Here is your short paragraph on Water Contamination!
Contaminants of water can be broadly classified into organic and inorganic.
Human and animal wastes contain organic matter that creates serious problems if it enters bodies untreated. Other kinds of organic matter such as leaves, grass clippings, trash etc. can enter water as a consequence of run-off.
With the exception of plastics and some man- made chemicals, such organic matter is biodegradable. As bacteria and detritus feeders decompose organic matter in water, they consume oxygen, which is dissolved as a gas in water. The amount of oxygen that the water can hold in solution is severely limited.
In cold water, dissolved oxygen (DO) can reach concentrations up to 10 ppm (parts per million): even less can be held in warm water. When we compare this ratio with that of oxygen in air, which is 200,000 ppm (20 per cent), we can understand why even a moderate amount of organic matter decomposing in water can deplete the water of its DO. Bacteria keep the water depleted in DO as long as there is dead organic matter to support their growth.
Thus, a common water-quality test is the biochemical oxygen demand (BOD), which is the measure of the amount of organic material in the water, in terms of how much oxygen will be required to break it down biologically, chemically or both. The higher the BOD measure, the greater is the likelihood that DO will be depleted in the course of breaking it down. A high BOD causes so much of oxygen depletion that animal life is severely limited or precluded, as in the bottom waters of Gulf of Mexico. Fish and shellfish are killed when they DO drops below 2 or 3 ppm; some are less tolerant at even higher DO levels.
If the system goes anaerobic (i.e., without oxygen), only bacteria can survive, using their abilities to switch to fermentation or anaerobic respiration (i.e., metabolic pathways that do not require oxygen). A typical BOD value of raw sewage would be around 250 ppm. Even a moderate amount of sewage, added to natural waters containing at most 10 ppm DO, can deplete the water of its oxygen and cause highly undesirable consequences.
Water-soluble inorganic chemicals constitute an important class of pollutants that include heavy metals (such as lead, mercury, cadmium and nickel), acids from mine drainage (such as sulfuric acid) and road salts employed to melt ice in colder climates.