Five years after the Fukushima Dai-ichi Nuclear Power Plant accident, the highest radiocesium ( 137 Cs) activities outside of the power plant site were observed in brackish groundwater underneath sand beaches. We hypothesize that the radiocesium was deposited on mineral surfaces in the days and weeks after the accident through wave- and tide-driven exchange of seawater through the beach face. As seawater radiocesium concentrations decreased, this radiocesium reentered the ocean via submarine groundwater discharge, at a rate on par with direct discharge from the power plant and river runoff. This new unanticipated pathway for the storage and release of radionuclides to ocean should be taken into account in the management of coastal areas where nuclear power plants are situated.

There are 440 operational nuclear reactors in the world, with approximately one-half situated along the coastline. This includes the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), which experienced multiple reactor meltdowns in March 2011 followed by the release of radioactivity to the marine environment. While surface inputs to the ocean via atmospheric deposition and rivers are usually well monitored after a nuclear accident, no study has focused on subterranean pathways. During our study period, we found the highest cesium-137 ( 137 Cs) levels (up to 23,000 Bq⋅m −3 ) outside of the FDNPP site not in the ocean, rivers, or potable groundwater, but in groundwater beneath sand beaches over tens of kilometers away from the FDNPP. Here, we present evidence of a previously unknown, ongoing source of Fukushima-derived 137 Cs to the coastal ocean. We postulate that these beach sands were contaminated in 2011 through wave- and tide-driven exchange and sorption of highly radioactive Cs from seawater. Subsequent desorption of 137 Cs and fluid exchange from the beach sands was quantified using naturally occurring radium isotopes. This estimated ocean 137 Cs source (0.6 TBq⋅y −1 ) is of similar magnitude as the ongoing releases of 137 Cs from the FDNPP site for 2013–2016, as well as the input of Fukushima-derived dissolved 137 Cs via rivers. Although this ongoing source is not at present a public health issue for Japan, the release of Cs of this type and scale needs to be considered in nuclear power plant monitoring and scenarios involving future accidents.

On March 11, 2011, a 9.0-magnitude earthquake triggered a 15-m tsunami that inundated the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), causing power loss, explosions, and reactor meltdowns, releasing a significant quantity of radionuclides into the atmosphere (1⇓–3). More than 80% of the atmospheric fallout occurred over the ocean, with the highest deposition in the nearshore marine environment (4). In addition, direct liquid discharge of contaminated cooling water flowed into the ocean, making the FDNPP disaster the largest accidental input of radionuclides to the ocean (5).

Cesium-137 is an abundant fission product of nuclear power generation and nuclear weapons testing, which when released to the environment persists for decades due to its long half-life (30.2 y). The largest releases of Fukushima-derived 137Cs took place within the first month of the accident. The ongoing sources to the ocean that are known include rivers and groundwater flow beneath the FDNPP, but these are by comparison more than 1,000 times smaller than the 2011 releases, although they have persisted nearly 6 y after the accident (4). Submarine groundwater discharge has been recognized as an important pathway for the transport of materials from the land to the ocean (6), yet this process has not been evaluated as an ongoing source of radionuclides to the coastal environment outside of the vicinity of the FDNPP.

Here, we present 137Cs activities measured in groundwater collected underneath beaches up to 100 km away from the FDNPP (Fig. 1A). Eight beaches were visited between 2013 and 2015, with a more intensive sampling survey conducted in 2016 at Yotsukura beach, 35 km south of the FDNPP. Additional fresh groundwater and river samples were collected in the vicinity of the sandy beaches.

Sample locations and 137 Cs activities near the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). (A) Sample locations in the vicinity of the FDNPP. The seawater data (open squares) are from the Japan Atomic Energy Agency online database. The beach groundwater (GW), surf zone, and freshwater samples were collected between 2013 and 2016. (B) 137 Cs activities determined in brackish groundwater underneath beaches, freshwater (irrigation wells and rivers), and seawater from the beach surf zones plotted vs. salinity. The error bars are smaller than the symbols. The lines denote the 137 Cs Japanese drinking water (DW) limit, the median 137 Cs activity in seawater after the FDNPP accident (excluding the FDNPP harbor), and the 137 Cs activity level in seawater before the FDNPP accident.

Results and Discussion

Dissolved (<0.45 µm) 137Cs activities in beach groundwater spanned three orders of magnitude, with a maximum value of 23,000 ± 460 Bq⋅m−3 (Fig. 1B and Table S1). For perspective, three of the Yotsukura beach groundwater samples were higher than the Japanese drinking water limit of 10,000 Bq⋅m−3, although no one is either exposed to, or drinks, these waters, and thus public health is not of primary concern here. Beach groundwater 137Cs activities were generally higher than in nearby seawater, rivers, natural spring, and groundwater wells used for irrigation. The 137Cs activity in seawater rapidly decreased after the accident (7), and for the period of our study, the median seawater 137Cs activity within 100 km of the coastline (excluding the FDNPP harbor) was 14 Bq⋅m−3 (Fig. S1). In freshwater, dissolved 137Cs activities ranged from below detection to 5.8 ± 0.2 Bq⋅m−3 (Table S1). Thus, the 137Cs activities in beach groundwater cannot be explained by conservative mixing between any known freshwater and seawater sources (8). It must therefore be sourced from 137Cs-enriched beach sands.

Table S1. Activities of dissolved 137Cs and 134Cs, and of 223Ra and 224Ra, in groundwater, river, irrigation well, natural spring, and surf zone samples collected between 2013 and 2016 in the vicinity of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP)

Fig. S1. 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.

Four sand cores were collected on Yotsukura beach (Table S2). In sand located between the beach surface and 40 cm, the 137Cs activity was relatively constant at 17 ± 4 Bq⋅kg−1. Below 40 cm, the 137Cs activity increased with depth, reaching maximum values of 700 ± 60 Bq⋅kg−1 (Fig. S2). The 137Cs inventory in the longest core was 4.8 ± 0.6 × 105 Bq⋅m−2, and is a minimum estimate since we did not reach the bottom of the high activity layer. This is one order of magnitude higher than the largest recorded marine sediment inventory [0.73 ± 0.02 × 105 Bq⋅m−2; offshore of the FDNPP (9)], within a factor of 4 of terrestrial soil cores (20 × 105 Bq⋅m−2) from the restricted access area, and in excess of soils in the Yotsukura region (105 Bq⋅m−2) (10). Furthermore, the deep enrichment of 137Cs is inconsistent with a Fukushima atmospheric fallout source as 137Cs would have likely been trapped in the upper layers of the sand horizon as demonstrated in land soils (10).

Table S2. Activities of 137Cs and 134Cs in sand

Fig. S2. 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.

Consequently, we need to consider an alternative source to explain the high 137Cs activities deeper in both beach sand and groundwater. In the days–weeks following the reactor meltdowns, dissolved 137Cs activities reached >60 × 106 Bq⋅m−3 in the ocean closest to the FDNPP (5). Numerical modeling demonstrated that a net southward flowing coastal current transported this highly radioactive seawater along the shoreline (11). We hypothesize that seawater intrusion, driven in part by waves and tides (12⇓⇓–15), led to the storage of 137Cs by adsorption onto beach sands. In the years since the accident, falling ocean 137Cs activities and similar groundwater–surface water exchange processes would have led to the reverse reaction (desorption) and the elevated beach groundwater 137Cs activities observed today (Fig. 2). The 137Cs-enriched groundwater is then available to be released to the ocean via submarine groundwater discharge (16, 17). The increasing 137Cs activities in the surf zone with the falling tide (Fig. S3) is one line of evidence that the groundwater 137Cs is discharged to the sea with tidal pumping.

Fig. 2. 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.

Fig. S3. 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.

