Nuclear accident-derived (3)H in river water of Fukushima Prefecture during 2011-2014.

During 2011-2014, we measured (3)H concentrations in river water samples collected during base flow conditions and during several flood events from two small rivers in a mountainous area in Fukushima Prefecture, which received deposition of (137)Cs from the Fukushima Dai-ichi Nuclear Power Plant accident. (3)H concentrations above background levels were found in water samples collected during both base flow conditions and flood events in 2011. The (3)H concentrations during flood events were generally higher than those during base flow conditions. The (3)H concentrations in both rivers during base flow conditions and flood events decreased with time after the accident and reached almost background levels in 2013. We also measured (3)H concentrations in freshwater samples from 16 other rivers and one dam in eastern Fukushima Prefecture from 2012 to 2014 during base flow conditions. The measured (3)H concentrations were higher than the background level in 2012 and decreased with time. The (137)Cs inventory in the catchment area at each sampling point was estimated from air-borne monitoring results in the literature and compared with the (3)H concentrations. We found surprisingly good correlations between (137)Cs inventories in the catchment areas and (3)H concentrations in the water samples. Further studies will be necessary to clarify the reason for the good correlation.