The Contribution of Sources to the Sustained Elevated Inventory of

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The Contribution of Sources to the Sustained Elevated Inventory of 137Cs in Offshore Waters East of Japan after the Fukushima Dai-ichi Nuclear Power Station Accident Hyoe Takata, Masashi Kusakabe, Naohiko Inatomi, Takahito Ikenoue, and Kazuyuki Hasegawa Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00613 • Publication Date (Web): 10 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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Environmental Science & Technology

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1

The Contribution of Sources to the Sustained Elevated Inventory of

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of Japan after the Fukushima Dai-ichi Nuclear Power Station Accident

Cs in Offshore Waters East

3 4

Hyoe Takata*, Masashi Kusakabe, Naohiko Inatomi, Takahito Ikenoue, and Kazuyuki Hasegawa,

5 6

Central Laboratory, Marine Ecology Research Institute, 300 Iwawada, Onjuku-machi, Isumi-gun,

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Chiba 299-5105, Japan

8 9

ACS Paragon Plus Environment


ABSTRACT: We have evaluated the contribution of sources of

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137

Cs to the inventory of

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radiocesium in waters (surface area: 6160 km2, water volume: 753 km3) off Fukushima Prefecture

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and neighboring prefectures from May 2011 to February 2015. A time-series of the inventory of

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137

14

February 2015 (1.89 TBq). The

15

approximately twice the background inventory of 1.1 TBq. We assumed that three sources

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contributed

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(FNPS) even after the massive discharge in late March 2011, desorption/dissolution from sediments,

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and fluvial input. The magnitudes of the

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periods of 182–183 days were estimated from the first period (1 October 2011 to 31 March 2012) to

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the last period (1 October 2014 to 31 March 2015). Quantification of these sources indicated that

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the direct discharge from the FNPS is the principal source of

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period) to 2.9 TBq (the first period) to maintain the relatively high inventory in the offshore area.

Cs in the offshore waters revealed a clearly decreasing trend from May 2011 (283.4 TBq) to 137

Cs inventory about four years after the accident was

137

Cs: continuous direct discharge from the Fukushima Dai-ichi Nuclear Power Station

137

Cs influxes from the sources into offshore waters for

137

Cs ranging from 0.7 TBq (the last

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■ INTRODUCTION

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As a result of the tsunami following the Great East Japan earthquake on 11 March 2011, the

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waters offshore of Fukushima and other nearby prefectures of Japan were contaminated by artificial

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radioactivity released by the damage to the Fukushima Dai-ichi Nuclear Power Station (FNPS).

29

137

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direct liquid releases has been estimated to be about 3.5 PBq.5 The total amount released to the

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atmosphere between 12 March and 30 April 2011 has also been estimated.6–10 Quite recently, more

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accurate estimate for 137Cs (15–20 PBq) released from FNPS to the atmosphere has been presented

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based on the meta data and model simulations.11 Of the

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PBq was deposited on the North Pacific Ocean.11 Thus, a large amount of FNPS-derived 137Cs was

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deposited directly in the ocean or transported later into the marine environment during a short

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period of time immediately after the accident.

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Cs released directly into seawater within one month after the accident has been reported.1–4 The

Oikawa et al.12 revealed that

137

137

Cs released to the atmosphere, 12–15

Cs concentrations in the waters in 30 km distance from the

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FNPS reached a maximum of 186,000 mBq/L in early April 2011 and thereafter decreased

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exponentially with time to about 100 mBq/L within a year. The inventory of

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Fukushima and neighboring prefectures also decreased from 8.4 ± 2.6 PBq to 2.0 ± 0.4 PBq

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between early April 2011 and mid May 2011 (the activity ratio of 134C/137Cs: ~1).13 The decrease of

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FNPS-derived contamination after the accident has been linked to the conservative behavior of

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cesium in seawater and thus the regional ocean circulation. Recent works studying the levels and

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transport pathways of FNPS derived contamination have shown that principal marine processes

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causing the decline of Cs have been: i) replacement of contaminated water by open ocean water

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with little or no FNPS influence, ii) eastward dispersion of surface waters by the Kuroshio-Oyashio

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current system, and iii) the subduction to subsurface depths as mode waters.12, 14-19 Despite the

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mechanisms listed above, four years after the FNPS accident, 137Cs concentrations are still at a level

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that is relatively high (the mean value at depths of 50% per month during the first six-month

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period (May–September 2011, Table 1), but it fell to about 20% per month after October 2011.

