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Question: What causes pollution?
Answer: Pollution is defined in a variety of ways, but generally involves the introduction of harmful substances into the environment. These substances can be harmful to humans or to other species, including plants and animals. Although we usually think about air and water pollution, land pollution is also a problem. The substances that pollute can come from both natural processes and human activities.
With respect to natural pollutants, eruptions of volcanoes put ash and gases in the atmosphere, and can affect local areas or regions much farther away, depending on how high the eruption carries the substances. There can be health impacts from the ash in the atmosphere, and environmental impacts from the sulfur dioxide that is associated with acid deposition. Forest fires are also a source of sulfur dioxide. Some scientists believe that natural sources of sulfur dioxide and nitrogen oxides are greater than human-caused sources. There are also natural sources of water pollution. The old saying 'what goes up must come down' applies here, as air pollution can become land and water pollution when precipitation carries atmospheric pollutants to the surface of the earth. Sediment is the largest water pollutant by volume. It is generated by the erosion of soil from the land, which can then be carried into water bodies as water runs off land and into rivers, streams, and lakes. It is not just the sediment that is a pollution problem. It is what the sediment carries with it that can create additional pollution. Animal wastes, plant residues, and other substances can be carried with the run off to cause pollution problems.
Human activities cause pollution and this can be a bigger problem than natural sources because many of the activities that cause the pollution tend to be concentrated where people are also concentrated. Industries, transportation facilities, and commercial and residential development are all concentrated in and around urban areas. Each of these activities produces both air and water pollution, though the type and amounts will vary between the activities and between urban areas. Industries are sources of sulfur and nitrogen oxides, and automobiles and other vehicles are sources of volatile organic compounds, carbon monoxide, and nitrogen oxides, which interact with sunlight to create smog. Indeed, the burning of fossil fuels in industries and vehicles contributes greatly to air pollution. The activities also contribute to water pollution. Automobiles can contribute through leakage of oil, antifreeze, and fuel onto the surface of the earth. Industrial processes frequently result in the production of wastewater that must be disposed of somewhere, usually following treatment of some kind. Residential and commercial uses create sewage that requires treatment. When we treat wastewater through treatment plants, the resulting effluent (what is put into the receiving water body) is not free on contaminants. We rely on natural processes of biodegradation and dilution to degrade or reduce the pollutants to safe levels. Of course, some pollutants are not easily biodegraded and others are dangerous in very small concentrations, so different methods of disposal or treatment are required.
It is not just urban areas that generate pollution. Agricultural activities can also result in air and water pollution. Dust that gets into the air during and after plowing can cause local air pollution problems. A bigger problem, however, is water pollution. Plowed fields, even when planted, provide channels for precipitation to run off into water bodies. This run off can carry animal waste, pesticides, and fertilizers along with sediment. All of these activities, both urban and agricultural, also create solid waste which must be disposed of somewhere - - typically buried in landfills or burned in incinerators. Unless landfills are completely contained, water, which comes in contact with the waste, can contaminate the land around the landfills, as well as groundwater, which in turn can contaminate surface water with which it comes in contact. When we burn wastes, some air pollution is created.
Water and air pollution can generally be put into one of two categories: point source pollution and nonpoint source pollution. Point source pollution comes from an identifiable source such as a power plant that burns coal and emits smoke and related pollutants out of a stack, or the discharge from a sewage treatment plant. Nonpoint source pollution is not easily identifiable. With air pollution, it can be vehicles; with water pollution, it is runoff that comes from land. In both cases, the precise sources of the pollution cannot be identified. Point sources are much easier to manage because the pollution can be collected and treated before it is released into the atmosphere or into the water body. Regulations exist for many point sources. With nonpoint sources, it is necessary to prevent the pollution from being generated in the first place, by putting pollution control devices on vehicles or by limiting the kinds of potential contaminants we put on land and by preventing erosion. It is much more difficult to develop and enforce regulations for nonpoint sources of pollution.
With increasing recognition of the adverse effects of pollution, we are beginning to take a more preventative approach. The old saying used to be 'Dilution is the solution to pollution.' Now, prevention is seen as the key.
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