Unexpected source of Fukushima-derived radiocesium to the coastal ocean of Japan

¹³⁷Cs activities in seawater during the study period. ¹³⁷Cs measured in seawater within the 100 km offshore from the FDNPP (excluding the harbor). The locations of the samples are shown in Fig. 1A. Because of the larger number of samples collected close to the FDNPP where the ¹³⁷Cs activities were higher, we used the median rather than the average. The median of the ¹³⁷Cs activity in seawater during 2013–2015 was 14 Bq⋅m⁻³. The median ¹³⁷Cs activity determined in the FDNPP harbor was 1,900 Bq⋅m⁻³ in Summer 2012 (green line) and 380 Bq⋅m⁻³ during the period 2013–2015 (red line). Data are from the Japan Atomic Energy Agency (JAEA), database for Radioactive Substance Monitoring Data. Available at emdb.jaea.go.jp/emdb/en/. No data were published by JAEA for the year 2016; however, according to the Nuclear Regulation Authority (NRA) (Japan), the ¹³⁷Cs activities remained constant between 2013 and 2016 in the surface offshore water (radioactivity.nsr.go.jp/en/contents/8000/7745/24/okiai.pdf). Similarly, no data were published by JAEA regarding the Cs harbor activity for the year 2016; however, the Tokyo Electric Power Company (TEPCO) reported relatively comparable Cs activities in the harbor between 2013 and 2016 (www.tepco.co.jp/en/nu/fukushima-np/f1/smp/indexold-e.html). Therefore, we assumed that the medians of ¹³⁷Cs activities for offshore seawater and for the harbor that we estimated based on JAEA data are valid for the period 2013–2016.

Vertical profiles of ¹³⁷Cs activities in beach sand. The sand cores were collected on November 16, 2016, at Yostukura beach, 35 km south of the FDNPP (Fig. 1A). The error bars are smaller than the symbols (3% on average). The bulk density of each sample, estimated based on the volume and the weight of the dry sand, was used to convert the ¹³⁷Cs activity from becquerels per kilogram into becquerels per cubic meter in order to estimate the inventory. The ¹³⁷Cs inventory of the deepest sand core (core 4) was 4.8 ± 0.6 × 10⁵ Bq⋅m⁻² determined by integrating the Cs activity over each 5-cm layer of sand and by summing the integrated activities. The uncertainty on the sand core Cs inventory results from the propagation of the error on each Cs activity in sand samples from the core, and is a minimum estimate as we did not reach ¹³⁷Cs-free sands below.

Sources of Fukushima-derived radiocesium to the coastal ocean off Japan in 2013–2016. As detailed in the text, the two known ongoing sources of dissolved ¹³⁷Cs include the FDNPP via flushing of its harbor (0.6 TBq⋅y⁻¹) and river runoff (0.2–1.2 TBq⋅y⁻¹). We report here a previously unknown source of dissolved ¹³⁷Cs to the ocean from submarine groundwater discharge (SGD) along the Japan coastline of between 0.2 and 1.1 TBq⋅y⁻¹ (average, 0.6 TBq⋅y⁻¹). The main driving forces of submarine groundwater from beaches are waves (W), hydraulic head (H), tidal pumping (T), and convection (C). The southward flowing coastal current, represented by the light blue arrow, would have carried extremely high ¹³⁷Cs, some fraction of which was sorbed onto beach sands and later released as indicated by this study.

Tide chart and ¹³⁷Cs, ²²³Ra, and ²²⁴Ra activities in surf zone samples collected at Yotsukura beach. The tide data are from the Onahama station (Tide Times and Tide Charts Worldwide; available at https://www.tide-forecast.com/). Seawater samples were collected from approximatively the same location at the surf zone of Yotsukura beach over a tide cycle during the November 2016 sampling trip. High Cs and Ra activities were measured at the surf zone during low tide, and lower Cs and Ra activities were measured at rising tide, demonstrating that groundwater is a source of Ra and Cs to the surf zone and the role played by the tidal pumping in the release of groundwater Cs to the ocean.

Desorption fraction as a function of the salinity of the seawater solutions. Details on ¹³⁷Cs activities and locations of the three sand samples used in the experiments are reported in Table S3.

²²⁴Ra and ²²³Ra activities in groundwater including freshwater, brackish, and surf zone seawater as a function of salinity. The radium data are from all of the groundwater samples collected in 2013–2016. The uncertainties are reported on the graph. The dashed line represents the conservative mixing line between freshwater and seawater.