Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 66

This quantity is often difficult to determine, and, as described in more detail below,
can represent a significant constrain on the use of 7 Be as a tracer as a result of its
relatively short half-life (Walling 2013 ).
Blake et al. ( 2009 ) applied the FRNbudgeting approach to a small (89ha), wildfire-
affected catchment in SE Australia. The objective of the study was to assess the
magnitude of the post-fire sediment and nutrient redistribution that had occurred.
Actually, three budgets were constructed using 210 Pb ex ,

137 Cs, and 7 Be, representing
average erosional and depositional processes over approximately the past 60years,
50years, and 3months, respectively (Fig. 3.8 )(Blakeetal. 2009 ). All three tracers
found that ridgetop and sideslope units were the primary sources of sediment to
the drainage network. However, notable differences exist between the constructed
budgets (Fig. 3.8 ). Blake et al. ( 2009 ) argued that these differences not only reflect
differences in the timeframe under consideration, but the materials with which the
isotopes were associated and the depth to which the radionuclides extend below the
surface. 137 Cs was associated with subsurface mineral matter, whereas 210 Pb ex and
7 Be were primarily associated with ash, litter, and soil organic matter at or near the
ground surface. Thus, 137 Cs tended to reflect the erosion of deeper, coarser grained
mineral matter, while 210 Pb ex and 7 Be reflected the movement of litter, ash and
burnt O-horizon materials. Estimated sediment export ratios for 210 Pb ex and 7 Be
suggest that 96-99% of the eroded organic material from the surface reached the
stream channel (Fig. 3.8 ). In contrast, only 67% of the deeper mineral matter was
transported to the channel, the remainder presumably deposited on the slope and/or
valley floor. The point to be made here is that the combined use of the three FRNs not
only provided insights into the magnitude of the erosion that occurred over different
timeframes and the primary sources of sediment to the channel network, but the
nature of the sediment remobilization processes.

Fig. 3.8 Schematic of annual average sediment budgets derived from 210 Pb ex ,

137 Cs, and 7 Be tracer
budgets for a small wildfire affected catchment in southeastern Australia. Thickness of arrows is
approximately proportional to total mass of sediment transferred over the respective timeframes
represented by the radionuclides. The budgeting approach allowed for the estimation of sediment
transfers between landscape units as well as the amount of sediment exported from the catchment
(from Blake et al. 2009 )