Eutrophication can be defined as enrichment of waters by inorganic plant nutrients, particularly nitrogen and phosphorous. This phenomenon can be either artificial (or cultural) or natural, depending on anthropogenic or natural causes. Sources of artificial pollution are either urban or rural.
Urban sources of eutrophication include domestic sewage, industrial wastes and storm drainage. The contribution of nitrogen and phosphorous per person is 10.8 g N and 2.2 g P on average . Industrial sources may be of local significance, depending on the type of industry, the volume of effluent and the amount of treatment it receives.
Rural sources include agriculture, forest management, and rural dwellings. Agriculture is a major contributor to nitrate pollution of freshwater; up of half of the nitrogen applied to crops is lost to groundwater. The loss of nitrate from agricultural land is largely caused by erosion. The other main source of agricultural eutrophication is livestock farming. The amount of phosphorous excreted by British livestock each year is four times that excreted by its human population. Forest management may have local effects on nutrient loading of rivers. In some countries forests are regularly fertilized, and this may result in local eutrophication. Rural dwellings often dispose off sewage into septic tanks, which may cause local pollution.
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