That 12 days is not a half life, but it is how long it stays in the body before you pee it out. This only matters if you had a single incident of drinking the water or eating contaminated food not if you are constantly exposed to it then each time you consume affected foods you know it stays with you for about 12 days and small part of it stays with you forever as your body doesn't see the difference between tritium and hydrogen, so it will be happy to use the radioactive version, which could increase your chances of cancer as well as your future generations.