Radiological Dose Rates to Marine Fish from the ... - ACS Publications

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Radiological dose rates to marine fish from the Fukushima Daiichi accident: the first three years across the North Pacific Mathew Paris Johansen, Elizabeth Ruedig, Keiko Tagami, Shigeo Uchida, Kathryn Higley, and Nick Beresford Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505064d • Publication Date (Web): 22 Dec 2014 Downloaded from http://pubs.acs.org on December 28, 2014

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Environmental Science & Technology

Radiological dose rates to marine fish from the Fukushima Daiichi accident: the first three years across the North Pacific Mathew P. Johansen,*† Elizabeth Ruedig,‡ Keiko Tagami,₸ Shigeo Uchida,₸ Kathryn Higley§ and Nicholas A. Beresfordǁ †

Australian Nuclear Science and Technology Organisation, NSW, Australia [email protected] ‡

Colorado State University, Colorado, USA

₸

National Institute of Radiological Sciences, Chiba, Japan

§

Oregon State University, Oregon, USA

ǁ

NERC Centre for Ecology & Hydrology, Lancaster, LA1 4AP, UK

KEYWORDS: Fukushima, radionuclides, marine, fish, dose rates, Pacific Ocean, isotopes, cesium, strontium, organ-specific, human dose, ingestion

TOC Graphic 134,137

Cs 73% muscle

11% gastrointestinal, 9% skin/scales, 3% skeleton, 2% gills, 1% liver, 1% testes/ovary.

38% skeleton 37% gastrointestinal 18% skin/scales

90 1% muscle, 0.1% liver, 5% other.

Sr

1 ACS Paragon Plus Environment


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ABSTRACT

A more complete record is emerging of radionuclide measurements in fish tissue, sediment, and seawater samples from near the Fukushima Daiichi Nuclear Power Plant (FDNPP) and across the Pacific Ocean. Based on publicly-available data, our analysis indicates the dose rates to the most impacted fish species near the FDNPP (median 1.1 mGy d-1, 2012–2014 data) have remained above benchmark levels for potential dose effects at least three years longer than was indicated by previous, data-limited, evaluations. Dose rates from 134,137Cs were highest in benthic species with sediment-associated food chains and feeding behaviors. In addition to 134,137Cs, the radionuclide 90Sr was estimated to contribute up to approximately one-half of the total 2013 dose rate to fish near the FDNPP. Mesopelagic fish 100–200 km east of the FDNPP, coastal fish in the Aleutian Islands (3300 km), and trans-Pacific migratory species, all had increased dose rates as a consequence of the FDNPP accident, but their total dose rates remained dominated by background radionuclides. A hypothetical human consumer of 50 kg of fish, gathered 3 km from the FDNPP in 2013, would have received a total committed effective dose of approximately 0.95 mSv a-1 from combined FDNPP and ambient radionuclides, of which 0.13 mSv a-1 (14%) is solely from the FDNPP radionuclides and below the 1 mSv a-1 benchmark for public exposure.


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INTRODUCTION

The March 2011 accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) produced the largest single-event (pulse) influx of radioactive cesium (Cs) isotopes into the Pacific marine ecosystem on record (estimates for 137Cs vary; 4-90,1 4–32,2 12–413 PBq). The transfer of Cs to organisms in the marine environment is well documented.4-8 When accumulated within fish, radionuclides emit energy that is absorbed into the organs and tissues, defined in terms of radiological dose rate (Gy d-1). Several evaluations have focused on the impacts to marine life during the initial weeks-months following the release.9-13 These suggested that the dose rates to fish near the FDNPP peaked rapidly (within weeks) 9, 11, 12 and only briefly remained above the benchmark levels for potential harmful effects that have been, based on experimental evidence, proposed for chronic exposures to wildlife.14-16

However, subsequent data have indicated highly elevated and persistent accumulation of Cs isotopes in fish samples, particularly from the ~30 ha FDNPP port,17 which has received ongoing releases of Cs18 and strontium (Sr)17 radioisotopes. The reports of the highly contaminated fish initiated discussion among researchers and the public about the persistence of the radioactive Cs and potential concerns over human consumption of fish (www.nytimes.com/2012/10/26/world/asia/fish-off-fukushima-japan-show-elevated-levelsof-cesium.htm). Vives i Batlle et al.11 commented on these new data (suggesting potential dose rates of approximately 1–3 mGy d-1) when summarising the evaluation of marine ecosystems by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR),19 and recommended further assessment, including consideration of food chains.



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Some of the released radionuclides are being carried long distances within the circulation of the North Pacific Gyre. Plume spreading is complex,20, 21 and although predictions vary, modelling suggests radionuclide concentrations will rise in the western North American coastal waters c. 2014,22-24 and in Australian and southeast Asian waters c. 2020 – c. 2030.22 To date, an evaluation of FDNPP-derived doses to marine fish on the Pacific scale has not been reported.

