Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 60

grazing areas). The point to be made is that characterization of source end members
may require the collection of more samples than originally thought, and it cannot be
assumed a priori that FRN concentrations are normally distributed within the source
and river sediments.
A commonly untested assumption is that 137 Cs and 210 Pb ex exhibit negligible
concentrations below the first 20-30 cm of the floodplain surface. In most studies,
FRN concentrations within the bank materials are determined on composite samples,
consisting of sediments collected from the bank. While 137 Cs and 210 Pb ex concen-
trations within these samples may be low, the observed values may result from the
mixing of upper bank sediments enriched in FRNs with lower bank sediments that
exhibit concentrations below detection. Studies that have used 137 Cs and 210 Pb ex to
date floodplain deposits illustrate, for example, that both radionuclides may extend
to greater depths below the ground surface and occur at higher concentrations than
found in upland soils. The higher concentrations result from the combined accumu-
lation of radionuclides in the floodplain from atmospheric fallout as well as from the
deposition of FRN-bearing sediments eroded from the surface of upland soils during
overbank events (He and Walling 1996 ; Stokes and Walling 2003 ). The maximum
concentrations of 137 Cs in the floodplain deposits will correspond to the deposition of
sediment in 1963 in the northern hemisphere (1964 in the southern). Thus, the depth
of maximum 137 Cs activity will vary as a function of the sedimentation rate on the
floodplain since that time. The activity of 210 Pb ex will likely decrease exponentially
below the floodplain surface, but because the surface layers may be episodically
buried during floods, the trend may be chaotic and extend to greater depths. During
bank erosion, these enriched surface deposits will add to the FRN activity measured
within the river sediment, which depending on their vertical distribution and con-
centration within the floodplain and the bank height, may lead to an overestimation
of sediment from upland sources.
While the above discussion focuses on 137 Cs and 210 Pb ex , recent studies have
shown that 7 Be can serve as an effective tracer. Nonetheless, Walling ( 2013 ) points
out that there are several factors that complicate its use as a fingerprinting parameter.
First, 7 Be has the potential to change rapidly through time in response to its fallout
during storm events and its relatively rapid radioactive decay. Thus, the characteri-
zation of 7 Be in the source materials will need to be done at or very close to the time
the river sediments are collected. This problem may pose considerable limitations
on the sources of suspended sediments by inhibiting its application to materials that
are no more than a few weeks old. Second, its concentration within the uppermost
soil materials (generally 1 cm) and its rapid decrease below the surface means that

the concentration of 7 Be within the eroded sediments will vary significantly as a
function of the depth of erosion. While this is also true for 137 Cs and 210 Pb ex ,the
variations are likely to be much more pronounced, thereby adding additional uncer-
tainty to the discrimination of sediment sources by means of 7 Be (Walling 2013 ).
Third, because a significant portion of the 7 Be may be intercepted by and stored on
vegetation, substantial variations in 7 Be, even within a single land-use category, may
occur as a result of variations in the vegetation cover. Finally, spatial variations in
7 Be may be initiated by the timing of human activities. Cultivation and mixing of

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