Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 75

'new' sediment, S new , is associated with a point source, and (2) the system is in an
equilibrium state in which there is no net storage or loss of sediment from the channel
bed (Bonniwell et al. 1999 ). In the study of the Gold Fork River, Bonniwell et al.
( 1999 ) argued that both assumptions were reasonably met allowing the equation to
be applied to their dataset. They found that particle transport distances varied from
about 60km near the peak of the snowmelt hydrograph to 12km near base flow, a
trend that matched expectations.
While the above methods have been used by a number of investigators (e.g.,
Bonniwell et al. 1999 ; Matisoff et al. 2005 ; Salant et al. 2007 ; Evrard et al. 2010 )
to determine sediment residence times, etc., a significant constraint on the approach
is that changes in the 7 Be/ 210 Pb ratio along the channel at a given time reflect both
the age of the sediment and dilution processes, a fact clearly recognized by the
investigators. Matisoff et al. ( 2005 ), for example, argued that the dual control of
sediment age and dilution on the ratios could be treated such that each represented
end-member states. Thus, by analyzing them separately, the two analyses provide a
unique perspective on the interpretation of the results. In reality, however, changes
in the ratio are likely to be due to both processes, leading to a nearly infinite set of
possible interpretations of the data. Determining which interpretation is correct will
likely be met with difficulty and lace the results with a large amount of uncertainty.
Other key assumptions upon which the approach is based may also lead to large
uncertainties in the results. For example, Walling ( 2013 ) points out that the assumed
similarity between recently eroded and tagged sediment and that in fallout (rainfall)
is unlikely to be met as the ratio will vary from year to year, seasonally, and from
one storm to the next. Thus, the use of a constant value for the sediment entering
the channel is unlikely to generate realistic results. Matisoff et al. ( 2005 ) suggest
that this problem may be overcome in part by measuring the 7 Be/ 210 Pb ratio in
precipitation. However, the 7 Be/ 210 Pb of eroded surface sediments will not only
reflect recent fallout, but the inventories accumulated over decades in the case of
210 Pb. Thus, the 7 Be/ 210 Pb measured in precipitation will likely differ from that of
the surface soils. The 7 Be/ 210 Pb ratio will also vary across the landscape as a result
of erosional and depositional processes that deplete or increase the FRN activities,
relative to that of an undisturbed site (as noted earlier). Sediment input into the river,
then, may exhibit a range of 7 Be/ 210 Pb ratios, depending on where the sediment was
derived. Finally, because the depth distribution of 7 Be and 210 Pb differ (Fig. 3.3 ),
the 7 Be/ 210 Pb ratio may vary through time and space as a result of the depth to
which erosion occurs (Bonniwell et al. 1999 ). Some of these problems in defining an
effective source signature may be minimized, as Matisoff et al. ( 2005 ) suggest, by
using the 7 Be/ 210 Pb ratio of the sediments eroded and suspended in runoff entering
the channel, although the effectiveness of this approach has yet to be demonstrated.
In light of the above, it appears that estimates of sediment age and residence
times are unlikely to provide meaningful results given the time and cost associated
with the analysis unless the boundaries of the study are highly constrained. Fisher
et al. ( 2010 ), for example, utilized 7 Be to quantitatively assess the storage times
of sediment in bars associate with in-channel obstructions including boulder and
large woody debris in the Ducktrap River of coastal Maine. Their study differs from