Its just been announced on NHK several minutes ago, 2:45 pm local time, that radiation testing of water purification plants has shown a reading of 210 Becquerels in the water for the 23 wards of central Tokyo as well as Western Tokyo. The cause is radioactive iodine. That includes Mitaka and Musashino City where I live.
100 Becquerels is considered over the safe level for infants and 300 is the safe level for older children and adults so a reading of 210 Bq per liter is pretty serious. The threat of iodine 131 is that it gathers in the thyroid and can cause cancer and other disorders as it decays. For developing children, the threat is much greater and less so for those over age 40. They are recommending that tap water not be given to infants under 12 months and not mixed with tap water.
The thing to think about is, while a reading of 210 Bq/liter is not threatening to health, continuous consumption will be a problem. I've pretty much started drinking bottled water since 3/16 but using tap water for cleaning should be safe since you are not ingesting it.
UPDATE: An article is now up about the issue on Japan Today, Radioactive iodine exceeding limit for infants found in Tokyo tap water.
In addition to tap water. raw milk, spinach and 10 other vegetables from Fukushima and Ibaraki Prefectures have been identified as having radioactive contamination.
Radiation discovery fans food fears in Japan - via Japan Today
Radiation monitoring online
I've been watching the online posts of radiation levels in my area of Mitaka, Musashino City and while its not immediately threatening to health, background radiation levels have been creeping up.
Japan's Ministry of Education. MEXT, is now providing daily graphs of radiation readings in Japan. The general website is: http://www.mext.go.jp/english/
And the link for Tokyo for the last week is here.
Another PDF for several prefectures in comparison for today is here.
Geiger Counter Readings with Counts Per Minute in Tokyo
There is a Geiger counter set up in Mitaka city that is updated more or less constantly:
Another online Geiger counter in another part of Tokyo is updated every ten minutes. (If you go to the site it is a mirror for, you can see a baseline graph recorded last year for comparison.)
And here is a 3rd site with a Geiger counter:
Count Per Minute, CPM & Microsievert, µSv: 100 CPM is equivalent to 1 microsievert or 0.1 millirem.
Sievert, Sv unit is used to measure the absorbed dose into biological material.
Becquerel, Bq is = 1 disintegration per second.
Here is a chart provided by MEXT to make sense of the readings and units of measure used:
Here is another useful chart on dosages for the human body.
Conversion charts are here for measures: