International Commission on Irrigation & Drainage Commission Interationale des Irrigation et du Drainage



Drainage and Water Quality

As the main functions of drainage is preventing salinization and removal of accumulated salts or toxic elements, drainage water contains high concentration of salts, nutrients and other crop-related chemicals in drainage discharge. This concentration varies with time and discharge rate. The use of fertilizers and pesticides in intensive agricultural production and the quality of effluent its impact on downstream aquatic ecosystems need to analysedfor nutrients and pesticides.

The nutrients of most concern are N and P. In addition, from time to time, natural trace elements from the soil itself may be harmful to the ecosystem. Effluent laden with N and P stimulates eutrophication in receiving water bodies. In addition to agricultural chemicals and trace elements, drainage water from irrigated areas frequently contains salts. The impact of salts on downstream users needs to be evaluated. Some soils are abundant in trace elements, and these could leach to the drainage system.

 

Small amounts of trace elements such as As, Cd, Hg, Pb, B, Cr and Se are harmful to aquatic species because of biological magnification. The environmental consequences of disposing of drainage water from California's irrigated San Joaquin Valley into the 470 ha Kesterson Reservoir (a closed basin) are well known. The reservoir was a waterfowl habitat, and concentrating Se in the drainage water caused fish species to disappear, and resulted in deformities in water bird embryos. The failure to construct an adequate outfall drain to the sea, the use of a waterfowl habitat for drainage water disposal, and the lack of proper water quality monitoring were the major reasons for these negative environmental impacts.

 

Care must be taken to ensure that the disposal of drainage water does not interfere with the ecosystem's aquatic and terrestrial species. Concern has been expressed about damage by agricultural drainage water to estuarine fisheries in some countries.

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