New Insights on using Fallout Radionuclides to Estimate Soil Redistribution Rates

The fallout radionuclides such as ¹³⁷Cs have been widely accepted and used in the past 40 yr to provide quantitative soil redistribution estimates at a point scale. Recently their usefulness has been questioned by a few researchers challenging the validity of the key assumption that the spatial distribution of fallout radionuclides in soil is initially uniform. The objective of this work is to reconcile the opposing views by examining the assumption and by developing a new concept for correctly interpreting soil redistribution rates estimated with the tracing technique. This review shows that the spatial uniformity assumption is invalid due to ¹³⁷Cs redistribution caused by spatial variations in rainfall interception, vegetation type and cover, surface residue cover, water infiltration rates, and micro-topography. Thus, the commonly used method of comparing a point ¹³⁷Cs inventory with the reference mean inventory for estimating soil redistribution rate for that point is flawed. Soil redistribution rate estimated for a sampling point is not the true erosion rate because random spatial variation of the radionuclide inventory at the point is erroneously attributed to an effect of soil redistribution. Fortunately, such spatial variations at points are typically random in nature, and therefore can be resolved statistically by increasing independent sample number and by interpreting soil redistribution rate in terms of mean value for a uniform area or slope position, as the random spatial errors of the radionuclide inventories tend to be averaged out.