Validating plutonium-239+240 as a novel soil redistribution tracer – a comparison to measured sediment yield

Quantifying soil redistribution rates is a global challenge addressed with direct sediment measurements (e.g. traps), models, and isotopic, geochemical, and radionuclide tracers. The isotope of plutonium, namely Pu239+240, is a relatively new soil redistribution tracer in this challenge. Direct validation of Pu239+240 as a soil redistribution tracer is, however, still missing. We used a unique sediment yield time series in southern Italy, reaching back to the initial fallout of Pu239+240 to verify Pu239+240 as a soil redistribution tracer. Distributed soil samples (n=55) were collected in the catchment, and at undisturbed reference sites (n=22), Pu239+240 was extracted, measured with inductively coupled plasma mass spectrometry (ICP-MS), and converted to soil redistribution rates. Finally, we used a generalized additive model (GAM) to regionalize soil redistribution estimates for the catchment. For the catchment sites, mean Pu239+240 inventories were significantly reduced (16.8 +/- 10.2 Bq m(-2)) compared to the reference inventory (40.5 +/- 3.5 Bq m(-2)), indicating the dominance of erosion. Converting these inventory losses into soil erosion rates resulted in an average soil loss of 22.2 +/- SD 7.2 t ha(-1) yr(-1). The uncertainties of the approach stemmed mainly from the high measurement uncertainties of some low-activity samples where samples have been bulked over depth. Therefore, we recommend taking incremental soil samples and extracting larger soil volumes (similar to 20 g). The geographic coordinates and the flow accumulation best described the spatial pattern of erosion rates in the GAM model. Using those predictors to upscale Pu-derived soil redistribution rates for the entire catchment resulted in an average on-site loss of 20.7 t ha(-1) yr(-1), which corresponds very well to the long-term average sediment yield of 18.7 t ha(-1) yr(-1) measured at the catchment outlet and to Cs-137-derived soil redistribution rates. Overall, this comparison of Pu-derived soil redistribution rates with measured sediment yield data validates Pu239+240 as a suitable retrospective soil redistribution tracer.