Radiocesium Distribution in the Core Sediments in Urban Tokyo Park

Abstract

We investigated the dynamics of air dose rate and radiocesium (¹³⁴Cs and ¹³⁷Cs) in pond sediments at urban environment, Hikarigaoka Park, Tokyo, where drained the water from the pond on Dec. 2017 for the first time after the Fukushima accident (Mar. 2011). The average of the air dose rates at the height of 1 m outside/inside the pond was 0.069±0.024 μSv/h (n=1926, Ave ±1σ), which did not conflict with the decontamination criteria. The distribution of radiocesium in sediment was totally uniform. The maximum radioactivity of pond sediments was 58.3 Bq/kg-wet, and that of average was 30.6±11.4 Bq/kg-wet (n=80, Ave ±1σ). The maximum inventory was 4.10 kBq/m², which was clearly less than the inventory (6.94 kBq/m²) when assuming the pond as Japanese paddy field. This fact showed that the pond had the small accumulation effect of radiocesium at the time of our survey. It is estimated that this effect is caused by a mechanism that constantly circulates the pond water.