Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 55

Fig. 3.2 Pathways of 137 Cs dispersal through the soil-plant system (from Parsons and Foster 2011 )

since the time of 137 Cs fallout will closely approximate the total atmospheric 137 Cs
flux to the Earth's surface.
A key assumption in the use of 137 Cs as a geochemical tracer is that it is strongly
bound to fine-grained particles in the upper 30 cm of the soil profile (Ritchie et al.
1974 ;Wise 1980 ; Walling and Quine 1992 ). This assumption has been supported by
a number of investigations (e.g., Lomenick and Tamura 1965 ) and is widely accepted
in the fingerprinting literature. However, study of the potential health effects of 137 Cs
following the Chernobyl incident has begun to question the assertion, noting that Cs
may be released from clay minerals by weathering processes and/or desorbed and
taken up by plant roots under some physiochemical conditions (Parsons and Foster
2011 ) (Fig. 3.2 ). In either case, 137 Cs typically exhibits an exponential decrease
in concentrations with depth in undisturbed areas, although slightly lower values
may be observed immediately below the ground surface (within the upper 5 cm)
(Fig. 3.3 a). The lower and/or near constant values at the surface reflect the cessation
of new 137 Cs fallout since the 1970s/1980s, dilution associated with the addition of
plant litter, and the influence of bioturbation processes that mix the upper soil layers
(Walling and Woodward 1995 ; Mabit et al. 2008 ). In agricultural areas, the 137 Cs
profile is altered by the mixing of surface and deeper soil materials, a process that
creates semi-uniform concentrations throughout the plough layer. Below the plough
layer, 137 Cs activity tends to decrease exponentially (Fig. 3.3 b).
In contrast to 137 Cs

210 Pb is a natural geogenic radioisotope produced as part of
the 238 U decay series. The half-life of 210 Pb is 22.26years, and the 210 Pb activity is
such that it can continue to be measured in soils for a period of about 4-5 half-lives,
or about 100years (Table 3.1 ). Its immediate parent along the decay chain is 222 Rn, a
gas formed from the decay of 226 Ra. Most of the produced 222 Rn remains in the soil
and decays to 210 Pb. Since this 210 Pb in the soil is created within the profile, and is
in equilibrium with 226 Ra, it is referred to as supported 210 Pb. A small portion of the
222 Rn, however, diffuses into the atmosphere where it decays to 210 Pb before being

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