.4: LITERATURE REVIEW
1.4.1: Waste Management (Landfills) in Lagos
The post independent periods in Nigeria has witnessed gradual growth of the nation’s economy through industrialization made possible from the benefits accruing to the country from mineral resource exploration and exploitation. As a result, old and new cities that are now state capitals in the country, expanded in size and population. These changes, particularly the demographic expansion of Nigerian cities, the urban centers, including the commercial and industrial activities have brought about phenomenal increase in the volume and diversity of solid waste generated daily in the country.
Lagos is Nigeria’s largest commercial center with large ports and industries. Industries include food processing, manufacturing of metal products, textiles, beverages, chemicals, pharmaceuticals, soaps and furniture. Due to high levels of industrialization and urbanization, the level of solid waste in Lagos has become offensive. These pollutants contaminate soil, leading to soil acidification, which causes nutrients to leach from the soil and subsequently damage trees and vegetation, (Adelekan, 2011).
Adelekan, (2011) reported the per capita solid waste generated per person in Lagos to be 0.3kg while Ogundira (2008) reported that every human population in Lagos generate over 0.45kg per person and the number is expected to increase. Heaps of refuse and garbage are a common sight in the state capitals and urban areas of the federation. The solid waste problem has today become number one serious environmental problem facing the country with consequent effect on the pollution of water, air and land, not to mention its hazards to health and other resources of social and economic importance.
Waste management operations in Lagos involve land filling which is the simplest and cheapest mode of municipal waste disposal followed by heating or burning of municipal solid waste, (LAWMA, 2004). This has led to the gradual depletion of the ozone layer, causing global warming (Amusan et al., 2005) and also affects the quality of local air. Leachate formed as a result of percolation of rainfall and moisture through waste in landfills which contains various contaminants and toxic substances especially heavy metals which can migrate, infiltrate and descend in the soil profile to contaminate the adjacent surface water and groundwater (Ogundira, 2008). Landfills are considered one of the major threats to groundwater (Ogundira, 2008).
1.4.2: Effects of Heavy Metals
Due to the toxicity and non-biodegradability of heavy metals, their introduction into the environment poses serious threat to life (Adjia, 2008) and unlike other kinds of pollution (atmosphere and water), the soil environment has a much lower ability to recover (Anetor et al., 2008). Furthermore, over 99% of environmental pollutants is bound to soil and sediment particles (Amusan et al., 2005), which interacts with clay minerals, humid substances, microorganisms, inorganic and organic ligands (Amusan et al., 2005).
The effects of heavy metals depend on their bio-availability and they have been extensively studied for their consequences on human health. Exposure to lead has been reported to cause mental retardation in children, kidney failure, anaemia and possibly may lead to lung cancer (Ogundira, 2008).
Cadmium is a priority environmental contaminant because it induces kidney damage and causes cancer (Wegwu et al., 2002). Copper, zinc, nickel and chromium have been studied to have adverse effects such as damage to the nervous membrane, kidney and liver failure, cancer and fatigue, (Wegwu et al., 2002). Mercury emission from municipal solid waste incinerators was found to contribute 20% of the overall background mercury concentration at locations surrounding the incinerator (Anetor et al., 2008). Incineration of these wastes also leads to toxic releases of dioxins and heavy metals into the surrounding soils and water bodies through rainfall, landslides and runoffs. This has raised concerns for determining the level of risk of residents living around dumpsites.
The toxicity of heavy metals are influenced by some factors which includes type and form of the metal, presence of other metals or toxins, environmental factors and condition of the target organisms of interest including its state of health and stages of life cycles. In general, the toxicity of heavy metals is mainly attributed to the capability of the metal ions to form stable complexes with the active site of proteins (Wegwu et al., 2002). Living organisms responds differently to chemical compounds including heavy metals. Differential responses are explicable in terms of differences in rate of metabolism and metabolic characteristics are species specific and even within a species, variants occur. Furthermore, factors like age, sex, size and life cycle stage can bring about intra specific differences in responses to toxicants in general (Onyelola, 2008). Heavy metal toxicity have been found to lead to damaged or reduced mental and central nervous functioning, damage of vital internal organs and lowering of energy levels in the body.
1.4.3: Effects of Landfills
According to Biwas et al., (2010) many cities in developing countries face serious environmental degradation and health risks due to the weakly developed municipal solid waste management system. Several studies have been conducted in order to examine the health and environmental effects arising from waste dumps. Such studies showed that a link exists between the two (Tahar, 2011). The conclusion from this and other studies has led to an increasing interest of researchers in the study relating to environmental pollution as well as its effects on plants and animals. Few of these studies examined the environmental and health implications of solid waste disposal to people living in close proximity of wastes dumpsites (Tahar, 2011).
