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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Mar. Sci.</journal-id>
<journal-title>Frontiers in Marine Science</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Mar. Sci.</abbrev-journal-title>
<issn pub-type="epub">2296-7745</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.3389/fmars.2024.1382229</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Marine Science</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Deciphering decadal observation of Fukushima-derived radiocesium in the most polluted port near the Fukushima Daiichi Nuclear Power Plant: from seawater to marine fish</article-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Lin</surname>
<given-names>Wuhui</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="author-notes" rid="fn001">
<sup>*</sup>
</xref>
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<role content-type="https://credit.niso.org/contributor-roles/investigation/"/>
<role content-type="https://credit.niso.org/contributor-roles/methodology/"/>
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</contrib>
<contrib contrib-type="author">
<name>
<surname>Zhang</surname>
<given-names>Yibang</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/2649495"/>
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</contrib>
<contrib contrib-type="author">
<name>
<surname>Du</surname>
<given-names>Jinqiu</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
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</contrib>
<contrib contrib-type="author">
<name>
<surname>Xuan</surname>
<given-names>Jiliang</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1129059"/>
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</contrib>
<contrib contrib-type="author">
<name>
<surname>Tuo</surname>
<given-names>Fei</given-names>
</name>
<xref ref-type="aff" rid="aff4">
<sup>4</sup>
</xref>
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<aff id="aff1">
<sup>1</sup>
<institution>Polar and Marine Research Institute, College of Harbour and Coastal Engineering, Jimei University</institution>, <addr-line>Xiamen</addr-line>, <country>China</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources (MNR)</institution>, <addr-line>Hangzhou</addr-line>, <country>China</country>
</aff>
<aff id="aff3">
<sup>3</sup>
<institution>National Marine Environmental Monitoring Center</institution>, <addr-line>Dalian</addr-line>, <country>China</country>
</aff>
<aff id="aff4">
<sup>4</sup>
<institution>National Institute for Radiological Protection, Chinese Center for Disease Control and Prevention</institution>, <addr-line>Beijing</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>Edited by: Ruijie Zhang, Guangxi University, China</p>
</fn>
<fn fn-type="edited-by">
<p>Reviewed by: Neven Cukrov, Ru&#x111;er Bo&#x161;kovi&#x107; Institute, Croatia</p>
<p>Wu Men, Nanjing University of Information Science and Technology, China</p>
</fn>
<fn fn-type="corresp" id="fn001">
<p>*Correspondence: Wuhui Lin, <email xlink:href="mailto:linwuhui8@163.com">linwuhui8@163.com</email>
</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>03</day>
<month>05</month>
<year>2024</year>
</pub-date>
<pub-date pub-type="collection">
<year>2024</year>
</pub-date>
<volume>11</volume>
<elocation-id>1382229</elocation-id>
<history>
<date date-type="received">
<day>05</day>
<month>02</month>
<year>2024</year>
</date>
<date date-type="accepted">
<day>02</day>
<month>04</month>
<year>2024</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2024 Lin, Zhang, Du, Xuan and Tuo</copyright-statement>
<copyright-year>2024</copyright-year>
<copyright-holder>Lin, Zhang, Du, Xuan and Tuo</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p>
</license>
</permissions>
<abstract>
<p>The biological concentration effect of radionuclides in marine fish has exacerbated public anxiety about seafood security in the context of Fukushima nuclear-contaminated water discharged into the ocean. However, the most polluted port near the Fukushima Daiichi Nuclear Power Plant (FDNPP) has seldom been investigated, especially for radioactivity in marine fish. In this study, decadal observations of radiocesium in marine fish and seawater from the most polluted port were simultaneously established after the Fukushima Nuclear Accident. We found a generally decreasing trend of historical <sup>137</sup>Cs activity in seawater, with seasonal variations modulated by precipitation. Seasonal variations were elucidated with finer detail and divided into exponential decline in the dry season and steady variation in the wet season. A novel method was proposed to estimate the continuing source term of <sup>137</sup>Cs derived from the FDNPP, which was 3.9 PBq in 2011 and 19.3 TBq between 2012 and 2022 on the basis of historical <sup>137</sup>Cs. The biological concentration effect of marine fish is quantitatively emphasized according to the higher ratio of over-standards for radiocesium in marine fish relative to that in seawater. Long-term observation and analysis of radiocesium in marine fish and seawater from the most polluted port would provide insights into the scientific evaluation of the effectiveness of the decommissioning of the FDNPP in the past and share lessons on the fate of Fukushima-derived radionuclides in the future.</p>
</abstract>
<kwd-group>
<kwd>marine fish</kwd>
<kwd>
<sup>137</sup>Cs</kwd>
<kwd>Fukushima</kwd>
<kwd>contaminated water</kwd>
<kwd>nuclear pollution</kwd>
</kwd-group>
<counts>
<fig-count count="11"/>
<table-count count="2"/>
<equation-count count="4"/>
<ref-count count="22"/>
<page-count count="10"/>
<word-count count="3974"/>
</counts>
<custom-meta-wrap>
<custom-meta>
<meta-name>section-in-acceptance</meta-name>
<meta-value>Marine Fisheries, Aquaculture and Living Resources</meta-value>
</custom-meta>
</custom-meta-wrap>
</article-meta>
</front>
<body>
<sec id="s1" sec-type="intro">
<label>1</label>
<title>Introduction</title>
