Xcel steps up pumping at Monticello nuke plant as radioactive water moves toward Mississippi River

Radioactive groundwater that’s the legacy of a 2022 spill at Xcel Energy’s Monticello nuclear plant is flowing toward the Mississippi River, but state agencies say there is no risk to human health or the environment.

Two and a half years ago, between 750,000 and 900,000 gallons of tritium-tainted water spilled from a broken pipe at the Monticello Nuclear Generating Plant. Since then, plant owner Xcel Energy has been working to contain the damage by pumping up contaminated water and building an underground containment wall between the plant and the river.

Now, a well next to the river showed tritium readings just below 20,000 picocuries per liter, the EPA‘s threshold for safe drinking water, Xcel announced Monday. A joint statement from Minnesota environmental and health agencies noted that the level was found in a well outside of Xcel’s containment wall.

It’s unclear if the new reading was deeper than the underground wall. Xcel spokesman Kevin Coss wrote that the same amount of tritium had not been found at shallower depths outside the barrier, and added, “The underground wall has been largely effective and has helped prevent groundwater from interacting with the river.”

Tritium, a radioactive form of hydrogen, can occur naturally but is also a byproduct of nuclear power generation, according to the Nuclear Regulatory Commission. Its radiation does not penetrate the skin.

“We are adjusting the amount of water we pump to pull the leaked tritium away from the vicinity of the river and recover it more effectively,” Coss wrote. “We expect measurements to fluctuate over time as we continue to retrieve the leaked water.”

Because the polluted plume was diluted by groundwater, Xcel has already pumped more than 9 million gallons to collect the tritium, which it says is about 90% of the original contamination.

The Mississippi provides drinking water for multiple downstream communities, including Minneapolis and St. Paul.

A statement from the Minnesota Pollution Control Agency, the Minnesota Department of Natural Resources and the Minnesota Department of Health also noted that tritium readings in the river have been below detection limits, but still said that, based on testing on Xcel’s property last month, “there has been a change” in the situation at Monticello.

Since April, “monitoring strongly suggests the plume of tritiated groundwater is moving toward the Mississippi River,” the statement sent by MPCA spokesman Dan Ruiter said.

However, the agencies also noted that the plume’s movement does not threaten human health or the environment.

Groundwater naturally flows towards rivers, and dry conditions can accelerate that movement. In 2023, Xcel reported that some of the plume may have reached the river, and Coss wrote then that a low water level might be contributing to the movement of the plume.

Now, Xcel is testing wells along the river more frequently and pumping more in the area where it has found the highest remaining tritium, Coss said.