The Tokyo metropolitan government announced on Mar. 23 that amounts of radioactive Iodine 131 of about twice the standard limit for infants were detected at the Kanamachi water purification plant located in Katushika Tokyo.

The government warned the residents living in the 23 wards and the cities of Musashino, Machida, Tama, Inagi, Mitaka, that infants should not be given water taken from water pipes.

The metropolitan water bureau explains that "health effects in case of long-term ingestion were considered in setting standard limits; In case alternatives to potable water cannot be secured, drinking [water of such level] is safe."  Governor Shintaro Ishihara said that they will continue to publish test results.  He appealed to residents to be calm.

As for the causal relationship with respect to the accident in the Daiichi Fukushima Nuclear Power Plant, the Water Bureau says it is hard to know how water resources were affected.

As stipulated under the Food Sanitation Law, the interim standard limit for infant drinks is 100 Becquerel per kilogram.  As of 9:00 AM March 22nd, an amount of 210 Becquerel was detected from tap water at the purification plant.  According to the metropolitan government, the level dropped to 190 Becquerel as of March 23, 9:00 AM (based on preliminary reports).  The detected amount is below the limit of 300 Becquerel for adults.  Radioactive Cesium 137 was not detected.

The levels in other water purification plants of the Water Bureau were lower than that of Kanamachi.  In the Asaka water plant (Asaka, Saitama Prefecture), radioactive iodine was not detected as of Mar. 22, 9AM.  In the Ozaku Water plant (Hamura Tokyo), only 32 Becquerel was detected.

Japanese source:
Yahoo news