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Water Pollution

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For at least two millennia natural water quality has deteriorated progressively and reached contamination levels where water uses are severely limited or the water can be harmful to humans. This deterioration is related to the socio-economic development within a river basin, but long-range atmospheric transport of contaminants has now changed this picture: even remote areas can be indirectly polluted (Meybeck and Helmer 1989).

Medieval reports and complaints about inadequate excreta disposal, foul and stinking water courses within overcrowded cities and other similar problems were an early manifestation of urban water pollution. The first time that a clear causal linkage between bad water quality and human health effects was established was in 1854, when John Snow traced back the outbreak of cholera epidemics in London to a particular drinking water source.

Since the middle of the twentieth century, and concurrent with the onset of accelerated industrial growth, various types of water pollution problems have occurred in rapid succession. Figure 1 illustrates the types of problems as they became apparent in European freshwaters.

Figure 1. Types of water pollution problems


In summarizing the European situation it can be stated that: (1) the challenges of the past (pathogens, oxygen balance, eutrophication, heavy metals) have been recognized, researched and the necessary controls identified and more or less implemented and (2) the challenges of today are of a different nature—on the one hand, “traditional” point and non-point pollution sources (nitrates) and ubiquitous environmental contamination problems (synthetic organics), and, on the other                                                                                                                       hand, “third generation” problems interfering with global cycles                                                                                                                      (acidification, climate change). 

In the past, water pollution in the developing countries resulted mainly from the discharge of untreated wastewater. Today it is more complex as a result of the production of hazardous wastes from industries and the rapidly increasing use of pesticides in agriculture. In fact, water pollution today in some developing countries, at least in the newly industrializing ones, is worse than in industrialized countries (Arceivala 1989). Unfortunately, developing countries, on the whole, are badly lagging behind in getting control over their major pollution sources. As a consequence, their environmental quality is gradually deteriorating (WHO/UNEP 1991).

Types and Sources of Pollution

There are a large number of microbial agents, elements and compounds which may cause water pollution. They can be classified as: microbiological organisms, biodegradable organic compounds, suspended matter, nitrates, salts, heavy metals, nutrients and organic micropollutants.

Microbiological organisms

Microbiological organisms are common in freshwater bodies polluted particularly by discharges of untreated domestic wastewater. These microbial agents include pathogenic bacteria, viruses, helminths, protozoa and several more complex multicellular organisms that can cause gastro-intestinal illness. Other organisms are more opportunistic in nature, infecting susceptible individuals through body contact with contaminated water or by inhalation of poor quality water droplets in aerosols of various origins.

Biodegradable organic compounds

Organic substances of either natural origin (allochthonous terrestrial detritus or autochthonous debris of aquatic plants) or from anthropogenic sources (domestic, agricultural and some industrial wastes) are decomposed by aerobic microbes as the river continues its course. The consequence is a lowering of the oxygen level downstream of the wastewater discharge, impairing the quality of the water and the survival of the aquatic biota, particularly of high-quality fish.

Particulate matter

Particulate matter is a major carrier of organic and inorganic pollutants. Most toxic heavy metals, organic pollutants, pathogens and nutrients, such as phosphorus, are found in suspended matter. An appreciable amount of the biodegradable organic material responsible for consumption of dissolved oxygen from rivers is also found in suspended particles. Particulate matter comes from urbanization and road construction, deforestation, mining operations, dredging operations in rivers, natural sources which are linked to continental erosion, or natural catastrophic events. Coarser particles are deposited on river beds, in reservoirs, in the flood plain and in wetlands and lakes.


The concentration of nitrates in unpolluted surface waters ranges from less than 0.1 to one milligrams per litre (expressed as nitrogen), so nitrate levels in excess of 1 mg/l indicate anthropogenic influences such as discharge of municipal wastes and urban and agricultural run-off. Atmospheric precipitation is also an important source of nitrate and ammonia to river basins, particularly in areas not affected by direct pollution sources—for example, some tropical regions. High concentrations of nitrate in drinking water may lead to acute toxicity in bottle-fed infants during their first months of life, or in the elderly, a phenomenon called methaemoglobinaemia.


