There is more concern than usual about radioactive water as authorities in Japan try to figure out how much radioactive contamination there is in water supplies and waterways from the ongoing nuclear disaster there. One of the latest reports is that Tokyo water is not necessarily safe to drink. The discussion of radioactive water is confusing to people, though. It is not the water itself that is the problem, but contaminants that may be in the water.
The main reason this is confusing is that most people know it is possible for hydrogen to be radioactive. Water is an oxide of hydrogen, so it is possible for a water molecule to be radioactive. However, the common hydrogen-1 isotope is not radioactive, nor is the hydrogen-2 isotope (deuterium) that occurs in large quantities in a nuclear reactor. It is only the hydrogen-3 isotope (tritium) that is radioactive. Tritium, though, occurs only in small amounts. A nuclear reactor and its cooling water contain less than 1 kilogram of tritium. A typical rain cloud, by contrast, contains 1 billion kilograms of water, enough to dilute tritium to the legal standards for drinking water. Further, water containing tritium doesn’t stay in the body for long, less than a month, so if you were to ingest water containing tritium, it wouldn’t create a long-term radiation exposure.
That doesn’t mean there isn’t a concern about rain water from a radioactive leak. The concern is not about the water itself, but about heavy radioactive contaminants, mostly metal oxides. Some of these form molecules in body tissues, including bones, staying there and releasing radiation inside the body for the rest of a person’s lifetime. It is especially important to avoid inhaling these metals in dust, and this is the main reason authorities have been urging people near Fukushima to stay inside when possible. Dust generally is absorbed by rainfall and may find its way into the water supply, and this is the main reason for the concern about the possibility of radioactive water following the release of nuclear materials into the air at Fukushima.
In the short term, radioactive iodine is also a concern. Iodine-131 is produced in very large amounts in a nuclear reactor, dissolves readily in water, sticks to body tissues if ingested, and produces damaging internal radiation for about a month. With a half-life of 8 days, only about 1/30 of the radioactive iodine remains a month later. The elevated levels of iodine found in some crops and drinking water supplies in Japan will be a concern for the next few weeks if it is verified to be iodine-131, and it could point to a longer-term problem with radioactive metals.