The ever increasing consumption of resources results in huge amounts of solid wastes from industrial and domestic activities, which pose significant threats to human health (Wegwu et al., 2002). However, the ills of inappropriately disposed municipal solid wastes are quite numerous to be mentioned. Health deterioration, accidents, flood occurrences, and environ- mental pressures are just a few of the negative effects. In many developing countries, solid waste disposal sites are found on the outskirts of urban areas. These areas become children’s sources of contamination due to the incubation and proliferation of flies, mosquitoes, and rodents. They, in turn, are disease transmitters that affect population’s health, which has its organic defenses in a formative and creative state. The said situation produces gastrointestinal, dermatological, respiratory, genetic, and several other kind of infectious diseases (Tahar, 2011). Open dumpsites in developing urban cities involve in- discriminate disposal of waste. They are uncontrolled and therefore pose major health threats which affect the landscape of urban cities (Obute et al., 2001). LAWMA, (2004) stated that wastes that are not managed properly, especially solid waste from households and the community, are a serious health hazard and lead to the spread of infectious diseases. The report further stated that unattended wastes lying around attract flies, rats, and other creatures that, in turn, spread diseases.
Normally, it is the wet waste that decomposes and releases a bad odor. The bad odor affects the people settled next to the dumpsite, which shows that the dumpsites have serious effects to people settled around or next to them. The group at risk from this un- scientific disposal of solid waste includes-the population in areas where there is no proper waste disposal method, especially the pre-school children, waste workers and workers in facilities producing toxic and infectious materials. Other high-risk group includes population living close to the waste dump (Ebong et al., 2008). In particular, organic domestic waste poses a serious threat, since they ferment creating conditions favorable to the survival and growth of microbial pathogens. Direct handling of solid waste can result in various types of infectious and chronic diseases with the waste workers and rag pickers being the most vulnerable (Esakku et al., 2003). Studies conducted by Tahar et al., (2011) show that exposure to hazardous waste in dumpsites can affect human health, children being the most vulnerable to these pollutants. Direct exposure can lead to diseases through chemical exposure as the release of chemical waste into the environment leads to chemical poisoning. (Esakku et al., 2003) in his studies to establish a connection between health and hazardous waste showed that waste from agriculture and industries can also cause serious health risks. Other than this, co-disposal of industrial waste with municipal waste can expose people to chemical and radioactive hazards.
Health care waste and other medical waste disposed in dumpsites, mixed with domestic waste, in- creasing the risk of infection with Hepatitis B and HIV, and other related diseases. Open dumpsites are a major problem to the environment especially to the air that we inhale. Dumpsites emit obnoxious odors and smoke that cause illness to people living in, around, or closer to them (Ikem et al., 2002). According to Alelekan (2011) pollution, a major environmental effect of dumpsites, is not directly transferred from land to people, except in the case of dusts and direct contact with toxic materials. Pollutants deposited on land usually enter the human body through the medium of contaminated crops, animals, food products, or water. Also, the dumpsite has smelly and unsightly conditions. These conditions are worse in the summer because of extreme temperatures, which speed up the rate of bacterial action on biodegradable organic material. Disposal sites can also create health hazards for the neighborhood (Ikem et al., 2002). Magaji, (2012) highlighted that in a number of health surveys a wide range of health problems, including respiratory systems, irritation of the skin, eyes and nose, gastrointestinal problems, psychological disorders, and allergies, have been discovered. In addition, dumpsites closer to residential areas are always feeding places for dogs and cats. These pets, together with rodents, carry diseases with them to nearby homesteads.
1.4.4: Bio-indicators of Pollution
Bio-indicators have been used to determine the condition of the soil and bio-availability of contaminants. They effectively indicate the conditions of their environments due to their response to environmental stress and give a quantitative determination of the levels of pollutants. It has been suggested that subterranean animals like earthworms are excellent bio-indicators of the relative health of the soil ecosystem (Magaji, 2012). They are a major component of the soil fauna, easily identified, relatively immobile and widely distributed. Subterranean animals are in full contact with the soil in which they live in, and swallow large amounts of earth. In addition, they do not add anything to the soil that was not already there. As a consequence, the effect on the level of contaminant is minimal or of negligible effect. The soil swallowed passes through the digestive tract and contaminants are adsorbed in the tissue of the earthworm, thereby accumulating these contaminants in its tissue.
