Marine Ecosystem Dynamics Modeling Laboratory

Fukushima radionuclide spreading

Fukushima_after_earthquake

The March 11, 2011 Tōhoku magnitude 9.0 and 7.9 earthquakes caused a massive tsunami with ~16 m wave height nearshore and tsunami-induced inundation that devastated the east coast of Japan. Unlike previous earthquake-induced tsunami events, the Fukushima Dai-ichi Nuclear Power Plant (FNPP) was seriously damaged, resulting in the leaking of large amounts of artificial radionuclides, mainly 131I (t1/2 = 8.02 days), 134Cs (t1/2 = 2.065 years) and 137Cs (t1/2 = 30.17 years), from several reactor units into the coastal ocean (Ohnishi, 2012). In this event, the planned dumping from the storage room contained low-level radioactive water, while the leaking from reactors contained high-level radioactive water, with a concentration of 9.4×1014 Bq for 137Cs and 134Cs as well as 2.8×1015 Bq for 131I from Unit-2 over the period of April 1-6 and of 9.8×1012 Bq for 137Cs, 9.3×1012 Bq for 134Cs, and 9.5×1012 Bq for 131I from Unit-3 over the period of May 10-11. In contrast to the Chernobyl disaster in 1986, this was not the most serious radionuclide-leakage in the past. The difference is that the Chernobyl Nuclear Power Plant was located inland and its impact on the Black and Baltic Seas was through deposition with a value of 105 Bq, much smaller than what happened at the FNPP. As a result, following the March 11, 2011 tsunami event, in addition to the wet and dry deposition from the atmosphere, the coastal water was contaminated by discharges of a large portion of high-level radioactive water out of FNPP from leaking sources (Honda et al., 2012) and from inland-polluted rivers (Oura and Ebihara, 2012).

Among these radioactive isotopes, 137Cs was of particular interest because of its long 30.2 year half-life. The accumulation of 137Cs in marine food chains could exert a profound impact on marine biota and human health and thus the local to regional ecosystem (Buesseler et al. 2011; Grossman, 2011). Determining accurately an initial dispersion of 137Cs off Japan’s coast was a prerequisite for assessing its long-term impacts in the interior of the Pacific Ocean. After leaking occurred, many efforts have been made to monitor the spread of 137Cs off Japan’s coast. Ministry of Education, Culture, Sport, Sciences and Technology (MEXT) and Tokyo Electric Power Company (TEPCO) started measuring the 137Cs concentration around the FNPP and in the offshore coastal waters. In addition to these two government-established monitoring programs, several field surveys were carried out in an offshore region to assess the spreading of 137Cs by oceanic currents, lateral diffusion and vertical mixing (e.g., Honda et al., 2012; Behrens et al., 2012; Dietze and Kriest, 2012). The research team led by K. Buesseler (Woods Hole Oceanographic Institution) made a comprehensive survey in the shelf and deeper waters off FNPP in June 2011 (Buesseler et al., 2012). Their survey measured 137Cs concentration over the inner-shelf area 30 km away from the coast and then along several transects across the Kuroshio pathway in the deep ocean. The data collected from these monitoring and field surveys have provided a direct assessment of temporal change and spatial distribution of 137Cs concentration in the coastal waters. Due to the complex nature of advection and mixing in this coastal region, however, these data cannot alone be used to predict the spreading processes of 137Cs from FNPP in the shelf waters after the leaking started. This is one of the key reasons why an ocean model was proposed for this purpose.

It is not a trivial task for a model to simulate and predict accurately the spatial distribution and temporal change of the 137Cs concentration off the Japan coast. Since advection and mixing are two key physical processes that control the spread of 137Cs in ocean waters, we need an ocean model that is capable of resolving an integrated coastal and regional circulation system over scales from a few meters (small scale, e.g. around FNPP) to a few kilometers (mesoscale) over the shelf. The flow around FNPP and near the coast is mainly controlled by tidal exchange, winds and local geometry. The circulation in this shelf region includes the Kuroshio Current on the south, the Oyashio Current on the north, the Tsugaru Current from Tsugaru Strait, and multiple eddies formed in the intersection area of these currents . To simulate the outflow from FNPP, we need a model with accurate fitting of complex coastal geometry within and around FNPP. The water over the shelf was always stratified so that water temperature and salinity must be included in the model simulation. Several regional-scale ocean model exercises have been made to simulate the 137Cs spread from FNPP, e.g., Kawamura et al. (2011) and Tsumune et al. (2012) with a spatial resolution of 2 km or larger, and Estournel et al (2012) with a resolution of 0.6 km. However, the water exchange between FNPP and the surrounding ocean is through a ~200-m wide narrow entrance between the two breakwaters. The FNPP seawall structure between the two discharging canals (namely, the north and south discharging canals) is ~1300 m. Without sufficient model resolution to accurately capture the complex pathways of 137Cs from FNPP, assessments made by these regional-scale models could be biased with large uncertainty. It is not clear, however, to what degree this bias could be. Could the bias caused by model resolution and geometric fitting issues led to a significant different conclusion about the dispersion of 137Cs off the Japan coast or reproduce the same distribution with just a small difference in accuracy? To our knowledge, this issue has not been well addressed yet in previous modeling experiments.

