Figure 9. This map shows the annual average precipitation in the contiguous 48 states. Compare this to locations of USGS National Water Quality Project sites that show selenium contamination (see
Figure 3). Regions of the arid and semi-arid West are the principal areas of concern for selenium contamination in soils and impounded water because selenium is not leached permanently out of the soil by rainfall. Instead, evaporation in these regions returns the selenium salts to the surface (Barnes, 1985).

http://geology.wr.usgs.gov/wreg/env/aridity.html
Contact: Margaret A. Keller (mkeller@usgs.gov)
Western Region Energy Group–Environmental Studies
Modified: July 31, 2002