<p>Large amounts of artificial radionuclides have been released into the atmosphere (e.g., ~160 PBq of <sup>131</sup>I, 15 PBq of <sup>137</sup>Cs, and 14,000 PBq of <sup>133</sup>Xe) and the marine environment (e.g., 11 PBq of <sup>131</sup>I and 4 PBq of <sup>137</sup>Cs) since the Fukushima Nuclear Accident (FNA) on 11 March 2011 (<xref ref-type="bibr" rid="B5">Lin et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B12">Povinec et&#xa0;al., 2021</xref>). It has been estimated that 74% of Fukushima-derived artificial radionuclides released into the atmosphere were deposited in the North Pacific Ocean (<xref ref-type="bibr" rid="B12">Povinec et&#xa0;al., 2021</xref>), probably contributing to the FNA being the most serious nuclear accident that directly pollutes the marine environment with radioactive material (<xref ref-type="bibr" rid="B5">Lin et&#xa0;al., 2016</xref>). Artificial radionuclides are continuously discharged from the most polluted port (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>) near the Fukushima Daiichi Nuclear Power Plant (FDNPP) (<xref ref-type="bibr" rid="B8">Machida et&#xa0;al., 2023</xref>). Subsequently, Fukushima-derived radionuclides have been widely elevated from the coastal sea to the open ocean (<xref ref-type="bibr" rid="B12">Povinec et&#xa0;al., 2021</xref>).</p>
<fig id="f1" position="float">
<label>Figure&#xa0;1</label>
<caption>
<p>Map of the most polluted port near the FDNPP.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g001.tif"/>
</fig>
<p>The most polluted port near the FDNPP serves as windows to reflect the progress and effectiveness of decommissioning of the FDNPP, which is inaccessible to the public and to many other counties around the world. Recently, extremely high <sup>134 + 137</sup>Cs activity of 1.8&#xd7;10<sup>4</sup> Bq/kg in marine fish from the most polluted port was reported to be 180 times higher than the Japanese regulatory limit (100 Bq/kg-wet) on 18 May 2023 (<xref ref-type="bibr" rid="B18">Tokyo-Electric-Power-Company, 2023a</xref>). Despite the continuous discharge of artificial radionuclides and frequent reports of abnormally high levels of radiocesium in marine fish, the most polluted port with the highest radioactivity near the FDNPP has seldom been investigated (<xref ref-type="bibr" rid="B4">Kanda, 2013</xref>; <xref ref-type="bibr" rid="B9">Machida et&#xa0;al., 2020</xref>, <xref ref-type="bibr" rid="B8">2023</xref>), especially with regard to radioactivity in marine fish.</p>
<p>The biological concentration effect of radionuclides in marine fish has exacerbated public anxiety in the context of nuclear-contaminated water with 64 radionuclides discharged into the ocean (<xref ref-type="bibr" rid="B1">Buesseler, 2020</xref>; <xref ref-type="bibr" rid="B6">Lin et&#xa0;al., 2021</xref>; <xref ref-type="bibr" rid="B7">Liu et&#xa0;al., 2022</xref>). In this study, we primarily focus on the long-term observation of radiocesium in marine fish because seafood consumption is the dominant pathway of ionizing radiation to humans and is a primary concern of the public and countries around the Pacific Ocean after the FNA. Additionally, historical levels of radiocesium in seawater are simultaneously discussed to reveal its effect on marine fish. We also attempted to determine the mechanisms of seasonal fluctuation of <sup>137</sup>Cs in seawater in order to identify potential leak events, estimate the continuing source terms of <sup>137</sup>Cs discharged from the port, and verify the effectiveness of multiple countermeasures in the most polluted port near the FDNPP.</p>
</sec>
<sec id="s2" sec-type="materials|methods">
<label>2</label>
<title>Materials and methods</title>
<sec id="s2_1">
<label>2.1</label>
<title>
<sup>134</sup>Cs and <sup>137</sup>Cs in seawater from the port</title>
<p>The most polluted port (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>) near the FDNPP is inaccessible to the public and many other countries around the world. The availability and transparency of historical radioactivity in the most polluted port near the FDNPP played a key role in verifying the progress after the decommissioning of the FDNPP. We downloaded 1,196 daily reports of radiocesium in seawater provided by the Tokyo Electric Power Company (TEPCO) from 2 April 2011 to 30 June 2014 and 114 monthly reports of radiocesium in seawater released by the Ministry of Economy, Trade and Industry (METI) from July 2013 to April 2023 (<xref ref-type="bibr" rid="B15">Tokyo-Electric-Power-Company, 2011</xref>; <xref ref-type="bibr" rid="B11">Ministry-of-Economy-Trade-and-Industry, 2023b</xref>, <xref ref-type="bibr" rid="B17">2018</xref>). The TEPCO&#x2019;s daily reports from 2 April 2011 to 30 June 2014 included 3&#x2013;14 monitoring stations in the port, while the METI&#x2019;s monthly reports from July 2013 to April 2023 encompassed 9&#x2013;14 monitoring stations in the port. In order to conservatively evaluate the radioactive level in seawater from the port, we chose and compiled the highest values of <sup>134</sup>Cs and <sup>137</sup>Cs activities among 3&#x2013;14 stations in the port from the above-mentioned daily/monthly reports. If the data in the daily/monthly reports were lower than the minimum detection activity (MDA), the MDA was utilized for discussion. The highest value of <sup>134</sup>Cs and <sup>137</sup>Cs activity in a typical report is selected and shown in <xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2</bold>
</xref>.</p>
<fig id="f2" position="float">
<label>Figure&#xa0;2</label>
<caption>
<p>The red ellipse indicates the highest value of <sup>134</sup>Cs (MDA, 370 Bq/m<sup>3</sup>) and <sup>137</sup>Cs (3,000 Bq/m<sup>3</sup>) activities in seawater from the port in April 2023 (<xref ref-type="bibr" rid="B10">Ministry-of-Economy-Trade-and-Industry, 2023a</xref>).</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g002.tif"/>
</fig>
</sec>
<sec id="s2_2">
<label>2.2</label>
<title>
<sup>134</sup>Cs and <sup>137</sup>Cs in marine fish from the port</title>
