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Radioactivity Dynamics in forests


When it rains, does radioactive cesium flow out of forests with soil? 【137Cs flow in heavy rain】

In times of heavy rain, radioactive cesium flows out of forests with soil. But the amount of cesium flowing out of forests is small compared with that deposited inside forests.

  • Investigations in Fukushima Prefecture have revealed that most radioactive cesium flowing out from forests is not dissolved in water (dissolved state), but adsorbed on soil particles (particulate state).
  • Cesium outflow from forests is mainly caused by typhoons and heavy rains that happen a few times each year.
  • According to investigations in Fukushima Prefecture, the ratio of cesium outflow to that deposited in forests is less than about 0.4% per a year even the heaviest rain in recorded history, Sep, 2015. Therefore, most of cesium remains in forests even now.

Outflowing soil in a mountain stream in forest

Fig.1 Outflowing soil in a mountain stream in forest

Table 1 Outflow behavior in forest area for cesium (137Cs) released by nuclear accident

Upstream of Uda River(For 36 months) Upstream of Ohta River(For 24months)
Deposited amount(kBq/m2) 170 1,900
Concentration of 137Cs from suspended sediments(kBq/kg) 6.8~9.3 61~130
Outflowing amount of 137Cs from suspended sediments(kBq/m2) 0.51 8.8
Proportion of outflowing 137Cs from suspended sediments(%) 0.30 0.46
Proportion of outflowing 137Cs in a year(%) 0.04~0.16 0.08~0.38

Observation period:
Upstream of Uda River:Sep 15, 2012-Sep 15, 2015
Upstream of Ohta River:Jan 1, 2014-Dec 31, 2015