To test this hypothesis, 137Cs adsorption and desorption experiments were conducted on Japanese beach sand samples(Fig. S4 and Table S3) with seawater solutions of varying salinities to reproduce the salinity gradient observed in beach groundwater. Seawater solutions were spiked with 137Cs standard to reproduce the concentration of 1 × 106 Bq⋅m−3 observed in April 2011 in the ocean off Iwasawa beach, 16 km south of the FDNPP (5). The experiments showed that beach sands have an adequate ion exchange capacity for this level of 137Cs, with an average adsorption fraction of 99% regardless of the salinity, sediment grain size, or mineralogy (Table S3). Desorption experiments involved recirculating 137Cs free seawater through the beach sand. The desorption fraction ranged from 1.4% to 11.4%, with the highest values found at intermediate salinities except for one low salinity treatment with 22% desorbed 137Cs (Fig. S4). These desorption rates were higher than those observed from riverine particles (18) or estuarine sediments (19), but consistent with a well-known property of Cs: decreasing solid–solution partitioning with increasing ionic strength (20). Therefore, beach sands appear to be capable of storing a large inventory of 137Cs at depth that over time may be remobilized by seawater intrusion into beach aquifers and released to the coastal environment via groundwater–surface water exchange processes.

Fig. S4. 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.

Table S3. Locations and 137Cs (becquerels per kilogram) activities of sand samples used in the adsorption–desorption experiments

In beach settings, seawater intrusion generally follows two pathways: (i) an upper saline plume in the intertidal zone set up by waves and tides and (ii) a saltwater wedge at depth, a function of the density difference between fresh groundwater and seawater (refs. 13 and 15 and Fig. 2). The sand cores and the groundwater samples were collected at depths less than 2 m, along the boundary of influence of a typical upper saline plume, which is characterized by dynamic mixing and exchange with seawater on timescales of days to weeks (13). The lower 137Cs concentration in the shallow cores could thus be the result of more frequent flushing with seawater in comparison with the deeper sand layer, or less exposure to high 137Cs activity seawater in 2011. The highly heterogeneous distribution of 137Cs in beach groundwater is also supported by the complexity of upper saline plume dynamics (15) as well as variability in sand physical and biogeochemical properties as shown by the desorption experiments.

Regardless of the mechanisms controlling the concentration of 137Cs in groundwater, beach aquifers in close proximity to FDNPP must be a source of 137Cs to the ocean, which is supported by surf zone 137Cs activities that were higher than offshore seawater during the study period (Fig. 1B). To quantify the magnitude of this 137Cs source, we used parallel measurements of radium isotopes (223Ra, 11.4 d; 224Ra, 3.66 d), which are a well-established tool for providing regional scale estimates of submarine groundwater discharge (21, 22). Radium isotopes are continuously produced in aquifer sediments by the decay of insoluble thorium parent isotopes. Ra has a similar geochemical behavior to Cs, and Ra activities were also significantly higher in brackish groundwater than freshwater or seawater (Fig. S5). Like 137Cs, Ra activities in the surf zone were inversely correlated with tidal stage (Fig. S3). These characteristics are why Ra isotopes can be used as tracers of the ocean input of 137Cs from beach groundwater.

Fig. S5. 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.

A Ra isotope mass-balance model (21⇓⇓–24) for the surf zone was used to estimate the volume of ocean water that exchanges with the beach aquifer on a daily basis. In the model, we assume that submarine groundwater discharge is the only source of Ra to the surf zone and that this source is balanced by Ra loss due to decay and mixing. The model was solved independently for the two Ra isotopes, which yielded a range of flux estimates between 0.07 and 0.51 m3⋅m−2⋅d−1 for multiple beaches (Table S4).

Table S4. Volumetric input of submarine groundwater discharge (V GW ) estimated using a 223Ra and 244Ra mass balance for the different beaches based on data collected in 2015 and 2016

To scale this estimate, we must make assumptions about the area over which the exchange of 137Cs-rich beach groundwater is occurring. The highest FDNPP 137Cs-contaminated marine sediments extend along 180 km of coastline (9). However, only ∼45% of the coastline is covered by sandy beaches (80 km). Assuming groundwater–surface water exchange occurs over a (shore perpendicular) intertidal zone width of 50 m (calculated from beach slope and tidal range), the exchange of water was estimated to be on the order of 3.0–20 × 105 m3⋅d−1 (mean, 9.9 × 105 m3⋅d−1). Using the statistical mean of 137Cs activity in beach groundwater 1,520 ± 570 Bq⋅m−3 (Fig. S6) and assuming a constant exchange of water over the period 2013–2016, we estimate that the amount of 137Cs delivered by submarine groundwater discharge along the Japanese coastline is on the order of 0.2–1.1 TBq⋅y−1, with an average of 0.6 TBq⋅y−1 (T = 1012).

