Distribution And Migration Characteristics Of Typical Radionuclides In China Coastal Environment After The Fukushima Nuclear Power Plant Accident

After Fukushima Nuclear Power Plant (FNPP) accident, the migration characteristics of typical artificial radionuclides (131Ⅰ and 134,137Cs) in atmospheric aerosols in Shanghai, and the distribution patterns of 137Cs in China Seas, as well as the distribution patterns of 129Ⅰ concentration and atomic ratios of 129Ⅰ/127Ⅰ in the East China Sea (ECS) were analyzed. The impacts on the environment radioactivity background of the China Seas from the FNPP were discussed and the main results are obtained as follows:The distribution patterns of 131Ⅰ,134,137Cs activities and their ratios in Shanghai aerosols were obtained. The date when 131Ⅰ could be detectable (2011/3/27) in Shanghai was earlier than 134Cs (2011/4/6) and 137Cs (2011/4/4). The activities of 131Ⅰ (0.01-1.20 mBq/m3) were 2-10 times higher than 134Cs (0.01-0.58 mBq/m3) and 137Cs (0.01-0.65 mBq/m3). Moreover, multiple peak values of 131Ⅰ were found within different time periods. The activity ratios of 131Ⅰ/137Cs (1.3-10.6) decreased after the date of April 5th,2011, while those of 134Cs/137Cs (0.8-2.9) varied at about 1.1. The radio-aerosols in shanghai during the FNPP accident were mainly transported from the northeast and northwest paths by using HYSPLIT model. Meanwhile, the northeast pathway was confirmed to be dominant by the analysis of the reported radio-aerosol data in the concerned cities of China during that time period. After the distributions of maximum values of 131Ⅰ/137Cs and 134Cs/137Cs activity ratios were summarized, we revealed the spreading processes of radio-aerosols in the northern hemisphereIn spring of 2011 (May),2 months after the FNPP accident, the 137Cs activities in surface water of the ECS were ranged of 0.66-1.36 Bq/m3 with an average value of 1.07±0.09 Bq/m3 (n=56).137Cs activities were low in nearshore areas and high in offshore areas. Two regions with high 137Cs activities appeared in the zones of Kuroshio Current Water (KCW) and the Yellow Sea Surface Current (YSSC). Low 137Cs activities appeared in the Zhejiang-Fujian Costal Current (ZFCC). Except for that,137Cs activities in most part of the ECS were around 1.10 Bq/m3. Vertical distributions of 137Cs activities in three water columns at stations G8, D9 and CJ along on the KCW path were similar with peak values showing at subsurface depths (G8: 1.14±0.07 Bq/m3 (200 m); D9:1.44±0.17 Bq/m3 (400 m); CJ:1.84±0.10 Bq/m3 (280 m)). The 137Cs inventories of these columns were 1219 Bq/m2 (G8),1182 Bq/m2 (D9) and 1283 Bq/m2 (CJ), respectively. Compared with historical data in the adjacent areas, there was no obvious input from the FNPP accident to the ECS. In summer of 2013 (August),2 years after the FNPP accident, the 137Cs activities in surface water of the ECS were ranged of 0.12-1.59 Bq/m3 with an average value of 0.63±0.12 Bq/m3 (n=52). High 137Cs activities appeared at the junction zone of the South Yellow Sea (SYS) and the ECS, and the area where the Taiwan Strait Warm Water (TSWW) and the KCW passed, which was similar with that in spring. The overall vertical distributions of 137Cs in water columns at station C12 and F8 (also on the KCW path) were similar with Station G8, D9 and CJ in the spring of 2011. The peak values, 1.92±0.36 Bq/m3 (C12) and 1.10±0.10 Bq/m3 (F8), also appeared at the subsurface water but with shallower water depths (100 m and 200 m). At the same time, the inventory of 137Cs at station F8 was 405 Bq/m2 which was much smaller than that of station G8 in Spring of 2011. Only surface water samples in the northeast part of the ECS were taken in autumn of 2013 (October), and the 137Cs activities were ranged of 0.26-0.72 Bq/m3 with an average value of 0.55±0.12 Bq/m3 (n=12) in autumn of 2013 (October). The spatial distributions of 137Cs followed the same patterns of those in summer.The 137Cs activities in surface water of the SYS, North Yellow Sea (NYS) and the Bohai Sea (BS) were ranged of 0.16-0.68 Bq/m3 with an average value of 0.46±0.06 Bq/m3 (n=11),0.08-0.56 Bq/m3 with an average value of 0.30±0.07 Bq/m3 (n=10) and 0.03-0.09 Bq/m3 with an average value of 0.05±0.01 Bq/m3 (n=9) in spring of 2014 (May), respectively.137Cs activities in the Yellow Sea (YS) and the BS were obviously smaller than those in the ECS, and the activity values decreased northward from the SYS, NYS to the BS. The vertical distributions of 137Cs in the YS and BS presented a slightly decreasing trend and showed poor correlation with salinity.In addition to the cesium isotopes, we also focused on the iodine isotopes which were more sensitive to the changes of marine environmental radioactivity during the study of offshore radioactive background level of the China Seas. The 129Ⅰ concentration in surface water of the ECS were ranged of (0.61-1.94)×107 atoms/L with an average value of (1.22±0.09)×107 atoms/L (n=26) in autumn of 2013 (October). High 129Ⅰ appeared in the north and nearshore areas, with low concentration in the south and offshore areas. In detail, high concentration of 129Ⅰ was observed in the Changjiang Estuary within 50 m isobath border and the junction zone between the YS and the ECS. The maximum 129Ⅰ appeared in surface water and presented a weakly decreasing trend from surface to bottom, and the vertical distribution showed no relationship with temperature and salinity. The 129Ⅰ/127Ⅰ atomic ratios in surface water of the ECS were ranged of (4.72-14.75) ×10-11 with an average value of (7.02±0.49) ×10-11 (n=26).129Ⅰ/127Ⅰ atomic ratios had a similar distribution with 129Ⅰ, with high ratios observed in the north part and the nearshore area, and low values in the south part and the offshore area. Especially, high 129Ⅰ/127Ⅰ atomic ratios were also found intensively within 50 m isobath border.Combining the lower limit of detection (LLD) of 134Cs (0.03 Bq/m3) with the observed 137Cs from three cruises in the ECS, we estimated that the FNPP accident contributed less than 3% of 137Cs to the ECS. This phenomenon illustrated that very limited amounts of 137Cs were inputted to the ECS by the atmospheric deposition and the oceanic transportation, i.e., the FNPP accident didn’t arouse a significant impact on background radioactivity in the ECS during our investigation periods. Meanwhile, our study further confirmed that the Fukushima accident didn’t impact the radioactive environment of the ECS by combining with the investigation of the concentration of 129Ⅰ and the atomic ratios of 129Ⅰ/127Ⅰ. Moreover, we found a certain relationship between the 137Cs distribution patterns and water mass structure of the ECS, especially in spring (May of 2011). But the corresponding relation was not remarkable between the YS and the BS.