You didn't say, but I'll wager you that no infants are drinking it.
That would mean it is under control.
If tap water is not now safe for babies in Tokyo, how long before it will be? The radiation thats in the ground water isn't going to go away any time soon. Also, with the situation at Fukushima still out of copntrol, tap water in Tokyo is going to be continually radiated for a while.
That may very well happen, but that hardly means that that Japan doesn't have it under control.
Actually, it could, depending on the isotopes. If it is 131I, the half-life is eight days, so you are down a factor of a thousand in about 80 days. If it is 137Cs or 90Sr, then you have to look at quantities and how much is likely to be absorbed. Of those, I'd be concerned about Sr, because it is a bone-seeker and substitutes for calcium, but if the quantities are small, you are probably okay, and water treatment facilities are very efficient at removing things like this. Charcoal filters used in municipal water filtration is quite efficient.
My guess is that contamination being found in Tokyo water is from streams and open reservoirs. Ground water contamination would take longer than a week. If the source of contamination in this event is stopped, the current contamination would dilute (and decompose as well) over a similar period of time, would it not?