Standing or flowing bodies of water are referred to as surface water. It is normally a mixture of groundwater and spring and rain water, although it may also contain waste water. Streams, rivers and lakes may contain minerals in widely varying proportions. Because of the numerous organisms living in and around bodies of water, many organic substances, mostly metabolic, can be found in the water. Frequently, surface water is contaminated by heavy metals and organic compounds introduced from industrial plants and households. A very problematic source of contamination is also agriculture, namely farming that is not ecologically friendly. In addition to pesticides and herbicides, whose active ingredients often contain halogenated hydrocarbons, large amounts of phosphate and nitrate (fertilizers) and organic substances (manure) may enter water from these sources. All surface water contains various bacteria, viruses and protozoans.