1-Causes of water pollution can be classified as follows:

    a- Pollution with microbes, viruses and other disease causing parasites.

    b- Pollution with organic compounds that may decay and consume the oxygen dissolved in water and so kill fish whilst producing nasty smells.

    c- Pollution with inorganic salts which are difficult to get rid of. As a result, the water becomes unsuitable for drinking, irrigation or use in industry.

    d- Pollution with oily substances which result from searching for petroleum under the sea. Also oil may leak out from huge tankers and then float on the water surface forming a layer that deprives water from air. In this way, it reduces the amount of oxygen in water and causes great harm to marine organisms.

    e- Pollution with industrial waste such as minerals and poisonous chemical compounds which cause the death of many organisms.

    f- Thermal pollution which results from the release of waste water from factories in rivers and seas leading to the death of many organisms.