Riverine 137Cs dynamics and remoralization in coastal waters during high flow events

Understanding riverine ¹³⁷Cs dynamics during high-flow events is crucial for improving predictability of ¹³⁷Cs transportation and relevant hydrological responses. It is frequently documented that the majority of ¹³⁷Cs is exported during high-flow events triggered by intensive rainfall. Studies on ¹³⁷Cs in coastal seawater suggested that a huge high-flow events resulted in high dissolved ¹³⁷Cs concentration in seawater. Different temporal patterns of ¹³⁷Cs concentrations in river water are found in the existing literature on ¹³⁷Cs dynamics during high-flow events. Although such differences may reflect catchment characteristics, there is no comprehensive analysis for the relationships. This study explores catchment characteristics affecting ¹³⁷Cs transport via river to ocean based on datasets obtained by sampling campaigns during high-flow events. ¹³⁷Cs datasets obtained at 13 points in 6 river water systems were subject to the analysis. The analyses intended to explore relationship between catchment characteristics (scale and land use composition) and ¹³⁷Cs dynamics in terms of variations in concentration, fluxes, and potential remobilization in seawater. We could not find any significant correlations between the parameters of catchment characteristics and mean values of normalized concentrations of ¹³⁷Cs and apparent K_d. However, when approximating ¹³⁷Cs concentrations and K_d value as a power function of suspended solid concentration (Y=α X^β), the power of β in the equations for dissolved ¹³⁷Cs concentration and K_d showed negative and positive correlations with the logarithm of the watershed area, respectively, and the positive β was found when the catchment area was on the order of 100 km² or larger and vice versa. This indicates that the concentration of dissolved ¹³⁷Cs tends to decrease with increased water discharge in larger catchments for smaller catchments. These results suggest that the temporal pattern of dissolved ¹³⁷Cs concentrations depends on watershed scale. ¹³⁷Cs flux during a single event ranged from 1.9 GBq to 1.1 TBq and accounted for 0.00074% to 0.22% of total ¹³⁷Cs deposited in relevant catchments. Particulate ¹³⁷Cs flux accounted for more than 92% of total ¹³⁷Cs flux, except for Ukedo River basin with a large dam reservoir. R-factor, an erosivity index in the Universal Soil Loss Equation model family, is a good parameter for reproducing sediment discharge and particulate ¹³⁷Cs flux. Efficiency of particulate ¹³⁷Cs flux, calculated by dividing the flux by R-factor of event, tended to be high in catchments with relatively low forest cover. Desorption ratio of ¹³⁷Cs, obtained by 1-day shaking experiment of SS in seawater, ranged from 2.8 to 6.6%. The ratio was almost proportional of ratio of exchangeable ¹³⁷Cs. The estimated amounts of desorbed ¹³⁷Cs, obtained by multiplying particulate ¹³⁷Cs and the desorption ratios, were greater than direct flux of dissolved ¹³⁷Cs. Reanalysis of riverine ¹³⁷Cs dataset in high flow events is revealing relationship between catchment characteristics and ¹³⁷Cs dynamics. Further analyses, such as evaluation of decontamination impacts and inter-catchment comparisons of ¹³⁷Cs fluxes, are required for better understanding.