Three dams were set up on the River Narew to draw the pollution off the surface of the water. Turbine oil leaked from the power plant in the city of Ostrołęka on Saturday.
Poland’s environment ministry said that an estimated 3 to 5 tonnes of oil had escaped. Dozens of firefighters worked to remove it.
A 35 km section of the river was polluted, state news agency PAP reported.
Environment Minister Henryk Kowalczyk said the local environmental protection authority would carry out checks at the power plant.