Sector of the environment affected by pollution
Classifying pollution by the sector of the environment affected – water, air, soil and land – is probably the most commonly used method.
Water pollution can affect surface water such as rivers and lakes, soil moisture and groundwater in aquifers, and the oceans. As you know from Study Session 4, the actions of the water cycle connect all these different reservoirs of water. For example, a polluted river will discharge into the ocean and could damage the marine environment. However, the volume of water in the ocean can disperse and dilute the pollutant so that its worst effects are only felt near the mouth of the river.
Water pollution is characterised by the presence of excess physical, chemical or biological substances that change the qualities of the water and are capable of causing harm to living organisms. We mentioned earlier that natural or unpolluted water is colourless, odourless and transparent. Water that tastes or smells bad or is cloudy can be said to have the symptoms of water pollution. However, some water pollutants cannot be seen or tasted, for example some chemicals, such as pesticides, and most of the micro-organisms that cause waterborne diseases. So, water pollution involves more than just the appearance of the water. Polluted water should not be used for drinking, washing, bathing or agriculture. If polluted water is used by humans, then it can adversely affect the body in different ways, depending on the type and concentration of pollutant.
You also read in Study Session 4 that most rivers and streams in developing countries contain significant quantities of suspended solids that are carried along with the flow and make the water look brown in colour, especially in the rainy season (Figure 7.9). Most of the solids are fine particles of soil that have been washed into the river from surrounding land by rain, often following cultivation or construction work. Large quantities of solids in the water can reduce light penetration into the water which can affect the growth of plants.
Biological water pollutants are micro-organisms that are harmful to humans and other forms of life. They are responsible for many different waterborne diseases. The original source of these pollutants is people or animals already infected with the micro-organisms concerned. If faeces from infected people are not correctly contained and treated, the pollutants can get into surface and groundwater. The main groups of biological pollutants are bacteria, viruses, protozoa and helminths (worms).
Chemical water pollutants take many different forms depending on their source. They include plant nutrients (compounds of phosphorus and nitrogen) used as fertilisers which, as you read earlier, can be washed from fields into rivers. These nutrients are also produced by the breakdown of human and animal wastes and are common pollutants of surface waters.
Chemical pollutants also include heavy metals, pesticides and other persistent pollutants. Heavy metals are a group of toxic chemical pollutants that contain compounds of persistent metals such as mercury, cadmium, lead and chromium. The presence of heavy metals in water in excess of acceptable levels can cause illness and death among animals and humans if consumed through drinking and food (Zinabu and Pearce, 2003).
Persistent organic pollutants (POPs) are also toxic to humans and wildlife. They include many different synthetic organic chemicals manufactured for use as pesticides and in industrial processes, e.g. DDT, aldrin and PCBs (polychlorinated biphenyls). Many of these persistent chemicals have been banned in some countries. Their persistence in the environment creates specific problems that are described in Study Session 8.
Air pollution can exist at all scales, from local to global, and can include gases and solid particles. It can affect you in your own home, or in your town or city, and can contribute to global atmospheric changes. The most common sources of air pollution in the urban centres include the burning of wood, charcoal and other biomass fuel by households, small businesses such as bakeries, manufacturing industries, and vehicles.
Air pollution is defined as the presence in the air of abnormal amounts of chemical constituents capable of causing harm to living organisms. Clean air consists of nitrogen (78% by volume), oxygen (21%) and trace gases (< 1%). Polluted air may contain particulate matter (such as black soot) and many different gaseous chemicals such as carbon monoxide, carbon dioxide, nitrogen oxides, sulphur oxides, ozone, nitrates, sulphates, organic hydrocarbons and many others. Many of these are also found in clean air as trace gases but they become pollutants if present in abnormal quantities.
The emission of black smoke is an indication of intense pollution. However, not all air pollution is visible or can be smelled. Gases such as carbon monoxide and carbon dioxide are invisible and odourless. Carbon monoxide is very dangerous to humans. It can be produced by inefficient burning of fuel (for example a charcoal stove in a home with inadequate air supply) and if breathed in large quantities it can be deadly. Carbon dioxide is an important pollutant that is involved in climate change. (You will learn about in climate change in Study Sessions 9, 10 and 11.)
Soil and land pollution
Soil pollution, also called land pollution, is linked to water pollution. Liquid wastes containing toxic chemicals or pathogenic micro-organisms on the surface of the land can seep slowly into the soil and may percolate down to contaminate groundwater, which can affect people using springs or wells in the area. Possible sources include open defecation, pit latrines or leaking storage containers for industrial chemicals and wastes.
Solid waste can cause soil pollution. A collection of solid wastes in one place or scattered around is unsightly and might smell bad to you as you pass by (Figure 7.10). Household waste typically consists mostly of food waste that will gradually decompose. This produces a bad odour and attracts insects and rats, both of which contribute to the transmission of disease. As the waste decomposes it produces a liquid called leachate which trickles down into the soil. Leachate is a highly concentrated liquid pollutant that may contain toxic chemicals and pathogenic micro-organisms as well as high levels of organic compounds. Rainwater falling on, and washing through, solid waste adds to the problem.