Transformation of Radioactive Contamination of Agricultural Land in the Upa River Basin Due to Erosion Processes

This study evaluates long-term changes (up to 2050) in 137 Cs contamination levels across agricultural lands within the Upa River basin (Tula Region, Russia). Following the 1986 Chernobyl disaster, this river basin remains one of the most heavily contaminated areas in the Central Russian Upland. Developing mid- and long-term predictive contamination maps is a critical environmental task. It requires accounting for all processes beyond radioactive decay that significantly influence radionuclide density. Due to the chemical properties of radiocesium, its lateral migration occurs primarily bound to fine sediments within sediment yield. Consequently, assessing soil erosion dynamics on arable lands is essential for accurate environmental forecasting. However, existing publicly available contamination atlases omitted the erosion factor due to the small scale of their mapping materials. In this study, we verified the radioactive contamination map using past field observations and refined the forecast of contamination density changes on agricultural lands by incorporating long- term average erosion rates modelled with the WaTEM/SEDEM model. Radioactive decay remains the dominant factor driving contamination density reduction across most of the studied area. However, identifying local hotspots of severe erosion and associated 137 Cs inventory redistribution serves as a key predictor of secondary environmental risks linked to the accumulation of legacy sediments and bound radionuclides within the fluvial network.