In Gumma Pref. near here, radioactive Cs is found in the filters at the water filtering facility as the web site of the prefectural office says;
The average value around 100Bq/kg depending on how long the filter is used. They say it is not detected in the supplied water at present.
It is no wonder that the filters are filtering it out at such content. Because the mountains in the northern part of the prefecture are highly polluted with radioactive substances, which are reported to be combined with the organic substances as fermented plants etc and to pour out into the rivers. However low the concentration of radioactives in water of the rivers may be, the filters would concentrate it in filtering process. Those radioactives combined to organic substances are deposited when being mixed with salt water. That is why the orifices of big rivers are mostly highly polluted in Kanto plain area. Tokyo is not free from this contamination at all.
Radioactives flowing out of the contaminated mountains may last for long time. It may go on contaminate the water for crops like rice. The contamination process is just starting at present. It may get worse in years. Water supply, either for industry or for civil use, could be polluted in the future even if it is free at present. It eventually pollute the sea water and the sea products. The nuclear power generation which proved to deteriorate our environment is against not only human being but also all the creatures and the nature in the earth.