<p>Radiocesium levels in marine fish were compiled from 130 monthly reports provided by TEPCO from December 2012 to May 2023 (<xref ref-type="bibr" rid="B17">Tokyo-Electric-Power-Company, 2018</xref>, <xref ref-type="bibr" rid="B19">2023b</xref>). There were seven fishing areas in the port (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>) (<xref ref-type="bibr" rid="B16">Tokyo-Electric-Power-Company, 2013</xref>). The typical species of marine fish from the port included <italic>Conger myriaster</italic>, <italic>Hexagrammos otakii</italic>, <italic>Microstomus achne</italic>, <italic>Paralichthys olivaceus</italic>, <italic>Pleuronectes yokohama</italic>, and <italic>Sebastes cheni</italic> (<xref ref-type="bibr" rid="B19">Tokyo Electric Power Company, 2023b</xref>). Detailed information on the size, diet, and habitat of marine fish was not provided in the monthly reports, probably because of the large number of fish and the temporal variations of fish species in the port. We conservatively selected and compiled the highest value of <sup>134</sup>Cs and <sup>137</sup>Cs activities in the muscle of marine fish caught from seven fishing areas in the port to conservatively evaluate the radiological impact. The value of MDA was utilized in the condition of the activity of radiocesium below the MDA. Only the maximum activity in marine fish was selected in order to discuss the relationship between radiocesium in seawater and marine fish; it was consistent with the highest value in seawater.</p>
<fig id="f3" position="float">
<label>Figure&#xa0;3</label>
<caption>
<p>Fishing areas from &#x201c;A&#x201d; to &#x201c;G&#x201d; in the port.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g003.tif"/>
</fig>
</sec>
<sec id="s2_3">
<label>2.3</label>
<title>Data on precipitation in Fukushima Prefecture</title>
<p>In order to investigate the mechanism of seasonal variation of <sup>137</sup>Cs in the port, we downloaded 145 monthly precipitation reports in Naniue and Tomioka near the FDNPP released by the Japan Meteorological Agency (JMA) from May 2011 to May 2023 (<xref ref-type="bibr" rid="B3">Japan-Meteorological-Agency, 2023</xref>). We calculated and compiled the mean value of monthly precipitation in Naniue and Tomioka because the FDNPP is located between them (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). The average monthly precipitation (119.8 mm) from May 2011 to May 2023 was calculated to define the wet season and the dry season in Fukushima Prefecture.</p>
<fig id="f4" position="float">
<label>Figure&#xa0;4</label>
<caption>
<p>Map of Naniue and Tomioka. The star in the red rectangle represents the location of the FDNPP (<xref ref-type="bibr" rid="B3">Japan-Meteorological-Agency, 2023</xref>).</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g004.tif"/>
</fig>
</sec>
</sec>
<sec id="s3">
<label>3</label>
<title>Results and discussion</title>
<sec id="s3_1">
<label>3.1</label>
<title>Historical observation of <sup>137</sup>Cs activity in seawater from the port</title>
<p>Historical <sup>137</sup>Cs activity in seawater (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>) from the most polluted port was systematically compiled from April 2011 to April 2023 based on 1,310 documents that were officially released by TEPCO and METI. Although the historical <sup>137</sup>Cs activity in seawater generally decreased due to the continuing decommissioning work at the FDNPP, the most recent monthly <sup>137</sup>Cs activity in seawater (3&#xd7;10<sup>3</sup> Bq/m<sup>3</sup> in April 2023) was still over 1,000 times higher than the background value (1&#x2013;2 Bq/m<sup>3</sup>) of <sup>137</sup>Cs before the FNA (<xref ref-type="bibr" rid="B12">Povinec et&#xa0;al., 2021</xref>). In this study, the historical <sup>137</sup>Cs activity in seawater was divided into three periods: April 2011 to June 2011 (purple area in region I), July 2011 to January 2016 (brown area in region II), and February 2016 to April 2023 (green area in region III).</p>
<fig id="f5" position="float">
<label>Figure&#xa0;5</label>
<caption>
<p>Historical <sup>137</sup>Cs activity in seawater from the most polluted port near the FDNPP from April 2011 to April 2023.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g005.tif"/>
</fig>
<p>Region I from April 2011 to June 2011 was recognized as the initial stage of FNA and was characterized by the direct discharge of contaminated water into the coastal sea. Two peaks of <sup>137</sup>Cs&#x2014;1.9&#xd7;10<sup>12</sup> Bq/m<sup>3</sup> on 2 April 2021 and 1.2&#xd7;10<sup>9</sup> Bq/m<sup>3</sup> on 12 May 2021&#x2014;were clearly recorded, corresponding to two leak events officially confirmed by TEPCO in 1&#x2013;6 April and 10&#x2013;11 May (<xref ref-type="bibr" rid="B4">Kanda, 2013</xref>). A sudden decline in <sup>137</sup>Cs activity was subsequently observed after TEPCO&#x2019;s operation to stop the leak in front of Unit 2 on 4 April 2016 (<xref ref-type="bibr" rid="B4">Kanda, 2013</xref>). The most significant phenomenon in region I was the appearance of an extremely high <sup>137</sup>Cs value, followed by a rapid exponential decline.</p>
<p>&#x201c;Region II from July 2011 to January 2016 was defined because of the completed construction of the seaside impermeable walls in February 2016 after carefully comparing the progress after the decommissioning in the monthly reports from METI in January and February 2016. The <sup>137</sup>Cs activity gradually decreased in region II due to the continuing decommissioning work, such as the relocation of the drainage channels from June 2014 to April 2015, the seabed covering of the port in April 2015, the removal of highly contaminated retained water in December 2015, the filling of tunnels and towers in December 2015, and the completed construction of seaside impermeable walls in February 2016 (<xref ref-type="bibr" rid="B9">Machida et&#xa0;al., 2020</xref>, <xref ref-type="bibr" rid="B8">2023</xref>). Seasonal fluctuation of <sup>137</sup>Cs activity was also observed, in addition to a decreasing trend at a slower rate in region II relative to a rapidly decreasing rate in region I. It was obvious that the average <sup>137</sup>Cs activity (7.0&#xd7;10<sup>3</sup> Bq/m<sup>3</sup>) in region III from February 2016 to April 2023 was approximately 40 times lower than that (2.7&#xd7;10<sup>5</sup>Bq/m<sup>3</sup>) in region II after the completed construction of seaside impermeable walls. However, a seasonal variation of <sup>137</sup>Cs without a significant decreasing trend is shown in region III.</p>
