Mechanical soil decontamination is an important tool in remediating contaminated soils. Remediation efforts following the Fukushima Daiichi disaster increased soil erosion and downstream sediment loads that showed reduced Cs-137 concentrations, but rapid revegetation quickly restored decontaminated landscapes.

In Fukushima, government-led decontamination reduced radiation risk and recovered Cs-137-contaminated soil, yet its long-term downstream impacts remain unclear. Here we provide the comprehensive decontamination impact assessment from 2013 to 2018 using governmental decontamination data, high-resolution satellite images and concurrent river monitoring results. We find that regional erosion potential intensified during decontamination (2013-2016) but decreased in the subsequent revegetation stage. Compared with 2013, suspended sediment at the 1-year-flood discharge increased by 237.1% in 2016. A mixing model suggests that the gradually increasing sediment from decontaminated regions caused a rapid particulate Cs-137 decline, whereas no significant changes in downstream discharge-normalized Cs-137 flux were observed after decontamination. Our findings demonstrate that upstream decontamination caused persistently excessive suspended sediment loads downstream, though with reduced Cs-137 concentration, and that rapid vegetation recovery can shorten the duration of such unsustainable impacts. Future upstream remediation should thus consider pre-assessing local natural restoration and preparing appropriate revegetation measures in remediated regions for downstream sustainability.