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Since removal of

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we assumed It negligible for this period. Thus, eq (3) can be simplified as follows:

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Cs in the Offshore Water from Early May 2011 to

Cs was overwhelmingly large compared to its inputs in May-September 2011,

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d(V·CCs)/dt = – (λp + λr)·V·CCs

(4)

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During these six months the inventory of

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Cs in the offshore water at any given time can be

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expressed mathematically by solving eq (4), subject to the condition that CCs = CCs0 at time zero to1

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May 2011. The solution is as follows:

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V·CCs = V·CCs0 ·exp {– (λp + λr)·t}

(5)

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The value of λp + λr was obtained by iterative calculations associated with the regression analyses

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that were performed, as shown in Figure 3. The value of λp + λr obtained by this analysis was 0.017

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d–1. Because

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negligible compared to λr. Thus, we use λr to estimate a residence time of 59 days for the water in

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the offshore area, for the period from May to September 2011. This value was relatively long,

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compared to the value of 32 days in the coastal area off the east coast of Japan2 and the published

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values in semi-closed coastal regions such as ca 30 days (Tokyo Bay),37 18–44 days (Sendai Bay),38

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and 26 and 53 days (Tampa Bay).39

137

Cs has a half-life of approximately 30 years, the value of λp, 6.3 × 10–5 d–1, was

171 172

Estimation of the Total Input Rate of

137

Cs into the Offshore Water from October 2011 to


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January–February 2015. The solution of eq (3) with the condition that the inventory (V·CCs)

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equals V·Ccs0 at time zero is as follows:

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V·CCs = It0/{(λp +λr)–λI}·exp(–λI·t) + V·CCs0·exp{– (λp +λr)·t}

(6)

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The regression analysis was performed applying the eq (6) to the observed inventories (Figure 3).

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The results were plugged in to the eq (2) to get the temporal change of It (TBq/day) as follows:

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It = 0.11·exp (–0.0031·(t – t0))

(7)

with t0 equal to 1 October 2011. We calculated the total released

137

Cs to be 15.3 TBq from 1

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October 2011 to 31 March 2012 and 7.4 TBq from 1 April to 30 September 2012, then, decreased

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below 4 TBq and reached 0.41 TBq in 1 October 2014- 31 March 2015 (Table 2). In the next

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section, we will evaluate how each source contributed to the estimated total input of

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offshore region.

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137

Cs to the

Direct Discharges from the FNPS. To evaluate the contribution of direct discharges from the 137

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FNPS to the offshore water, we calculated the daily release of

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monitoring data from TEPCO33 from five stations (i.e. ULD, U6, U1-4N, U1-4N-2, U1-4S) located

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within the harbor. These stations represent the most heavily impacted area by FNPS, with

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concentrations of 137Cs 1- 3 orders of magnitude higher than at stations (T-1, T-2, MH, M-101~104,

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T-D1, T-D5, and T-D9)32,33 outside the harbor (Figure 5) (Figure S2 in Supporting Information). The

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eq (6) was fitted to the observed data to get parameters in the same way for the derivation of eq (7)

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(Figure 5). A daily release of

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water volume (0.00188 km3) of the harbor21:

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IFNPS = 0.018·exp (–0.0013·(t – t0))

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Cs. For the calculation we used

137

Cs to the harbor, IFNPS, is, thus, expressed as follows based on a

The temporal change of total amount of

(8) 137

Cs entering to the nearshore area from the FNPS for

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about 6-month period is summarized in Table 2. During the first period (1 October 2011 – 31 March

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2012), the amount of

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estimated total input of 137Cs that entered the offshore water (15.3 TBq in third column in Table 2).

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Kanda (2013)

21

137

Cs input was calculated to be 2.9 TBq, which accounts for 19 % of the

calculated the amount in a similar manner: recalculation of his data to the first ACS Paragon Plus Environment


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period shown above resulted to be 6.1 Bq listed also in Table 2. Apparently small contribution of

200

the FNPS-derived

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first massive discharge in late March 2011 may affect the calculation leading to the relatively low

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contribution.

137

Cs to the total input to the offshore water implies that possible remains of the

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Continuous direct discharges primarily contributed to the maintenance of the relative high

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inventory in the offshore water until the last one-year period. The input from FNPS decreased to 2.3

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TBq (1.5 TBq based on Kanda (2013)’s calculation21) during the second period, and to ~0.8 TBq in

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the last two periods of time. The contribution of FNPS to the total input to offshore waters, however,

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increased from ~20% to >100%, from the first to the last period of time. It is noted that the

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contributions from the FNPS during the last period were estimated to be greater than 100% (Table

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2). Since temporal change of 137Cs concentrations inside the harbor has been highly variable with its

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correlation coefficient of 0.4 (Fig.5), the estimated contributions include significant error leading to

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the value greater than 100 %.

