Uranium (U) is the third element in the actinide series of the periodic table. It has an atomic number of 92, an atomic weight of 238.04, and valences of 3, 4, and 6. The average abundance of U in the earth’s crust is 2.3 ppm, and in soils it is 1.8 ppm. Concentrations of uranium in drinking waters usually are expressed in terms of picocuries per liter, but that is now being replaced by Becquerel per liter (Bq/L). The approximate conversion factor, assuming equilibrium between 234U and 238U, is 1 μg uranium equals 0.67 pCi. The mean concentration of uranium in drinking water is 1.8 pCi/L. The chief ore is uraninite, or pitchblende, uranous uranate [U(UO4)2]. Uranium is known mainly for its use in the nuclear industry, but has also been used in glass, ceramics, and photography.
Uranium compounds are radioactive and are thereby toxic by inhalation and ingestion. There are 3 natural radioisotopes of uranium. Uranium-238 has a half-life of 4.5 × 109 years, represents 99% of uranium’s natural abundance, and is not fissionable; however, it can be used to form plutonium-239, which is fissionable. Uranium-235 has a half-life of 7.1 × 108 years, represents 0.75% of uranium’s natural abundance, is readily fissionable, and was the energy source in the original atomic bombs. Uranium-234 has a half-life of 2.5 × 105 years and represents only 0.006% of uranium’s natural abundance.
The common forms in natural water are U4+ and UO22+. In natural waters below pH 5, UO22+ dominates; in the pH range of 5 to 10, soluble carbonate complexes predominate. Although there is no U.S. EPA drinking water standard MCL for uranium, an analysis for uranium is required if the gross alpha activity of a water sample is greater than 15 pCi/L.
Perform analyses by the inductively coupled plasma/mass spectrometry method (Section 3125) or by one of the methods in Section 7500-U (for regulatory compliance purposes).