</sec>
<sec id="s3_2">
<label>3.2</label>
<title>Estimation of continuing source terms of <sup>137</sup>Cs based on the wet&#x2013;dry season model</title>
<p>Although the key feature of the seasonal variation of <sup>137</sup>Cs has been observed after the initial stage of FNA, the factors influencing the seasonal variation of <sup>137</sup>Cs have not been discussed in detail (<xref ref-type="bibr" rid="B9">Machida et&#xa0;al., 2020</xref>). To determine the seasonal variations of <sup>137</sup>Cs activity in seawater from July 2011 to April 2023 in detail, <sup>137</sup>Cs activity and monthly precipitation were simultaneously displayed to reveal the contrasting patterns of the exponential decrease of <sup>137</sup>Cs in the dry season (light blue in <xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>) and the steady variation of <sup>137</sup>Cs in the wet season (yellow in <xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>).</p>
<fig id="f6" position="float">
<label>Figure&#xa0;6</label>
<caption>
<p>Seasonal variations of <sup>137</sup>Cs in seawater from the port and monthly precipitation in Fukushima Prefecture from July 2011 to January 2016 <bold>(A)</bold> and February 2016 to April 2023 <bold>(B)</bold>. The average monthly precipitation is presented with a dotted line to distinguish the wet season (yellow) from the dry season (light blue). Effective half-lives (EHLs) and average <sup>137</sup>Cs activity are quantitatively displayed for the dry season and wet season, respectively.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g006.tif"/>
</fig>
<p>It was reasonable to expect the exponential decline in <sup>137</sup>Cs activity from the port, analogous to previous studies on the exponential decrease of <sup>137</sup>Cs activity in river water, seawater, sediment, and marine biotas because of the decommissioning work and stabilization at the FDNPP (<xref ref-type="bibr" rid="B12">Povinec et&#xa0;al., 2021</xref>). The exponential decline in <sup>137</sup>Cs activity in the dry season is well-fitted and quantitatively depicted with effective half-lives (EHLs) in different time intervals in <xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref> and <xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>. Two groups of EHLs have been quantified: 0.14 &#xb1; 0.03 a (seven time intervals fitted with red lines) and 0.41 &#xb1; 0.26 a (six time intervals fitted with green lines). The EHLs of <sup>137</sup>Cs in region II and region III were much longer than those in region I (1.58 d) at the initial stage of the FNA (<xref ref-type="bibr" rid="B4">Kanda, 2013</xref>), implying multiple continuing source terms of <sup>137</sup>Cs to lengthen EHLs and slow down the decreasing trend in the port after the initial stage of the FNA. The apparent decreasing rate (<italic>k</italic>
<sub>2</sub>) in the dry season is calculated in <xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>. Combined with the exchange rate (<italic>k</italic>
<sub>1 =</sub> 0.44 d<sup>&#x2212;1</sup>) and seawater volume (2.78&#xd7;10<sup>5</sup> m<sup>3</sup>) in the port from <xref ref-type="bibr" rid="B4">Kanda (2013)</xref>, the source terms of <sup>137</sup>Cs in the dry season could be quantified according to <xref ref-type="disp-formula" rid="eq2">Equations 3, 4</xref>.</p>
<disp-formula id="eq1">
<label>(1)</label>
<mml:math display="block" id="M1">
<mml:mrow>
<mml:mi>V</mml:mi>
<mml:mfrac>
<mml:mrow>
<mml:mtext>d</mml:mtext>
<mml:msub>
<mml:mi>A</mml:mi>
<mml:mrow>
<mml:mn>137</mml:mn>
<mml:mi>C</mml:mi>
<mml:mi>s</mml:mi>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mrow>
<mml:mtext>d</mml:mtext>
<mml:mi>t</mml:mi>
</mml:mrow>
</mml:mfrac>
<mml:mo>=</mml:mo>
<mml:msub>
<mml:mi>S</mml:mi>
<mml:mrow>
<mml:mtext>dry</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mo>&#x2212;</mml:mo>
<mml:msub>
<mml:mi>k</mml:mi>
<mml:mn>1</mml:mn>
</mml:msub>
<mml:msub>
<mml:mi>A</mml:mi>
<mml:mrow>
<mml:mn>137</mml:mn>
<mml:mtext>Cs</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mi>V</mml:mi>
<mml:mo>=</mml:mo>
<mml:mo>&#x2212;</mml:mo>
<mml:msub>
<mml:mi>k</mml:mi>
<mml:mn>2</mml:mn>
</mml:msub>
<mml:msub>
<mml:mi>A</mml:mi>
<mml:mrow>
<mml:mn>137</mml:mn>
<mml:mtext>Cs</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mi>V</mml:mi>
</mml:mrow>
</mml:math>
</disp-formula>
<disp-formula id="eq2">
<label>(2)</label>
<mml:math display="block" id="M2">
<mml:mrow>
<mml:msub>
<mml:mi>S</mml:mi>
<mml:mrow>
<mml:mtext>dry</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mo>=</mml:mo>
<mml:mrow>
<mml:mo stretchy="false">(</mml:mo>
<mml:mrow>
<mml:msub>
<mml:mi>k</mml:mi>
<mml:mn>1</mml:mn>
</mml:msub>
<mml:mo>&#x2212;</mml:mo>
<mml:msub>
<mml:mi>k</mml:mi>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
<mml:mo stretchy="false">)</mml:mo>
</mml:mrow>
<mml:msub>
<mml:mi>A</mml:mi>
<mml:mrow>
<mml:mn>137</mml:mn>
<mml:mtext>Cs</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mi>V</mml:mi>
</mml:mrow>
</mml:math>
</disp-formula>
<p>where <italic>A</italic>
<sub>137Cs</sub> refers to the <sup>137</sup>Cs activity in seawater. <italic>V</italic> and <italic>S</italic>
<sub>dry</sub> are the mean volume of seawater and the source terms of <sup>137</sup>Cs in the port during the dry season, respectively. <italic>k</italic>
<sub>1</sub> and <italic>k</italic>
<sub>2</sub> are the exchange rate of the port with outer seawater and the apparent decreasing rate of <sup>137</sup>Cs in the port, respectively (see values in <xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>).</p>
<table-wrap id="T1" position="float">
<label>Table&#xa0;1</label>
<caption>
<p>Exponential fitting, EHLs, and apparent decreasing rate (<italic>k</italic>
<sub>2</sub>) of time intervals in dry seasons.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="center">Group</th>
<th valign="middle" align="center">Time</th>
