Nature and origins of pollution of aquatic systems by pesticides

Pesticides reach aquatic systems by direct application, spray drift, aerial spraying, washing from the atmosphere by precipitation, erosion and run-off from agricultural land, by discharge of effluent from factories, and in sewage. The relative importance of these sources are discussed and evaluated; it is concluded that run-off from agricultural land is the main source of gradual pollution, with direct application to water and discharge of effluent into aquatic systems causing more serious, but localised contamination. The pesticides that cause most pollution are the organochlorine insecticides and certain herbicides. In water, pesticides become bound to organic matter in mud and sediment quite rapidly, only small amounts remaining in solution. There is a continuous interchange between sediments and water, influenced by water movement, turbulence and temperature. Pesticides are also taken up into the biota but the overall amounts stored in this way are small relative to the overall amounts in aquatic systems. Pesticide residues are largest in rivers, less in estuaries and least in the oceans.