A multi-radionuclide approach to evaluate the suitability of ^239 + 240Pu as soil erosion tracer

Fallout radionuclides have been used successfully worldwide as tracers for soil erosion, but relatively few studies exploit the full potential of plutonium (Pu) isotopes. Hence, this study aims to explore the suitability of the plutonium isotopes ²³⁹Pu and ²⁴⁰Pu as a method to assess soil erosion magnitude by comparison to more established fallout radionuclides such as ¹³⁷Cs and ²¹⁰Pb_ex. As test area an erosion affected headwater catchment of the Lake Soyang (South Korea) was selected. All three fallout radionuclides confirmed high erosion rates for agricultural sites (> 25 t ha^− 1 yr^− 1). Pu isotopes further allowed determining the origin of the fallout. Both ²⁴⁰Pu/²³⁹Pu atomic ratios and ^239 + 240Pu/¹³⁷Cs activity ratios were close to the global fallout ratio. However, the depth profile of the ^239 + 240Pu/¹³⁷Cs activity ratios in undisturbed sites showed lower ratios in the top soil increments, which might be due to higher migration rates of ^239 + 240Pu. The activity ratios further indicated preferential transport of ¹³⁷Cs from eroded sites (higher ratio compared to the global fallout) to the depositional sites (smaller ratio). As such the ^239 + 240Pu/¹³⁷Cs activity ratio offered a new approach to parameterize a particle size correction factor that can be applied when both ¹³⁷Cs and ^239 + 240Pu have the same fallout source. Implementing this particle size correction factor in the conversion of ¹³⁷Cs inventories resulted in comparable estimates of soil loss for ¹³⁷Cs and ^239 + 240Pu. The comparison among the different fallout radionuclides highlights the suitability of ^239 + 240Pu through less preferential transport compared to ¹³⁷Cs and the possibility to gain information regarding the origin of the fallout. In conclusion, ^239 + 240Pu is a promising soil erosion tracer, however, since the behaviour i.e. vertical migration in the soil and lateral transport during water erosion was shown to differ from that of ¹³⁷Cs, there is a clear need for a wider agro-environmental testing.