Water samples collected off the coast of Canada had revealed traces of radionuclides linked with the radiation leak at the crippled Japanese nuclear plant Fukushima.
The good news was that the concentrations of the two forms of radioactive cesium detected by scientists from the Woods Hole Oceanographic Institution (WHOI), cesium-134 and cesium-137, were too low to represent a radiological hazard.
This, as the Tokyo Electric Power (Tepco), operator of Fukushima nuclear power plant, disclosed on Tuesday that radiation levels from previous readings have been "significantly undercounted."
"These errors occurred during a time when the number of the samplings rapidly increased as the result of a series of events since last April, including groundwater reservoir leakage and a major leak from a storage tank," Tepco told Bloomberg.
Scientists from WHOI had forecast that radionuclides from the Fukushima plant devastated by a tsunami in 2011 will reach Alaska and British Columbia in early 2014. It will travel south over two years with its trace amounts reaching the coast of Hawaii.
"The reason why we see such low levels of radiation in these samples is because the plume is not here yet. But it's coming. And we'll actually be able to see its arrival," Ken Buesseler, a chemical oceanographer at the WHOI, said in a statement. "That baseline data is critical."
Meantime, the levels of radioactive cesium will arrive at the U.S shores by April, the scientists said on Monday.
"We expect over the rest of 2014, levels will become detectable starting first along the northern coastline. But the complex behavior of coastal currents will likely result in varying intensities and changes that cannot be predicted from models alone," Mr Buesseler said.
Researchers said they have detected radioactive cesium-137 in the ocean but this should not be surprising, according to Mr Buesseler. He explained the radionuclide has actually been present since the 1950s-1960s nuclear weapons tests.
WHOI said the incoming radionuclides will be below the safety limit set by the U.S. Environmental Protection Agency, which is 7,400 Becquerels per cubic meter.
"It's not here yet," Mr Buesseler said. "When we're talking about the arrival of the plume - and, you know, I'm the first person to say radioactivity can be quite dangerous, we should be concerned - but maybe not at the levels we're going to expect coming across from Japan."
Tepco meantime said it will be conducting new tests on the water samples taken from April to September 2013, and will endeavour to release corrected beta radiation readings.
It added it had called on outside experts to double check its results and analysis as well as review the company's measurement methods.
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