Investigating the source of radiocesium contaminated sediment in two Fukushima coastal catchments with sediment tracing techniques

The Fukushima Dai-ichi nuclear power plant accident resulted in the fallout of significant quantities of radiocesium. After deposition on the soil surface, rainfall and spring flood events transfer radiocesium downstream. Identifying the source of contaminated sediment is important for managing potential downstream radiocesium contamination. Soil (n = 37) and sediment (n = 211) were sampled from November 2011 to May 2014 in the Mano and the Niida coastal catchments. Two sediment fingerprinting approaches quantified the source of radiocesium contaminated sediment. First, cesium-137 activities in surface soil and sediment samples were modelled to determine the contribution of upstream, more contaminated areas, to sediment transiting the more densely populated coastal plain. Second, elemental geochemistry of three major soil types (zs, Cambisols and Fluvisols) was used to model the relative contribution of these soils to sediment sampled throughout the catchments. In the Niida catchment, 47% (σ 19%) of sediment sampled in the coastal plain was modelled to be derived from the upstream area whereas, it was only 19% (σ 19%) in the Mano catchment. The main factor driving this difference is the presence of a large dam on the main stem of the Mano River. Geochemical modelling results indicated that Fluvisols, an alluvial soil type on which paddy fields are typically cultivated, supply the majority of sediment (μ 76, σ 14%). The results confirm that the management of dams is fundamental to radiocesium migration. Moreover, this research indicates that Fluvisols and concomitantly, rice paddies on this soil type, supply a disproportionate amount of sediment. Managing sediment derived from Fluvisols, while incorporating potential impacts from major dams, should help mitigate the downstream migration of radiocesium contaminated sediment.