Effects of radiocesium inventory on 137Cs concentrations in river waters of Fukushima, Japan, under base-flow conditions

To investigate the behavior of nuclear accident-derived Cs-137 in river water under base-flow conditions, concentrations of dissolved and particulate Cs-137 were measured at 16 sampling points in seven rivers of Fukushima Prefecture, Japan, in 2012 and 2013. The concentration of dissolved Cs-137 was significantly correlated with the mean Cs-137 inventory in the catchment area above each sampling point in both sampling years. These results suggest that the concentration of dissolved Cs-137 under base-flow conditions is primarily determined by the Cs-137 inventory of the catchment area above the sampling point. However, the concentration of particulate Cs-137 did not show a clear relationship with either the mean Cs-137 inventory or the dissolved Cs-137 concentration, thus indicating that particulate and dissolved forms do not effectively interact in rivers. To evaluate the contribution of the Cs-137 inventory within catchment areas, we analyzed relations between the Cs-137 concentration and the mean Cs-137 inventory over the area within certain flow path lengths that were traced along the river and slope above the sampling point. Coefficients of determination for dissolved Cs-137 concentrations were highest for the longest flow path, i.e., the whole catchment area, and lower for shorter flow paths. Coefficients of determination for particulate Cs-137 concentrations were only moderately high for the shortest flow path in 2012, whereas the values were quite low for all flow paths in 2013. These results suggest that dissolved Cs-137 can originate from a larger area of the catchment even under base-flow conditions; however, particulate Cs-137 did not show such behavior. The results also show that under base-flow conditions, dissolved and particulate Cs-137 behave independently during their transport from river catchments to the ocean. (C) 2015 Elsevier Ltd. All rights reserved.