210Pb as a tracer of soil erosion, sediment source area identification and particle transport in the terrestrial environment

Although 137Cs has been used extensively to study soil erosion and particle transport in the terrestrial environment, there has been much less work using excess or unsupported 210Pb (210Pbxs) to study the same processes. Furthermore, since 137Cs activities in soils are decreasing because of radioactive decay, some locations have an added complication due to the addition of Chernobyl-derived 137Cs, and the activities of 137Cs in the southern hemisphere are low, there is a need to develop techniques that use 210Pbxs to provide estimates of rates of soil erosion and particle transport. This paper reviews the current status of 210Pbxs methods to quantify soil erosion rates, to identify and partition suspended sediment source areas, and to determine the transport rates of particles in the terrestrial landscape. Soil erosion rates determined using 210Pbxs are based on the unsupported 210Pb (210Pbxs) inventory in the soil, the depth distribution of 210Pbxs, and a mass balance calibration ('conversion model') that relates the soil inventory to the erosion rate using a 'reference site' at which neither soil erosion nor soil deposition has occurred. In this paper several different models are presented to illustrate the effects of different model assumptions such as the timing, depth and rates of the surface soil mixing on the calculated erosion rates. The suitability of model assumptions, including estimates of the depositional flux of 210Pbxs to the soil surface and the post-depositional mobility of 210Pb are also discussed. 210Pb can be used as one tracer to permit sediment source area identification. This sediment 'fingerprinting' has been extended far beyond using 210Pb as a single radioisotope to include numerous radioactive and stable tracers and has been applied to identifying the source areas of suspended sediment based on underlying rock type, land use (roads, stream banks, channel beds, cultivated or uncultivated lands, pasture lands, forested lands, construction sites, undisturbed lands) or style of erosion (sheet wash, rills, bank). The transport time of particles in the terrestrial system can be estimated using 7Be/210Pbxs radionuclide ratios and from mass balance models of 210Pbxs and/or 7Be in streams. Watershed residence times can be calculated from the radionuclide inventory and the erosional loss rate.