*5.1. Dispersion of High 90Sr and 134, 137Cs Activity Plume*

The high activities of 90Sr and 137Cs in the coastal region (Figure 3) can be explained by the release at the time of the FDNPP accident and the physical processes that later occurred in the ocean. In the coastal region, high 90Sr activity seawater samples with high 134Cs and 137Cs activities were mainly collected from south of the FDNPP (NP2, and AN7) to off Iwaki (S12 and S6), which reflects the southward transport of seawater along the Fukushima coast by the coastal currents. Higher 90Sr activity in seawater (>8 mBq L<sup>−</sup>1) was found where the salinity was 33.23–33.33 psu and 11.95–12.41 psu. In the coastal region, no clear correlation between 90Sr and salinity or 90Sr and temperature was observed.

**Figure 3.** Distribution of 90Sr and 137Cs activities in surface seawater collected in May 2013.

The distributions of 90Sr and 137Cs activities in May 2013 observed in this study correspond to those of a model simulation of the direct-release event between 26 March and 6 April 2011 [20]. The 137Cs released from the FDNPP from 26 March was initially advected southward, then transported to the Ibaraki coast. This simulation suggested that the 137Cs concentration decreased in May due to advection and diffusion in the open ocean. The coastal currents are variable in this region and sometimes flow northward. Oikawa et al. [16] compiled monitoring data for seawater in the coastal region by the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and suggested that the activities of 90Sr in surface water decreased slowly over time in 2011 and reached the background level by the end of December 2011. However, because of a lack of sampling sites for 90Sr, the 90Sr plume could have been missed in previous observations. The distribution of our results indicates that the released 90Sr plume from the FDNPP site (leakage of contaminated water from storage tanks) could move to the coastal region south of the FDNPP, carried by the southward coastal current.

Both activities decreased rapidly from NP2 toward the eastern sites (S6, NP1, and NP3), which indicate that the eastward dispersion was limited because of the effect of the southern coastal current during this sampling period. Compared with the pre-Fukushima accident, offshore 90Sr activities in the north Pacific Ocean (1.0 ± 0.1) [3] and the activities measured in May 2013 indicate that the influences of the Fukushima-derived 90Sr on open ocean sites in the mixed region between the Oyashio and Kuroshio currents were negligibly small, as were those of 134Cs and 137Cs. However, if any accidental releases from the FDNPP site were to occur during the decommissioning of the reactors, coastal areas could be exposed to a high activity plume.

In September 2017, low 90Sr (1.0−1.8 mBq L−1) and high 137Cs (9−43 mBq L−1) activities were obtained at low salinity (4–28 psu) in groundwater and beach seawater samples from Sendai Bay, located north of the FDNPP [17]. 90Sr/ 137Cs ratios ranged from 0.036 to 0.19. The 137Cs activity in low salinity samples was affected by atmospheric fallout from the FDNPP accident that was deposited on land, while 90Sr activity was not sensitive to terrestrial input. Therefore, the relationship between 90Sr and 137Cs can be a useful indicator for river input.

The mouth of the Ukedo River is located between collection sites NP2 and AN6. 137Cs activity at NP2 (39.0 mBq L−1) was more than five times higher than that at AN6 (6.9 mBq L−1), though their salinities were comparable (33.24 and 33.26 psu). Dissolved 137Cs activity in the Ukedo River, which drains a highly contaminated area, ranged from 200 to 1100 Bq L−<sup>1</sup> in August and November 2012 [6]. 90Sr activity in the Ukedo River was not available but the reported 90Sr/ 137Cs ratio for river water in the Fukushima Prefecture [6] was less than 0.04. The contribution to 90Sr activity in seawater by input from the Ukedo River should be minor.

#### *5.2. 90Sr*/ *137Cs Activity Ratios Derived from the FDNPP Accident*

The 90Sr/ 137Cs activity ratios in seawater are different according to timing of any release or leakage event (e.g., direct discharge event from late March to early April 2011 [3,15]). Since Sr and Cs are highly soluble in seawater, the 90Sr/ 137Cs activity ratio depends on the source, which could be a useful tracer for the source. The most possible source of 90Sr and 137Cs is stagnant water in the reactor building of unit 2. The 90Sr/ 137Cs ratio of open ocean seawater [3], seawater monitoring data near the FDNPP [6], stagnant water [4], atmospheric input [1], seabed sediment [24], and river water [6] are summarized with our data in Figure 4.

