There are three types of impurities which can be found in water supplies: physical, chemical and biological.
Physical impurities are usually in the form of suspended solids such as dirt, clay or silt. These solids can come from a variety of sources, including runoff from fields or construction sites, urban stormwater run-off, or even erosion of natural deposits. Chemical impurities can come from a wide range of sources as well, including factories and sewage treatment plants. Common chemical contaminants include pesticides, herbicides, industrial chemicals and heavy metals. Biological impurities are usually in the form of microorganisms such as bacteria, viruses or cysts. These contaminants can enter the water supply through sewage overflows or agricultural run-off contaminated with animal waste.