<th valign="middle" align="center">Exponential fitting</th>
<th valign="middle" align="center">
<italic>r</italic>
</th>
<th valign="middle" align="center">
<italic>p</italic>
</th>
<th valign="middle" align="center">EHL<break/>(a)</th>
<th valign="middle" align="center">
<italic>k</italic>
<sub>2</sub>
<break/>(a<sup>&#x2212;1</sup>)</th>
<th valign="middle" align="center">Mean value of EHL</th>
<th valign="middle" align="center">Mean value of <italic>k</italic>
<sub>2</sub>
</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" rowspan="7" align="center">1</td>
<td valign="middle" align="center">20111002&#x2013;20120225</td>
<td valign="middle" align="center">
<italic>A</italic> = 1.2&#xd7;10<sup>6</sup>&#xd7;e<sup>-4.6&#xd7;(T-2011.10.2)</sup>
</td>
<td valign="middle" align="center">0.80</td>
<td valign="middle" align="center">6.2&#xd7;10<sup>-27</sup>
</td>
<td valign="middle" align="center">0.15 &#xb1; 0.01</td>
<td valign="middle" align="center">4.56 &#xb1; 0.34</td>
<td valign="middle" rowspan="7" align="center">0.14 &#xb1; 0.03 a</td>
<td valign="middle" rowspan="7" align="center">4.95 &#xb1; 1.06a<sup>&#x2212;1</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">20121016&#x2013;20130307</td>
<td valign="middle" align="center">
<italic>A</italic> = 3.1&#xd7;10<sup>5</sup>&#xd7;e<sup>-6.0&#xd7;(T-2012.10.16)</sup>
</td>
<td valign="middle" align="center">0.75</td>
<td valign="middle" align="center">5.9&#xd7;10<sup>-23</sup>
</td>
<td valign="middle" align="center">0.11 &#xb1; 0.01</td>
<td valign="middle" align="center">6.08 &#xb1; 0.51</td>
</tr>
<tr>
<td valign="middle" align="center">201411&#x2013;201502</td>
<td valign="middle" align="center">
<italic>A</italic> = 2.5&#xd7;10<sup>4</sup>&#xd7;e<sup>-4.2&#xd7; (T-2014.11)</sup>
</td>
<td valign="middle" align="center">0.99</td>
<td valign="middle" align="center">6.4&#xd7;10<sup>-3</sup>
</td>
<td valign="middle" align="center">0.17 &#xb1; 0.01</td>
<td valign="middle" align="center">4.18 &#xb1; 0.34</td>
</tr>
<tr>
<td valign="middle" align="center">201609&#x2013;201702</td>
<td valign="middle" align="center">
<italic>A</italic> = 1.2&#xd7;10<sup>4</sup>&#xd7;e<sup>-5.1&#xd7; (T-2016.9)</sup>
</td>
<td valign="middle" align="center">0.80</td>
<td valign="middle" align="center">0.06</td>
<td valign="middle" align="center">0.14 &#xb1; 0.05</td>
<td valign="middle" align="center">5.06 &#xb1; 1.99</td>
</tr>
<tr>
<td valign="middle" align="center">201710&#x2013;201802</td>
<td valign="middle" align="center">
<italic>A</italic> = 1.3&#xd7;10<sup>4</sup>&#xd7;e<sup>-8.2&#xd7; (T-2017.10)</sup>
</td>
<td valign="middle" align="center">0.94</td>
<td valign="middle" align="center">0.04</td>
<td valign="middle" align="center">0.08 &#xb1; 0.02</td>
<td valign="middle" align="center">8.18 &#xb1; 2.25</td>
</tr>
<tr>
<td valign="middle" align="center">201911&#x2013;202003</td>
<td valign="middle" align="center">
<italic>A</italic> = 1.1&#xd7;10<sup>4</sup>&#xd7;e<sup>-5.6&#xd7; (T-2019.11)</sup>
</td>
<td valign="middle" align="center">0.87</td>
<td valign="middle" align="center">0.06</td>
<td valign="middle" align="center">0.12 &#xb1; 0.04</td>
<td valign="middle" align="center">5.57 &#xb1; 1.95</td>
</tr>
<tr>
<td valign="middle" align="center">202110&#x2013;202203</td>
<td valign="middle" align="center">
<italic>A</italic> = 5.0&#xd7;10<sup>3</sup>&#xd7;e<sup>-3.9&#xd7; (T-2021.10)</sup>
</td>
<td valign="middle" align="center">0.86</td>
<td valign="middle" align="center">0.06</td>
<td valign="middle" align="center">0.18 &#xb1; 0.06</td>
<td valign="middle" align="center">3.90 &#xb1; 1.36</td>
</tr>
<tr>
<td valign="middle" rowspan="6" align="center">2</td>
<td valign="middle" align="center">20131021&#x2013;20140122</td>
<td valign="middle" align="center">
<italic>A</italic> = 8.6&#xd7;10<sup>4</sup>&#xd7;e<sup>-1.4&#xd7;(T-2013.10.17)</sup>
</td>
<td valign="middle" align="center">0.32</td>
<td valign="middle" align="center">2.2&#xd7;10<sup>-3</sup>
</td>
<td valign="middle" align="center">0.50 &#xb1; 0.16</td>
<td valign="middle" align="center">1.40 &#xb1; 0.44</td>
<td valign="middle" rowspan="6" align="center">0.41 &#xb1; 0.26 a</td>
<td valign="middle" rowspan="6" align="center">1.69 &#xb1; 1.07a<sup>&#x2212;1</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">201509&#x2013;201601</td>
<td valign="middle" align="center">
<italic>A</italic> = 1.7&#xd7;10<sup>4</sup>&#xd7;e<sup>-1.9&#xd7; (T-2015.9)</sup>
</td>
<td valign="middle" align="center">0.55</td>
<td valign="middle" align="center">0.38</td>
<td valign="middle" align="center">0.36 &#xb1; 0.36</td>
<td valign="middle" align="center">1.90 &#xb1; 1.87</td>
</tr>
<tr>
<td valign="middle" align="center">201602&#x2013;201605</td>
<td valign="middle" align="center">
<italic>A</italic> = 3.4&#xd7;10<sup>3</sup>&#xd7;e<sup>-1.9&#xd7; (T-2016.2)</sup>
</td>
<td valign="middle" align="center">0.49</td>
<td valign="middle" align="center">0.47</td>
<td valign="middle" align="center">0.36 &#xb1; 0.42</td>
<td valign="middle" align="center">1.94 &#xb1; 2.21</td>
</tr>
<tr>
<td valign="middle" align="center">201809&#x2013;201904</td>
<td valign="middle" align="center">
<italic>A</italic> = 7.6&#xd7;10<sup>3</sup>&#xd7;e<sup>-1.6&#xd7; (T-2018.9)</sup>
</td>
<td valign="middle" align="center">0.82</td>
<td valign="middle" align="center">0.09</td>
<td valign="middle" align="center">0.43 &#xb1; 0.17</td>
<td valign="middle" align="center">1.57 &#xb1; 0.63</td>
</tr>
<tr>
<td valign="middle" align="center">202007&#x2013;202012</td>
<td valign="middle" align="center">
<italic>A</italic> = 5.2&#xd7;10<sup>3</sup>&#xd7;e<sup>-1.5&#xd7; (T-2020.7)</sup>
</td>
<td valign="middle" align="center">0.75</td>
<td valign="middle" align="center">0.06</td>
<td valign="middle" align="center">0.46 &#xb1; 0.19</td>
<td valign="middle" align="center">1.51 &#xb1; 0.62</td>
</tr>
<tr>
<td valign="middle" align="center">202210&#x2013;202302</td>
<td valign="middle" align="center">
<italic>A</italic> = 3.5&#xd7;10<sup>3</sup>&#xd7;e<sup>-2.1&#xd7; (T-2022.10)</sup>
</td>
<td valign="middle" align="center">0.59</td>
<td valign="middle" align="center">0.28</td>
