Drop in water temperature causes fish kill near Monticello nuclear power plant

Electrical lines and towers frame a power plant
The Xcel Energy nuclear generating plant near Monticello, Minn. is pictured on Friday.
Ben Hovland | MPR News file

Authorities say fluctuating water temperatures in the Mississippi River following the temporary shutdown of Xcel Energy’s Monticello nuclear power plant have killed more than 200 fish.

Xcel and the Minnesota Pollution Control Agency said the fish kill is not the result of tritium, the radioactive material found in water that leaked at the plant and led to the shutdown.

“In Minnesota, our nuclear and thermal plants use once-through cooling. This process continuously draws water from nearby rivers for cooling steam and equipment and then returns the water,” Xcel said in a statement Monday night. “When a plant is running, it warms the water in the nearby river, keeping fish active. When a plant is taken offline, the river cools down. Fish are affected by the change in water temperature.”

As of Monday afternoon, at least 230 fish had died in the Mississippi River near the plant, including bass, channel catfish, common carp and suckers.

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“The fish kill is unfortunate but not unexpected given the significant temperature change that can occur when warm water from the plant stops flowing to the river during a shut down in operations,” the MPCA reported Monday night.

Xcel publicly reported earlier this month that a leak of 400,000 gallons of water containing mildly radioactive tritium was discovered last November. It’s believed the water leaked from a pipe connecting two buildings at the Monticello plant.

Then last week, the utility reported that a container being used to catch the contaminated water as a short-term solution had spilled over, resulting in a new leak of several hundred gallons of water.

In the wake of that, Xcel opted to temporarily shut down the plant so crews can more quickly repair the original leak.

The MPCA said Monday that it, along with the state Department of Health, is “monitoring samples taken from groundwater wells and have no evidence that the tritium has reached the Mississippi River or contaminated drinking water sources.”