Contamination in Water
The measurement strategy in Japanese waters and the wider Pacific is being progressively refined in order to obtain the most complete picture possible of contamination in water and sediments. The measurements now reach out into the deep waters of Pacific where the detection thresholds have been reduced in order to take account of the lower levels – currently about 10 times background level – in the deep Pacific.
The table below, produced by the Japan Atomic Energy Agency using the Japanese government's (MEXT's) data, summarises the current measurements of 137Cs in the seawater and sediments in 2012.
Near nuclear plant discharge ports
Near nuclear plant - Iwasawa coast (15km south of nuclear plant)
Coastal area within 30km of Miyagi, Fukushima and Ibaraki prefectures (including river outlets)
Offshore area - within about 30 to 90km from the coastline
Outer sea area – within about 90 to 280km and 280km or forther from the coastline
background – 0.01
Contamination in seawater near the stricken nuclear plant dropped till September 2011 and has thereafter remained constant. IRSN thought that, provided there are no new releases, 137Cs concentrations would return to pre-accidental conditions by 2013 but this will depend on the balance between dilution by mixing, resuspension from sediments and input from rivers.
Levels in sediments remain above 100 Bq/k in a number of areas within 90km of Fukushima with the highest levels – up to 3000 Bq/kg near the plant itself. This persistence is consistent with measurements offshore of Sellafields off the UK and Cap de la Hague where there had been discharges years ago.
Contamination in Fish
Concentration factors are a measure of the relationship between equilibrium concentrations in water measured in Bq/litres with concentrations in marine life measured in Bq/kg. In commercial fish this factor is generally taken as 100. i.e. 1Bq/litre in water leads to 100 Bq/kg in the tissue of fish. The concentration factors for crustaceans, molluscs, cephalopod and seaweed are lower. So except near the plant itself, the levels in the water would not themselves be enough to raise the contamination to above the minimum level for human consumption which in April 2011 was lowered from 500Bq/kg to 100 Bq/kg – both in Japan and the EU. However, it takes some time to reach this equilibrium between concentrations in seawater and marine life. Typically the concentration difference between fish and seawater decays with a biological half-time of approximately 50 days.
Although the pathways for individual marine species to absorb and release radioactive isotopes will depend on movements during breeding, spawning and feeding and are not known in detail, one might expect that those fish that are bottom –dwelling near the Fukushima plant would be more contaminated than those in the water column and further from the plant.
Indeed this is what has been observed. The Japanese Fisheries Agency in the Ministry of Agriculture, Forestry and Fisheries has regularly published measurements of contamination of landed fish on its web-site and these have been analysed; both by the Agency itself and by the Commission's Joint Research Centre (JRC)
The figure above, produced by JRC, shows the results for the different groups of fish. The horizontal line shows the maximum permissible limit for consumption in Japan or export to the EU.
There has been some concern that migratory tuna might feed in the contaminated zone and be caught elsewhere. Indeed this has happened. Nick Fisher, from Stonybrook University, by analyzing the ratio of relatively short-lived 134Cs that could only have come from Fukushima to longer-lived 137Cs remaining in the oceans after bomb testing in the atmosphere fifty years ago showed that blue fin tuna caught off California, had levels of contamination consistent with this. However, he also showed that the levels of radioactivity in tuna was well below maximum permissible levels wherever they are caught. This was consistent with the Japanese measurements.
Measures in Japan to prevent consumption of contaminated fish
If there is a risk that fish might be above the limit for consumption then fishing is immediately suspended by the Japanese authorities. Currently all fishing in the Fukushima area has been stopped (except trial fisheries) as it has for certain species in neighbouring areas– for instance seabass and cod in Miyagi, Japanese sand lance and brown hakeling in Ibraki and Pacific cod in Aomori.
The origin of fish caught in the North Pacific for domestic consumption is clearly labelled. The Ministry of Health and some municipalities in addition monitor fishery products sold in stores. Some supermarkets make measurements themselves.
Measures to prevent consumption of contaminated fish in EU
Regulation 184/2012 stipulates that all Japanese food products entering the EU from contaminated prefectures be certified as having been tested for radioactivity. Following careful review of measurements in these prefectures, the Commission has proposed that the number of prefectures designated as contaminated be reduced from 12 to 10 and that, with the exception of Fukushima, pretesting be restricted to certain named foodstuffs including fish and fishery products. It is further envisaged to reduce the checks at import to 5%.
This reflects confidence in the measures taken by the Japanese authorities. Measurements of imports over the past year by EU authorities have shown that contamination is far below the maximum permitted level. No non-compliant samples have been measured. Imports of fish and fishery products from Japan in 2011 were only 6% below their 2010 level, reflecting the confidence of EU consumers in the measures taken.
Conclusion of meeting
The experts concluded that:
- whilst there is still some uncertainty about the amount of radioactivity that entered the Pacific and on mechanisms for transfer of contamination between seawater, sediments and marine products, the current state of the marine environment outside the Fukushima plant and in the wider Pacific is now well understood;
- this improved understanding is due to the careful and transparent monitoring undertaken by the Japanese authorities;
- the measures in place to stop contaminated fish and fisheries foodstuffs entering the EU food chain are effective;
- further measurements and analysis of contamination should be submitted to the European Commission at the beginning of January in order to feed the interim review of import measures planned for 31 March 2013;
- there is no risk of fish caught outside Japanese waters being contaminated above the levels deemed fit for human consumption so monitoring of imports from the non-Japanese Pacific is no longer recommended.
The presentations and a list of participants can be found here: