Аннотация:INTRODUCTION
Processes of vertical and lateral migration lead to gradual reduction in contamination of catchment soil,
particularly its top layer. This, in turn, results in a gradual decrease of radiocesium activity concentrations
in the surface runoff and river water, in both dissolved and particulate forms. The purpose of this research
is to study the dynamics of Fukushima-derived radiocesium in undisturbed soils and floodplain deposits
exposed to erosion and sedimentation during floods.
METHODS
Seven cross-sections for sampling and sediment traps installation were selected on the Niida River
floodplain. One cross-section characterized the headwaters of the Hiso River basin upstream of its most
polluted part (site N7). Three cross-sections integrate the 137Cs concentration in sediments delivered from
headwaters of the main uplands rivers (Upper Niida, Iitoi and Hiso, sites N4, N5 and N6 respectively).
Two cross-sections are in lower reaches of the Niida River (sites N1 and N2). Site N3 was selected for
integration of 137Cs concentration in sediments transported from both the Iitoi and the Upper Niida Rivers
at the boundary between headwaters uplands and mid-basin rangelands (Konoplev et al., 2018).
Fig. 1 demonstrates dynamics of 137Cs vertical distribution in the upper 30-cm layer of soil/sediments at
four selected observation sites on the Niida River floodplain.
CONCLUSIONS
Combined observations of radiocesium vertical distribution in soil and sediment deposition on artificial
lawn-grass mats on the Niida River floodplain allowed us to estimate both annual mean sediment
accumulation rates and maximum sedimentation rates corresponding to an extreme flood event during
Tropical Storm Etau, 6-11 September 2015. Dose rates were reduced considerably for floodplain sections
with high sedimentation because the top soil layer with high radionuclide contamination was eroded
and/or buried under cleaner fresh sediments produced mostly due to bank erosion and sediments
movements. Sediment deposition for downstream section of Niida river floodplain reached 20 cm after the
event, and for upstream section in the area of confluence of Hiso river with basic Niida river – up to 40
cm. Extreme flood events during typhoons result in fast and efficient natural attenuation. Generally, due to
higher precipitation, steeper slopes, higher temperatures and higher biological activities in soils, selfpurification
and natural attenuation of radioactive contamination in Fukushima associated with vertical
and lateral radionuclide migration is essentially faster than in Chernobyl. Accounting for soil erosion
and sediment accumulation within river catchment and in particular, river floodplain, is key for
predicting redistribution of radioactive contamination after the FDNPP accident on the
contaminated territories, as well as for decision making about remediation and clean-up of
contaminated territories.