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 137 Cs in river water under base-flow conditions, concentrations of dissolved and particulate 137 Cs were measured at 16 sampling points in seven rivers of Fukushima Prefecture, Japan, in 2012 and 2013. The concentration of dissolved 137 Cs was significantly correlated with the mean 137 Cs inventory in the catchment area above each sampling point in both sampling years. These results suggest that the concentration of dissolved 137 Cs under base-flow conditions is primarily determined by the 137 Cs inventory of the catchment area above the sampling point. However, the concentration of particulate 137 Cs did not show a clear relationship with either the mean 137 Cs inventory or the dissolved 137 Cs concentration, thus indicating that particulate and dissolved forms do not effectively interact in rivers. To evaluate the contribution of the 137 Cs inventory within catchment areas, we analyzed relations between the 137 Cs concentration and the mean 137 Cs 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 137 Cs 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 137 Cs 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 137 Cs can originate from a larger area of the catchment even under base-flow conditions; however, particulate 137 Cs did not show such behavior. The results also show that under base-flow conditions, dissolved and particulate 137 Cs behave independently during their transport from river catchments to the ocean. [ABSTRACT FROM AUTHOR]