Level, Distribution, and Soil Redistribution Assessment of Plutonium Isotopes in the Desert-Loess Critical Zone in China

Soil erosion in hilly and gully areas of the Loess Plateau has attracted considerable attention in recent decades. However, the understanding of soil redistribution in the desert-loess critical zone, where wind and water erosion interact, remains limited. Plutonium (Pu) isotopes have received growing interest in environmental level assessments and as tracers because of their distinct geochemical characteristics and ideal tracer properties. In this study, eight soil profiles representing different land use types were collected from Jingbian County. The activity concentrations of ^239+240Pu and ²⁴⁰Pu/²³⁹Pu atom ratios in soil samples were determined using inductively coupled plasma mass spectrometry (ICP-MS). The soil redistribution rates under different land use patterns were assessed using Pu isotopes as tracers, and the vertical distribution of ^239+240Pu activity concentration was analyzed. Additionally, the migration behavior of Pu isotopes across temporal scales was modeled using the Convection-Dispersion Equation (CDE) model. The results show the ²⁴⁰Pu/²³⁹Pu atom ratios (0.166–0.202, mean 0.175) indicate that the global fallout from atmospheric nuclear weapons testing is the primary source of Pu in the study area; The ^239+240Pu activity concentrations ranging from 0.002 to 0.470 mBq/g, consist with background values in China; The CDE model predictions indicate that the distribution of Pu isotopes in the soil profile will become increasingly uniform over the next 100–200 years. Based on the Pu tracing model, the estimated soil redistribution rates vary from -3.87 to 39.74 t/(ha·yr), where positive values indicate net erosion and negative values indicate net sedimentation. Soil erosion rates follow the order: planted woodland (1.71 t/(ha·yr)) < sunflower field (28.19 t/(ha·yr)) < cornfield (29.69 t/(ha·yr)). Severe erosion occurred predominantly in cultivated areas with low vegetation cover and high elevations. In contrast, grassland and woodland were more effective in soil and water retention, mitigating erosion intensity. These findings improve the understanding of Pu isotopes behavior and soil redistribution processes in the desert-loess critical zone and provide a scientific basis for optimizing land use patterns. In practice, expanding grassland and woodland are effective measures for soil and water conservation.