[Water pollution, self-purification and restoration possibilities].

Organic pollution is still the most important anthropogenic stress factor influencing the usability of ground and surface water as drinking water for cattle. Organic pollution is decomposed in several distinct heterotrophic processes that cause oxygen deficit, and finally end in the production of ammonia, hydrogen sulphide or methane. Even after the oxygen balance being restored higher nutrient concentrations (eutrophication) will remain. Eutrophication itself often leads to secondary pollution processes that adversely affect oxygen availability. Toxins may be generated by certain bluegreen bacteria growing under highly eutrophic conditions. Due to the concentrations of organic substances bacterial growth is forced, sometimes including faecal indicators and pathogenic species originating from the effluents of most of the communal sewage treatment works. Another kind of man made pollution concerns certain toxic substances. Disregarding the effects of accidental spills, the industrial production and use of pesticides in agriculture as well as other industrial chemicals or heavy metals may cause concentrations beyond acceptable limits. The use of surface water for cattle may become a problem for this reason. Accidental spills must be reckoned with as well, so the technical equipment for providing alternative sources of drinking water in emergencies must be available. Whereas the use of groundwater is normally not affected by toxic substance or pathogenic bacteria, high concentrations of iron, humic acids or salination may affect the suitability of water for cattle.