Contaminants in the water are substances that make water unfit for use. Contaminants can be easily identified by assessing the taste, odor, and turbidity of the water. Most, however, cannot be easily detected and require testing to reveal whether or not water is contaminated. Contaminants are segregated in 2 major categories: BOD (Biological Oxygen Demand) & COD (Chemical Oxygen demand)
BOD is a measure of, the amount of oxygen that require for the bacteria to degrade the organic components present in water. BODs are either man-made or naturally occurring. Some contaminants are organisms that include pathogens like bacteria, viruses, and parasites such as microscopic protozoa and worms. These living organisms can be spread by human, animal waste, or industrial waste. Sanitation and hygiene can help to stop the spread of these organisms.
COD or Chemical Oxygen Demand is the total measurement of all chemicals (organics & in-organics) in the water.Other contaminants are the man-made byproducts of industry and agriculture including heavy metals like lead, arsenic, uranium, mercury etc., and hazardous chemicals and compounds like medical waste, insecticides and fertilizers.
Naturally occurring elements can contaminate water as well. Toxins such as the highly poisonous metal arsenic may be naturally present at unacceptable levels. Depletion of ground water is leading to the oxidation of inner crust of Earth and is a major reason for increase of arsenic contamination.