17.2.5 Types of pollutant defined by their source
Pollutants from certain sources may be a mixture of the types described above and therefore need a separate category because they combine several possible impacts. Municipal wastewater and agricultural wastes are in this category.
Municipal wastewater is generated from residential areas and often contains high concentrations of organic matter, phosphorus and nitrogen, pesticides, toxic chemicals, salts, inorganic solids such as silt, as well as pathogenic bacteria and viruses.
Agricultural wastes are generated from livestock and poultry farming and from growing crops. They can be the source of many organic and inorganic pollutants in surface waters and groundwater. Agricultural wastes include sediment from erosion of cropland, and phosphorus and nitrogen compounds that originate in animal wastes and commercial fertilisers. Animal wastes require oxygen to be broken down in water bodies and can also harbour pathogenic organisms. The extensive use of fertilisers and pesticides in agricultural regions means that both surface and groundwater are affected by these pollutants.
What is likely to happen when fertiliser is washed off agricultural fields into a lake?
Fertiliser contains nitrate and phosphate, which are plant nutrients. If these are washed into a lake the water will become eutrophic (high concentrations of nutrients).