About 400,000 gallons of radioactive water has leaked from a nuclear power plant in Monticello, the midwestern U.S. state of Minnesota, regulators revealed recently.

The Minnesota Pollution Control Agency (MPCA) said this week that state agencies are monitoring Xcel Energy's efforts to clean up "a release of water contaminated with tritium" detected at the company's Monticello nuclear-generating plant.

Xcel Energy reported the leak to the Minnesota Duty Officer and the U.S. Nuclear Regulatory Commission in late November 2022 after finding unusual results during routine groundwater monitoring. The four-month delay in making public the leak of radioactive water has sparked concerns about public safety and transparency.

The MPCA said that the leak has been stopped and has not reached the Mississippi River or contaminated drinking water sources. "There is no evidence at this time to indicate a risk to any drinking water wells in the vicinity of the plant," it stated.

Tritium is a naturally occurring radioactive form of hydrogen that is produced in the atmosphere. It is a byproduct of the production of electricity by nuclear power plants.

Xcel Energy said the leak came from a water pipe running between two buildings at its Monticello facility and was stopped. The company is monitoring the groundwater plume through two dozen wells while pumping contaminated groundwater through extraction wells.

"We are working to ensure this cleanup is concluded as thoroughly as possible with minimal or no risk to drinking water supplies," said Kirk Koudelka, MPCA assistant commissioner for land and strategic initiatives.

Xcel Energy is said to be exploring building aboveground storage tanks or installing a retention pond to store water containing tritium that has been collected during ongoing recovery activities.