Water pollution is considered all activities that will lead to the decline of water quality. Agriculture, industry and households are all responsible for water pollution. The sewerage can run over in rivers, lakes or oceans. Pollution can cause severe damage to the water's ecosystem. Results could be that sources of drinking water will be affected, making it harder and more costly to purify them.

The waters usually are being tested on pollution. Usually somebody from the government takes samples from the water. These will get tested and analysed in a laboratory. Sometimes fish will be used as sample. Specified characteristics of the fish give information about the kind of pollution. Sometimes computer programs are used to make models about the dangers in the water.

There are different levels of pollution. The first are substances that stimulate illness. These substances are bacteria, viruses and monocellular which live in sewage and waste. The second grade in pollution is oxygen consumption pollution. This is digested by oxygen using bacteria. When these bacteria convert the pollution, the oxygen disappears from the water. All organisms in such water will die because of oxygen shortage. The third grade encloses the inorganic pollution that is water-soluble. Examples of these are acids, salt, and metals. When there's a large number of this pollution in the water, it can’t be used as drinking water and the vegetation in the water will die. Even nutrient will pollute the water. Some substances such as nitrates and phosphates will stimulate the grow of algae and water vegetation, and using all the oxygen. This causes fishes to die. Organic substances can pollute the water, examples are oil, plastic and pesticide. These are harmful for humans, animals and vegetation in the water. The most dangerous grade of pollution is sediment. The sediment will prevent the water to absorb the light. Radioactive substances which are water-soluble, can cause cancer, birth defectss or genetic disorder.

Water pollution could be traced to the outlet. There are deliberate dumpings by factories. Manure that is used for the agriculture is harmful for the groundwater. An oil tanker that loses oil is also a polluter. Water pollution without outlet is acid rain and gases that are released by the industry and transport. They penetrate the earth and run to the groundwater. Acid rain (or more accurately acid precipitation) occurs when sulphur dioxide and nitrogen oxides are emitted into the atmosphere, undergoing chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams. The emission of sulphur dioxide(SO2), nitrogen oxides (Nox) and ammonia (NH3) form in the air acids (the nitric acid and the sulphuric acid), which through wet precipitate (such as rain, snow and hailstone), dry precipitate (such as gasses) or trough fog come down. You also could be polluting the water, by using some sorts of cleaning products and pesticides, and flushing paint of bleaches.

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Environment Canada - Water pollution
Natural Resources Defence Council - Water pollution
Great Lakes 
Water Pollution Guide