The leak occurred after a drain overflowed during a maintenance exercise while workers were transferring water, which has high levels of radioactive contamination, said Neil Sheehan, a spokesman for the Nuclear Regulatory Commission. First of all, this water's not going anywhere immediately ... and, again, because of the dilution factor, you wouldn't even be able to detect it were you to take a direct sample. An Associated Press investigation in 2009 showed three-quarters of America's 65 nuclear plant sites have leaked tritium, a radioactive form of hydrogen that poses the greatest risk of causing cancer when it ends up in drinking water.