Water salinization may be caused by natural conditions, such as geochemical interaction of waters with salty soils or by anthropogenic activities, including irrigated agriculture, sea water intrusion due to excessive pumping of groundwaters in islands and coastal areas, disposal of industrial wastes and of oilfield brines, highway de-icing, landfill leachates and leaking sewers.

While hampering beneficial uses, particularly for irrigation of sensitive crops or for drinking, salinity in itself may not, at even quite high levels, be directly harmful to health, but the indirect effects can be dramatic. The loss of fertile agricultural land and reduced crop yields caused by waterlogging and soil salinization of irrigated areas destroy the livelihood of whole communities and cause hardships in the form of food shortages.

Heavy metals

Heavy metals such as lead, cadmium and mercury are micro-pollutants and of special interest as they have health and environmental significance due to their persistence, high toxicity and bio-accumulation characteristics.

There are basically five sources of heavy metals contributing to water pollution: geological weathering, which provides the background level; industrial processing of ores and metals; the use of metal and metal compounds, such as chromium salts in tanneries, copper compounds in agriculture, and tetraethyl lead as an anti-knock agent in gasoline; leaching of heavy metals from domestic wastes and solid waste dumps; and heavy metals in human and animal excretions, particularly zinc. Metals released to the air from automobiles, fuel burning and industrial process emissions may settle on land and ultimately run off to surface waters.


Eutrophication is defined as the enrichment of waters with plant nutrients, primarily phosphorus and nitrogen, leading to enhanced plant growth (both algae and macrophytes) which results in visible algae blooms, floating algal or macrophyte mats, benthic algae and submerged macrophyte agglomerations. When decaying, this plant material leads to the depletion of the oxygen reserves of water bodies, which, in turn, causes an array of secondary problems such as fish mortality and liberation of corrosive gases and other undesirable substances, such as carbonic gas, methane, hydrogen sulphide, organoleptic substances (causing taste and odour), toxins and so on.

The source of phosphorus and nitrogen compounds is primarily untreated domestic wastewater, but other sources such as drainage of artificially fertilized agricultural land, surface run-off from intensive livestock farming and some industrial wastewaters can also substantially increase the trophic level of lakes and reservoirs, particularly in tropical developing countries.

The main problems associated with eutrophication of lakes, reservoirs and impoundments are: oxygen depletion of the bottom layer of lakes and reservoirs; water quality impairment, leading to treatment difficulties, particularly for the removal of taste- and odour-causing substances; recreational impairment, increased health hazards to bathers and unsightliness; fisheries impairment due to fish mortality and the development of undesirable and low-quality fish stocks; ageing and reducing the holding capacity of lakes and reservoirs by silting; and increase of corrosion problems in pipes and other structures.

Organic micropollutants

Organic micropollutants can be classified in groups of chemical products on the basis of how they are used and consequently how they are dispersed in the environment:

  • Pesticides are substances, generally synthetic, that are deliberately introduced into the environment to protect crops or control disease vectors. They are found in various distinct families, such as organochloride insecticides, organophosphate insecticides, herbicides of the plant hormone type, triazines, substituted ureas and others.
  • Materials for widespread household and industrial use comprise volatile organic substances used as extraction solvents, solvents for degreasing metals and dry-cleaning clothes, and propellants for use in aerosol containers. This group also includes halogenated derivatives of methane, ethane and ethylene. As they are widely used their rates of dispersion in the environment, compared with the amounts produced, are generally high. The group also contains the polycyclic aromatic hydrocarbons, whose presence in the environment results from the extraction, transport and refining of petroleum products and the dispersion of combustion products resulting from their use (petrol and heating oil).
  • Materials used essentially in industry include substances which are direct or intermediate agents of chemical synthesis, such as carbon tetrachloride for synthesizing freons; vinyl chloride for polymerizing PVC; and chlorinated derivates of benzene, naphthalene, phenol and aniline for manufacturing dyestuffs. The group also contains finished products used in closed systems, such as heat-exchange fluids and dielectrics.

