Residents rest along Cheonggyecheon in Seoul. [Yonhap]
Residents rest along Cheonggyecheon in Seoul. [Yonhap]

White foam spotted floating across Cheonggyecheon in recent weeks prompted a wave of complaints about water quality, but city authorities say the phenomenon is entirely natural.

The Seoul Facilities Corporation said Friday that a string of complaints had been filed on its website claiming the stream's upper reaches were "contaminated with foam."

One resident, identified only by the surname Lee, said the foam extended from the upper to the middle stretches of the stream. "It's something you notice right away, and I'd like to know why nothing has been done about it even after watching for several days," Lee said.

Another resident, surnamed Jo, said the water looked murky and unkempt and that the stream's overall environment appeared poorly maintained. "Cheonggyecheon is one of Seoul's most iconic landmarks, visited by many tourists from home and abroad," Jo said. "If the current condition continues, I think it could have a negative impact on Seoul's image."

In response, the Cheonggyecheon Management Office under the Seoul Facilities Corporation said the foam was not the result of water pollution but a natural occurrence caused by the large temperature swings typical of autumn.

"The foam seen in streams is generally a natural phenomenon that occurs in spring and autumn when the gap between the daily low and high temperatures widens to around 10 degrees Celsius or more," the office said. "In the case of Cheonggyecheon, an on-site visual inspection conducted during a patrol in mid-September confirmed that the phenomenon was not attributable to any external source of contamination at the time."

The office said that during seasonal transitions, large daily temperature swings cause aquatic plants — including algae and plankton — and organic matter to die off and decompose in the water, generating natural surfactants such as proteins and fatty acids. "These substances lower the surface tension of the water, and as air mixes in through the stream's eddies and currents, white air bubbles that do not burst easily form in clusters — in other words, foam," it said.

The office also said naturally occurring foam, unlike artificial foam from synthetic detergent spills, leaves no stickiness or odor when touched and disappears on its own once water temperatures stabilize and algae populations settle. "This is a natural phenomenon that can appear in any stream during periods of sharp temperature differences between morning and afternoon, like now," it said, adding that the foam would dissipate naturally after rainfall or once daily temperature swings ease.

The office said there is no problem with water quality. "Cheonggyecheon is supplied with purified Han River water and is managed to maintain clean water quality targeting a Grade Ib (good) rating under stream water standards," an official said. "Monthly water quality measurements confirmed that the stream met the Grade Ib (good) standard in September as well."


bbo@heraldcorp.com