Is that because it's going to be diluted to unnoticeable levels even near the origin?

Yes

Unless, of course, you're a homeopath. On a slightly more serious note, I would expect to see a variety of illnesses occurring at a statistically ordinary rate around the variously-projected times of arrival to be blamed on radioactive seawater regardless of the actual concentrations (which will probably be in the high-C, or incredibly powerful because it's incredibly weak, homeopathic dilution range).