Arrival of the Fukushima radioactivity plume in North American continental waters

The comparison of the Fukushima radioactivity signal with the fallout background is straightforward because ¹³⁷Cs has been tracked quite extensively in the Pacific Ocean since the peak period of atmospheric weapons testing in the early 1960s (11, 12). Levels in the region east of Japan have decreased from 10–20 Bq/m³ in 1960 to 1.5 Bq/m³ on average in 2010 (13). The decrease in ¹³⁷Cs during this 50-y period reflects both radioactive decay of ¹³⁷Cs and removal from the surface layer of the ocean by mixing. In the present study, measurements of ¹³⁷Cs on Line P, focusing particularly on stations P4 and P26, are intended to intercept the eastward flow of Fukushima radioactivity in the North Pacific and Alaska currents at the eastern edge of the subpolar gyre (Fig. 1). Station P4 is situated at the edge of the continental shelf at a water depth of 1,300 m and provides a sampling perspective for flow onto or adjacent to the shelf, whereas station P26, located at a depth of 4,250 m, anchors Line P offshore and is the location of a time series site for observing ocean processes.

The distributions of ¹³⁷Cs concentrations with water depth at stations P4 and P26 for the June 2011 CCGS John P. Tully mission are illustrated in Fig. 2A and given in Table 1. At both stations, ¹³⁷Cs concentrations in 2011 were 1–1.5 Bq/m³ in the upper 100 m of the surface mixed layer, decreasing to values of about 0.1 Bq/m³ at 1,000 m. Levels of ¹³⁴Cs were below the detection limit of 0.13 Bq/m³ in all samples, indicating that the observed ¹³⁷Cs was entirely derived from fallout and that no detectable contamination from the Fukushima accident was present at that time. The first observations of detectable ¹³⁴Cs on Line P were made at station P26 in June 2012 (Fig. 2A). ¹³⁴Cs levels of 0.2–0.4 Bq/m³ [decay-corrected to April 6, 2011 (4)] were measured in the upper 100 m, clearly indicating the presence of Fukushima-derived radioactivity. The Fukushima ¹³⁴Cs signal had not yet traveled sufficiently eastward to be detectable at station P4 by June 2012. However, by June 2013, ¹³⁴Cs was detectable in the upper 100 m at all stations sampled on Line P (Fig. 2 A and B), thereby signaling the arrival of the Fukushima radioactivity plume of ¹³⁴Cs and ¹³⁷Cs in North American continental waters. ¹³⁴Cs concentrations in surface water were 0.8 and 0.4 Bq/m³ at stations P26 and P4, respectively, in June 2013. Because the initial ¹³⁴Cs/¹³⁷Cs ratio in Fukushima-derived radioactivity was 1 (3), the measured ¹³⁴Cs concentration on Line P, decay-corrected to April 6, 2011, is directly equivalent to the ¹³⁷Cs concentration discharged from Fukushima and is hereafter referred to as the Fukushima ¹³⁷Cs concentration (Table 1). Between June 2013 and February 2014, the Fukushima-derived ¹³⁷Cs concentration in the surface mixed layer at station P26 continued to increase to a level of about 2 Bq/m³, resulting in an increase in total ¹³⁷Cs levels (Fukushima plus fallout ¹³⁷Cs) to 3.6 Bq/m³. However, only smaller or even negligible increases were observed in the 2014 Fukushima ¹³⁷Cs signal at stations, such as station P4 (Fig. 2A), that are located proximal to the continental shelf.

The cross sectional distribution of the Fukushima ¹³⁷Cs concentration along Line P in June 2013 is illustrated in Fig. 2B. The Fukushima ¹³⁷Cs signal was restricted to the upper 150 m of the water column, which is the approximate depth of the winter mixed layer in the eastern North Pacific (14). The decreasing gradient in the Fukushima ¹³⁷Cs surface mixed layer concentration (Fig. 2B) extending from station P26 to station P1 reflects the eastward circulation of Fukushima radioactivity from the ocean interior. However, most of the eastward decrease in the Fukushima ¹³⁷Cs concentration both in 2013 and 2014 occurred in the region between station P20 and station P16 that is heavily influenced by the northward flowing Alaska Current (Fig. 1).

