Nearly 4,000 gallons of water containing "small amounts" of tritium and "trace amounts" of other chemicals have been released by Prairie Island nuclear plant since November, plant spokeswoman Mary Sandok said Wednesday.

The most recent leak, occurring when 27 gallons of water that had condensed from heating system steam overflowed a holding tank, happened Feb. 3.

A pump failed to regulate levels within the tank, Sandok said, and the water overflowed out a vent pipe and onto the ground. The heating system has been shut down while the cause of the pump failure is investigated and corrected.

The plant notified the Minnesota Pollution Control Agency and the Nuclear Regulatory Commission of the release Monday.

In late November, about 3,900 gallons of water containing small amounts of tritium was released "under similar circumstances," Sandok said Wednesday.

That overflow occurred over several days, leading to the large volume of water released. The most recent release was identified and stopped more quickly, Sandok said.

Neither release posed a threat to the public or employees, Sandok said, adding that the amount of tritium in the released water falls below the U.S. Environmental Protection Agency's limit for safe drinking water.

The plant monitors groundwater at Prairie Island and has seen no increase of tritium in the groundwater, Sandok said. Still, she added that any impact from the spills would not have shown up yet.

The most recent spill comes just before the release of a report on the risk nuclear power plants pose to Minnesota's drinking water. Representatives from Environment Minnesota and the Minnesota Public Interest Research Group will present the report at 2 p.m. today at the Red Wing Public Library.