Comparison of coastal area sediment-seawater distribution coefficients (Kd) of stable and radioactive Sr and Cs

Values of the sediment-seawater distribution coefficient (Kd) in Japanese coastal areas were compared for stable and radioactive Sr and Cs, respectively, in order to clarify whether or not the constant exchangeable fraction in sediment of 20% for the ocean margin (the IAEA recommended method) for all elements was reasonable. Global fallout origin 90Sr and 137Cs concentrations in sediment and seawater were obtained from open data sources as collected in 1964-2010 and 319 and 1506 Kd values were calculated, respectively. When the values were classified into about 10 y intervals from the 1960's to 2000's, Kd values of the two most recent decades did not show any difference for 90Sr and 137Cs. The geometric means of the most recent Kd values in 2000-2010 were 1.2 × 102 L kg−1 for 90Sr and 5.8 × 102 L kg−1 for 137Cs. The stable Sr and Cs concentration data in sediment and seawater samples collected in 2000-2011 were used to calculate stable Sr and Cs Kd values based on the IAEA's recommended method and values were compared. It was found that the stable Sr Kd (geometric mean = 5.3) was much less than that of Kd-90Sr, while stable Cs Kd (geometric mean = 1.7 × 103) was higher than that of Kd-137Cs. Thus, it was judged to be unsuitable to estimate the Kds of these radionuclides by apply a factor of 0.2 to the total concentration of Sr and Cs in Japanese coastal sediment, assuming 20% was the exchangeable fraction. It is likely that applying the constant exchangeable fraction percentage to all elements will sometimes lead to overestimated or underestimated values.