Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 51

Chapter 3
Fallout Radionuclides

Abstract A number of short-lived radioactive isotopes of both natural and
anthropogenic origins which are (or were) atmospherically deposited over the land-
scape have been extensively utilized as sediment tracers in riverine environments.
The three most extensively utilized isotopes, which are often referred to as fallout
radionuclides (or FRNs), include in decreasing order of application, 137 Cs

210 Pb ex ,
and 7 Be. Herein we examine the primary ways in which these three isotopes have
been applied to gain insights into the riverine sediment system. More specifically,
we explore the strengths and weaknesses of using FRNs in combination with mixing
models to determine sediment provenance at the catchment scale, particularly with
regards to determining whether the sediment was derived by means of sheet, rill,
gully, or bank erosion. The nuclide inventory approach is also examined for its abil-
ity to characterize other components of the sediment system at much smaller spatial
scales, including the redistribution of sediment on hillslopes and between landscape
units. Our discussion concludes by examining the ability of 7 Be to document dynamic
processes operating along the channel bed by determining sediment residence times,
scour and fill depths, particle filtration, and sediment travel distances.

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Keywords Fallout radionuclides Hillslope sediment redistribution Sediment

residence time

3.1 Introduction

In the previous chapter we examined the use of geochemical data to quantitatively
determine the relative contributions of sediment from defined sources at the catch-
ment scale using inverse modeling techniques. A number of short-lived radioactive
isotopes also have a long history of being used as tracers to address various compo-
nents of the sediment system.
The field of isotope geochemistry was founded shortly after 1886 when Henri
Becquerel discovered 'radioactivity' (a term put forth by Marie Curie) (Baskaran
2011 ). Since then about 300 stable and over 1,200 unstable isotopes have been iden-
tified (Hoefs 2010 ), and their use has become a central component of the analysis of
Earth and environmental systems. Of particular importance has been the use of iso-
topes to constrain the age and rates of geological processes and as tracers to assess a