The main objectives of this study are to provide an initial evaluation of the accumulation of radionuclides, and resultant whole-body and organ-specific dose rates, in the most exposed fish nearest to the FDNPP, and compare them to those at more distant locations along the predicted decadal pathway of the contaminant plume. We assess whether dose rates to fish have been sustained above benchmark levels, using the available direct-measurement data rather than relying on modelling involving assumptions on environmental transport and transfer, and estimate doses to the potential human consumers of fish.

MATERIALS AND METHODS

For this evaluation, an individual dose rate (mGy d-1) to fish was calculated for each of 1064 reported tissue samples (Bq kg-1 fresh mass, FM). This approach of using measured fish tissue activity concentrations, along with co-located sediment and seawater data, provides the most accurate practicable calculation of the dose rates. Data that were available through to March 2014 were used in this study. Most (82%) were from locations within the FDNPP port (Figure S1), and the nearest consistently sampled location outside of the port, 3 km east of the FDNPP. With the exception of the Australian data, which are new, all of the data used here are publicly available. Most were sourced from the Japan Nuclear Regulation Authority (JNRA)17 and the Tokyo Electric Power Company25 4 ACS Paragon Plus Environment

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(TEPCO; see supporting information (SI), Table S1, for monitoring plans, sampling and analysis descriptions, and quality assurance evaluation). Further data on mesopelagic fish from 100–200 km east and southeast were available,26 as well as coastal fish data from the Aleutian islands (Amchitka Island, 3300km to the northeast of the FDNPP; Figure S1).27 The data from Amchitka Island included the same Hexagrammos (greenling) and Sebastes (rockfish) genera that were sampled near the FDNPP. Other data were drawn from scientific journals and government reports (Table S1), and are considered to be representative of available data on the Pacific scale through March 2014. Data from various marine sampling locations along the coast north and south of the FDNPP site are not included here as this paper focuses on Pacific-scale comparisons, and other papers have examined the variation along the eastern coast of Japan.28, 29

Data on ambient (background) radionuclide levels in fish are from pre-event sampling reported for Japan,30, 31 Amchitka Island32, 33 and various Pacific locations.32, 34, 35 The background isotopes of natural origin included here (40K, 210Po, 226Ra, 228Ra, 228Th, 238U) are consistent with those identified as key contributors of dose to marine organisms by Hosseini, et al. 36 and Yamamoto et al.31 Data for ambient isotopes of anthropogenicdominated origin (e.g. 99Tc, 239Pu, 240Pu, 241Am) are from the references in Table S1. When available, the ambient background measurements from Japan and the Pacific Ocean were prioritised for use here over data from other marine systems.

Conventional (whole-organism) and organ-specific dose rates

Whole-organism dose rates to fish were calculated using the standard dosimetric approach for non-human biota in the ERICA-Tool.37, 38 This analysis used realistic (not overly conservative) exposure parameters (see SI for dose conversion coefficients, sediment and 5 ACS Paragon Plus Environment


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water occupancy factors, and muscle:whole-organism factors). The absorbed dose rates were weighted by emission type using factors of α=10, low energy β=3, other β and γ=137 as used in previous FDNPP evaluations.11, 39 This approach assumes the receptor fish is represented by a simplified ellipsoid in which the isotopes are homogenously distributed (i.e. there is no differentiation of activity concentrations among tissue/organ types).

This conventional approach was augmented with organ-specific (voxel) modelling as radionuclides are known to accumulate disproportionately among the various tissue types in fish (most 137Cs occurs in muscle, most 90Sr occurs in bone, see SI discussion).40, 41 In voxel modelling, organs (e.g. liver) or tissues (e.g. muscle), are represented as three-dimensional volumes based upon the contours revealed in imaging data (typically computed tomography or magnetic resonance imaging).42 For this study, the three-dimensional data were imported into voxelizer software (Human Resources Lab, Canada). The N-Particle eXtended code (MCNPX, Los Alamos National Laboratory, USA) was employed to perform particle transport simulation, and ultimately used to derive absorbed fraction curves for a range of emission energies. Such data are not available for the specific marine species considered and the absorbed fraction data used here were from the dosimetric voxel work on a fish of similar general dimensions, Oncorhynchus mykiss (rainbow trout),42 a species falling within the definition of an International Commission on Radiological Protection (ICRP) Reference Animal and Plant (RAP) 14(see supporting text and Ruedig et al.42).

Potential committed effective doses to human consumers of fish were estimated using standard exposure-to-dose dose conversion factors for ingestion sourced from the ICRP,43 and various intake rates for adult consumers as discussed below and in the SI.

