Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 68

It is generally accepted, however, that over time differences in rainfall patterns
between precipitation events will reduce or eliminate the variations in fallout
(Walling and Quine 1992 ). This 'smoothing' effect will be most effective when
deposition occurs over periods of years (e.g., for 137 Cs derived from bomb tests)
in comparison to deposition over periods of months (e.g., 137 Cs from Chernobyl
or 7 Be). The degree of heterogeneity is also likely to vary as a function of spatial
scale, becoming more significant at larger spatial scales, where large differences
in elevation exist, or where reference sites are located far from the study area.
Other factors cited by Parsons and Foster ( 2011 ) that may lead to spatial variations
in FRN content include (1) the indirect transfer of FRNs from the atmosphere to
the soil as a result of the existing vegetation cover (Fig. 3.2 ) (Dörr and Münnich
1987 ; Wallbrink and Murray 1996 ), and (2) the redistribution of FRNs by flow
processes prior to attaching to the soil particles (particularly after the infiltration
capacity of the soil has been exceeded) (Lance et al. 1986 ; Foster et al. 1994 ).
2. FRNs are rapidly, strongly, and irreversible bound to sediment and organic matter
An important assumption in the use of FRNs as tracers is that they are rapidly,
strongly and irreversibly fixed to soil particles (Walling and He 1999 ). This
assumption is required to attribute spatial variations in FRN inventories to the
redistribution of soil particles by means of erosional and depositional processes.
While Cs mobility in soils is a complex process controlled by multiple parame-
ters (e.g., pH, organic matter content, particle mineralogy, CEC), the assumption
has been widely accepted by the tracer community (e.g., Ritchie et al. 1974 ;
Walling and Quine 1992 ; Nouira et al. 2003 ;Mabitetal. 2008 ). Parsons and
Foster ( 2011 ), however, suggested using data collected primarily to assess the
potential ecological and human health effects of 137 Cs from environmental inci-
dents (e.g., Chernobyl) that 137 Cs is not as rapidly, strongly, or irreversibly bound
to sediment or organic matter as originally assumed. Their argument is based on
studies/data that suggest (1) the sorption of 137 Cs to clay minerals varies from one
mineral type to another, and may be “a rather slow process which extends over
many years” (Bunzl et al. 1995 ). Thus, at least some of the 137 Cs that reaches the
soil may remain in the soil solution and be redistributed with the migrating fluids;
(2) 137 Cs is partly associatedwith exchangeable sorption sites on particle surfaces,
indicating that it is not permanently bound to the particles (Livens et al. 1996 ),
and (3) the nature of the sorption sites to which 137 Cs is bound changes through
time, initially being associated with more available planar sites and becoming
less bioavailable as it becomes bound in the interlayer sites in a process referred
to as radiocesium ageing (Staunton 1994 ). This ageing process may be counter-
acted by mineral weathering that releases the 137 Cs from the strongly bound sites
(Wendling et al. 2005 ). Desorption from the soil particles may also be promoted
by a decrease in Cs concentrations within the soil solution, a process that may
occur in the vicinity of plant roots as Cs is taken up by plants (Fig. 3.2 ). The net
effect of these processes is that cycling of 137 Cs in the near surface environment
may lead to their redistribution by processes other than particle movement, or
may result in the loss of FRNs from the system, producing either an overestimate
of the initial inventory or an underestimation of the inventory within the disturbed