Organic micropollutants are generated from point and diffuse sources, either urban or rural. The largest part originates in major industrial activities such as petrol refining, coal mining, organic synthesis and the manufacture of synthetic products, the iron and steel industries, the textile industry and the wood and pulp industry. Effluents from pesticides factories may contain considerable quantities of these manufactured products. A significant proportion of organic pollutants are discharged into the aquatic environment as run-off from urban surfaces; and in agricultural areas, pesticides applied to crops may reach surface waters through rainwater run-off and artificial or natural drainage. Also, accidental discharges have led to severe ecological damage and temporary closure of water supplies.

Urban Pollution

Owing to this continuously expanding, aggressive and multi-faceted pollution scenario, the problem of maintaining the quality of water resources has become acute, particularly in the more urbanized areas of the developing world. Maintaining water quality is hampered by two factors: failure to enforce pollution control at the main sources, especially industries, and inadequacy of sanitation systems and of garbage collection and disposal (WHO 1992b). See some examples of water pollution in different cities in developing countries.


Examples of water pollution in selected cities

Karachi (Pakistan)

The Lyari river, which runs through Karachi, Pakistan’s largest industrial city, is an open drain from both the chemical and the microbiological point of view, a mixture of raw sewage and untreated industrial effluents. Most industrial effluents come from an industrial estate with some 300 major industries and almost three times as many small units. Three-fifths of the units are textile mills. Most other industries in Karachi also discharge untreated effluents into the nearest water body.

Alexandria (Egypt)

Industries in Alexandria account for around 40% of all Egypt’s industrial output, and most discharge untreated liquid wastes into the sea or into Lake Maryut. In the past decade, fish production in Lake Maryut declined by some 80% because of the direct discharge of industrial and domestic effluents. The lake has also ceased to be a prime recreational site because of its poor condition. Similar environmental degradation is taking place along the seafront as a result of the discharge of untreated wastewater from poorly located outfalls.

Shanghai (China)

Some 3.4 million cubic metres of industrial and domestic waste pour mostly into the Suzhou Creek and the Huangpu River, which flows through the heart of the city. These have become the main (open) sewers for the city. Most of the waste is industrial, since few houses possess flush toilets. The Huangpu has essentially been dead since 1980. In all, less than 5% of the city’s wastewater is treated. The normally high water table also means that a variety of toxins from industrial plants and local rivers find their way into groundwater and contaminate wells, which also contribute to the city water supply.

São Paulo (Brazil)

The Tiete River, as it passes through Greater São Paulo, one of the world’s largest urban agglomerations, receives 300 tonnes of effluents each day from 1,200 industries located in the region. Lead, cadmium and other heavy metals are among the main pollutants. It also receives 900 tonnes of sewage each day, of which only 12.5% is treated by the five sewage treatment stations located in the area.

Source: Based on Hardoy and Satterthwaite 1989.


Health Impacts of Microbial Pollution

Diseases arising from the ingestion of pathogens in contaminated water have the greatest impact worldwide. “An estimated 80% of all diseases, and over one-third of deaths in developing countries are caused by the consumption of contaminated water, and on average as much as one-tenth of each person’s productive time is sacrificed to water-related diseases” (UNCED 1992). Water-borne diseases are the largest single category of communicable diseases contributing to infant mortality in developing countries and second only to tuberculosis in contributing to adult mortality, with one million deaths per year.

The total annual number of cholera cases reported to the WHO by its member states has reached levels unprecedented during the seventh pandemic, with a peak of 595,000 cases in 1991 (WHO 1993). Table 1 shows the global morbidity and mortality rates of the major water-related diseases. These figures are, in many cases, grossly underestimated, since reporting of disease cases is done quite erratically by many countries.

