First it was "it cannot be from Fukushima I Nuke Plant" when the radiation measured was 57.5 microsieverts/hour.
Then it was "it may be from Fukushima I Nuke Plant, and it may not be" when the density turned out to be 276,000 becquerels/kg of radioactive cesium. But since the density was very high, the suspicion was voiced all around (including me) that someone secretly dumped the radioactive sludge or dirt, possibly from cleaning out his yard, onto that location.
The Ministry's current position: "It is highly likely that radioactive cesium is from Fukushima I Nuke Plant, and radioactive cesium has been condensed at that particular location because the storm drain nearby was broken."
So radioactive cesium from Fukushima I Nuke Plant has been naturally concentrated by elements, 200 kilometers away from the plant, to the density level on par with those found in Fukushima. The Kashiwa City's dirt contains almost as much radioactive cesium in the dirt in the location in Watari District (link is in Japanese) in Fukushima City where 300,000 becquerels/kg of radioactive cesium was found (also near the drain).
Points from the Ministry's announcement on October 23, 2011 "Result of the investigation of Kashiwa City's location with high air radiation" (my translation, not the Ministry's):
On receiving the result of the soil analysis on October 22 where maximum 276,000 becquerels/kg of radioactive cesium was detected, the Ministry sent two officials, along with three experts from Japan Atomic Energy Agency on October 23 to Kashiwa City to consult with the local officials in charge of radiation countermeasures.
The Ministry's survey of the location included measuring air radiation levels at the location and nearby, and investigating the surroundings to figure out what caused the high radiation level.
- Maximum 2.0 microsieverts/hour 1 meter off the ground
- Maximum 4.5 microsieverts/hour 50 centimeter off the ground
- Maximum 15 microsieverts/hour on the ground
Typical air radiation levels in the surrounding area at 1 meter off the ground was 0.3 microsievert/hour.
A 50-centimeter wide breach was found in the side drain (30-centimeter deep) next to the location with the high air radiation. This breach was close to the spot where cesium-134 (half life 2 years) was detected. We therefore presume that the rainwater containing radioactive cesium that came from the Fukushima I Nuclear Power Plant accident flowed in from the breach in the side drain, and radioactive cesium was condensed and accumulated in the soil at that particular location.
Asahi Shinbun (10/23/2011) has the picture of the broken side drain.