Cesium-137’s half-life: 30 years, which means it loses half its radioactivity every 30 years, leaving it dangerous for 10 to 20 half-life cycles, or 300 to 600 years. It can accumulate in increasing concentrations through the food chain. At this point (April 10, 2011 in Japan) is is already showing up in young sand lance 1 km offshore and 30 km south from the troubled Fukuchima Daiinchi plants. What strikes me most is that so few samples were taken. No samples were mentioned for the northern Perfectures Aomori, Iwate and Miyagi, as well as further out east in the Pacific. While there is a weak south ocean current off Fukuchima, the wind has blown most radionuclides east and northeast; possibly far enough for them to cycle around to the northern Perfectures. Also, a strong current runs east just north of Tokyo. See images further below in this post for ocean currents maps. The contamination is going mostly unmonitored (or unreported) and thus may be much worse elsewhere.
- TOKYO, April 10, Kyodo: Excessive radioactive cesium found in fish sampled off Fukushima: “Radioactive cesium above the legal limit for consumption was detected Saturday in young sand lance caught off Fukushima Prefecture, as the prefecture took samples amid a voluntary ban on fishing there in the wake of the ongoing nuclear crisis. One of the four sample fish had a level of cesium of 570 becquerels per kilogram on Thursday about 1 kilometer off the city of Iwaki, and the other three measured 480 to 500 becquerels. The limit is 500 becquerels under the Food Sanitation Law.
[…] Radioactive iodine measured 1,100 to 1,700 becquerels in the samples against the legal limit of 2,000 becquerels. […] ” [= per kilogram / my empahsis] SOURCE and full article: http://english.kyodonews.jp/news/2011/04/84289.html (The map I adapted from outdated NY Time article. Click on map to access image source)
- Dominant Ocean Currents off Eastern Japan: