Groundwater contaminated with highly radioactive substances has been detected from a monitoring well just 27 meters from the seashore within the compound of the crippled Fukushima No. 1 nuclear plant, Tokyo Electric Power Co. said Wednesday.

Wednesday’s announcement is the latest in a series of difficulties Tepco has faced as it struggles to manage contaminated water at the wrecked plant, posing a great risk to the environment.

Testing revealed strontium-90 readings of 1,000 becquerels per liter, 33 times more than the legal limit, as well as tritium readings of 500,000 becquerels per liter, 8.3 times the limit.

Tepco said it believes the radioactive groundwater has yet to reach the ocean, as radiation readings in seawater samples from near the shore have not shown significant shifts.

Tepco first found a spike in the readings of radioactive strontium-90 and tritium on May 24. The readings in the previous study in December was 8.6 becquerels per liter and 29,000 becquerels per liter, respectively, both well below the legal limits.

Tepco will soon begin building a bank protection along the shore that will be strengthened with waterproof liquid glass in an effort to prevent the contaminated groundwater from reaching the sea.

The utility plans to start construction by the end of this month and finish the project in about 90 days, a Tepco spokesman told reporters at the firm’s Tokyo head office.

If introduced into the food chain, radioactive strontium-90, with a half-life of 28.8 years, can remain in the human body for long periods and eventually cause cancer. Tritium is discharged from the body much quicker and is believed to pose less of a threat in general, but could still pose risks to human health.

Tepco has not determined where those radioactive materials came from, but believes they were in water discharged in April 2011 from a damaged pit located south of the monitoring well.

In March 2011, the Fukushima plant suffered meltdowns at three of its six reactors. On April 2, Tepco found highly contaminated water of 1,000 millisieverts per hour flowing into the sea from the pit, which was connected to a turbine building via a covered trench.

The utility managed to stop the flow by injecting liquid glass into the soil under the pit.

Tepco theorized that radioactive materials from that leak might still remain in the soil and could have seeped into the monitoring well in May.

Tepco plans to dig four other monitoring wells around the one in question to assess the environment.

The well in question is located about 28 meters north of the pit from which highly radioactive water leaked in April 2011.

“The density of radioactive materials in the seawater are within the fluctuation range of the past. We don’t think (contaminated water) leaked into the sea,” Toshihiko Fukuda, a Tepco executive and spokesman, said during the news conference.

The Nuclear Regulation Authority plans to order Tepco to beef up monitoring and take measures to prevent the water from reaching the Pacific, according to Kyodo News.