Actually, geometric fitting of complex coastlines around FNPP and in coastal regions is a critical factor to resolve multi-scale geometrically-controlled near-shore advection while sufficient model resolution is prerequisite of capturing a realistic lateral dispersion. Chen et al. (2008) conducted a model-dye comparison experiment over Georges Bank, with an aim of examining the impact of model resolution on lateral dispersion in the coastal ocean. They found that in order to simulate accurately the observed lateral dispersion within a tidal mixing front with a spatial variation scale of a few kilometers, model resolution down to ~500 m or less was required. Overestimation of lateral dispersion due to model resolution varied in space and time, which could be 4-10 times larger as the model grid size is bigger than 2-4 km. As a result, the model could point to an unrealistic conclusion that differed significantly from the dye observations. The dynamical processes off the FNPP coast are more complicated than on Georges Bank, so that failure to adequately resolve the important spatial scales in this region might lead to a large lateral dispersion rate and thus overestimate the offshore spreading of 137Cs over the eastern Japan shelf.

The biggest challenge for a model to provide an accurate simulation of the spatial distribution and temporal change of 137Cs over the Japan shelf is the large uncertainty in the estimation of the total amount of 137Cs leaking into the water. The leaking lasted for months, so that the source was both spatially- and time-dependent (Estournel et al., 2012). One approach to solve this problem is to treat 137Cs as a conservative tracer and inversely determine its source amount by tracking it in the flow field for a relatively short period during which the model-predicted tracer field had the best match to observations. This method was used to evaluate the total amount of 137Cs from FNPP in the previous modeling experiments made by Kawamura et al. (2011), Tsumune et al. (2012), and Estournel et al. (2012). This method is generally sound, but an adjustment in this type of inverse tracking could vary from model to model, particularly for the case with different model resolutions and setups. Due to this uncertainty, the more interesting model problem, to our opinion, is on gaining knowledge of the sensitivity of the model assessment results to model skill and configuration rather than on evaluating how well a model simulates the observed 137Cs concentration.

We, an international research team with members from the University of Massachusetts-Dartmouth, Woods Hole Oceanographic Institution and Yokohama National University, have developed a high-resolution global-regional-coastal integrated seismic-ocean-tracer FVCOM model system to simulate the March 11 earthquake-induced tsunami, coastal inundation and initial spread of 137Cs. Taking advantage of the geometric flexibility of the unstructured triangular grid, the model has a local resolution of up to 5 m around FNPP and near the coast. Nesting with the global-FVCOM hindcast field with data assimilation of satellite-derived sea surface temperature and sea surface height, the high-resolution regional-coastal FVCOM model not only resolved a realistic regional circulation but also provided a better representation of the water exchange between FNPP and the surrounding ocean. Built on our success in simulating the observed tsunami and coastal inundation, we applied this model system to track 137Cs over the period of March 26 – August 31, 2011. Our studies aimed at assessing the impact of multi-scale physical processes on the initial spread of 137Cs in the coastal region of Japan, which form the foundation for more realistic simulations with the inclusion of biogeochemical processes. A high-resolution global-coastal nested ocean model was first constructed to simulate the March 11 tsunami and coastal inundation. Based on the model’s success in reproducing the observed tsunami and coastal inundation, model experiments were then conducted with differing grid resolution to assess the initial spread of 137Cs over the east shelf of Japan. The 137Cs was tracked as conservative tracer in the three-dimensional model flow field over the period of March 26-August 31, 2011. The results clearly show that for the same 137Cs discharge, the model-predicted spreading of 137Cs was sensitive not only to model resolution but also the FNPP seawall structure. A coarse-resolution (~2 km) model simulation, which were similar to other assessment modeling experiments, could lead to an overestimation of lateral diffusion and thus faster dispersion of 137Cs from the coast to the deep ocean, while advective processes played a more significant role when the model resolution at and around the FNPP was refined to ~ 5 m. By resolving the pathways from the leaking source to the southern and northern discharge canals, the high-resolution model better predicted the 137Cs spreading in the inner shelf where in situ measurements were made at 30 km off the coast. The overestimation of 137Cs concentration near the coast is thought to be due to the omission of sedimentation and biogeochemical processes as well as uncertainties of leaking amounts from the sources in the model. The sedimentation was evident in sediment measurements taken after accident.

Our assessment results have been published on Biogeociences. Click here to view or download the paper.

concentration-in-Japan-coastal-region

This figure shows the distributions of the high-resolution nested model computed surface 137Cs concentration in Japan’s coastal region at 15:00GMT 15 April; 15:00GMT 15 May; 00:00GMT 1 July; and 00:00GMT 1 August 2011.

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Note: A regional-scale model with a horizontal resolution of ~2 km significantly overestimated the size of the 137Cs concentration plume. As a result, the model-predicted 137Cs concentration were significantly lower than observations at both nearshore and offshore monitoring sites. Due to overestimation of lateral diffusion, the coarse-resolution model failed to reproduce the temporal variation of the 137Cs concenteation in the shelf region.

Click the play button or link to play the animation

Note: With resolving of coastal geometry around the power plant, the high-resolution model-predicted 137Cs concentration showed a much better agreement with observations. The spread of 137Cs is controlled mainly by current advection and dispersion processes. Considering a dye patch that moves conservatively in the ocean, the total amount of the dye remains unchanged, but its concentration can change significantly as a a result of deformation of the dye patch due to vertical and lateral dispersions that are related to velocity shears and turbulent diffusion. In order to capture the dye spreading,  it is critical to resolve the realistic vertical and lateral diffusion processes.

Posted on January 10, 2014