Radiation exceeding government safety limits has seeped into groundwater under a tsunami-crippled Japanese nuclear plant, according to the operator, but has not affected drinking supplies.
But the leak could pose a long-term problem and at the very least it is a concerning indicator of how far Tokyo Electric Power Co (Tepco) is from bringing its plant under control.
Workers have been battling to stabilise dangerously overheating reactors after cooling systems were knocked out in the March 11 tsunami.
Tepco has increasingly asked for international help in its uphill battle, most recently ordering giant pumps from the US that were to arrive later this month to spray water on the reactors.
The groundwater contamination - 10,000 times higher than the government standard for the plant - is the latest setback at the Fukushima Dai-ichi complex, where leaks have already contaminated food and hindered workers’ ability to bring the plant under control.
Iodine-131, a radioactive substance that decays quickly, was found 15M below one of the reactors, according to Tepco spokesman Naoyuki Matsumo.
While the contamination does not appear to have caused an immediate problem, there are two ways it could eventually affect drinking water if concentrations were high enough.
One is if it were to seep into wells in the area. For now, a 12-mile radius around the plant has been cleared, though residents are growing increasingly frustrated with evacuation orders and have been sneaking back to check on their homes.
The other concern is whether contaminated water from the plant could seep into underground waterways and eventually into rivers used for drinking water. It is not yet clear if this is possible.
Seiki Kawagoe, an environmental science professor at Tohoku University, also noted that radiation tends to dissipate quickly in the ground, as it does in the ocean.
Although the company has acknowledged that it was initially slow to ask for help in dealing with the nuclear crisis, experts from around the world are now flooding in.
French nuclear giant Areva, which supplied fuel to the plant, is helping work out how to dispose of contaminated water and American nuclear experts are joining Japanese on a panel to address the disaster.
Japan has also ordered two giant pumps, typically used for spraying concrete, from the US. They are being retrofitted to spray water first, according to Kelly Blickle, a spokeswoman at Putzmeister America in Wisconsin. At least one similar pump is already in operation at the plant.