<td valign="middle" align="center">0.33 &#xb1; 0.25</td>
<td valign="middle" align="center">2.06 &#xb1; 1.60</td>
</tr>
<tr>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B4">Kanda, 2013</xref>
</td>
<td valign="middle" align="center">20110406&#x2013;20110419</td>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="top" align="center"/>
<td valign="middle" align="center">1.58 d</td>
<td valign="middle" align="center">0.44 d<sup>&#x2212;1</sup>
</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>By contrast, the steady variation of <sup>137</sup>Cs without the decreasing trend in the wet season is illustrated in <xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>. We found that the average <sup>137</sup>Cs activities in wet seasons from July 2011 to January 2016 also gradually decreased from 1.0&#xd7;10<sup>6</sup> Bq/m<sup>3</sup> to 3.9&#xd7;10<sup>4</sup> Bq/m<sup>3</sup> in <xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6A</bold>
</xref>. The average <sup>137</sup>Cs activities in wet seasons from February 2016 to April 2023 varied from 5.1&#xd7;10<sup>3</sup> Bq/m<sup>3</sup> to 2.6&#xd7;10<sup>4</sup> Bq/m<sup>3</sup> without a decreasing trend in <xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6B</bold>
</xref>. Previous studies have pointed out that the <sup>137</sup>Cs activity in the river and coastal sea has been significantly elevated during the flood season, especially with the additional influences of typhoons and storms (<xref ref-type="bibr" rid="B14">Tanaka et&#xa0;al., 2022</xref>; <xref ref-type="bibr" rid="B21">Uchiyama et&#xa0;al., 2022</xref>). The positive relationship between <sup>137</sup>Cs activity and monthly precipitation (<italic>r</italic> = 0.44, <italic>p</italic>&lt; 0.0001) from February 2016 to April 2023 is also shown in <xref ref-type="fig" rid="f7">
<bold>Figure&#xa0;7</bold>
</xref>. Precipitation probably contributed to the additional input of <sup>137</sup>Cs into the port via leaching and erosion of <sup>137</sup>Cs from a terrestrial environment in the wet season, resulting in the steady variation of <sup>137</sup>Cs activity in the wet season in contrast to a decreasing trend of <sup>137</sup>Cs in the dry season. The source terms of <sup>137</sup>Cs in the wet season could be quantified according to <xref ref-type="disp-formula" rid="eq3">Equation 3</xref>, <xref ref-type="disp-formula" rid="eq4">Equation 4</xref>.</p>
<disp-formula id="eq3">
<label>(3)</label>
<mml:math display="block" id="M3">
<mml:mrow>
<mml:mi>V</mml:mi>
<mml:mfrac>
<mml:mrow>
<mml:mtext>d</mml:mtext>
<mml:msub>
<mml:mi>A</mml:mi>
<mml:mrow>
<mml:mn>137</mml:mn>
<mml:mi>C</mml:mi>
<mml:mi>s</mml:mi>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mrow>
<mml:mtext>d</mml:mtext>
<mml:mi>t</mml:mi>
</mml:mrow>
</mml:mfrac>
<mml:mo>=</mml:mo>
<mml:msub>
<mml:mi>S</mml:mi>
<mml:mrow>
<mml:mtext>wet</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mo>&#x2212;</mml:mo>
<mml:msub>
<mml:mi>k</mml:mi>
<mml:mn>1</mml:mn>
</mml:msub>
<mml:msub>
<mml:mi>A</mml:mi>
<mml:mrow>
<mml:mn>137</mml:mn>
<mml:mtext>Cs</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mi>V</mml:mi>
<mml:mo>=</mml:mo>
<mml:mn>0</mml:mn>
</mml:mrow>
</mml:math>
</disp-formula>
<disp-formula id="eq4">
<label>(4)</label>
<mml:math display="block" id="M4">
<mml:mrow>
<mml:msub>
<mml:mi>S</mml:mi>
<mml:mrow>
<mml:mtext>wet</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mo>=</mml:mo>
<mml:msub>
<mml:mi>k</mml:mi>
<mml:mn>1</mml:mn>
</mml:msub>
<mml:msub>
<mml:mi>A</mml:mi>
<mml:mrow>
<mml:mn>137</mml:mn>
<mml:mtext>Cs</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mi>V</mml:mi>
</mml:mrow>
</mml:math>
</disp-formula>
<p>where <italic>S</italic>
<sub>wet</sub> refers to the source terms of <sup>137</sup>Cs in the wet season. Other parameters are the same as in <xref ref-type="disp-formula" rid="eq1">Equations 1</xref>, <xref ref-type="disp-formula" rid="eq2">2</xref>.</p>
<fig id="f7" position="float">
<label>Figure&#xa0;7</label>
<caption>
<p>Positive relationship between <sup>137</sup>Cs activity from the port and monthly precipitation from February 2016 to April 2023.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g007.tif"/>
</fig>
<p>Therefore, historical <sup>137</sup>Cs activity and its associated mechanisms are suggested to be delicately classified into the wet season and the dry season for quantitative discussion. Our study also implied that the source terms of radionuclides derived from the FDNPP to the Pacific Ocean should be different between the wet season (<xref ref-type="disp-formula" rid="eq2">Equation 2</xref>) and the dry season (<xref ref-type="disp-formula" rid="eq4">Equation 4</xref>) rather than a simple assumption of steady state in previous studies (<xref ref-type="bibr" rid="B4">Kanda, 2013</xref>; <xref ref-type="bibr" rid="B9">Machida et&#xa0;al., 2020</xref>, <xref ref-type="bibr" rid="B8">2023</xref>). The annual flux of <sup>137</sup>Cs discharged from the port to outer seawater was 3.9 PBq in 2011 and 19.3 TBq from 2012 to 2022, which is generally consistent with previous studies, as shown in <xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref> and <xref ref-type="fig" rid="f8">
<bold>Figure&#xa0;8</bold>
</xref> (<xref ref-type="bibr" rid="B9">Machida et&#xa0;al., 2020</xref>).</p>
<table-wrap id="T2" position="float">
<label>Table&#xa0;2</label>
<caption>
<p>Comparison of the annual flux of the <sup>137</sup>Cs discharged from the port.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" rowspan="3" align="center">Year</th>
<th valign="middle" colspan="6" align="center">Annual flux of <sup>137</sup>Cs (Bq/a)</th>
</tr>
<tr>
<th valign="middle" rowspan="2" align="center">Tsumune et&#xa0;al (<xref ref-type="bibr" rid="B20">Tsumune et&#xa0;al., 2020</xref>)</th>
<th valign="middle" colspan="2" align="center">Machida-V method <break/>(<xref ref-type="bibr" rid="B9">Machida et&#xa0;al., 2020</xref>)</th>
<th valign="middle" colspan="2" align="center">Machida-K method  <break/>(<xref ref-type="bibr" rid="B9">Machida et&#xa0;al., 2020</xref>)</th>
<th valign="middle" rowspan="2" align="center">This study</th>
