Today I read in a discussion thread that 150 tonnes of toxic waste per day was dumped into the Pacific at Fukushima. The storyteller got his 150 tonnes from here, from which he changed 'contaminated water' into 'toxic waste' with typical poetic flourish. Deliberately misrepresenting the article.
Is there a risk posed by contaminated water at Fukushima?
In fact, the water is contaminated mainly with a little bit of tritium. The same tritium already present in all water. Tritium is perhaps the mildest radioactive substance I know of. It undergoes beta decay to helium-3. No gamma is emitted. The electron emitted has an average energy = 5.7 keV.
Compare that to 89% of potassium-40 decays, which are beta decays with maximum energy = 1330 keV. The ratio 5.7 : 1330 = 1:233. Potassium-40 radioactivity is up to 233 times more energetic than tritium. The cell damage inflicted by radiation is directly proportional to the energy. A typical human body internally experiences about 500 million radioactive decays per day, about half of them potassium-40.
Water contaminated with tritium is only harmful (in theory) if you drink it, and we drink sea water all the time don't we? Not quite. Unless eaten or drunk, tritium radiation will not penetrate into your body. If you were swimming in the sea you'd find water an excellent radioactive shield. Beta radiation does not penetrate air either very far, only about 2 metres. Externally all beta radiation will be stopped by your skin. So the effect of tritium beta radiation is something like strong UV sunlight, but much less intense. Because there's masses of sunlight and tiny amounts of tritium to worry about. Ooop, there's not actually any tritium to worry about.