Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 74

includes 210 Pb from both lithogenic and atmospheric sources, the activity of which
decreases with increasing grain size (Salant et al. 2007 ). However, the associated
errors are not so significant that they are likely to invalidate the results.
Since the 7 Be

210 Pb ratio measured in a river's suspended load or channel bed
sediment will reflect the time that has elapsed since the tagged sediment has entered
the flow, the ratio can be compared to the signature of the sediment that enters the
water to determine the sediments age, t . Age, in this case, is measured relative to the
time the sediment first entered the water, and can be calculated using the following
equation (Matisoff et al. 2005 ):

/

log log

7 Be sample
210 Pb sample

7 Be source
210 Pb source

− 1 1

t = + (3.2)

ʻ − ʻ ʻ − ʻ

7 Be 210 Pb 7 Be 210 Pb

where 7 Be source and 210 Pb source are the 7 Be and 210 Pb activities in the sourcematerial,
7 Be sample and 210 Pb sample are the activities in the sampled river sediment, and

ʻ 7 Be and

ʻ 210 Pb are the decay constants for 7 Be 01300 d − 1 and 210 Pb 10 − 5 d − 1 ( 0 . ) ( 8 . 50999 × ) ,

respectively. Spatial and temporal changes in the 7 Be

210 Pb in river sediments will
also reflect dilution processeswhere 'new' sediment is exchanged for 'dead' sediment
in the channel bed, or 'new' sediment is diluted with 'old' sediment eroded from
the channel bed and banks. This relationship between 7 Be

/

210 Pb ratios and dilution
processes allows for an estimation of the relative percent of 'new' sediment, S new
within the load or deposit to be calculated by (Matisoff et al. 2005 ):

/

7 Be sample / 210 Pb sample

7 Be source /

S new = 100 · (3.3)

210 Pb source

or

e − ʻ 7 Be − ʻ 210 Pb t

S new = 100 · (3.4)

In addition to the estimation of sediment age and dilution effects, the analysis can
be further extended to assess particle travel distances over specific time intervals by
documenting downstream trends in either the 7 Be/ 210 Pb ratio or the percent of 'new'
sediment within the suspended or bedload. For example, Bonniwell et al. ( 1999 ),
following the approach provided by Cushing et al. ( 1993 ), found that 7 Be/ 210 Pb
ratios collected on a given day varied semi-systematically along the Gold Fork River
of Idaho. The data could be expressed by a non-linear function of the form:

F 0 e − k 1 x F ( x ) = (3.5)

is the measure 7 Be/ 210 Pb ratio in the sediment, F 0 is the 7 Be/ 210 Pb ratio
of the source materials at the point of input, x is distance downstream, and k 1 is the
rate of change along the river. Once F 0 and k 1 are determined from an equation fit to
the measured data, the average transport distance can be calculated as

where F ( x )

1
k 1

. The use
of the equation is constrained, however, by two assumptions: (1) that the input of