Impact of wildfire on 137Cs and 90Sr wash-off in heavily contaminated forests in the Chernobyl exclusion zone

Wildfires may play a role in redistributing radionuclides in the environment in combination with hydrological processes such as surface runoff and soil erosion. We investigated plot-scale radionuclide wash-off at forest sites affected by wildfires in the Chernobyl Exclusion Zone (CEZ). We also compared speciation of the washed-off radionuclides with those in previous studies conducted just after the accident in 1986. We observed the surface runoff and the radionuclide wash-off with a soil erosion plot at forest and post-fire sites during May–September 2018. In the post-fire site, 2.81 mm of surface runoff was observed in at least three flow events resulting from 285.8 mm total rainfall. The fluxes of dissolved and particulate ¹³⁷Cs were estimated as 4.9 and 161 Bq m⁻², respectively. The dissolved phase ⁹⁰Sr flux was estimated as 214 Bq m⁻². At the forest site, a single surface runoff (0.67 mm) event was generated by rainfall of 182.2 mm. The fluxes of dissolved and particulate ¹³⁷Cs wash-off values were 6.2 and 8.6 Bq m⁻², respectively. The flux of dissolved ⁹⁰Sr wash-off from the forest was estimated as 45.1 Bq m⁻². The distribution coefficient, which indicates the dissolved-particulate form of radionuclides, in the post-fire site was 30 times higher than that in the forest site, indicating the importance of particulate ¹³⁷Cs wash-off after fire in the CEZ. The entrainment coefficients for dissolved and particulate ¹³⁷Cs concentrations were around 50 times lower than those obtained in the corresponding position within the CEZ immediately after the accident in 1987. The effect of downward migration of ¹³⁷Cs over 30 years led to decreased entrainment coefficients for dissolved and particulate ¹³⁷Cs. The effect of downward migration of radionuclides was considered sufficient to indicate changes in normalized liquid and solid radionuclides wash-off entrainment coefficient and the distribution coefficient in this study.