Water quality is affected by a wide range of natural and human influences. The most
important of the natural influences are geological, hydrological and climatic, since these
affect the quantity and the quality of water available. Their influence is generally greatest
when available water quantities are low and maximum use must be made of the limited
resource; for example, high salinity is a frequent problem in arid and coastal areas. If the
financial and technical resources are available, seawater or saline groundwater can be
desalinated but in many circumstances this is not feasible. Thus, although water may be
available in adequate quantities, its unsuitable quality limits the uses that can be made of it.
Although the natural ecosystem is in harmony with natural water quality, any significant
changes to water quality will usually be disruptive to the ecosystem.