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Dissolution from Sediments. The inventory of

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Cs that accumulated in sediments was

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estimated to be 100–200 TBq for the coastal regions shallower than 800 m depth off the east coast

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of Japan after the FNPS accident.23, 40 A

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of surface sediments in the offshore water,35 that is, 19–38% of sedimentary

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surface layers as a result of the accident.23, 40

137

Cs inventory of 37.8 TBq was found in the upper 3 cm 137

Cs present in the

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In late 2012 the sedimentary 137Cs accounts for a large percentage of the total inventory (water

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and sediment) off the east coast of Japan40 because inventory of 137Cs is much lower in the offshore

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water (2.2–3.5 TBq in Table 1) and of the east coast off Japan (15 TBq)41 than in the sediments

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(100–200 TBq).24,25 Recently, Kusakabe et al.23 have demonstrated that the inventory in the

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sediments has been declining with time. They have proposed several mechanisms to explain the

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decline such as downward migration of surface

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mixture, resuspension and subsequent lateral transportation, and dissolution of Cs from the

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sediment. Only the last one could be the mechanism to supply 137Cs to the offshore water so that it

137

Cs to the depth by biological activity/physical


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is worthwhile to evaluate its contribution to mass balance in the offshore water. Based on the formulation for the temporal change of the

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137

Cs in the surface sediments

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presented by Kusakabe et al.,23 the temporal change of net reduction rate (TBq/day) of sedimentary

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137

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derived from the following equation;

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Cs inventory (Insediment) from the sediment in east coast off Japan (surface area: 25,200 km2) is

Insediment = 48.01 exp(–0.000854·(t – t0))

(9)

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Assuming that the net reduction of the term, Insediment, is released from the sediment to the

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bottom water in the nearshore and offshore areas (~12,000 km2: 47% of the area of the east coast off

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Japan), and that 15% of removed

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amount of dissolved

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amount of 137Cs released from sediments in each half-year period decreased from 0.38 TBq to 0.14

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TBq in the four years after the accident. In contrast, the percentage contribution of this release to

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the total input of 137Cs to the offshore water increased from 3% to 34%. Some FNPS-derived 137Cs

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would have adsorbed loosely onto the surface of sediments under non-equilibrium conditions during

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the first year after the accident because that 137Cs was recently precipitated in sediments. However,

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the value of 0.14 TBq of dissolved forms of 137Cs released from sediments during the last half-year

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period (1 October 2014 to 31 March 2015) might be an upper estimation because it is unlikely that

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15% of all 137Cs released from sediment to water column as dissolved form42 in the nearshore area

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is then transferred to the offshore water (Figure 4). Transport of

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deeper sediments would reduce the contribution of the sedimentary 137Cs input to the water column.

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Furthermore, the content of exchangeable

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accident due to destruction of fuel particles, ageing of

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strength of the soil solid phase.42 Taking account of this fact would reduce the total percentage of

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dissolved forms of 137Cs released from sediments during the most recent half-year period below the

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value of 34% (Table 2). Thus, our calculation would provide the ‘upper estimate’. However,

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understanding of the details of the process of dissolution of

137

Cs from the sediments was in dissolved form,39 the total

137

Cs from the sediment to the bottom water were estimated (Table 2). The

137

137

Cs in surface sediment into the

Cs in soils reportedly decreased after the Chernobyl 137

Cs, and changes in the

137

137

Cs sorption

Cs from sediments requires further



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studies, including, for example, laboratory experiments focusing on the desorption-sorption

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interactions between sediments and seawater and dissolution processes in sediments.

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Fluvial Inputs: Case Study of the Abukuma River. The monitoring data of radiocesium in 137

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soil has indicated that 1.5 PBq of

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FNPS during the three months after the accident.43 Eventually, the Cs deposited on land could reach

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the marine environment through rivers, but most of the Cs discharged from the rivers could be

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deposited onto sediments in estuarine and coastal regions because particulate

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large percentage of total

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In addition, there are few data on amounts of dissolved

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data on

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particulate).44

137

137

Cs was accumulated on land within an 80-km radius of the

137

Cs represents a

Cs export from the river water during, for example, the rain events.26,27 137

Cs in river waters because most of the

Cs concentrations (Bq/L) were obtained by assaying unfiltered river water (dissolved +

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The distributions of FNPS-derived radiocesium in both dissolved and particulate phases in

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Abukuma River system have been well surveyed.27,28,29,45 The Abukuma River flows through the

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Fukushima and Miyagi Prefectures and discharges into the northeastern sector of Sendai Bay in an

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area north of the FNPS. The River has the largest catchment area in the Fukushima Prefecture, and

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its catchment area, total length, and mean annual freshwater discharge are ~5400 km2, 239 km and

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52 m3/s, respectively. In addition, the catchment area of the Abukuma River system is largely

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affected by atmospheric deposition of

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area to evaluate the contribution of the fluvial 137Cs input.

