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One major pressure is pollution. Pollution is harmful to aquatic plants and animals, and may threaten drinking water and water supplies. Pollution can be anything from hazardous substances to a nutrient which can result in excessive plant growth or even silt that can smother fish spawning beds. Pollution comes from one of two types of source:

• point sources, e.g. pipes discharging effluents from urban wastewater treatment plants, industrial sites, or mines; and
• diffuse sources, e.g. land use activities such as farming, forestry and urban areas.

During the last decades, significant progress has been made in reducing the point source pollution: improved wastewater treatment, reduced volume of industrial effluents, and reduced or banned phosphate content in detergents as well as reduced atmospheric emissions. Over the last 30 years the urban and industrial wastewater treatment has progressively improved and in many parts of Europe a large proportion of the pollutants are today removed (see chapter 7). However, pollution caused by inadequately treated wastewater is still in some areas an important source of river pollution and an important source for transitional and coastal waters.  Main impacts related to point source pollution are organic pollution, nutrient enrichment and contamination by hazardous substances. Severe organic pollution may lead to rapid de-oxygenation of water, a high concentration of ammonia and the disappearance of fish and aquatic invertebrates.

Diffuse water pollution is a serious problem in many parts of Europe (see chapter 7). Diffuse sources of pollution include run-off from farmland, run-off from roads or scattered dwellings. Diffuse pollution is closely linked to land use (e.g. the application of fertiliser or pesticides to farmland; livestock manure; use of chemicals and leakage from old waste storage and polluted industrial sites). Diffuse pollution is also linked to air emissions, for example acid rain or deposition of nitrogen, impacts of traffic emissions or other air transported pollutants.

Some of the main impacts related to diffuse pollution are high levels of nutrients in rivers, lakes, estuaries and coastal waters, which can cause eutrophication; nitrate and pesticide contamination of groundwater; hazardous chemicals leaking into rivers, lakes and groundwater from industrial sites; and air pollution causing acid rain, deposition of nitrogen on sensitive waters and deposition of hazardous chemicals (e.g. mercury and PAHs).