After recalculating the radiation level, Tepco said the water contained 280 million becquerels per liter of beta ray-emitting radioactive materials such as strontium-90, instead of 80 million becquerels.
A total of 300 tons of toxic water was found to have leaked at that time, part of which is believed to have flowed into the adjacent Pacific Ocean. The Nuclear Regulation Authority assessed the severity of the incident to be level 3 on an eight-point international scale.
Tepco decided to review data on 173 water samples it took until last October, as it found readings may be lower than actual figures due to improper measurement formulas.
Related article (the Atomic Age): Tank Has Leaked Tons of Contaminated Water at Japan Nuclear Site via the New York Times