Officials of Tokyo Electric Power Company say water taken from a well near the coastal embankment on Monday contained 251,000 becquerels of cesium per liter.
The level is the highest observed in well water near the embankment. It was more than 3 times the level of the sample taken from the same well 4 days earlier.
The well for observing groundwater is located on the coastal side of the No.2 reactor building.
Highly contaminated water that flowed into an underground tunnel in the 2011 accident seeped into soil in the area.
Officials say the heavy rainfall from Typhoon Phanfone likely caused radioactive substances in the soil to flow into the groundwater. The typhoon passed off the coast of Fukushima Prefecture on October 6th.
High levels of radioactive substances were found in groundwater at other locations in the plant on October 8th and 9th. The water samples were taken near tanks storing contaminated water and near the embankment.
The officials plan to increase the frequency of water sample tests at 3 wells near the underground tunnel from twice weekly to every day to monitor the effects of the heavy rainfall.
But they say it will be difficult to take drastic steps because they do not know how far the contaminated water seeped into the ground after the accident.