Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 52

wide range of topics in almost every geological setting (Porcillie and Baskaran 2011 ),
including the reconstruction of paleoclimates and sea level change, the paleochem-
istry of the oceans, the rates of weathering and erosion, the analysis of magmatic and
tectonic processes, and the evolution of Earth and planetary systems, to mention just
afew.
With regards to sediment and sediment-associated contaminants in river systems,
the fallout radionuclides (FRNs) (e.g.,

210 Pb ex , and 7 Be) have been most
extensively utilized, often in combination with inverse mixing models, for source
ascription purposes. The use of FRNs, however, is not restricted to the determination
of sediment provenance by means of geochemical fingerprinting. Rather, they have
been extensively used to characterize other important components of the sediment
system. In the following sections we examine the general characteristics and spatial
distribution of selected FRNs. We then turn our attention to the use of FRNs as a
geochemical tracer of sediment within a catchment.

137 Cs

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3.2 Lead-210, Cesium-137 and Beryllium-7:
General Characteristics

The potential to use 137 Cs to indirectly determine average rates of soil loss was
recognized as early as the 1960s (Menzel 1960 ; Rogowski and Tamura 1965 ). Since
then, FRNs, particularly 137 Cs

210 Pb ex and 7 Be, have been extensively investigated
and used as geochemical tracers. Cesium-137 has been the most widely studied
and applied FRN; in fact, a bibliography produced by Ritchie and Ritchie ( 2008 )
identified approximately 4,500 papers related to the transport and fate of Cs in the
environment, a significant number of which pertain to the use of 137 Cs as a tool for
analyzing erosional and depositional processes.
Cesium-137 is a 'man-made' or artificial radionuclide with a half-life of 30.2years
that primarily owes its presence in the environment to nuclear weapons testing
and, to a much lesser extent, accidents at nuclear power plants (e.g., Chernobyl
in 1986) (Ritchie and McHenry 1990 ; Mabit et al. 2008 ) (Table 3.1 ). Although the
first weapons tests were carried out in 1945, the released 137 Cs was primarily retained
within the region. In 1952, however, the testing of thermonuclear weapons injected
137 Cs into the stratosphere where it was distributed globally before being redeposited
over the landscape (Perkins and Thomas 1980 ). Total fallout of 137 Cs was signif-
icantly (about an order of magnitude) greater in the northern hemisphere than in
the southern hemisphere, reflecting the distribution of testing activities, and reached
detectable concentrations in many areas in 1954 (Ritchie and McHenry 1990 ). Mea-
surable concentrations in the southern hemisphere did not occur until about 1958.
Concentrations in both hemispheres began to decrease following the 1963 Test Ban
Treaty. By the mid-1970s, fallout had declined below detectable levels in the south-
ern hemisphere, whereas in the northern hemisphere it could no longer be measured
after 1983/1984 (Ritchie and McHenry 1990 ). The atmospheric deposition of 137 Cs
occurs by both wet and dry processes, but the majority is associated with rainfall.

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