Determining soil loss using the inventory ratio of excess lead-210 to cesium-137

The anthropogenic nuclide Cs-137 is widely used to measure soil movement. However, changes in Cs-137 areal concentrations may not result from soil redistribution alone. There can be considerable variabilty in Cs-137 fallout due to rain shadowing and small-scale runon-runoff processes at the time of deposition. Fallout Pb-210 is also affected by these processes and thus areal concentrations of the two should be correlated. The ratio of fallout (210)pb to (CS)-C-137 reduces variability by up to a factor of two in undisturbed control forest sites at St. Helens, Tasmania. In addition, these two nuclides penetrate to different depths, thereby producing a varying activity ratio with depth, and this gives rise to a new method for determining soil loss. The average inventory ratios of fallout Pb-210 to Cs-137 from plots adjacent to the controls were measured; these had undergone ''normal'' and ''minimal impact'' logging procedures. The average core inventory ratio at the control locations was 2.24 +/- 0.14 (n = 18), compared with means of 0.74 +/- 0.09 (n = 10) and 1.73 +/- 0.29 (n = 10) from the normal and minimal impact sites. The average depth of soil removed from the logged sites was then calculated by comparing these ratios with the inventory activity ratio curve from their respective control sites (40 +/- 6 mm, normal site; 17 +/- 5 mm, minimal site). We concluded that, compared with areal concentrations alone, ratios of fallout nuclides are likely to provide a less randomly variable (and thus more sensitive) method for investigating surface erosion in landscapes where vertical soil mixing is not sufficiently recent to be of concern.