Table 1. Global morbidity and mortality rates of main diseases related to water


Number/Year or Reporting Period




Cholera - 1993












Diarrhoeal disease (under 5 years)



Dracunculiasis (Guinea Worm)






Source: Galal-Gorchev 1994.

Health Impacts of Chemical Pollution

The health problems associated with chemical substances dissolved in water arise primarily from their ability to cause adverse effects after prolonged periods of exposure; of particular concern are contaminants that have cumulative toxic properties such as heavy metals and some organic micropollutants, substances that are carcinogenic and substances that may cause reproductive and developmental effects. Other dissolved substances in water are essential ingredients of dietary intake and yet others are neutral with regards to human needs. Chemicals in water, particularly in drinking water, may be classified into three typical categories for the purpose of health impact (Galal-Gorchev 1986):

  • Substances exerting an acute or chronic toxicity upon consumption. The severity of the health impairment increases with the increase of their concentration in drinking water. On the other hand, below a certain threshold concentration no health effects can be observed—that is, the human metabolism can handle this exposure without measurable long-term effects. Various metals, nitrates, cyanides and so on fall within this category.
  • Genotoxic substances, which cause health effects such as carcinogenicity, mutagenicity and birth-defects. According to present scientific thinking there is no threshold level which could be considered safe, since any amount of the substance ingested contributes to an increase in cancer and similar risks. Complex mathematical extrapolation models are used to determine such risks, since very little epidemiological evidence exists. Synthetic organics, many chlorinated organic micropollutants, some pesticides and arsenic fall within this category.
  • For some elements, such as fluoride, iodine and selenium, the contribution made by drinking water is crucial and, if deficient, causes more or less severe health effects. At high concentrations, however, these same substances cause equally severe health effects, but of a different nature.


Environmental Impacts

The impacts of environmental pollution on freshwater quality are numerous and have existed for a long time. Industrial development, the advent of intensive agriculture, the exponential development of human populations and the production and use of tens of thousands of synthetic chemicals are among the main causes of water quality deterioration at local, national and global scales. The major issue of water pollution is the interference with actual or planned water uses.

One of the most severe and ubiquitous causes of environmental degradation is the discharge of organic wastes into watercourses (see “Biodegradable organic compounds” above). This pollution is mainly of concern in the aquatic environment where many organisms, for example fish, require high oxygen levels. A serious side effect of water anoxia is the release of toxic substances from particulates and bottom sediments in rivers and lakes. Other pollution effects from domestic sewage discharges into watercourses and aquifers include the build-up of nitrate levels in rivers and groundwaters, and the eutrophication of lakes and reservoirs (see above, “Nitrates” and “Salts”). In both cases, the pollution is a synergistic effect of sewage effluents and agricultural run-off or infiltration.

Economic Impacts

The economic consequences of water pollution can be rather severe due to detrimental effects on human health or on the environment. Impaired health often lowers human productivity, and environmental degradation reduces the productivity of water resources used directly by people.

The economic disease burden can be expressed not only in costs of treatment, but also in quantifying the loss of productivity. This is particularly true for primarily disabling diseases, such as diarrhoea or Guinea Worm. In India, for example, there are about 73 million workdays per year estimated to be lost due to water-related diseases (Arceivala 1989).

Deficiencies in sanitation and the resulting epidemics can also lead to severe economic penalties. This became most apparent during the recent cholera epidemic in Latin America. During the cholera epidemic in Peru, losses from reduced agricultural exports and tourism were estimated at one billion US dollars. This is more than three times the amount that the country had invested in water supply and sanitation services during the 1980s (World Bank 1992).

Water resources affected by pollution become less suitable as sources of water for municipal supply. As a consequence, expensive treatment has to be installed or clean water from far away has to be piped to the city at much higher costs.

In the developing countries of Asia and the Pacific, environmental damage was estimated by Economic and Social Commission for Asia and the Pacific (ESCAP) in 1985 to cost about 3% of the GNP, amounting to US$250 billion, while the cost of repairing such damage would range around 1%.



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