Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 73

Sediments that entered the flow recently will exhibit higher activities than those that
have remained in the channel bed and have been inundated for an extended period
of time (Kasprak et al. 2013 ). If the sediments remain in the channel bed, or are
buried within alluvial deposits for a long-enough period of time, the FRN activity
will decrease below detectable levels and the sediment is considered to be dead. In the
case of 7 Be, radionuclide activity can be measured in sediments that have entered the
flowwithin about 265days, whereas it is about 112years for 210 Pb characterized by a
much longer half-life. Any re-exposure of the bed sediments, such as those associated
with the surface of channel bars that are periodically inundated during floods, will be
retagged with the FRNs. In theory, then, the atmospherically tagged sediments make
an ideal tracer as the signature of the recently eroded and tagged particles differ sig-
nificantly in radionuclide activity from the sediment already in the channel allowing
the particles to be tracked through the drainage network (Bonniwell et al. 1999 ).
A number of studies have attempted to examine the time it takes for particles
eroded from upland areas to move downstream through the drainage network. The
estimation of sediment residence time, travel distances, and particle exchange rates
with the channel bed are of particular concern. A central tenet of the approach is that
the depositional fluxes to the ground surface are uniform over the catchment so that
the eroded particles possess the same signature before they enter the flow no matter
where they are derived. However, over short periods, depositional fluxes are highly
variable, raising questions as towhether 7 Be can be used as an independent dating tool
(Matisoff et al. 2005 ). Moreover, FRN activity can vary between grain size fractions.
Thus, hydraulic sorting and other processes that partition sediments according to their
size and composition into different parts of the channel may affect the measured 7 Be
signature. To reduce the effects of varying atmospheric fluxes and sedimentological
characteristics on the study results, it is common for the 7 Be activity to be normalized
by 210 Pb activity. This normalization by 210 Pb is based on the logic that because 7 Be
and 210 Pb are both derived fromatmospheric fallout, are strongly bound to particulate
matter, and exhibit a 2 valance state in aqueous solutions, they should exhibit a sim-

ilar distribution in the environment. In fact, their activity should co-vary as a function
of grain size and compositional differences. Thus, normalizing 7 Be activity by the
activity of 210 Pb

+

7 Be 210 Pb

should both decrease the spatial variability observed
for the individual isotopes (Matisoff et al. 2005 ), and partially correct for variations
in activity associated with compositional differences (Bonniwell et al. 1999 ; Salant
et al. 2007 ). The argument of a reduced spatial variability in the 7 Be

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210 Pb ratio in
comparison to that of the individual isotopes is consistent with the observations made
by Baskaran et al. ( 1993 ) and Koch et al. ( 1996 ). In addition, because the half-life of
210 Pb is much larger than that of 7 Be, changes in the ratio will predominately reflect
the radioactive decay of 7 Be and the age of the sediment.
It is important to note that the total 210 Pb activity, as opposed to 210 Pb ex ,is
typically used for normalization. This stems from the fact that the amount of 210 Pb
that is supported by 222 Rn cannot be determined in river systems where the loss
of 226 Ra by advection in pore-waters migrating through the sediments inhibits a
determination of the degree to which disequilibrium between 210 Pb and 226 Ra may
occur (Salant et al. 2007 ). The use of total 210 Pb may be problematic because it

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