Uranium in Drinking Water

The EPA, in their final rule revising the MCLs for drinking water, has established a Maximum Contamination Level of 30 µg/L for uranium in drinking water (see attached EPA announcement). The standard uranium method used by most laboratories measures the total uranium activity (in picocuries/liter) in water and is not adequate to accurately determine the mass concentration listed in the new rule. The reason for this requires some background knowledge about natural uranium.

Natural uranium consists of three isotopes, 238U, 235U, and 234U. It is a primordial radionuclide found in all soils in concentrations of 1 to over 400 pCi/g. It is the dominant radionuclide found in Southwest drinking water as it is reasonably soluble in water.

Isotope Half Life Specific Activity
238U 4.51 x 109 years 0.336 µCi/g
235U 7.1 x 108 years 2.14 µCi/g
234U 2.47 x 105 years 6,220 µCi/g

Uranium is nephrotoxic (causes kidney damage), so the new EPA limit is based on the mass of uranium in the water, rather than the radioactivity. Consequently, the mass of uranium in the water is needed to demonstrate compliance with the new rule, and the activity is needed to determine the adjusted gross alpha level in the water (adjusted gross alpha is total gross alpha minus the uranium measured in pCi/L). Most methods measure either the mass or the activity of the total uranium. Efforts to estimate the mass from the total activity, or the total activity from the mass, are prone to significant errors. Only methods that measure concentration of each individual isotope of uranium can provide both needed parameters accurately.

The mass of uranium in the water is largely determined by the 238U, due to its longer half-life, while the total activity in the water is determined by the activity of all uranium isotopes. In soils and rocks, the activity of 234U and 238U is identical; they are said to be in secular equilibrium. In natural waters, however, the 234U is slightly more soluble and the activity ratio of 234U to 238U varies from 1:1 to more than 20:1. Consequently, the conversion from activity to mass or vice versa, requires knowledge of the concentration of each of the three uranium isotopes.

Clients who want to analyze for uranium to demonstrate compliance with the new uranium in water standard should ask for isotopic uranium rather than total uranium. The results of the isotopic uranium test (example below) shows the mass and activity of the individual uranium isotopes in the water as well as the mass and activity totals.

Sample No. 238U 235U 234U Total
WS1234 4.8 ± 0.5 0.22± 0.02 14.5± 0.9 19.4± 1.1 Activity(pCi/L)
14.3±1.6 0.10± 0.01 0.002± 0.0002 14.4± 1.6 Content(µg/L)