Radioactive material has been found in the groundwater near the Fukushima Daiichi nuclear power plant.

Several news outlets noted that groundwater contamination was found in concentrations 10,000 times higher than the government standards. The substance is iodine-131, which decays quickly. It was found nearly 50 feet (15 meters) below one of the reactors, according to a statement from Tokyo Electric Power Corp. Thus far the groundwater has not entered any water supplies, officials said.

The Japanese Nuclear Industrial Safety Agency said that it was reserving judgment on the measurements, however, because TEPCO had made errors before. Earlier this week the company reported radiation levels millions of times greater than allowable limits, but revised that figure downward after it was discovered that the measurements were wrong.

Radioactive water leaking into the ground is a sign there may be more damage to the reactors than previously thought. Iodine-131 has a half life of about eight days, which means that the only way for it to show up in those amounts is for the water to come in contact with either spent fuel rods or the reactor core.

On Wednesday, TEPCO took measurements of the water in the drains of the turbine buildings, finding high levels of radioactive iodine, cesium, lanthanum, niobium and tellurium. All but the cesium-137 have half lives measured in days or weeks, with niobium-95 the longest at 35 days. Cesium-137 has a half-life of about 30 years. Short half-life elements are more dangerous to be exposed to, as they are more highly radioactive.

Contamination levels in the seawater near the damaged plant's discharge canals are already elevated. TEPCO reported Thursday that levels were thousands of times legal limits. Water collected some 50 meters north of the outlets from reactor Units 5 and 6 showed concentrations of 57 becquerels per cubic centimeter of iodine-131, about 1,425 times the regulatory limits. Cesium-134 and cesium-137 were measured at 15 becquerels, 250 and 166 times the limits, respectively.

The previous afternoon the concentrations were 51 becquerels for the iodine-131, and 12 becquerels for the two cesium isotopes. Wednesday's radiation levels were more than twice as high as those taken on March 26. A becquerel (Bq) is a unit of radioactivity that measures the number of decay events per second, adjusted for the element one is measuring. Limits for safe exposure will differ depending on the element involved.

The Nuclear Industrial Safety Agency confirmed that the two possible sources for the contaminants are the spent fuel rods or the reactor core. But there is no certainty yet, the agency said, as to which it is.