[ RadSafe ] Fractionation of Uranium Isotopes in Nature

[A Karam]

Leo,
 
You're correct that, at times, U-234 and U-238 appear in groundwater in non-equilibrium concentrations, and thanks for pointing this out.  However, this is due to physical, not chemical processes and it is not fractionation on account of mass.  What happens is that the alpha decay of the U-238 damages the crystal holding the uranium.  This makes the progeny nuclides more likely to escape into the water than the parent U-238.  Faure (among many others) describes this nicely.  So, yes, there can be a difference in isotopic abundance, even though the chemistry of both isotopes is identical.
 
Andy

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Further to the recent discussions on the chemistry of natural uranium, DU
or enriched uranium, the fractionation of uranium isotopes does occur in
nature.  For example, despite the equal radioactivity concentrations of
U-238 and U-234 in rocks and soils, the U-234/U-238 ratio in groundwater
generally unity, and can reach values of 10 or more.  The US EPA drinking
water guidelines use a U-234/U-238 ratio of 1.3 when converting between
mass and activity concentrations of uranium in water.

Of course, this should not be taken to imply that the chemistry or
biokinetics of natural uranium, DU  and enriched uranium differ
significantly when discussing the potential non-radiological health impacts
of uranium.

Regards,
Leo M. Lowe, Ph.D., P.Phys.