Namyangju City, a satellite city northeast of Seoul, illegally released wastewater into the Han River, a water source for 25 million people, a government environmental office said Tuesday.

The city has discharged up to 10,000 tons of untreated water a day since at least late 2005 into a tributary of the river, according to the Han River Basin Environmental Office, a subsidiary office of the Ministry of Environment.

“The city has released sewage into the river on quite a regular basis for years,” said Song Chae-young, an official from the office.

The office said the city seems to have released the untreated water from the Hwado integrated sewage disposal plant as the amount of sewage exceeded the plant’s 43,000-ton capacity.

The sewage flowed to the Paldang Dam, a water source for Seoul and the area affected by the recent blue-green algae advisory.

Algal blooms are caused by nutrient enrichment, little sunlight, and warm water temperatures, and the office suspects the wastewater discharge might have exacerbated the situation.

“The illegally released sewage could have aggravated the algal blooms in the Han River. Since the city is located in the upper part of the river, the discharge might have greatly affected the water sources,” the official said.

From early Aug, the blue-green algae has appeared in the capital’s river, raising fears that the algae-contaminated water could have made its way into the tap water.

The environmental office said it was investigating the amount and impact of the discharged wastewater and is considering filing a lawsuit against Namyangju City.

By Kim Young-won (wone0102@heraldcorp.com)