Testing conducted in February and March of 2015 found cesium in many tap water samples collected around Japan. In the readings, Tokyo had higher tap water contamination levels than Fukushima City.
One reading that may cause confusion is the reading for Ichihara Chiba. It indicated iodine 131 was found in that tap water sample. The reading is isolated, no other cities found iodine 131 in their water.
This same problem was encountered in the US soon after the disaster were iodine 131 was being found in some east coast municipal water supplies. What may be happening is linked to medical treatments.
People receiving iodine 131 radiation therapy treatments, used for certain thyroid disorders excrete the substance in their urine. This is intended to allow any iodine 131 in the waste water to decay away. If a containment system is not used or not properly operating, iodine 131 can flow into waste treatment plants and eventually to the waterways. This directly adds contamination to waste water.
If that treated waste water is released into a water way and picked up soon enough downstream it could then contaminate public drinking water supplies.