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Table 2 lists the fluvial inputs of

137

Cs,27,28 so this river system is the most important study

137

Cs based on the flux of dissolved 137

137

Cs from Abukuma

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River during each half-year period and application of the dissolved

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normal flow conditions. During the first half-year period, the contribution to the influx of 137Cs into

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the offshore water was 6%, but after that time, the contribution was less than 1%. It is noteworthy

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that this value excluded contributions from other small rivers such as the Niida and Natsui

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Rivers,26,46 where the depositions of

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Abukuma River catchment. If we take into consideration the results during high-flow conditions,

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Cs data obtained under

Cs in their catchments were comparable with that in the


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137

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the transport of high

Cs contents in all the rivers near the FNPS or located on the east coast of

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Japan to the offshore water could increase. Furthermore, salinity-induced desorption of

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fluvial particles heightens the export flux of dissolved

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desorption process must be taken into account when estimating the radiocesium fluxes from rivers

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to coastal regions near the FNPS. Thus, continuous monitoring of the fluvial input of radiocesium to

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coastal regions is highly recommended, considering the fact that watersheds in the regions

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surrounding the FNPS accumulated high levels of radiocesium.

137

137

Cs from

Cs from rivers.30 In particular, the

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■ ASSOCIATED CONTENT

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Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website.

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Supplementary methods, figures, and tables (PDF).

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■ AUTHOR INFORMATION

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Corresponding Author

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*Phone:+81-470-68-5111; fax: +81-470-68-5115; e-mail: [email protected].

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Notes

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The authors declare no competing financial interest.

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■ACKNOWLEDGEMENTS

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We thank the captains and crew of research vessels of the Sanyo Techno Marine Co. and Kaiyo

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Engineering Co. Ltd. for their help in the sampling. We also thank KANSO Technos and Japan

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Chemical Analysis Center for their analysis of radiocesium in seawater. The marine environmental

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radioactivity survey is part of a research project contracted from the Japanese Ministry of Education, ACS Paragon Plus Environment


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Culture, Sports, Science and Technology (May 2011 –March 2013) and the Secretariat of the

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Nuclear Regulation Authority (April 2013 – present).

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the export flux to the marine environment. Mar. Chem. 2015, 176, 51–63.

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(31)Kakehi, S.; Kaeriyama, H.; Ambe, D.; Ono, T.; Ito, S. I.; Shimizu, Y.; Watanabe, T.

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Regulatory

Agency–NRA.

Database

of

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Power

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Monitoring

by

sampling.

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Power Plant accident. Jpn. J. Environ. Radioact. 2013, 118, 96–104. (46) Nagao, S.; Kanamori, M.; Ochiai, S.; Inoue, M.; Yamamoto, M. Migration behavior of

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Cs and

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Cs in the Niida River water in Fukushima Prefecture, Japan during 2011–2012. J.

446 447 448 449 450 451 452



453

Figure captions

454

Figure 1. The area used to calculate the inventory of

455

For the calculation, the 27 stations indicated by filled circles were used.

456

Figure 2. The

457

stations within the 30 km radius from the FNPS. Cs-137 data from March 2011 to February 2012

458

were taken from Oikawa et al.12 Data represented in the figure also include data from stations, not

459

used for the inventory calculations, and shown in Table S1 and Figures S1A.

460

Figure 3. The temporal change of

461

fits to data according to eq 5 and eq 6, and whose coefficients (r2) are to be 0.93 and 0.90,

462

respectively. The second component in the exponential fitting on the right hand represents the

463

second term of eq 6.

464

Figure 4. Schematic illustration of the model of the offshore water with input and removal

465

indicated by arrows.

466

Figure 5. Cs-137 concentrations in seawater samples near the FNPS. Red solid line is best fit to

467

data according to the equation from Kanda21 which is the same equation with eq. 6 in this study, and

468

whose coefficients (r2) is to be 0.16. Calculation of export flux from the FNPS in described in

469

Supporting Information.

137

137

Cs in offshore seawater (Light blue area).

