This will dilute massively as soon as it enters the ocean. The active chemical of concern will be back to normal crustal abundancy levels in the fine deep water sediment within days.
Strangely, the more the radioactive elements spread north, south and east, the better the situation becomes in terms of concentration.
Water is an amazing solvent. That is why washing your hands with water and soap is so effective. It dilutes literally everything.
We did an experiment once in Chemistry at University about how much water dilutes chemicals, and after two washes of a solvent, the numbers remaining were ridiculously low.
Of course, Japan wouldn't have been doing this if they didn't already know it was safe.
One of the key criticisms of environmental groups like Greenpeace is that they neglect the science. We have so many environmental science projects on the planet now, there is no way a country would do this without proper studies and consideration.
I wouldn't put it past China or Russia to do this without proper consideration. Japan though, I agree.
