The threat of contamination and pollution poses a major threat to water supplies.

"Contamination is the presence of a minor constituent in another chemical or mixture, often at the trace level. In chemistry, the term usually describes a single chemical, but in specialized fields the term can also mean chemical mixtures, even up to the level of cellular materials. All chemicals contain some level of contamination. Contamination may be recognized or not and may become an issue if the contaminated chemical is mixed with other chemicals or mixtures and causes additional chemical reactions. The additional chemical reactions can sometimes be beneficial, in which case the label ‘contaminant’ is replaced with reactant or catalyst. If the additional reactions are detrimental, other terms are often applied such as toxin, poison or pollutant depending on the chemistry involved. A major fraction of chemistry is involved with identifying, isolating, and studying contaminants."

"Pollution is the introduction of contaminants into an environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances, or energy, such as noise, heat, or light energy. Pollutants, the elements of pollution, can be foreign substances or energies, or naturally occurring; when naturally occurring, they are considered contaminants when they exceed natural levels. Pollution is often classed as point source or non-point source pollution. The Blacksmith Institute issues annually a list of the world's worst polluted places. In the 2007 issues the ten top nominees are located in Azerbaijan, China, India, Peru, Russia, Ukraine and Zambia."

Water pollution, by the release of waste products and contaminants into surface runoff into river drainage systems, leaching into groundwater, liquid spills, wastewater discharges, eutrophication and littering."

Supply of fresh water decrease as pollution and contamination increase.