Cs concentrations in the water columns all the stations in Figure S1 (except for

137

Cs inventories in the offshore water. Red solid lines are best


Page 20 of 28

Page 21 of 28


Table 1. Inventory of 137Cs in offshore water East of Japan in each Sampling 137 Time Cs (TBq) Before the accident 1.1 Early May 2011 283.4 Late May 2011 231.3 Early June 2011 239.5 Late June 2011 123.1 Early July 2011 90.9 Late July 2011 54.3 September 2011 28.9 October 2011 14.9 December 2011 11.5 February 2012 6.1 May 2012 9.2 August 2012 3.5 October 2012 2.2 January 2013 2.3 May 2013 2.5 August 2013 1.8 October 2013 2.1 January 2014 2.6 May 2014 2.3 August 2014 1.5 October 2014 1.87 January-February 2015 1.89



Page 22 of 28

Table 2. Total input of 137Cs into the offshore water and inputs to the nearshore area from three sources during each period

Period

days

Total input to sustain the inventory in the offshore watera TBq

Source* FNPSb

Sedimentc

Abukuma River

TBq d

d

1-Oct-11 — 31-Mar-12

183

15.3

2.9-6.1

(19-40)

0.38 (3)

1-Apr-12 — 30-Sep-12

183

7.4

1.5d-2.3 (20-31)

0.32 (4)

1-Oct-12 — 31-Mar-13

182

4.2

1.8 (43)

0.27 (6)

1-Apr-13 — 30-Sep-13

183

2.3

1.4 (61)

0.23 (10)

1.07e (6) 0.02–0.08f (0.2–0.8) 0.04g (1) —

— h

1-Oct-13 — 31-Mar-14

182

1.3

1.1 (86)

0.20 (15)

1-Apr-14 — 30-Sep-14

183

0.91

0.89 (98)

0.17 (19)

—

—

1-Oct-14 — 31-Mar-15

182

0.41

0.70 (>100)

0.14 (34)

—

—

*

0.01

(0.8)

The numbers within brackets represent the percentage contribution (%) of each source to the total input (column 3) Calculated based on eq (7) for each period b Calculated based on eq (8) for each period c Calculated based on eq (9) for each period. See the text for detail. d The numbers include the data from Kanda (2013).21 See the text for detail. e Total amount of dissolved 137Cs released, which was estimated to be about 20%30 of all released 137Cs (5.34 TBq) from 10 Aug 2011 to 10 May 2012.29 f Estimated from the published data of 460 MBq/day in Sep 2012 28 and 157 MBq/day on 5-Aug-12 .27 g Estimated from the published data of 240 MBq/day in Jan 2013.28 h Estimated from the published data of 240 MBq/day in Dec 2013.30 a


Page 23 of 28


Nearshore

FNPS

Open Ocean

Offshore Water Area: 6160 km2 Water volume: 753 km3

Figure 1.



Page 24 of 28

500,000

Sampling depth 012345611 789122 0 0 m 12 13 14 15 16 17 18 19 20 21 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

100,000

50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300

137

Cs (mBq/L)

10,000

300 m >300 m

1,000 100 10 1.0 0.1

011 2 . Mar

r. Ma

2 201

013 2 . Mar

014 2 . Mar

Figure 2.


015 2 . Mar

Page 25 of 28


300 137

Cs = 283.4e-0.017t

250

15 137

Cs = 8.8e-0.0031t +5.2e-0.017 t(R = 0.9)

137

150

Cs (TBq)

10

137

Cs (TBq)

200

5

100 0

50

1O

0 11 -220 . ct

1

12 20 . t Oc

1

13 20 . t Oc

1O

4 01 2 . ct

0

1 1 4 2 2 3 15 14 13 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 2 2 2 2 2 2 2 2 2 . . . . . . r. r p p. r. p r p r a a e e a e a e a M M S S M S M S M Figure 3.



Page 26 of 28

FNPS Rivers

Nearshore

Offshore

Dispersion

Total input (It) to sustain the inventory

Net sediment input= Dissolution/desorption - Deposition

Sediments Figure 4.


Open ocean

Page 27 of 28


106

Inside of the harbour Station ULD Station U6 Station U1-4N Station U1-4N-2 Station U1-4S

137

Cs = 42100e-0.0013t +150e

-0.22t

137

Cs (mBq/L)

105 104 103 102 101 100

Outside of the harbour Station MH Station T1 Station T2 Station T2-1 Stations M-101,M-102

Stations M-103,M-104 Stations T-D1,T-D5,T-D9

4 5 3 4 2 3 2 011 2 201 201 201 201 201 201 201 . . . . . . . r. Oct Oct Apr Oct Apr Oct Apr Ma Figure 5. 27


(R =0.4)


TOC/Abstract Art

28


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The Contribution of Sources to the Sustained Elevated Inventory of

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