Migration of Fallout Radionuclides and Soil Erosion of Hongsongwa Mountainous in China

To study the effect of soil erosion on the distribution and migration pattern of radionuclides, the levels of ^239+240Pu and ¹³⁷Cs in alpine meadow soil were measured in the Hongsongwa Nature Reserve, Hebei Province. The measured activities of ^239+240Pu and ¹³⁷Cs in surface soil ranged from 0.028 to 2.781 Bq/kg and from 1.3 to 59.8 Bq/kg, respectively. The distribution of ¹³⁷Cs and ^239+240Pu is uneven and significantly correlated with the organic matter content and altitude variations within the study area. Core samples were collected from both ridge and valley locations to assess erosion rates, revealing that ridge areas experienced approximately 2.5 times higher erosion rates (18.0 t ha^–l a ^–1) compared to valleys (7.3 t ha^–l a ^–1). The vertical migration behavior of ^239+240Pu and ¹³⁷Cs was quantitatively described by a convection-diffusion equation model. Results indicated that core samples taken from the ridge displayed significantly higher apparent diffusion coefficients (D_Cs = 4.57 cm²/y; D_Pu = 2.42 cm²/y) as well as apparent convection coefficient (ν_Cs = 0.31 cm/y; ν_Pu = 0.43 cm/y), which were approximately 10 and 2 times those observed in reference sample (D_Cs = 0.33 cm²/y; D_Pu = 0.32 cm²/y; ν_Cs = 0.16 cm/y; ν_Pu = 0.17 cm/y), respectively. The migration rate of ^239+240Pu is accelerated by 39% compared to that of ¹³⁷Cs due to soil erosion. The diffusion and convection rates of both isotopes in the valley sample are similar to those in the reference sample. In general, soil erosion significantly affects the horizontal and vertical migration of ^239+240Pu and ¹³⁷Cs.