The level stood at 8.7 million becquerels per liter of water, which was 145 times that of the permissible level stipulated under Japanese law, officials with plant operator Tokyo Electric Power Co. said Sunday.
The officials said radioactive water apparently flowed into the pit on the ocean side of the turbine building for the No. 2 reactor immediately following the March 2011 nuclear accident and has remained there, The Asahi Shimbun reported.
It is believed the pit holds 5,000 tons of radioactive water that flowed from the No. 2 reactor building to the turbine building and then into the pit, Tepco said.
In June, Japan's Nuclear Regulation Authority expressed concerns high tritium levels detected in the sea near the plant were a result of contaminated water leaking from the site, damaged in the 2011 earthquake and tsunami.
Tepco has acknowledged some radioactive water is leaking into the sea from the compound of the nuclear plant, and it is suspected some of the water in the pit could have permeated the surrounding soil and leaked into the sea.