Introduction


You may already know that water is a good solvent. In many ways this is a really useful property. However, it also means that lots of toxic substances can dissolve in water and pollute this most precious resource. In this unit we will find out about some of the pollution problems that affect our water.

Farmers across the world spread millions of tonnes of fertilizer on their land each year. This provides plants with the essential elements they need to grow well, such as nitrogen from ammonium nitrate fertilizer. These fertilizers must be soluble in water so that plants can draw up the nutrients through their roots. However, because it is soluble, the fertilizer can be washed from the soil in heavy rain. We say that it is leached out of the soil. And that's when our pollution problems start.

The nitrate fertilizers drain from groundwater into rivers and streams. Once in a river, the fertilizer promotes rapid growth of algae (a tiny plant) on the surface of the water. This stops light reaching other water plants, and also reduces the amount of dissolved oxygen in the water. Fish rely on this dissolved oxygen: without it, they die.

Of course, plants produce oxygen when they photosynthesize. So why do the levels of dissolved oxygen decrease as the mass of plants increases? The algae thrive at first, but when they die, micro-organisms have a feast feeding on and decomposing the dead algae. The micro-organisms multiply rapidly with so much food available, and their activity uses up the oxygen in the water – unfortunately for the fish.

This process is called eutrophication. You can see this in Fig.1 below.



Nitrates from fertilizers can also find their way into our drinking water. There are limits set for the amount of nitrate that we can drink safely. Many scientists are worried that nitrates may cause stomach cancers and 'blue baby' syndrome (in which the blood of new-born babies is unable to transport oxygen efficiently). For this reason, it seems sensible to control carefully the amounts of nitrates in drinking water.



Detergents washed into rivers often contain phosphates. Phosphates contain another essential

element

An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.

element for plant growth (phosphorus). For this reason, detergents also cause eutrophication in rivers and lakes. In addition, untreated sewage can cause eutrophication because it contains many nutrients useful to growing plants.

Many power stations are built near rivers because they need plenty of water to convert to steam in the process of generating electricity. Water returned to the river is often at a higher temperature than normal river water. This can affect the natural balance of ecosystems in the river. For example, oxygen dissolves better in cold water than in hot water. So water at a higher temperature does not contain as much oxygen for aquatic life to use.

Many lakes in Scandinavia and Canada are badly affected by

acid rain

Acid rain is so called because it has a pH value below 5 due to acidic pollutant gases in the atmosphere.

acid rain. The acid rain increases the acidity of the water in the lake, killing all aquatic life. The acid rain also leaches aluminium ions from soils. These ions enter the lake water and poison the fish. Powdered limestone (calcium carbonate) can be used to neutralize affected lakes, which can then be restocked with fish.

Summary


Water is a precious resource but it is being polluted in a variety of ways.

Fertilizers, detergents, and sewage cause eutrophication.

Power stations discharge hot water into rivers causing thermal pollution.

Acidic gases (such as sulfur dioxide) from power stations and other sources cause acid rain, which has killed the aquatic life in some lakes.