USE OF FALLOUT RADIONUCLIDES AS INDICATORS OF EROSION PROCESSES

The different depth penetration characteristics of Cs-137, Be-7 and Pb-210 excess in undisturbed soils can be used to identify erosion processes by analysis of sediments derived from surface erosion. Caesium-137 concentrations (half-life 30 years) typically decrease to half the surface value at between 30 and 50 mm. Beryllium-7 (half-life 53 days) has half-penetration depths of between 0.7 and 10 mm, whereas Pb-210 excess (half-life 20.2 years) has half-penetration depths between 10 and 30 mm. Experiments designed to determine the applicability of these depth penetration characteristics to soil erosion studies are reported. Surface runoff was artificially generated at two locations in a grazed paddock using a rainfall simulator. Suspended sediment was extracted from runoff and analysed for natural and artificial gamma emitting radionuclides. Suspended sediment derived from sheet flow contained initially high values of Cs-137, Be-7 and Pb-210 excess. As the experiment continued Cs-137 concentrations remained high, but Be-7 and Pb-210 excess value decreased with time. This is interpreted as indicating a change from sheet dominated erosion to rill dominated erosion. During a second experiment artificial rain was allowed to fall onto an eroded gully wall. The derived suspended sediment contained no detectable Cs-137, Be-7 or Pb-210 excess. Overland flow from above the gully wall was then allowed to run down the gully face and mix with the water falling directly onto the gully wall. There was no detectable change in the radionuclide signature, showing that the gully wall was the predominant source of sediment. This was tested independently by mass balance and Ra-226 to Th-232 ratios. The good correlation between Pb-210 excess and Be-7 at this site suggests that the differential technique described here may be applicable over time-scales longer than are possible with Be-7. It may therefore be practical to examine catchment erosion history through analysis of Pb-210 excess and Cs-137 in sediment cores.