The 90Sr activity of 0.80 <sup>±</sup> 0.11 mBq L−<sup>1</sup> was obtained at the offshore sites, S1, S2, S3, and N01 (Table 2). To evaluate FDNPP site-derived 90Sr, measured 90Sr activity was subtracted from this value as the background value for North Pacific seawater. The measured 134Cs was a pure FDNPP site-derived component because of its short half-life (*T*1/<sup>2</sup> = 2.06 yr). The FDNPP site-derived 134Cs/ 137Cs ratio was reported to be 0.99 <sup>±</sup> 0.03 [11] in March 2011. FDNPP site-derived 137Cs was calculated on the basis of measured 134Cs activity and the FDNPP site-derived 134Cs/ 137Cs ratio.


**Table 2.** FDNPP site-derived 90Sr (90Srcorr.) and 137Cs (137Cscorr.) activities and 90Srcorr./ 137Cscorr. activity ratios.

The 90Srcorr/ 137Cscorr ratio estimated from the slope of a linear regression fitting was 0.66 <sup>±</sup> 0.05 in Figure 5. 90Srcorr activities strongly correlated with those of 137Cscorr (R<sup>2</sup> = 0.919), as described in similar contour maps of 90Sr and 137Cs (Figure 3). The high correlation between 90Sr and 137Cs indicates that 90Sr and 137Cs were derived from a common source. However, the low 90Sr activity samples (<10 mBq L−1) showed larger variability in 90Sr/ 137Cs activity ratio (0.34 <sup>±</sup> 0.14) relative to those for high 90Sr activity samples (>10 mBq L<sup>−</sup>1: ratio of 0.56 <sup>±</sup> 0.08). If multiple sources to seawater exist, such as stagnant water, storage water, and groundwater, contributions from each source could yield temporal and spatial variations. To distinguish these components, detailed 134,137Cs and 90Sr distributions should be investigated. Castrillejo et al. [17] found a short-term transition of 90Sr/ 137Cs ratio from 0.14 to 0.36 and an abrupt increase in 137Cs activity in the vicinity of the FDNPP (observation site St. 1 (or NP0)) in September 2013. 90Sr and 137Cs release from the FDNPP could be related to the tidal cycle and weather conditions, which caused a temporal variation of the released 90Sr/ 137Cs ratio from the FDNPP site.

**Figure 4.** The 90Sr/ 137Cs activity ratios in the environment and the FDNPP site. The 90Sr/ 137Cs activity ratio in seawater near the FDNPP was consistent with those from the monitoring points, T1 and T2 [12]. Soil [4] and sediment [24] samples had extremely low 90Sr/ 137Cs activity ratios.

The slope of a linear regression fitting (0.66) was similar to the reported 90Sr/ 137Cs activity ratios in stagnant water of 0.78 and 0.88, respectively, in July and August of 2013 [5]. The stagnant water samples were collected from the sampling line behind the mixing point of water from each reactor building [4,5] (Figure 5). These radionuclides were thought to mainly be derived from the reactor building of unit 2 on the basis of the initial data for stagnant water in the unit 2 turbine building [2], which was severely damaged. The 90Sr/ 137Cs activity ratio in stagnant water varied depending on the decontamination of 134, 137Cs, and gradually increased from the direct-release event in March 2011 (0.0256 <sup>±</sup> 0.0006 [15]). The 90Sr/ 137Cs activity ratio of seawater in this study is slightly lower than that of stagnant water, although the most possible source candidate is the continuous release of stagnant water from the FDNPP.

The discrepancy between our data and monitoring data at T1 (1.25 ± 0.71) implies multiple sources exist at the FDNPP site. The higher 90Sr/ 137Cs at the T1 site could reflect a contribution from 90Sr-rich groundwater. Groundwater around the reactor buildings had a 90Sr/ 137Cs activity ratio (2.4 <sup>×</sup> 104 [5]) that was 5 orders of magnitude higher than the seawater value observed in this study. The decontaminated water in storage tanks in the FDNPP was also observed to have high 90Sr/ 137Cs activity ratios.

**Figure 5.** FDNPP site-derived 90Sr and 137Cs activities in surface seawater with 90Sr/ 137Cs ratio for possible sources. The lower and upper red solid lines show 90Sr/ 137Cs ratios for stagnant water in the reactor building in July 2013 (0.78) and August 2013 (0.88), respectively [6]. Core inventory (0.74) was estimated by the ORIGEN2 code [2]. Shaded areas were averaged monitoring data at T1 (1.25 ± 0.71) and T2-1 (0.31 ± 0.14) sites from January 2013 to October 2013.

