The weir was constructed in 1988 to control the water level, control water flow and prevent floods. It also aimed at playing a security role in preventing underwater infiltration by North Korean spies.

Since last month, algal bloom spread has been detected in the Hangang River, with the Seoul Metropolitan Government issuing an algal bloom warning on June 30 in the downstream section. The government expanded the alert later to nearly all sections of the river in the capital. It was the first such warning in 15 years.

The green tide poses harmful ecological impacts as it hinders the water flow and causes odors. It also contains toxic substances such as microcystin-LR, which can cause respiratory problems or even paralysis in humans.

While the continuous drought and high temperatures are generally suspected as the main causes, some experts argue that Singok weir is actually a major culprit behind the green tide, citing the unusual starting point of the algae.

“If there’s enough water flow, the problematic phenomenon does not occur. The algal bloom usually starts from the Paldang Dam in Gyeonggi Province and spreads to the Hangang River. This time, however, the green tide started from downstream where the weir is located,” said KFEM director Lee Sae-gul.

According to the city government, the green tide was first concentrated in the 5-kilometer-long section around the Singok weir, spanning from Haengju Bridge to Banghwa Bridge this year.

While the long-awaited rain showers last weekend mitigated the algal bloom, the city government has not lifted the warning alert yet, saying that the green tide level is likely to go up again. A day after the rain, the level started to rebound, the city said.

“Once the green tide settles in a certain spot, it makes its periodical appearance,” said Kim Jung-wook, the president of the Korean Society of Limnology, warning that the algal bloom around Singok weir will occur annually. 

Since 1998, experts have warned that the weir could seriously slow down the water flow, worsening the ecological system.

They argued that the deaths of numerous fish near the weir late last month was caused by the green tide as well. Over 600 kilograms of dead fish were found June 27, three days before the algal bloom warning was issued.

“The small dam slows down the water flow speed by two times, making it easier for the green tide to grow as the water pollutants remain stagnant,” Lee added.

Fishermen in the neighboring area, however, refute such a scenario, arguing that the fish deaths were not caused by the algal bloom, but wastewater.

“If the green tide was the real cause, then all the fish in the Hangang River would have had to die. The fish died only on June 27. I believe the illegal effluent, which massively flew toward the Singok weir region by those who attempted to throw away the wastewater on a rainy day, affected it,” said Park Chan-soo, the representative of a fisherman group in Goyang, Gyeonggi Province. 

About 40 fishermen who work near Singok weir claimed that the water level will go down if the weir is removed and that would pose difficulties to their means of living.

“The weir supplies crucial water for farmers in our province. Plus, if the water level is not controlled, there will be highs and lows in the river. All fishermen will flock to the high water level areas, letting all fish get caught in a few years,” he added.

Some experts also sided with the fishermen.

“The small dam safely prevents the mud from coming up the river. If there’s no weir, the mud can affect the river because of the tidal current,” said civil engineering professor Jang Suk-hwan at Daejin University.

The Ministry of Environment also remains cautious of linking the weir to the problematic green tide.

“It’s very difficult to say that the small dam contributed to the ecological problem. Drought is more likely to have affected the phenomenon. Since last summer, the discharged water from Paldang Dam nearly halved because of the drought, having the Hangang River stay stagnant,” said a ministry official in a press conference Thursday.

While the controversy goes on, a half-year study conducted by Seoul Metropolitan Government in 2013 has shown that it would be economically beneficial to tear down the small dam.

The study showed that pulling it down would boost biodiversity and river quality as it would drop the biochemical oxygen demand and chlorophyll-A by up to 4 percent and 19 percent, respectively. The removal project was anticipated to cost about 17 billion won ($15 million).

The city, however, decided in April to conduct the study again as some questioned the result, claiming that it was exaggerated.

Even if the second study finds similar results, the city does not have the discretion to tear down the concrete construction, as the small dam is owned by the Ministry of Land, Infrastructure and Transport.

“(We) know that the city doesn’t have any authority to unilaterally get rid of the weir. The study just aims at gathering the basic information as part its efforts to come up with solutions,” city officials said.

Despite the growing concerns over the weir, the Land Ministry has been reluctant to remove the small dam, citing its benefits.

The ministry said that pulling down the weir would not ensure high enough water level, but only break the ecological system that has long been settled around the weir.

A decade after the weir was built, a 7.5-square-kilometer-large wetland was created around Singok weir. Being a habitat for various plants and animals, it was designated as a “protection region” by the government.

As long as the small dam remains, ensuring good river water quality is the best solution, experts said.

“The climate issue is a constant variable.

The temperature will continue to go up while the drought will be more serious. If we cannot improve the climate conditions, the best solution for now is to strengthen the river water quality management,” said researcher Kim Young-ran at public think-tank Seoul Institute. “The green tide will likely become a daily life issue for us.”

By Lee Hyun-jeong (rene@heraldcorp.com)