Time changes in suspended sediment radiocesium concentration in rivers in Fukushima affected by the Fukushima Daiichi NPP accident

Abstract:

Due to Fukushima Daiichi Nuclear Power Plant accident, radioactive materials including Cs-134 and Cs-137 were widely distributed in surrounded area. The radiocesium deposited in Fukushima area have been transported in river networks. The monitoring started at 6 sites from June 2011. Subsequently, additional 24 monitoring sites were installed between October 2012 and January 2013. Flow and turbidity (for calculation of suspended sediment concentration) were measured at each site, while suspended sediments and river water were collected every one or half month to measure Cs-134 and Cs-137 activity concentrations by gamma spectrometry.

Our monitoring result data demonstrated that the Cs-137 activity concentration in sediment eroded from the runoff-erosion plot, has been almost constant for the past 3 years, however the Cs-137 concentration of suspended sediment from the forested catchment showed a slight decrease through with time. On the other hand, the suspended sediment from paddy fields and those in river water from large catchments exhibited rapid decrease in Cs-137 activity concentration with time. The decreasing trend of Cs-137 activity concentration was fitted using a two-component exponential model. Differences in the exponential reduction rates of the model were compared and discussed with respect to various land uses and catchment scales. Such analysis can provide important insights into the future prediction of radiocesium wash-off from catchments with different land uses.