If it's not radioactive cesium, it's something else like formaldehyde...
Authorities haven't identified the source of formaldehyde, but judging from the density they suspect straight formaldehyde liquid leaking from a plant somewhere in Tochigi Prefecture along the Tone River that runs through Gunma, Saitama, and Chiba Prefectures.
The safety limit for formaldehyde in drinking water is 0.080 milligram/liter.
From Japan Times quoting Kyodo News (5/19/2012):
Formaldehyde found at water plants in Saitama, Chiba
Saitama Prefecture said Friday night it has halted water intake and supply at one of its filtration plants after hazardous formaldehyde exceeding limits was detected in tap water there, while neighboring Chiba Prefecture said it has stopped water intake at two plants after detecting the substance.
The water at the plants was taken from the Tone River and one of its branches, the Edo River, the prefectural governments said.
According to the Saitama prefectural government, more than twice the amount of formaldehyde permitted under national water quality standards was detected in water at its plant in the city of Gyoda.
On Saturday morning, however, the Saitama government said it resumed water intake and supply at the plant after the figures for the substance dropped and remained stably below the standard set by the central government.
The Chiba prefectural government said Friday it has stopped intake at its plants in the cities of Noda and Nagareyama. Early Saturday, it also halted water intake at another plant in the city of Matsudo.
The local governments are now examining the water quality upstream of the Tone River with cooperation from Gunma Prefecture, where the upstream water is located, to identify the source of the contamination, they said.
Formaldehyde could be generated by a combination of organic substances included in the drainage from chemical plants and chlorine, according to the governments.
As with radioactive cesium in food, the Japanese authorities are repeating the mantra that "there is no effect on health" even if you drank the formaldehyde-laced water.
NHK Update: 68,000 households in Nagareyama City in Chiba Prefecture are without water to their homes, as of 2PM on May 19, 2012. 46,000 households in Noda City and 35,000 households in Abiko City in Chiba are also without water. The so-called "tokatsu" region of Chiba (northwestern corner of the prefecture) along the Tone River is hit hard.