The causes of water pollution

The causes of water pollution are:

1. Sewage and wastewater

Sewage is created by residential, institutional, and commercial and industrial establishments and includes household waste liquid from toilets, baths, showers, kitchens, sinks and so forth that is disposed of via sewers. in fact, sewage contains hundreds of toxic chemicals dumped into the sewage system. Sewage disposal is a major problem in developing countries as many people in these areas don’t have access to sanitary conditions and clean water. Untreated sewage water in such areas can contaminate the environment and cause diseases such as diarrhoea.....

Sewage in developed countries is carried away from the home quickly and hygienically through sewage pipes. Sewage is treated in water treatment plants and the waste is often disposed into the sea. Sewage is mainly biodegradable and most of it is broken down in the environment. In developed countries, sewage often causes problems when people flush chemical and pharmaceutical substances down the toilet. When people are ill, sewage often carries harmful viruses and bacteria into the environment causing health problems.

2. Industrial waste

Industry is a huge source of water pollution, it produces pollutants that are extremely harmful to people and the environment. Many industrial facilities use freshwater to carry away waste from the plant and into rivers, lakes and oceans. Pollutants from industrial sources include:
a. Asbestos
This pollutant is a serious health hazard and carcinogenic. Asbestos fibres can be inhaled and cause illnesses such as asbestosis, mesothelioma, lung cancer, intestinal cancer and liver cancer.
b. Lead
This is a metallic element and can cause health and environmental problems. It is a non-biodegradable substance so is hard to clean up once the environment is contaminated. Lead is harmful to the health of many animals, including humans, as it can inhibit the action of bodily enzymes.
c. Mercury
This is a metallic element and can cause health and environmental problems. It is a non-biodegradable substance so is hard to clean up once the environment is contaminated. Mercury is also harmful to animal health as it can cause illness through mercury poisoning.
d. Nitrates
The increased use of fertilisers means that nitrates are more often being washed from the soil and into rivers and lakes. This can cause eutrophication, which can be very problematic to marine environments.
e. Phosphates
The increased use of fertilisers means that phosphates are more often being washed from the soil and into rivers and lakes. This can cause eutrophication, which can be very problematic to marine environments.
f. Sulphur
This is a non-metallic substance that is harmful for marine life.
g. Oils
Oil does not dissolve in water, instead it forms a thick layer on the water surface. This can stop marine plants receiving enough light for photosynthesis. It is also harmful for fish and marine birds.
h. Petrochemicals
This is formed from gas or petrol and can be toxic to marine life.

3. Oil spills

Oceans are polluted by oil on a daily basis from oil spills, routine shipping, run-offs and dumping. Oil spills make up about 12% of the oil that enters the ocean. Oil spills happen when people make mistakes or are careless and cause an oil tanker to leak oil into the ocean. Oil spills cause a very localised problem but can be catastrophic to local marine wildlife such as fish, birds and sea otters. Oil cannot dissolve in water and forms a thick sludge in the water. This suffocates fish, gets caught in the feathers of marine birds stopping them from flying and blocks light from photosynthetic aquatic plants.

4. Atmospheric desposition

Atmospheric deposition is the transfer of substances from the air to the surface of the earth, either in wet form (rain, fog, snow, dew, frost, hail) or in dry form (gases, aerosols, particles).  This includes acidic deposition, or "acid rain", which is a common term used to reference the acidic components found in rain, snow, fog, dew, or dry particles.  Acid precipitation best describes this atmospheric deposition, which results from a reaction of air pollutants, primarily sulfur oxides and nitrogen oxides, with water in the air to form strong acids (like sulfuric and nitric acid). The main sources of these pollutants are vehicles and industrial and power-generating plants.

5. Eutrophication

Eutrophication can be human-caused or natural. Untreated sewage effluent and agricultural run-off carrying fertilizers are examples of human-caused eutrophication. However, it also occurs naturally in situations where nutrients accumulate (e.g. depositional environments), or where they flow into systems on an ephemeral basis. Eutrophication generally promotes excessive plant growth and decay, favouring simple algae and plankton over other more complicated plants, and causes a severe reduction in water quality. Enhanced growth of aquatic vegetation or phytoplankton and algal blooms disrupts normal functioning of the ecosystem, causing a variety of problems such as a lack of oxygen needed for fish and shellfish to survive. The water becomes cloudy, typically coloured a shade of green, yellow, brown, or red. Eutrophication also decreases the value of rivers, lakes, and estuaries for recreation, fishing, hunting, and aesthetic enjoyment. Health problems can occur where eutrophic conditions interfere with drinking water treatment. (Mariz_08-06)

References:

Taken from:

http://www.greenmuze.com/nature/sewer/280-why-sewage-is-harmful.html
http://en.wikipedia.org/wiki/Sewage_treatment
http://www.water-pollution.org.uk/causes.html
http://en.wikipedia.org/wiki/Eutrophication
http://library.thinkquest.org/CR0215471/oil_spills.htm
http://biology.usgs.gov/ecosystems/atmospheric_deposition.html