Radioactivity Dynamics in forests

（2016）

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

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.

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