River water quality and pollution
| A river is defined as a large natural stream of water emptying into an ocean, lake, or other body of water and usually fed along its course by converging tributaries. Rivers and streams drain water that falls in upland areas. Moving water dilutes and decomposes pollutants more rapidly than standing water, but many rivers and streams are significantly polluted all around the world. A primary reason for this is that all three major sources of pollution (industry, agriculture and domestic) are concentrated along the rivers. Industries and cities have historically been located along rivers because the rivers provide transportation and have traditionally been a convenient place to discharge waste. Agricultural activities have tended to be concentrated near rivers, because river floodplains are exceptionally fertile due to the many nutrients that are deposited in the soil when the river overflows. |  |
 | Sources of pollution Farmers put fertilizers and pesticides on their crops so that they grow better. But these fertilizers and pesticides can be washed through the soil by rain, to end up in rivers. If large amounts of fertilizers or farm waste drain into a river the concentration of nitrate and phosphate in the water increases considerably. Algae use these substances to grow and multiply rapidly turning the water green. This massive growth of algae, called eutrophication, leads to pollution. When the algae die they are broken down by the action of the bacteria which quickly multiply, using up all the oxygen in the water which leads to the death of many animals. Chemical waste products from industrial processes are sometimes accidentally discharged into rivers. Examples of such pollutants include cyanide, zinc, lead, copper, cadmium and mercury. These substances may enter the water in such high concentrations that fish and other animals are killed immediately. Sometimes the pollutants enter a food chain and accumulate until they reach toxic levels, eventually killing birds, fish and mammals. |
| Factories use water from rivers to power machinery or to cool down machinery. Dirty water containing chemicals is put back in the river. Water used for cooling is warmer than the river itself. Raising the temperature of the water lowers the level of dissolved oxygen and upsets the balance of life in the water. People are sometimes careless and throw rubbish directly into rivers. Water quality The quality of natural water in rivers, lakes and reservoirs and below the ground surface depends on a number of interrelated factors. In its movement on and through the surface of the heart, water has the ability to react with the minerals that occur in the soil and rocks and to dissolve a wide range of materials, so that its natural state is never pure. It always contains a variety of soluble inorganic, soluble organic and organic compounds. In addition to these, water can carry large amounts of insoluble materials that are held in suspension. Both the amounts and type of impurities found in natural water vary from place to place and by time of year and depends on a number of factors. These factors include geology, climate, topography, biological processes and land use. The impurities determine the characteristics of a water body. The chemical concentration of some substances in water rivers is shown in the following table in mg/liter: | ||||||||||||||||||||||||||||||||||||||||||||||||
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| Oxygen is the most well-established indicator of water quality. Dissolved oxygen is in fact essential for the survival of all aquatic organisms. Moreover, oxygen affects a vast number of other water indicators, not only biochemical but aesthetic ones like odor, clarity and taste. Economic analyses seem to indicate that higher levels of income tend to improve oxygen levels. Oxygen levels of some of the major rivers have nowadays returned to their previous high levels after decades of low levels. This has improved the possibility of life. Rivers in the richer countries have become steadily cleaner over the past decade. But when measured for nitrates, fewer than one in ten European rivers is any longer natural: most have nitrate levels four times the norms found in nature. As cities expand to support larger populations, roofs, highways and parking lots increasingly replace permeable soils and vegetation. Rain water in urban areas is channeled into sewers and drain systems instead of filtering into the ground to raise the water table. In developing countries the picture is very different. Rivers in the poorest countries have shown a substantial drop in the level of dissolved oxygen. Nine-tenths of all sewage in developing countries runs directly into rivers, lakes and seas without treatment. | ||||||||||||||||||||||||||||||||||||||||||||||||
 | Many types of pollution are discharged into rivers, and the purification processes remove them at various speeds. Some heavy metals, for example, are removed relatively quickly because suspended clay and organic particles have a slight electric charge and adsorb the metal atoms. When the clay or organic particles settle out of the water, they take the metal atoms with them. Unfortunately some pollutants are very persistent in the water and can accumulate downstream, causing great hazard. |
| Suspended solids in a moving body of water will settle out at a various points or be carried longer distances, depending on their size and the rate of the flow. The higher the amount of suspended solids is, the cloudier or more turbid is the water. Suspended matter can affect the amount of light entering water and therefore restrict the amount of photosynthesis that can occur and therefore the growth of plants. Small particles settling out in large amount on the bottom of a water body can prevent some organisms from living there as well as preventing green plants from photosynthesising. How fast the water body moves affects the degree of mixing of water and how much dioxygen it will carry. Thus, fast-flowing highly agitated streams will not only be saturated with oxygen but also carry well-mixed nutrients, which will be ultimately carried to a river. The temperature of a water body is crucial to the amount of dissolved dioxygen it can contain. The warmer the water, the less dioxygen it contains. (2) - John Wright 'Environmental Chemistry', p. 285 | |