Agriculture changes the environment in many ways, both locally and globally.  This section should foster your understanding of the environmental problems that result from agriculture, particularly irrigation and the degradation of water resources due to salinization and accumulation of toxic organic compounds. 

A CLOSER LOOK  - Kesterson National Wildlife Refuge: Undesirable Effects of Irrigation.  

   An important example of chemical concentration as a result of heavy irrigation is found in the area near the Kesterson National Wildlife Refuge in California.  In the Spring of 1985, the U.S. Bureau of Reclamation announced that it was closing the Kesterson National Wildlife Refuge, a 17,000 ha (42,000 acre) preserve in the San Joaquin valley of California and that it was also closing a 132 km- (82-mi-) long irrigation drainage canal.  The canal drained water that had been used in farm irrigation into the refuge, where it provided water for a wetland habitat for birds.

   Why were the wildlife refuge and the canal closed?  In 1983, biologists began to discover birth defects in water birds born in the refuge.  These defects were due to a high concentration of the chemical element selenium, which, although harmless in the small concentrations normally found in soils and waters, causes genetic changes when present in high concentrations.  The selenium was carried into the refuge in the irrigation water flowing from the drainage canal.  Slenium concentration was low in the original irrigation water, but in the dry California valley, water used in irrigation evaporated quickly from the soil, concentrating the selenium.

   The drainage water used by farmers to leach the soil of salts was high in many chemical elements, including selenium.  A building of selenium in the refuge resulted from saline water that was transported by the canal for the pupose of providing a wetland habitat for waterfowl and to dispos of the water.  In addition to its damage to life in the refuge, there is concern that selenium pollution might spread to thousands of acres of nearby marsh and farmlands, where it could poison livestock and enter the comestic water supply.