The fitted regression line had an x-intercept of 5.8 mBq L<sup>−</sup>1. A very low 90Sr/ 137Cs ratio (0.16) was observed at S6 without a change in salinity. These results indicate that there is a missing source for the site with a low 90Sr/ 137Cs ratio. In the coastal region, salinities ranged from 33.2 to 33.3 psu and showed no correlation with activities of 90Sr and 134, 137Cs. Atmospherically deposited 90Sr on land soil in March 2011 was at a lower level (<1.1 Bq g<sup>−</sup>1) than 137Cs, where the 90Sr/ 137Cs activity ratio was considered to be 0.00008−0.017 [1] (Figure 5). Higher mobility of 90Sr has been recognized, but 90Sr activity in water of the Fukushima River was less than 4 mBq L−<sup>1</sup> in 2012 [6]. 137Cs activity ranged from 12 to 190 mBq L<sup>−</sup>1, which yielded a low 90Sr/ 137Cs activity ratio of 0.01–0.04 [6]. Considering the 90Sr/ 137Cs activity ratio in seawater, riverine input from the land to the ocean was minor for 90Sr, though dissolved 90Sr activity in the Ukedo River was never reported.

A possible supply process for 137Cs is the release from seabed sediments. Some amount of Cs could be scavenged by seabed sediments through adsorption onto particles, such as clay minerals [25–27] during the direct discharge event. The sedimentary 137Cs inventory of 100–200 TBq represents only 1%–3% of the total discharge from the FDNPP to the Pacific Ocean in 2011 [28,29]. Approximately 80% of the total 137Cs sedimentary inventory was found in coastal sediments at less than 150 m water depth [29]. The highest 90Sr activity of 63 Bq kg-dry−<sup>1</sup> in seabed sediments was observed near the south discharge gate of the FDNPP site (T2 monitoring point) in September 2011 [6]. Sedimentary 90Sr/ 137Cs activity ratios observed after the accident ranged from 0.001 to 0.08, which were lower than those in seawater. 137Cs could be attributed to the direct discharge event in late March to April 2011 [20]. The extremely low 90Sr/ 137Cs ratio indicates that the contribution of 90Sr in seawater from the soil and seafloor sediments is less than that of 137Cs, even if there is a higher mobility for Sr than for Cs in the soil and sediments.

Another possible low 90Sr/ 137Cs source is contaminated water that remained in a tunnel for pipes and cables, which were connected to the turbine buildings of units 2 and 3. Contaminated water in the turbine and reactor buildings was released into the ocean via the tunnel and cracks resulting from the earthquake and tsunami. During this direct release event, the 90Sr/ 137Cs ratio was very low 0.0256 <sup>±</sup> 0.0006 [15] (Figure 5). 137Cs activity at T1 reached 68 kBq L−<sup>1</sup> [20]. After the direct release was stopped in early April 2011 by sealing cracks and the tunnel entrance, contaminated water could have been left in the tunnel until July 2015. Such highly contaminated water could be the source of the low 90Sr/ 137Cs ratio.

The 90Sr/ 137Cs activity ratio of 0.66 <sup>±</sup> 0.05 observed in this study was higher than data at the monitoring point, T2-1, near the south discharge gate (0.31 ± 0.14) [6] from January to December 2013, but was lower than that at T1 near the north discharge gate (1.25 ± 0.71) [6] (Figure 5). A large variation of 90Sr/ 137Cs at T1 was observed, which might reflect the local input processes of 90Sr and 137Cs. A much higher 90Sr/ 137Cs activity ratio (e.g., 90Sr activity of 7.5 Bq L−<sup>1</sup> and 90Sr/ 137Cs activity ratio of 3.2 on 26 June 2013) was observed by TEPCO in the harbor [6] than the coastal region as observed in this study. The variation in 90Sr/ 137Cs activity ratios might reflect the spatial and temporal heterogeneities of released water.

As mentioned above, the similarity of the 90Sr/ 137Cs activity ratio between seawater and the stagnant water supported the idea that the most likely candidate was the continuous release from the reactor buildings of the FDNPP. Both high 90Sr activity and 90Sr/ 137Cs activity ratio in the coastal region reflect the input of the stagnant water. Variability of 90Sr/ 137Cs activity ratios in seawater is an important indicator to understand the status of the release of contaminated water from the FDNPP. Unfortunately, the contribution of underground water near the reactor buildings, and released from sediments to the harbor water, could not be distinguished from the release of the reactor buildings on the basis of seawater obtained from outside of the harbor. More detailed temporal and spatial data in the harbor and for other radionuclides such as tritium (3H) and iodine-129 (129I) are necessary.
