(Clarification: Plutonium was measured by the Ministry, but it was below the detection limit in the 10 locations measured, as described in the article.)
The Ministry of Education and Science (and the media reporting the news) is spinning it as "good news" that radioactive materials detected in river water and well water in Fukushima Prefecture are "far less than the provisional safety limit".
If you compare the measured level to the provisional safety limit for water which is high as 200 becquerels/liter for radioactive cesium for adults, well yes, it is far less.
If you compare the level to the one before the Fukushima I Nuke Plant accident, it is a different story altogether. The highest strontium-90 level in the Ministry's survey is 5.14 times the highest level measured in 2009, and the highest cesium-137 level is 6,500 times the highest level measured in 2009.
The Ministry's announcement (10/20/2011) is here (in Japanese, PDF).
From Asahi Shinbun (10/20/2011):
The Ministry of Education and Science announced the result of the survey of water contamination in rivers and wells in Fukushima Prefecture, except in the 20-kilometer radius from the Fukushima I Nuclear Power Plant. Nuclides such as cesium and strontium were tested, but according to the Ministry there was no detection of radioactive materials exceeding the standard for drinking water.
The Ministry did the survey twice in June and August. It selected the survey locations from the areas that showed relatively high level of cesium deposition in soil in the Ministry's aerial survey after the accident. 50 river locations and 51 wells were selected. Radioactive cesium and iodine-131 were measured in all 101 locations. Strontium and plutonium were measured in 10 river locations where the air radiation was high. Similarly, at 6 wells, only strontium was measured.
The highest cesium-137 (half life 30 years) for the river water was detected in Mano District in Minami Soma City (37 kilometers north by northwest from the nuke plant), at 2.0 becquerels/kg. The average amount of cesium-137 in river water was 0.58 becquerels/kg. The highest cesium-137 for the well water was detected in Nukazawa in Motomiya City (54 kilometers west of the plant), at 1.1 becquerels/kg. The average for well water was 0.49 becquerels/kg.
According to the Ministry of Education and Science, "Radioactive materials in both river water and well water are far below the provisional safety limit of 200 becquerels/kg". However, according to the Ministry's national survey in 2009, the highest level in river water was found in Akita Prefecture at 0.00037 becquerels/kg (ND in Fukushima). So, 2.0 becquerels/kg of cesium-137 detected this time in Fukushima is 5,400 times as much as the highest level in 2009 in river water. As to 1.1 becquerels/kg of cesium-137 from the well water, it is 6,500 times as much as the highest level detected in tap water in 2009.
The largest amount of strontium-90 (half life 30 years) was detected in a river in Onahama in Iwaki City, at 0.018 becquerels/kg, 5.14 times the level detected in the 2009 survey. Strontium-90 in well water was the same level as before the accident. Plutonium and iodine-131 were below the detection limit.
According to the Ministry's calculation on the internal radiation if one drinks the river water that had the maximum amount of radioactive materials for one year, cesium-137 would result in 0.025 millisievert, and strontium-90 in 0.00049 millisievert.
Hmmm. They tested an alpha emitter (plutonium) and a beta emitter (strontium) in water in locations with high air radiation? What does high air radiation have to do with alpha and beta emitters? And what about other nuclides, like cobalt-60?
The Ministry of Education tested water at these locations twice: first in late June to early July, then in early August. Looking at the result, there are two locations where the amount of radioactive cesium significantly INCREASED during the one month, indicating perhaps the inflow of radioactive materials from the surrounding mountains.
The Ministry's document has very poor resolution, but here's the page that shows charts of cesium-137 detections (page 19 in the document):
The detection limit for the Ministry of Education's survey was meaningfully low. For radioactive cesium and iodine-131, the detection limit was 0.1 becquerel/kg. For plutonium, it was 8 x 10^-6 becquerel/kg (0.000008 becquerel/kg), strontium-89 at 4 x 10^-3 becquerel/kg (0.004 becquerel/kg), and strontium-90 at 6 x 10^-4 becquerel/kg (0.0006 becquerel/kg).
Compare that to the survey by the Ministry of the Environment in May, where the detection limit for radioactive materials in river water was 10 becquerels/kg. The Ministry proudly declared on the announcement that no radioactive cesium or iodine was detected.
The first measurement was at the end of June, so there was no danger of detecting high level of iodine-131 in the water. (How convenient.) I wonder if I can find the data of the measurement that the Ministry did back in March and April in the areas now admitted by the government to have been heavily contaminated, such as Namie-machi and Iitate-mura.