The use of subterranean animals like earthworm as a bio- indicator for heavy metal pollution has been documented by many authors, (Adjia et al., 2008) in a study of heavy metals in dumpsites in Abeokuta, Nigeria, showed that levels of metal concentrated in earthworms to soil is less than one with the exception of cadmium and chromium. They explained that the observation may be due to chemical changes which occur in the alimentary tract of the earthworms, rendering various metals available to plants. Data on earthworms collected near roadsides show an accumulation of heavy metal above the amount found in earthworms from non-polluted sites (Aboyale, 2004). Amusan et al. (2005) reported a considerable amount of cadmium, nickel, lead and zinc in the tissues of earthworms from soil adjacent to roads. Amounts decreased with increasing distance from the roadside. The correlation between residues in earthworms and soil decreased with decreasing atomic weight (Pb>Cd>Zn>Ni). Higher concentrations were found in the soil and earthworms in sites closer to the smelting complex (Biwas et al., 2010). Earthworm population count has been said to decrease with exposure to higher heavy metal concentrations (Tahar, 2011).
Ikem et al. (2002) also studied the effect of heavy metals on survival and respiration rate of tubificid worms. Earthworms that tolerate high environmental concentrations of toxic heavy metals, do not absorb the metal, accumulate it in a non-toxic form or excrete it efficiently. The influence of heavy metal on earthworm is complex and needs further study, (Miroslav, 2005). Lagos State health and environmental officials acknowledge that most of the garbage and sewage collected by private operators, as well as the effluents from industries ends up in the lagoons and creeks. Much of the rest is burnt either in numerous illegal open dumps that dot the city or in the three official dumps operated by the government. Most of the land biomass is represented by terrestrial invertebrate of which subterranean animals including earthworm has the greater proportion (>80%). They play an important role in structuring and increasing the nutrient content of the soil (Ndukwu et al., 2008). Therefore, they can be suitable bio-indicators of chemical contamination of the soil in terrestrial ecosystems providing an early warning of deterioration in soil quality.
1.4.5: Sources of Heavy Metals in Contaminated Soils
Heavy metals occur naturally in the soil environment from the pedogenetic processes of weathering of parent materials at levels that are regarded as trace (<1000 mg kg−1) and rarely toxic (Ndukwu et al., 2008). Due to the disturbance and acceleration of nature’s slowly occurring geochemical cycle of metals by man, most soils of rural and urban environments may accumulate one or more of the heavy metals above defined background values high enough to cause risks to human health, plants, animals, ecosystems, or other media (Ndukwu et al., 2008).
The heavy metals essentially become contaminants in the soil environments because (i) their rates of generation via man-made cycles are more rapid relative to natural ones, (ii) they become transferred from mines to random environmental locations where higher potentials of direct exposure occur, (iii) the concentrations of the metals in discarded products are relatively high compared to those in the receiving environment, and (iv) the chemical form (species) in which a metal is found in the receiving environmental system may render it more bioavailable (Miroslav, 2005). It is projected that the anthropogenic emission into the atmosphere, for several heavy metals, is one-to-three orders of magnitude higher than natural fluxes (Miroslav, 2005).
Heavy metals in the soil from anthropogenic sources tend to be more mobile, hence bioavailable than pedogenic, or lithogenic ones (Biwas et al., 2010). Metal-bearing solids at contaminated sites can originate from a wide variety of anthropogenic sources in the form of metal mine tailings, disposal of high metal wastes in improperly protected landfills, leaded gasoline and lead-based paints, land application of fertilizer, animal manures, biosolids (sewage sludge), compost, pesticides, coal combustion residues, petrochemicals, and atmospheric deposition (Biwas et al., 2010) are discussed hereunder.
Historically, agriculture was the first major human influence on the soil (Biwas et al., 2010). To grow and complete the lifecycle, plants must acquire not only macronutrients (N, P, K, S, Ca, and Mg), but also essential micronutrients. Some soils are deficient in the heavy metals (such as Co, Cu, Fe, Mn, Mo, Ni, and Zn) that are essential for healthy plant growth (Amusan et al., 2005) and crops may be supplied with these as an addition to the soil or as a foliar spray. Cereal crops grown on Cu-deficient soils are occasionally treated with Cu as an addition to the soil, and Mn may similarly be supplied to cereal and root crops.
Large quantities of fertilizers are regularly added to soils in intensive farming systems to provide adequate N, P, and K for crop growth. The compounds used to supply these elements contain trace amounts of heavy metals (e.g., Cd and Pb) as impurities, which, after continued fertilizer, application may significantly increase their content in the soil. Metals, such as Cd and Pb, have no known physiological activity. Application of certain phosphatic fertilizers inadvertently adds Cd and other potentially toxic elements to the soil, including F, Hg, and Pb (Amusan et al., 2005).
Several common pesticides used fairly extensively in agriculture and horticulture in the past contained substantial concentrations of metals. For instance in the recent past, about 10% of the chemicals have approved for use as insecticides and fungicides in UK were based on compounds which contain Cu, Hg, Mn, Pb, or Zn. Examples of such pesticides are copper-containing fungicidal sprays such as Bordeaux mixture (copper sulphate) and copper oxychloride (Biwas et al., 2010).
Lead arsenate was used in fruit orchards for many years to control some parasitic insects. Arsenic-containing compounds were also used extensively to control cattle ticks and to control pests in banana in New Zealand and Australia, timbers have been preserved with formulations of Cu, Cr, and As (CCA), and there are now many derelict sites where soil concentrations of these elements greatly exceed background concentrations. Such contamination has the potential to cause problems, particularly if sites are redeveloped for other agricultural or nonagricultural purposes. Compared with fertilizers, the use of such materials has been more localized, being restricted to particular sites or crops (Biwas et al., 2010).
Biosolids and Manures
The application of numerous biosolids (e.g., livestock manures, composts, and municipal sewage sludge) to land inadvertently leads to the accumulation of heavy metals such as As, Cd, Cr, Cu, Pb, Hg, Ni, Se, Mo, Zn, Tl, Sb, and so forth, in the soil (Adjia et al., 2008). Certain animal wastes such as poultry, cattle, and pig manures produced in agriculture are commonly applied to crops and pastures either as solids or slurries (Magaji, 2012). Although most manures are seen as valuable fertilizers, in the pig and poultry industry, the Cu and Zn added to diets as growth promoters and As contained in poultry health products may also have the potential to cause metal contamination of the soil (Adjia et al., 2008).The manures produced from animals on such diets contain high concentrations of As, Cu, and Zn and, if repeatedly applied to restricted areas of land, can cause considerable buildup of these metals in the soil in the long run.
Land application of biosolids materials is a common practice in many countries that allow the reuse of biosolids produced by urban populations (Magaji, 2012). The term sewage sludge is used in many references because of its wide recognition and its regulatory definition. However, the term biosolids is becoming more common as a replacement for sewage sludge because it is thought to reflect more accurately the beneficial characteristics inherent to sewage sludge. It is estimated that in the United States, more than half of approximately 5.6 million dry tonnes of sewage sludge used or disposed of annually is land applied, and agricultural utilization of biosolids occurs in every region of the country. In the European community, over 30% of the sewage sludge is used as fertilizer in agriculture (Skye, 2006). In Australia over 175 000 tonnes of dry biosolids are produced each year by the major metropolitan authorities, and currently most biosolids applied to agricultural land are used in arable cropping situations where they can be incorporated into the soil (Helmenstine, 2014).
There is also considerable interest in the potential for composting biosolids with other organic materials such as sawdust, straw, or garden waste. If this trend continues, there will be implications for metal contamination of soils. The potential of biosolids for contaminating soils with heavy metals has caused great concern about their application in agricultural practices (Skye, 2006). Heavy metals most commonly found in biosolids are Pb, Ni, Cd, Cr, Cu, and Zn, and the metal concentrations are governed by the nature and the intensity of the industrial activity, as well as the type of process employed during the biosolids treatment (Helmenstine, 2014). Under certain conditions, metals added to soils in applications of biosolids can be leached downwards through the soil profile and can have the potential to contaminate groundwater. Recent studies on some New Zealand soils treated with biosolids have shown increased concentrations of Cd, Ni, and Zn in drainage leachates (Helmenstine, 2014).
The application of municipal and industrial wastewater and related effluents to land dates back 400 years and now is a common practice in many parts of the world (Lenntech, 2014). Worldwide, it is estimated that 20 million hectares of arable land are irrigated with waste water. In several Asian and African cities, studies suggest that agriculture based on wastewater irrigation accounts for 50 percent of the vegetable supply to urban areas (Lenntech, 2014). Farmers generally are not bothered about environmental benefits or hazards and are primarily interested in maximizing their yields and profits. Although the metal concentrations in wastewater effluents are usually relatively low, long-term irrigation of land with such can eventually result in heavy metal accumulation in the soil.
