Water pollution is caused by a wide range of contaminants including chemicals, pathogens, and changes in water temperature or clarity. Many chemicals occur naturally in small amounts but become pollutants at higher concentrations. Oxygen-depleting substances and turbidity can disrupt ecosystems. Toxic chemicals and pathogens can sicken humans or animals. Water pollution alters pH, conductivity, and nutrients and can lead to eutrophication and reduced water quality.

1. Causes of water pollution<br />The specific contaminants leading to pollution in water include a wide spectrum of chemicals, pathogens, and physical or sensory changes such as elevated temperature and discoloration. While many of the chemicals and substances that are regulated may be naturally occurring (calcium, sodium, iron, manganese, etc.) the concentration is often the key in determining what is a natural component of water, and what is a contaminant.<br />Oxygen-depleting substances may be natural materials, such as plant matter (e.g. leaves and grass) as well as man-made chemicals. Other natural and anthropogenic substances may cause turbidity (cloudiness) which blocks light and disrupts plant growth, and clogs the gills of some fish species.[10]<br />Many of the chemical substances are toxic. Pathogens can produce waterborne diseases in either human or animal hosts.[11] Alteration of water's physical chemistry includes acidity (change in pH), electrical conductivity, temperature, and eutrophication. Eutrophication is an increase in the concentration of chemical nutrients in an ecosystem to an extent that increases in the primary productivity of the ecosystem. Depending on the degree of eutrophication, subsequent negative environmental effects such as anoxia (oxygen depletion) and severe reductions in water quality may occur, affecting fish and other animal populations.<br />Overview Water pollution is contamination of water by foreign matter that deteriorates the quality of the water. Water pollution covers pollutions in liquid forms like ocean pollution and river pollution. As the term applies, liquid pollution occurs in the oceans, lakes, streams, rivers, underground water and bays, in short liquid-containing areas. It involves the release of toxic substances, pathogenic germs, substances that require much oxygen to decompose, easy-soluble substances, radioactivity, etc. that becomes deposited upon the bottom and their accumulations will interfere with the condition of aquatic ecosystems. For example, the eutrophication: lack of oxygen in a water body caused by excessive algae growths because of enrichment of pollutants.Water Cycle and PollutionWater cycle is, simply saying, the circulation of water in earth. In fact, the water in the earth's biosphere is used and reused again and again. This is called water cycle or continuous movement of water between the earth and the atmosphere. It involves the following mechanisms:Evaporation: changing of water from liquid to gas Transpiration: Release of water vapor from plant leaves Condensation: changing of vapor to liquid (cooled down) Precipitation: Water that returns to the earth (water droplets in clouds become large enough and there comes the rain). What's the relation of water cycle and pollution? According to the water cycle, naturally, water around us will be absorbed to the land (soil) and rivers will stream from the upstream to the downstream and released to the sea. In normal situation organic pollutants are biodegraded by microbes and converted to a form that brings benefits to the aquatic life. And for the inorganic pollutants, in the same situation, don't bring to much hazards because they are widely dispersed and have almost no effect to the environment which they are released to.In a small scale, both inorganic and organic pollutants safely decompose throughout the stream, their concentration decrease in the sea, and they don't harm the sea ecosystem and its distribution. But in an excessive scale, communities in beach and estuary will be affected by the pollutants, and can heavily harm them. Sources and Methods We can classify major sources that lead to water pollution to the following categories:petroleum products synthetic agricultural chemicals heavy metals hazardous wastes excess organic matter sediment infectious organisms air pollution thermal pollution soil pollution <br />