</tr>
<tr>
<th valign="middle" align="center">Min</th>
<th valign="middle" align="center">Max</th>
<th valign="middle" align="center">Min</th>
<th valign="middle" align="center">Max</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="center">2011</td>
<td valign="middle" align="center">3.55&#xd7;10<sup>15</sup>
</td>
<td valign="middle" align="center">1.9&#xd7;10<sup>15</sup>
</td>
<td valign="middle" align="center">1.9&#xd7;10<sup>15</sup>
</td>
<td valign="middle" align="center">2.3&#xd7;10<sup>15</sup>
</td>
<td valign="middle" align="center">2.3&#xd7;10<sup>15</sup>
</td>
<td valign="middle" align="center">3.9&#xd7;10<sup>15</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2012</td>
<td valign="middle" align="center">1.63&#xd7;10<sup>13</sup>
</td>
<td valign="middle" align="center">2.1&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">2.2&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">2.9&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">2.9&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">9.9&#xd7;10<sup>12</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2013</td>
<td valign="middle" align="center">7.81&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">7.0&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">8.1&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">1.4&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">1.4&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">4.5&#xd7;10<sup>12</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2014</td>
<td valign="middle" align="center">3.76&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">3.0&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">4.5&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">7.1&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">7.4&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">1.5&#xd7;10<sup>12</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2015</td>
<td valign="middle" align="center">1.81&#xd7;10<sup>12</sup>
</td>
<td valign="middle" align="center">2.4&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">4.1&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">3.2&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">6.4&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">1.2&#xd7;10<sup>12</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2016</td>
<td valign="middle" align="center">8.75&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">2.0&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">2.5&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">2.4&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">2.8&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">4.8&#xd7;10<sup>11</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2017</td>
<td valign="middle" align="center"/>
<td valign="middle" align="center">1.3&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">1.6&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">1.9&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">2.4&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">3.1&#xd7;10<sup>11</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2018</td>
<td valign="middle" align="center"/>
<td valign="middle" align="center">4.4&#xd7;10<sup>10</sup>
</td>
<td valign="middle" align="center">6.4&#xd7;10<sup>10</sup>
</td>
<td valign="middle" align="center">9.2&#xd7;10<sup>10</sup>
</td>
<td valign="middle" align="center">1.2&#xd7;10<sup>11</sup>
</td>
<td valign="middle" align="center">2.1&#xd7;10<sup>11</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2019</td>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center">3.7&#xd7;10<sup>11</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2020</td>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center">2.0&#xd7;10<sup>11</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2021</td>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center">3.5&#xd7;10<sup>11</sup>
</td>
</tr>
<tr>
<td valign="middle" align="center">2022</td>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center"/>
<td valign="middle" align="center">2.9&#xd7;10<sup>11</sup>
</td>
</tr>
</tbody>
</table>
</table-wrap>
<fig id="f8" position="float">
<label>Figure&#xa0;8</label>
<caption>
<p>Comparison of the annual flux of the <sup>137</sup>Cs discharged from the port from 2011 to 2022.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g008.tif"/>
</fig>
</sec>
<sec id="s3_3">
<label>3.3</label>
<title>Delayed increase of <sup>134 + 137</sup>Cs in marine fish following the <sup>134 + 137</sup>Cs peak in seawater from the port</title>
<p>The long-term monthly <sup>134 + 137</sup>Cs activity in marine fish and the corresponding value in seawater from the most polluted port are simultaneously shown in <xref ref-type="fig" rid="f9">
<bold>Figure&#xa0;9</bold>
</xref> from December 2012 to May 2023. A clear decline in <sup>134 + 137</sup>Cs activity in marine fish was observed before February 2016, consistent with the decreasing trend of <sup>134 + 137</sup>Cs activity in seawater (<xref ref-type="fig" rid="f9">
<bold>Figures&#xa0;9</bold>
</xref>, <xref ref-type="fig" rid="f10">
<bold>10A</bold>
</xref>). In contrast, periodic increases in <sup>134 + 137</sup>Cs activity without a decreasing trend in marine fish and seawater were observed after February 2016 (<xref ref-type="fig" rid="f10">
<bold>Figure&#xa0;10B</bold>
</xref>). The Japanese regulatory limit for marine fish (100 Bq/kg-wet) and the WHO guidance levels for drinking water (10,000 Bq/m<sup>3</sup>) for <sup>134 + 137</sup>Cs were adopted to evaluate the radioactive level. We found that the ratio of over-standard for <sup>134 + 137</sup>Cs in marine fish (&gt;100 Bq/kg-wet) was 100% from December 2012 to January 2016 and 59% from February 2016 to May 2023. Meanwhile, the ratio of over-standard for <sup>134 + 137</sup>Cs in seawater (&gt;10,000 Bq/m<sup>3</sup>) was 89.5% from July 2014 to January 2016 and 20.5% from February 2016 to April 2023. Obviously, the ratio of over-standard for <sup>134 + 137</sup>Cs in marine fish was higher than that in seawater, probably attributed to the biological concentration effect of marine fish.</p>