Line P is situated in the vicinity of the bifurcation of the North Pacific Current, where the large-scale circulation diverges into the northward-flowing Alaska Current and the southward-flowing California Current (Fig. 1). These flows are subject to pronounced variability on interannual to decadal time scales (15). Time-averaged streamlines representing the mean dynamic height field for 2002–2012 calculated from Project Argo float data (www.meds-sdmm.dfo-mpo.gc.ca/isdm-gdsi/argo/canadian-products/index-eng.html) are illustrated in the inset for Fig. 1 (14). The mean streamlines are concentrated on the western part of Line P (west of station P15), which on average intercepts the northward geostrophic transport of the Alaska Current with flow speeds of 5–10 cm/s. The streamlines diverge markedly on the eastern side of Line P, which lies generally within the bifurcation zone. The flow in this region is highly variable, and mean currents are weak and difficult to define. The decreasing ¹³⁷Cs tracer gradient in the surface mixed layer eastward along Line P (Fig. 2B) represents a transition from higher levels in the northward-flowing core of the Fukushima tracer plume to lower levels in the weaker, transitional flow field of the bifurcation zone. This slower eastward flow of the Fukushima signal onto the shelf may explain why the Fukushima ¹³⁷Cs signal had yet to be detected by mid-2014 in Pacific coastal regions off British Columbia by a Woods Hole Oceanographic Institution crowd sourcing program (www.ourradioactiveocean.org/). Seasonally variable winds are also a factor in the exchange of water between the open ocean and the shelf, resulting in a downwelling regime that dominates through most of the year off British Columbia (16). Downwelling tends to enhance, rather than weaken, offshore transport and likely does not contribute to the delayed transport of the Fukushima ¹³⁷Cs signal onto the shelf along Line P.

In contrast to the North Pacific results, ¹³⁷Cs concentrations measured in Pacific water collected in the upper 170 m of the Arctic Ocean in September 2012 were in the range (1.1–1.8 Bq/m³) associated with fallout (Table 2). ¹³⁴Cs levels were below the detection limit (0.13 Bq/m³) at all depths at stations A, BL, CAP-10, and TU-1, located in the inflow region for Pacific water entering the Beaufort Sea (Fig. 1). These results indicate that as of September 2012, detectable Fukushima radioactivity had yet to reach the Arctic Ocean by ocean current transport through the Bering Sea. This observation is consistent with the view that the Bering Sea is downstream of Line P in the large-scale ocean circulation pathway of the North Pacific subpolar gyre (17). The higher ¹³⁷Cs levels (>2 Bq/m³) measured at depths below 200 m (Table 2) represent radionuclide contaminants transported into the Arctic Ocean from the North Atlantic Ocean that were discharged from European nuclear fuel reprocessing plants (18).

Ocean circulation models (6–8) indicate that the initial spreading of the Fukushima tracer signal was governed by the large-scale horizontal currents and mesoscale eddy fields off Japan in 2011, in conjunction with vertical turbulent mixing. These model simulations reveal a postaccident, broadening tracer patch propelled across the central North Pacific at about 40°N by the North Pacific Current (Fig. 1). The principle component of the tracer field in these simulations reaches the coastal waters of North America in several years and eventually occupies a broad region of the eastern North Pacific from Alaska to California. The Line P time series for surface water concentrations of Fukushima ¹³⁷Cs at stations P4 and P26 is compared with the results of two model simulations (6–8) of the lateral dispersion of the Fukushima tracer plume off British Columbia in Fig. 3. Behrens and colleagues (6) predicted Fukushima ¹³⁷Cs concentrations to first become measurable in the surface mixed layer of the area defined by Box B in Fig. 1 in 2015, 2 y after Fukushima ¹³⁷Cs was detected at station P4. In contrast, Rossi and colleagues (7, 8) predicted the arrival of Fukushima ¹³⁷Cs in surface water at station R, a 300-km-wide coastal band at 49°N (Fig. 1), to occur in early 2013. The model simulation reported by Rossi and colleagues (7, 8) is in good agreement with the timing of the initial detection of the Fukushima ¹³⁷Cs signal at the nearby location of station P4 (Fig. 3). Rossi and colleagues (7) had initially predicted Fukushima ¹³⁷Cs concentrations to increase rapidly to a value of 27 Bq/m³ at station R by 2015. However, they have recently downscaled their results by a factor of 10 (8) to levels that are in good agreement with the Line P measurements (Fig. 3). The time series of Rossi and colleagues (7, 8) for Fukushima ¹³⁷Cs slightly lags the measured values at the ocean interior location, station P26, and slightly leads the time series at station P4. The revised simulation of Rossi and colleagues (7, 8) indicates that a maximum Fukushima ¹³⁷Cs level of 2.8 Bq/m³ will be attained at station R in 2015.

