Mass balance models for use with ~(210)Pb, measurements to estimate soil loss from cultivated land

摘要

~(210)Pb (half-life 22.3 years) is a natural product of the ~(235)U decay series that is derived from the decay of gaseous ~(222)Kn. (half-life 3.8 days), the daughter of ~(226)Ra (half-life 1622 years). Like fallout ~(37)CS,~(210)Pb_(ex) reaching the land surface as fallout from the atmosphere will be rapidly adsorbed by the surface soil and its subsequent redistribution within the landscape will reflect the movement of soil and sediment particles associated with soil erosion and sediment transport processes. The radionuclide therefore offers considerable potential for use as a tracer for estimating soil erosion rates. It can be used in regions where ~(137)Cs inventories are low, and therefore difficult to measure, or where they are complicated by additional Chernobyl inputs. Furthermore, due to its essentially constant input over time, it can provide information on erosion rates over a longer period of time. Conversion models utilizing a mass balance approach have been developed for deriving estimates of soil redistribution rates on cultivated land from ~(210)Pb,measurements. There is, however, a need to refine these models to take account of key features of the redistribution of ~(210)Pb_(ex)no in the soil profile. In most cultivated soils ~(210)Pb_(ex), is evenly distributed within the plough layer, but a small proportion of the ~(210)Pb_(ex) inventory is commonly found below the plough layer, forming a tail to the depth distribution. This tail reflects the downward movement of ~(210)Pb(ex)into the soil below the plough layer by a range of mechanisms that can be represented as diffusion and migration processes. Existing conversion models ignore this feature of the depth distribution of ~(210)Pb_(ex) in cultivated soils and therefore overestimate rates of soil loss. This paper reports the development of a revised mass balance model that takes account of the tail of the depth distribution. Soil losses estimated by using the revised model are greater than those estimated by the existing models and the extent is determined by the soil properties, which are related to transportation of the ~(210)Pb_(ex) nuclide from the plough layer to the plough pan layer, such as the plough depth (H), the ~(210)Pb_(ex) depth distribution coefficient (h_0) in the plough pan layer and the ratio of soil bulk densities between the plough pan layer and plough layer (y_(pp)/y_p).