Applicability of reconstruction of temporal changes in 137Cs concentrations in pond water based on the vertical distribution of bottom sediments: a case in three ponds in the Fukushima evacuation zone.

Long-term changes in particulate and dissolved 137Cs concentrations in rivers and dam reservoirs have been reconstructed using 137Cs depth distributions in bottom sediments near Chornobyl and Fukushima. However, few studies have applied this method to pond water. This study tested the applicability of this reconstruction method in ponds within the Fukushima evacuation zone. Bottom sediments in ponds were collected in November 2019. Our reconstruction of a shallow closed pond disagreed with our observations. Even in ponds where the reconstruction agreed well with observations, underestimations may have occurred because the time constant for the decrease in 137Cs concentration was lower than those previously reported. We conclude that the present reconstruction method may be applied to deeper Fukushima ponds with inflow and outflow, but underestimations of initial 137Cs concentrations in water may lead to uncertainties. For realistic reconstruction, attention should be paid to pond characteristics, catchment conditions, and sediment core selection.