The quality of any body of surface or ground water is a function of either or both natural influences and human activities. Without human influences, water quality would be determined by the weathering of bedrock minerals, by the atmospheric processes of evapotranspiration and the deposition of dust and salts by wind. By the natural leaching of organic matter and nutrients from soil, by hydrological factors that lead to runoff, and by biological processes within the aquatic environment that can alter the physical and chemical composition of water. As a result water in the natural environment contains many dissolved substances and non-dissolved particulate matter. Dissolved salts and minerals are necessary components of good quality water as they help maintain the health and vitality of the organisms that rely on this ecosystem service. The distribution of water hardness, a water quality parameter that is most influenced by the geology of the surrounding drainage basin, in lake and river monitoring stations worldwide. Water can also have certain substances that are harmful to life. These include metals such as mercury, lead and cadmium, pesticides, organic toxins and radioactive contaminants. Water from natural sources almost always contains living organisms that are integral components of the biogeochemical cycles in aquatic ecosystems. However, some of these particularly bacteria, protists, parasitic worms, fungi, and viruses, can be harmful to humans if present in water used for drinking. Heavy metals are natural component of Earth's crust. They can not be degraded. To a small extent they enter our bodies via food, drinking water and air. Some metals are essential to maintain the metabolism of the human body. However at higher concentration they can lead to poisoning. The harmful effect to human life is mainly from drinking water which is contaminated naturally or from human activities
