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Fig. 3.3 Variations in fallout radionuclides with depth in soils from Morocco. a 137 Cs in an undis-
turbed soil; b 137 Cs in a ploughed agricultural soil; c 210 Pb ex in an undisturbed soil; d 210 Pb ex in
a ploughed agricultural soil; e 7 Be in an undisturbed soil; f
7 Be in a ploughed agricultural soil
(adapted from Mabit et al. 2008 )
returned to the Earth's surface within a few days (Fig. 3.4 ). This atmospherically
deposited 210 Pb is referred to as unsupported or excess 210 Pb ex to distinguish it
from the 210 Pb created by, and in equilibrium with, 226 Ra in soil. The amount of
supported 210 Pb in the soil can be determined on the basis of the 226 Ra activity and,
thus, removed from the total to determine the unsupported 210 Pb ex in the soil derived
from atmospheric deposition.
Like 137 Cs, an undisturbed soil profile that has been subjected to no erosion or
deposition is thought to reflect the atmospheric 210 Pb ex flux to the Earth's surface.
Unlike 137 Cs, however, the 210 Pb ex is essentially constant through time (although
minor variations in the flux of 210 Pb ex to the Earth's surface have been noted; Preiss
et al. 1996 ). Thus, while the inventory of 137 Cs in an undisturbed soil that has been
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