<fig id="f9" position="float">
<label>Figure&#xa0;9</label>
<caption>
<p>Simultaneous analysis of <sup>134 + 137</sup>Cs in marine fish (green circle) from December 2012 to May 2023 and seawater (blue triangle) from July 2014 to April 2023. The solid symbols mean the sum of detectable <sup>134</sup>Cs and measurable <sup>137</sup>Cs, while the open symbols refer to the sum of measurable<sup>137</sup>Cs and the MDA of <sup>134</sup>Cs. The brown and red dotted lines refer to the Japanese regulatory limit for marine fish (100 Bq/kg-wet) and the WHO guidance levels for drinking water (10<sup>4</sup> Bq/m<sup>3</sup>) for <sup>134 + 137</sup>Cs.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g009.tif"/>
</fig>
<fig id="f10" position="float">
<label>Figure&#xa0;10</label>
<caption>
<p>Delayed increase of <sup>134 + 137</sup>Cs in marine fish following the <sup>134 + 137</sup>Cs peak in seawater from July 2014 to January 2016 <bold>(A)</bold> and February 2016 to May 2023 <bold>(B)</bold>. The orange rectangles indicate the corresponding <sup>134 + 137</sup>Cs peak in marine fish and seawater.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g010.tif"/>
</fig>
<p>Additionally, the delayed increase in <sup>134 + 137</sup>Cs activity in marine fish following the <sup>134 + 137</sup>Cs peak in seawater is also depicted by the orange shadow in <xref ref-type="fig" rid="f10">
<bold>Figures&#xa0;10A, B</bold>
</xref>. The positive relationship of peak <sup>134 + 137</sup>Cs activity between marine fish and seawater is well-fitted in <xref ref-type="fig" rid="f11">
<bold>Figure&#xa0;11</bold>
</xref> (<italic>r</italic> = 0.99, <italic>p</italic>&lt; 0.0001) on the basis of the corresponding peaks of <sup>134 + 137</sup>Cs activity in marine fish and seawater. The concentration factors of <sup>134 + 137</sup>Cs in marine fish ranged from 29 L/kg to 514 L/kg, with an average value of 136 L/kg from February 2016 to May 2023. The average value of 136 L/kg in marine fish was consistent with the recommended value of 100 L/kg provided by the <xref ref-type="bibr" rid="B2">IAEA (2004)</xref>, confirming the corresponding relationship of <sup>134 + 137</sup>Cs peaks between marine fish and seawater.</p>
<fig id="f11" position="float">
<label>Figure&#xa0;11</label>
<caption>
<p>Positive relationship between the corresponding peak of <sup>134 + 137</sup>Cs activity in marine fish and seawater.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1382229-g011.tif"/>
</fig>
<p>It was noted that radiocesium in sediment also contributed to the elevated activity of radiocesium in marine fish (<xref ref-type="bibr" rid="B22">Wang et&#xa0;al., 2018</xref>; <xref ref-type="bibr" rid="B13">Song et&#xa0;al., 2020</xref>). Unfortunately, radiocesium in the sediment from the most polluted port was not available from TEPCO and METI, limiting our discussion of the pathway of sediment ingestion by marine fish. However, the continuing source of radiocesium derived from the FDNPP should increase radiocesium levels in seawater followed by those in sediment and marine fish. The corresponding peaks of radiocesium between seawater and marine fish should be logically correlated. Even so, radiocesium in the sediment from the most polluted port should be measured for a better understanding of frequent reports of extremely high radiocesium levels in marine fish from the port in the future.</p>
</sec>
</sec>
<sec id="s4" sec-type="conclusion">
<label>4</label>
<title>Conclusion</title>
<p>Overall, we revealed the distinct seasonal patterns of an exponential decline in the dry season and steady variation in the wet season in detail based on the historical <sup>137</sup>Cs activity in seawater and proposed a novel method to quantify the continuing source terms of <sup>137</sup>Cs derived from the FDNPP (3.9 PBq in 2011 and 19.3 TBq from 2012 to 2022). Moreover, the biological concentration effect of marine fish was quantitatively emphasized on the basis of the higher ratio of excess <sup>134 + 137</sup>Cs in marine fish compared to seawater. Long-term observation and analysis of radiocesium in marine fish and seawater from the most polluted port would benefit the scientific evaluation of the decommissioning of the FDNPP, and share lessons on the fate of Fukushima-derived radionuclides in the marine environment for the prediction and assessment of nuclear-contaminated water discharged into the ocean.</p>
</sec>
<sec id="s5" sec-type="data-availability">
<title>Data availability statement</title>
<p>The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.</p>
</sec>
<sec id="s6" sec-type="author-contributions">
<title>Author contributions</title>
<p>WL: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing &#x2013; original draft. YZ: Methodology, Software, Visualization, Writing &#x2013; original draft. JD: Data curation, Formal analysis, Resources, Visualization, Writing &#x2013; original draft. JX: Funding acquisition, Methodology, Resources, Software, Writing &#x2013; original draft. FT: Methodology, Resources, Visualization, Writing &#x2013; original draft.</p>
</sec>
</body>
<back>
<sec id="s7" sec-type="funding-information">
<title>Funding</title>
<p>The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This work was supported by the National Natural Science Foundation of China (42276044); the Natural Science Foundation of Guangxi Province (2021GXNSFAA220053), and the open fund of the State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, MNR (QNHX2320).</p>
</sec>
<ack>
<title>Acknowledgments</title>
<p>The authors would like to thank TEPCO, METI, and the Japan Meteorological Agency for sharing data on the websites.</p>
</ack>
<sec id="s8" sec-type="COI-statement">
<title>Conflict of interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationship that could be construed as a potential conflict of interest.</p>
</sec>
<sec id="s9" sec-type="disclaimer">
<title>Publisher&#x2019;s note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
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