Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 76

earlier investigations in that (1) the predominant input of FRNs to the river was the
wet deposition of the radionuclides to the surface of emergent bars located along the
channel margins and not from external sources, and (2) storage times were based on
the constant initial activity (CIA) decay model where the initial activities, A 0 ,were
defined by a range of values equal to one standard deviation above and below the
mean activities observed on adjacent thalweg and emergent bars at a site. The CIA
model is expressed as:

log A 0
− ʻ

t = (3.6)

where A is the observed activity of the sample and A 0 is the initial activity at the time
of deposition. They found that sediment storage times were influenced by reach-scale
variations in unit stream power and the frequency of large woody debris. Sediment
storage associated with in-channel obstructions were generally longer ( 100days)

than along supply limited reaches which exhibited sediment storage times generally
<

>

100days. Use of the CIA model for these types of in-channel assessments require
that a significant portion of the stored sediments are exposed to the atmospheric depo-
sition of 7 Be and that the initial activities of the sediments can be estimated (Fisher
et al. 2010 ). In addition, greater uncertainties may occur where dilution of bed or bar
materials by dead sediment make it more difficult to define the initial 7 Be activities.
A different approach to the determination of sediment residence times utilizes
a mass balancing method carried out at the catchment scale (Dominik et al. 1987 ;
Le Cloarec et al. 2007 ; Evrard et al. 2010 ). Originally put forth by Dominik et al.
( 1987 ), the model subdivides sediment movement within a catchment into two com-
ponents (referred to as boxes), a soil box in which sediment transfers are slow and
residence times are long, and a river box inwhich sediment transfers occur rapidly and
sediment residence times are short (Fig. 3.10 ). Data required for the model include
an understanding of the atmospheric fluxes of 7 Be

137 Cs, and 210 Pb, which repre-
sents radionuclide inputs to the catchment, and the 7 Be

,

137 Cs, and 210 Pb activities
within sediments exported from the basin via the basin mouth. The residence time
and radionuclide inventory in each box is then determined using a series of equations
that estimate the input and output fluxes from each box. The model developed by
Dominik et al. ( 1987 ) assumed that the transfer of FRNs was entirely in association
with particulate matter, but the model was later revised by Le Cloarec et al. ( 2007 )
to account for the transport of dissolved radionuclides through the system using
established Kd values. Le Cloarec et al. ( 2007 ) suggest that uncertainty in the model
results are primarily associated with errors in (1) the measurement of FRN fluxes
out of the basin with suspended particulate matter, and (2) the determination of the
atmospheric fluxes of 7 Be and 210 Pb. Perhaps a more important source of uncertainty
is the assumption that 7 Be export from the soil box (upland areas) to the river box
is negligible as it will be lost as a result of decay. This assumption contradicts the
studies described earlier that suggest eroded surface sediments tagged with 7 Be can
be used to determine the age of the sediment relative to the time it enters the channel.
Less uncertainty may be involved with the use of FRNs to assess the relative
timing and magnitude of deposition and mobilization during flood events on the

,