Tepco detected 400,000 becquerels per liter of beta ray-emitting substances, such as strontium, on Thursday in water samples taken from the observation well located 15 meters from a storage tank, which leaked about 300 tons of highly radioactive water in August, the Japan Times reports.
The level of becquerels was up 6,500-fold from Wednesday sampling, which stood at 61 becquerels, the report added.
According to the report, Tepco has been planning to pump groundwater from different wells, at a distance of about 100 meters from the leaky tank, for release into the Pacific Ocean, before the water flows into the damaged reactor buildings and becomes heavily contaminated with radioactive materials.
However, the sharp increase in the levels of radioactive materials in the observation well suggests that the radioactive groundwater is spreading at a faster pace.
According to Japanese law, water containing beta particle-emitting substances exceeding certain levels cannot be released into the sea.
Tepco also found out from the water samples taken on Wednesday at a point in the drainage ditch contained 34,000 becquerels of beta particle-emitting substances per liter, compared to 2,300 becquerels a day before Typhoon Wipha hit the area.