Estimates of the total Fukushima ¹³⁷Cs input into the ocean vary widely from 3.5 PBq (19) to 27 PBq (20), most of which are based on a combination of numerical analyses and direct observations. Recent estimates of the Fukushima release into the ocean of 14.5 PBq (21) and 16.2 PBq (22), based on higher-resolution simulations, tend to favor a source strength intermediate between those used in the model simulations reported by Behrens and colleagues (6) and those reported by Rossi and colleagues (7, 8). The agreement of the magnitude of the ¹³⁷Cs signal in the model simulations with the experimental results on Line P (Fig. 3) is consistent with the latter estimates (21, 22) of the Fukushima inputs.

The Line P data generally conform to measurements of the magnitude and timing for the eastward transport of the main Fukushima radioactivity plume by Aoyama and colleagues (5). They defined the leading edge of the Fukushima ¹³⁷Cs plume by the 10 Bq/m³ iso-concentration front based on samples collected in the central North Pacific in 2011–2012. Their results revealed a Fukushima ¹³⁷Cs signal decreasing approximately exponentially with time by mixing with a time constant of 6 mo as the radioactivity plume was transported eastward across the Pacific at a speed of about 8 cm/s (5). This propagation speed is comparable to zonal geostrophic current velocities in the core of the North Pacific Current of 5–6 cm/s (15). At this rate of transport, the leading edge of the Fukushima plume would have arrived at Line P several months after the June 2013 Line P sampling mission, with a ¹³⁷Cs concentration reduced by mixing to a level of 1–3 Bq/m³, which is in general agreement with the range of concentrations measured at stations P4 and P26 in 2013 and 2014 (Table 1).

Public concerns have focused on the eventual magnitude of the Fukushima radioactivity signal in the ocean and the effect of this radioactivity on marine organisms. Given that the ¹³⁷Cs fallout background averages about 1.2 Bq/m³ in surface water on Line P, levels of Fukushima-derived ¹³⁷Cs in February 2014 can be viewed as ranging from 170% of the fallout background at station P26 to 75% of fallout levels at station P4. Comparison with the history of atmospheric fallout in surface water in the North Pacific (inset, Fig. 3) indicates that total ¹³⁷Cs values (Fukushima-derived plus fallout ¹³⁷Cs) predicted for the models of Behrens and colleagues (6) and Rossi and colleagues (7, 8) with maximum values in the 3–5 Bq/m³ range would return ¹³⁷Cs levels in continental shelf regimes in the northeast Pacific Ocean to those fallout levels that prevailed during the 1980s. On the basis of a comparison of results between stations P26 and P4 (Fig. 2 and 3), it appears that ¹³⁷Cs levels in the interior of the Northeast Pacific may approach values greater than those on the continental shelf. However, these concentrations of ¹³⁷Cs in the Northeast Pacific Ocean are well below Canadian guidelines for drinking water quality, for which the maximum acceptable concentration of ¹³⁷Cs in drinking water is 10,000 Bq/m³.

