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The contaminated water may contain large amounts of radioactive carbon-14 and other radioactive isotopes. It takes tens or even hundreds of thousands of years for some atomic isotopes to decay.

Fukushima nuclear contamination

Japan plans to dump nuclear-contaminated wastewater into the sea despite widening international concerns. Among others, the US has endorsed Japan's decision regardless of the possible havoc.
Condones Radioactive Water Dump

Fukushima water safe