"Bark samples collected from the Peak District National Park in Derbyshire (U.K.) exhibited a natural 235U/238U isotope ratio value (0.0072) whereas samples from Sellafield, West Cumbria (U.K.) showed depletion in 235U (235U/238U = 0.0053-0.0064)."

Isotopic analysis of uranium in tree bark by ICP mass spectrometry: a strategy for assessment of airborne contamination, by Ma R, Bellis D, McLeod CW; Anal Chem 2000 Oct 15; 72(20):4878-81