The potential effect of these predicted increases in ¹³⁷Cs seawater concentrations on marine organisms can be evaluated using the concentration factor approach used by Kryshev and colleagues (23) in the postaccident marine environment at Fukushima. Radioactive cesium in fish is excreted through osmotic pressure regulation and elimination, so it does not bioaccumulate indefinitely. Instead, the ¹³⁷Cs concentration in fish tissue attains a steady-state value under conditions in which the ¹³⁷Cs concentration in seawater remains constant. The ¹³⁷Cs concentration in fish tissue can then be characterized by a concentration factor which is a dimensionless parameter defined as the ¹³⁷Cs concentration (Bq/kg) in the fish tissue divided by the ¹³⁷Cs concentration (Bq/kg) in ambient seawater. The recommended literature value for the concentration factor for ¹³⁷Cs in fish of 100 (24) can be used together with the maximum projected seawater concentration for ¹³⁷Cs of 5 Bq/m³ to give a predicted ¹³⁷Cs concentration in fish of 0.5 Bq/kg (wet weight) or 2.5 Bq/kg (dry weight). This predicted level is several times greater than the fallout background levels of ¹³⁷Cs in fish in the North Pacific typified by the pre-Fukushima value of 1.0 Bq/kg (25) for Bluefin tuna off California. The internal radiation dose rate to fish is the product of the ¹³⁷Cs concentration in fish and the internal dose conversion factor [1.8 × 10⁻⁴ µG/h/Bq/kg (26)]. The internal radiation dose calculated using the earlier predicted ¹³⁷Cs concentration in fish for maximum Fukushima levels in seawater yields a value of 4.5 × 10⁻⁴ µGy/h. (450 × 10⁻¹² Gy/h) for fish in the eastern North Pacific. This predicted exposure level is many orders of magnitude less than the baseline safe level of 420 µGy/h, below which harmful effects are not expected at either the aquatic ecosystem or the population level (27). Fisher and colleagues (28) calculated the effective radiologic dose to humans from the consumption of Bluefin tuna having levels of about 6 Bq/kg of ¹³⁷Cs resulting form contamination from Fukushima. They noted that the dose to humans was only about 7% and 0.2% of the dose from the natural radionuclides ⁴⁰K and ²¹⁰Po in the fish, which is comparable to the dose commonly received from naturally occurring radionuclides in many other food items, and only a small fraction of doses from other background sources. These results indicate that future projected levels of ¹³⁷Cs in seawater in the Northeast Pacific Ocean are well below levels posing a threat to human health or the environment.

During oceanographic missions of the CCGS John P. Tully and the CCGS Louis S. St. Laurent in the North Pacific and Arctic oceans, respectively, large-volume (≅ 60 L) water samples were collected to depths of 1,000 m and then passed through potassium cobalt ferrocyanide resin columns to selectively extract Cs isotopes from seawater (10). Column extraction efficiencies are generally greater than 96%, as determined using spiked yield tracers (10, 18), with resin columns arranged in series. The isotopes ¹³⁷Cs and ¹³⁴Cs were subsequently measured on the oven-dried potassium cobalt ferrocyanide resins in the laboratory, using high-purity Ge well detectors (10). All data were decay-corrected to the time, April 6, 2011, of maximum discharge from Fukushima, following Buesseler and colleagues (4). Detection limits for ¹³⁷Cs and ¹³⁴Cs were generally 0.10 and 0.13 Bq/m³, respectively. Detector efficiencies were measured using National Institute of Standards and Technology and National Bureau of Sciences (NBS) calibration standards (e.g., NBS 4350B river sediment) and International Atomic Energy Agency reference materials. Hydrographic results for the Line P cruises are available at linep.waterproperties.ca.

We thank the officers and crews of the CCGS John P. Tully and CCGS Louis S. St. Laurent for their assistance in sample collection and G. Forwarczna (Fisheries and Oceans Canada) and R. Kelly (University of Rhode Island) for their laboratory and field assistance. We also thank two anonymous reviewers and P. Cummins (Fisheries and Oceans Canada) for their insights and helpful comments. S.B.M. acknowledges funding support from the National Science Foundation (OCE-0926311).