The Swiss Federal Nuclear Safety Inspectorate (ENSI) on Monday downplayed the presence of Caesium-137 in sediments of the lake, which provides drinking water for two-thirds of the city of Biel.
The finding of the radioactive substance dating from 2000 was reported by two Swiss newspapers — SonntagsZeitung and Le Matin Dimanche — on Sunday.
The discovery was made by University of Geneva geologists, who believe it leaked from the Mühleberg nuclear power station, located on the Aar River, which flows into Lake Biel.
The revelation raised concerns among residents in Biel and among environmentalists, including Greenpeace, who wanted to know why people weren’t informed before about the radioactive leak.
ENSI said the findings were not surprising.
Caesium-137 can be found in sediments of all Swiss lakes, it said, coming from a variety of sources, including industry, medicine and research, as well as nuclear power stations.
The substance can also spread through the atmosphere from foreign countries, it noted, adding that the biggest impact on Caesium levels in Lake Biel came from fallout from the nuclear disaster in Chernobyl.
The finding of the radioactive isotope in Lake Biel sediments dating from 2000 corresponds with a slight increase in Caesium-137 emissions from the Mühleberg nuclear plant noted in 1998 and 1999, ENSI said.
These emissions were well documented in annual reports, the inspectorate said.
It pointed out that University of Geneva scientists also found traces of Caesium-137 in other Swiss bodies of water, such as Lake Brienz and Lake Thun.
Even higher levels were found in Ticino lakes, ENSI said.
It said the contaminated sediment did not pose a health risk and because Caesium is fixed it cannot contaminate drinking water.
Switzerland has four nuclear power plants, which produce 40 percent of the electricity produced in the country.
However, the federal government, prompted by ongoing safety concerns, decided in 2011 to phase out such plants by 2034.