A popular stream running through downtown Seoul, which is visited by millions of people annually, has been severely polluted by bacteria, a report by the city government showed Tuesday.

According to the report, submitted to the Seoul council member Jang Hwan-jin of the Democratic Party, parts of the Cheonggyecheon stream contain as many as 53 times the legal limit of colon bacillus. The standard for the number of bacteria for a body of water suitable for swimming is 1,000 per 100 milliliters.

The study checked for water quality from January to September this year at three different locations along the stream, which stretches for 8.4 kilometers under more than a dozen bridges through Seoul.

The water under the Mojeon Bridge upstream had 7,201 bacteria per 100 milliliters, more than seven times the limit. But farther downstream toward the eastern end, water samples contained 53,303 bacteria.

A Seoul official said the stream may have been polluted during the rainy season, when pollutants could have entered Cheonggyecheon through rainwater pipes. The stream was flooded on a few occasions under heavy monsoon rains this summer.

Jang said he wasn't buying that argument.

"There were nine checks for quality this year and samples exceeded the legal limit on three to six occasions, depending on their locations," the councilor said. "The argument for rainwater pollution isn't very convincing. There should be an all-out review of the water quality management for the Cheonggyecheon."

Seoul says up to 17 million people visit the stream each year.

Once covered with asphalt for decades, Cheonggyecheon transformed into a popular recreation spot following a US$900 million restoration project, initiated by then Seoul Mayor and current President Lee Myung-bak in 2003. (Yonhap News)