An Indirect Look at Radioactive Material Leaks from Fukushima Dai-ichi Nuclear Power Plant

It's been four years since Fukushima Dai-ichi Nuclear Power Plant was ravaged by a tsunami that was triggered by a historic earthquake in March 2011. Radioactive materials leaked from the accident are forecasted to raise the radiation levels in coastal waters of North America as high as 3–5 becquerels per cubic meter (Bq/m³) during 2015-2016 (Smith et al., 2014). The same study is expecting the seawater radiation levels will drop to normal levels (1 Bq/m³) in 2021. Then can we expect that there will be no more leaking of radioactive materials from the Fukushima Dai-ichi Nuclear Power Plant?
The following figure visually summarizes the levels of radioactive cesium in seawater, sediment, and fish tissue that had been sampled at the Fukushima Dai-ichi Nuclear Power Plant port from the pre-accident period to three years after the accident.
Firstly, before the accident, seawater radiation levels resulting from radioactive cesium were lower than 0.01 becquerel per liter (Bq/liter). In 2014, the activity concentrations of ^134,137Cs were still around 10 Bq/liter, more than 1,000 times the pre-accident radiation levels.
Secondly, fishes caught before the 2011 accident showed radioactive cesium concentrations of lower than 0.1 becquerel per kilogram (Bq/kg). Even three years after the accident, the fish tissue radioactivity levels were recorded between 10 to 100,000 Bq/kg.
Thirdly, radioactive cesium activity concentrations in sediment didn't drop significantly at least until 2014, compared to the post-accident 2011 levels, either. Only highest activity concentrations seem to have declined.
All in all, leaking of radioactive materials from Fukushima Dai-ichi Nuclear Power Plant is not being controlled yet, apparently. I expect more thorough monitoring data will come out soon. Until then, let us not be so decisive about the effects of the tragic accident, whether being optimistic or pessimistic.

Figure. ^134,137Cs activity concentrations in seawater, sediment, and muscle tissue samples from four species, of similar trophic levels (TLs), from the Fukushima Dai-ichi Nuclear Power Plant port. Lines represent means of data over 60-day intervals.

Source: Johansen, et al., 2015


References:

Johansen, M. P., et al. (2015). Radiological Dose Rates to Marine Fish from the Fukushima Daiichi Accident: The First Three Years Across the North Pacific. Environmental Science & Technology, 49, 1277–1285. [Full-text at http://dx.doi.org/10.1021/es505064d]

Smith, J. N., et al. (2015). Arrival of the Fukushima radioactivity plume in North American continental waters. Proceedings of the National Academy of Sciences, 112(5), 1310–1315. [Full-text at http://dx.doi.org/10.1073/pnas.1412814112]