A plume of radioactive water that has been lingering under Xcel Energy’s nuclear plant in Monticello may have seeped into the Mississippi River, the utility said Thursday — but the amount is so low, it hasn’t been detected in the river.

In a news release Thursday, Xcel said that tritium — a mildly radioactive form of hydrogen — had been detected in low levels in a monitoring well 30 feet from the edge of the Mississippi. The Environmental Protection Agency’s health limit for tritium in drinking water is 20,000 picocuries per liter; the sample taken from the well along the river showed 1,000 picocuries, according to Xcel.

State health and environmental regulators said in their own statement that no tritium had shown up in river testing just downstream of the Monticello plant.

The situation “does not present a threat to public health, and there are no immediate impacts to the safety of drinking water or private wells,” Andrea Cournoyer, a spokesperson for the Minnesota Pollution Agency, wrote in an email.

Still, the potential release into the river shows the challenge of managing the underground plume that persists at Monticello, some seven months after Xcel first found and patched a leaking pipe.

The utility revealed last month that tritium was drifting in the direction of the river, but utility spokesman Kevin Coss wrote then that “we are confident in our ability to keep the extent of the affected groundwater limited to the site’s property.”

Xcel originally notified the Nuclear Regulatory Commission of a likely leak in November 2022. A month later, it found the underground pipe that was leaking and installed a temporary fix — in a hard to reach area between two buildings. The public did not broadly learn of the leak until months later, in March.

Tritium is sometimes produced naturally in the environment, but mostly a product of nuclear power processes. It cannot harm people unless they ingest it in significant concentrations.

As it’s lingered underground, the tritium has been significantly diluted. Xcel originally said that 400,000 gallons of contaminated water leaked; to date, it has pumped 2.7 million gallons of water back above ground for storage and re-use in the plant, recovering about 75% of the tritium that escaped.

In part, Xcel is battling underground pressure that is pushing groundwater towards the river, which happens during drought conditions like those that are worsening across Minnesota. In wetter times, groundwater tends to be pushed away from rivers and streams — a condition that partially helped Xcel as it was vacuuming up tritiated water this spring.

Now, the pressure is in an unfavorable direction. “The low water level in the river has contributed to the rate at which [tritium is] moving towards the river,” Coss said Thursday.

This is a developing story. Check startribune.com for updates.