RESULTS AND DISCUSSION 6 ACS Paragon Plus Environment

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Maximally exposed fish near the FDNPP

The highest reported activity concentration of 134,137Cs in fish muscle was 740 kBq kg-1FM in an Hexagrammos otakii (greenling; February 2013). This represents an increase of more than six orders of magnitude over the pre-event levels of 0.03–0.15 Bq kg-1 FM in coastal fish of Japan30 (Figure 1). The elevated activity concentrations were not isolated to one sample, or one species. In 2013, activity concentrations of 134,137Cs exceeding 100 kBq kg-1 FM were measured in more than 100 fish from ten species sampled from FDNPP port (Conger myriaster, H. otakii, Microstomus achne, Paralichthys olivaceus, Pleuronectes yokohama, Sebastes cheni, Sebastes schlegeli, Sebastes oblongus, Sebastes pachycephalus, Sebasticus marmoratus). Of the H. otakii and S. pachycephalus (spotbelly rockfish), approximately 40% had muscle tissue levels exceeding 100 kBq kg-1 FM; 80% of S. oblongus samples were higher than this level.

Due to the ongoing persistent accumulation of 134,137Cs in their tissues, the dose rates to some FDNPP port fish have exceeded a generic screening assessment no-effects benchmark of 0.24 mGy d-1 (10µGy h-1)16, 44 over a period extending to at least three years after the accident (to March 2014, the last date for which we had data), and longer than had been previously modelled (for the general area near the FDNPP and before the fish tissue data from the port had become available; Figure 2). The median dose rates from 134,137Cs alone were 0.34 mGy d-1 for the fish sampled from within the port (806 samples from 32 species during 12/2012–3/2014; Table 1) indicating that more than half of the fish in the port had dose rates surpassing 10µGy h-1. While most intraspecies variation included some dose rates to individuals above the 10µGy h-1, some species had no occurrences above the threshold, including species more likely to have entered the port, prior to capture, from less 7 ACS Paragon Plus Environment


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contaminated areas (e.g. Gadus macrocephalus). The maximum dose rate of 3.1 mGy d-1 occurred in the H. otakii with 740 kBq kg-1 FM of 134,137Cs, which, after three years of monitoring, represents the maximally exposed fish reported, when examining data from within the port as well as along the coast north and south of the FDNPP.28 The three individual fish from the port with the highest dose rates were all H. otakii. However, the highest median dose rates were to the Sebastes genera (Figure 2; S. pachycephalus, 0.6 mGy d-1, n=78; and S. oblongus, 0.7 mGy d-1, n=17; 12/2012–3/2014 data).

The persistent elevation of 134,137Cs in certain species (Figures 1, 2) is likely mainly due to their sediment-associated food chains45, 46 and feeding behaviors. H. otakii and S. pachycephalus are carnivorous foragers that feed mostly on sediment-associated prey (~80% macrobenthos and decapod crustaceans5). Activity concentrations of 134,137Cs in benthic invertebrates near the FDNPP have remained elevated with effective half-lives similar to those of sediments.46 In the port, the 134,137Cs activity concentrations in local sediments have remained highly elevated (November 2011, median 103 kBq kg-1 FM; August 2012, median 48 kBq kg-1 FM; and February 2014, median 32 kBq kg-1 FM).17

However, the degree of transfer of the sediment-associated 134,137Cs to fish varies among the species that live and feed at the floor of the contaminated port. Of the four carnivorous demersal species for which substantial data are available, order-of-magnitude lower 134,137

Cs burdens are indicated in the two species, C. myriaster (conger) and Paralichthys

spp. (flatfish), which have little sediment association in their diets (Figure 1). The C. myriaster diet contains essentially no macrobenthos and ~10% decapod crustaceans.5 Although Paralichthys spp. often rest on the sediment surface, as ambush predators they feed mainly on fish and decapoda in the water column which generally have less sediment association and some of which could originate from less contaminated areas outside of the 8 ACS Paragon Plus Environment

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port. The lower 134,137Cs activity concentrations in C. myriaster and Paralichthys spp. are only marginally higher than would be predicted from the port water concentrations using the range of fish-to-water concentration ratios for Japanese coastal fish from before the accident event (from this study; geometric mean=45, arithmetic mean=50), or using international database concentration ratio values (geometric mean=48, arithmetic mean=8447 48).

In contrast, 134,137Cs activity concentrations in H. otakii and Sebastes spp. are orders of magnitude higher than predicted using these ratios from water concentrations. However, the accumulation of 134,137Cs through sediment-associated diet selection does not, by itself, fully explain the elevated levels in H. otakii and Sebastes spp. (Figure 1). A source of further accumulation may be explained by specific feeding behaviors. Foraging by Hexagrammos in the sea of Japan has been described in terms of “pecking” at prey in the substratum and “search digging” where the fish filled its mouth with sediment to sift for prey.49 Such feeding behavior not only can lead to increased routine ingestion of 134,137Cscontaminated sediments and organic matter (along with the prey), but also routine exposure of the gills as a portion of the sifted sediment is expelled through the gill opening.49 Transfer via sediment routes would require that a fraction of the Cs is bioavailable. Extraction experiments on post-event marine sediments (0-3 cm; collected July 2011January 2012)50 indicated that a fraction (

Radiological Dose Rates to Marine Fish from the ... - ACS Publications

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