Metal Mining and Milling Processes and Industrial Wastes
Mining and milling of metal ores coupled with industries have bequeathed many countries, the legacy of wide distribution of metal contaminants in soil. During mining, tailings (heavier and larger particles settled at the bottom of the flotation cell during mining) are directly discharged into natural depressions, including onsite wetlands resulting in elevated concentrations (Zhu et al., 2008). Extensive Pb and zinc Zn ore mining and smelting have resulted in contamination of soil that poses risk to human and ecological health. Many reclamation methods used for these sites are lengthy and expensive and may not restore soil productivity. Soil heavy metal environmental risk to humans is related to bioavailability. Assimilation pathways include the ingestion of plant material grown in (food chain), or the direct ingestion (oral bioavailability) of, contaminated soil (Zhu et al., 2008).
Other materials are generated by a variety of industries such as textile, tanning, petrochemicals from accidental oil spills or utilization of petroleum-based products, pesticides, and pharmaceutical facilities and are highly variable in composition. Although some are disposed of on land, few have benefits to agriculture or forestry. In addition, many are potentially hazardous because of their contents of heavy metals (Cr, Pb, and Zn) or toxic organic compounds and are seldom, if ever, applied to land. Others are very low in plant nutrients or have no soil conditioning properties (Zhu et al., 2008).
Airborne sources of metals include stack or duct emissions of air, gas, or vapor streams, and fugitive emissions such as dust from storage areas or waste piles. Metals from airborne sources are generally released as particulates contained in the gas stream. Some metals such as As, Cd, and Pb can also volatilize during high-temperature processing. These metals will convert to oxides and condense as fine particulates unless a reducing atmosphere is maintained (Tahar et al., 2011).
Stack emissions can be distributed over a wide area by natural air currents until dry and/or wet precipitation mechanisms remove them from the gas stream. Fugitive emissions are often distributed over a much smaller area because emissions are made near the ground. In general, contaminant concentrations are lower in fugitive emissions compared to stack emissions. The type and concentration of metals emitted from both types of sources will depend on site-specific conditions.
All solid particles in smoke from fires and in other emissions from factory chimneys are eventually deposited on land or sea; most forms of fossil fuels contain some heavy metals and this is, therefore, a form of contamination which has been continuing on a large scale since the industrial revolution began. For example, very high concentration of Cd, Pb, and Zn has been found in plants and soils adjacent to smelting works. Another major source of soil contamination is the aerial emission of Pb from the combustion of petrol containing tetraethyl lead; this contributes substantially to the content of Pb in soils in urban areas and in those adjacent to major roads.
1.4.6: The relationships between subterranean animals and heavy metal concentrations
In a study carried out by Awokunmi et al. (2010) earthworms (Lumbricus rebellus and Dendrodrilus rubidus) were sampled from uncontaminated and known metal-contaminated sites. Significant positive correlations were found between the earthworms and ‘total’ (conc. nitric acid-extractable) soil cadmium, copper, lead and zinc concentrations. The relationships were linear, and the accumulation patterns for both species were similar as far as a single metal was concerned, even though there was a species difference in the mean metal concentrations. Earthworm cadmium concentration exceeded that of the soil, home to the worms; in contrast, the earthworm lead levels were lower than the soil lead concentration in all but one (acidic, low soil calcium) site. Soil pH coupled with a cation-exchange capacity and soil calcium had a major influence on lead accumulation and cadmium accumulation may be suppressed in extremely organic soils (Awokunmi et al., 2010).
Heavy metal concentrations in organic rich soils close to the emission sources (1, 2, and 4 km) were high enough to have harmful effects on earthworms and their environments. In general, diversity, total numbers, and biomass of earthworms increased with increasing distance from the emission sources. Positive correlations between metal concentrations in the earthworms and those in the soils were observed (Oyelola, 2008 and Awokunmi et al., 2010). Generally earthworms were observed to increase the mobility and availability of metals and metalloids in soils (Awokunmi et al., 2010). This resulted in greater concentrations of metals leaching out of the soil into ground water or greater uptake into plants (Wegwu et al., 2002) and soil animals (Aboyade, 2004). Also, earthworms reduced the efficiency of soil remediation by mobilising recalcitrant metals (Miroslav, 2005). The mechanisms for earthworms increasing metal mobility and availability are unclear, but may involve changes in microbial populations, pH, dissolved organic carbon or metal speciation (Awokunmi et al., 2010).