pollutionThe contamination of air, ground, snow or water by man-made substances such as gases and liquids. 
The main sources of pollution.
When poor people live close to a river they often deposit all of their wastes into the river because they have no alternative. This pollutes the rivers. This is Indonesia.
In industrialised countries rivers can become polluted by industrial wastes. This is Germany. You can see the purification works in the background.
Much mountain pollution comes in from the air in the form of gases that can cause acid rain or acid snow. They can kill lakes unless the lakes are sprayed with lime, as is being done here in Scandinavia.
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