Fig. S6. Distribution of 137Cs activity in groundwater. A bootstrapping method was used to determine the statistical mean 137Cs activities in groundwater underneath sand beaches. The solid line and the dashed line represent the average and the SD of the bootstrap mean distribution, respectively. The bootstrap method was run 1,000 times on groundwater data displaying a salinity between 5 and 30 to limit the influence from estuary or seawater. The groundwater samples were collected randomly and are uniformly distributed across the salinity gradient (Fig. 1B). The distribution of 137Cs in groundwater is a combination of several factors including the 137Cs activity in beach sand, the minerology of the sand layer, and groundwater salinity. All of these vary spatially and temporally. Using a bootstrap method to calculate the statistical mean is the best approach for representing the average Cs activity.

Terrestrial groundwater discharge to the ocean from the main island of Japan (Honshu) has been estimated at 44.5 × 103 m3⋅d−1⋅km−1 (25), which is equivalent to 3.5 × 106 m3⋅d−1 when scaled to our 80-km shoreline length. The 137Cs activities were relatively low in terrestrial groundwater, 2.3 Bq⋅m−3 on average in irrigation wells and in the natural spring, likely due to the strong affinity of Cs for clay particles in low ionic strength fluids, which limits its mobility in most inland freshwater aquifers (20). We estimate the amount of 137Cs carried to the ocean by terrestrial groundwater to be on the order of 3 × 10−3 TBq⋅y−1, which is <1% of the flux of 137Cs from brackish groundwater discharge. Thus, outside of the FDNPP site, the beach groundwater source dominates the flux of 137Cs released to the coastal ocean via submarine groundwater discharge.

In comparison with other sources, the ongoing releases of 137Cs at the FDNPP harbor were estimated to be 3 TBq⋅y−1 for the summer of 2012 (26). Harbor 137Cs activities decreased by a factor of 5 between 2013 and 2016 (27) (Fig. S1). Therefore, assuming that the flushing rate of the harbor has remained constant, the present-day FDNPP harbor flux should be ∼0.6 TBq⋅y−1. Another ongoing source of FDNPP-derived 137Cs is from rivers, with release estimates for total 137Cs ranging from 2 to 12 TBq⋅y−1 (28⇓⇓–31). Typhoons and heavy rain events have been shown to increase the river runoff flux; the 137Cs input from this source is largely in the particulate phase (32), of which only a small amount is capable of entering the dissolved phase via desorption in the estuarine mixing zone (19). Thus, considering that ∼90% of the Cs is irreversibly bound to riverine suspended sediments (19), the riverine flux of dissolved 137Cs is only ∼0.2–1.2 TBq⋅y−1. Hence, the ocean input of 137Cs from groundwater below the sandy beaches is similar in magnitude as the other two major sources, namely ongoing FDNPP and dissolved river sources. Similar to projected decreases in 137Cs carried by runoff from land (30) and decreasing concentrations in the harbor at the FDNPP, the concentrations of 137Cs in sand will likely diminish over time due to desorption, and thereby the beach source to the ocean is expected to become depleted.

This study demonstrates that, aside from the aquifer beneath the FDNPP, the highest recorded present-day activities of 137Cs in the aqueous environment in Japan are associated with brackish groundwater underneath beaches. This finding suggests that the beach sands served as a reservoir for 137Cs, which is subsequently released via submarine groundwater discharge to the ocean. Using Ra isotopes, we were able to make an estimate of the magnitude of this flux and found that it is similar to other ongoing sources, including export from the FDNPP harbor. This unexpected and ongoing 137Cs source requires further investigation, in particular more systematic sampling in space and time given the variability in our groundwater and sand 137Cs observations. The implications of our study extend well beyond the FDNPP event to siting of all coastal NPPs, and this source will need to be considered when evaluating the fate of radionuclides in the ocean from both intentional (e.g., Sellafield) and unanticipated releases.