Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 61

the upper soil horizons, for example, may 'reset' the 7 Be inventory to non-detectable
values. Thus, the 7 Be inventory of recently cultivated fields may differ significantly
from fields that were cultivated some weeks in the past.
Although the use of a single FRN has proven to be effective at determining sedi-
ment provenance, the combined use of 137 Cs

210 Pb ex and/or 7 Be can often lead to an
improved ability to distinguish between sediment sources.Wilkinson et al. ( 2009 ), for
example, effectively combined the use of 137 Cs and 210 Pb ex to distinguish between
burnt and unburnt surface and subsurface sediments within a eucalypt-forested catch-
ment near Sydney, Australia (Fig. 3.5 a). Similarly, Walling ( 2013 ) illustrated how
the combined use of 7 Be and 137 Cs could be used to more effectively distinguish
between cultivated and uncultivated fields eroded by sheet, rill, and bank processes.
In this latter case, 7 Be was able to discriminate between channel bank sediments,
rill-eroded cultivated lands, eroded trackways, and the sheet erosion of pasture and
cultivated soils. However, a distinction could not be made between sediments eroded
by sheet erosion on pastures and cultivated lands without the additional use of 137 Cs
(Fig. 3.5 b).

,

3.3.2 Determination of Sediment Redistribution
and Erosion Rates

3.3.2.1 Methods and Approach

One of the most widely used applications of 137 Cs as a geochemical tracer is to
quantify the amount of erosion and deposition that has occurred at a specific location,
that when combined with data from other sites can be used to assess the spatial
redistribution of sediment across the landscape. The use of tracers other than 137 Cs
have also been explored for this purpose during the past several decades, including
134 Cs (e.g., Syversen et al. 2001 ), 59 Fe (e.g., Wooldridge 1965 ), 239 , 240 Pu (Everett
et al. 2008 ; Dong et al. 2010 ; Smith et al. 2012 ), 210 Pb ex (Wilkinson et al. 2009 ;
Smith et al. 2012 ), and 7 Be (e.g. Blake et al. 1999 ; Walling et al. 1999 ; Schuller et al.
2006 ; Wilson et al. 2003 ; Kaste et al. 2011 ). The latter two ( 210 Pb ex and 7 Be) have
received the most attention and will be focused upon here with 137 Cs.
The approach is similar for each of the above mentioned radionuclides. The total
nuclide inventory for an undisturbed site that has experienced no erosion or deposi-
tion, called the reference site , is determined and compared to inventories measured
at sites where the amount of soil erosion or deposition is in question. Since ero-
sion or deposition has not occurred at the reference site, the inventory is assumed
to be a function of the total atmospheric flux of the nuclide to the ground surface
and its subsequent radioactive decay. In contrast, the inventory at a geomorphically
disturbed site is a function of three primary factors: the atmospheric flux to the site,
radioactive decay, and the loss or gain of a radionuclide associated with soil parti-
cles that are either eroded from or deposited at the site. Thus, negative or positive
differences between the inventories of the reference and non-reference sites reflect