Uraniferous Waters from Irrigated Lands in the Western United States

David L. Naftz, U.S. Geological Survey



Because of concern about possible adverse effects from irrigation drainage 
in the Western United States, the U.S. Department of the Interior (DOI) 
implemented the National Irrigation Water Quality Program (NIWQP) in 1985. 
Beginning in 1992, a study team comprised of scientists from the U.S. 
Geological Survey and U.S. Fish and Wildlife Service was formed to construct a 
cumulative data base of detailed geochemical and biological data from 26 
irrigation project areas. Elevated concentrations of selenium in water and 
biota have been the primary focus of the study team; however, plans by the 
U.S. Environmental Protection Agency (USEPA) to implement a drinking-water 
standard of 20 micrograms per liter (ug/L) for uranium in 1996 has renewed 
concern about concentrations of uranium in irrigation drainwater. The 
cumulative NIWQP data base was used to (1) determine the magnitude and 
extent of elevated uranium concentrations in irrigation drainwater in the arid 
and semi-arid lands of the Western United States and (2) determine geochemical 
processes controlling uranium in drainwater. 

More than 30 percent of the water samples collected in areas that receive 
irrigation drainage exceed the proposed USEPA drinking-water standard for 
uranium. Study areas where at least 10 samples exceeded the proposed USEPA 
drinking-water standard for uranium are generally associated with shale 
bedrock and derivative soils.

Bivariate plots comparing uranium and alkalinity concentrations to the 
concentrations expected from dissolution of limestone were used to 
differentiate possible weathering processes that con-tribute to elevated 
uranium concentrations in water samples. Samples with elevated uranium 
concentrations have significantly higher uranium-to-alkalinity ratios than 
is compatible with limestone dissolution, indicating an alternate uranium 
source(s) such as shale bedrock. Other areas developed for agriculture in arid 
and semi-arid regions of the world with similar geology will probably 
contain uranium concentrations in irrigation drainwater that cause public 
health concerns.