Green tide returned to major water sources of the Seoul metropolitan area this week amid ongoing controversy over the ecological impact of a small dam in the Hangang River.

The Environment Ministry issued a green tide advisory Wednesday for Paldang Lake in Gwangju, Gyeonggi Province, as the water’s chlorophyll-A concentration reached 55.8 milligrams per cubic meter along with over 4,000 blue-green algal cells per milliliter.

The green tide advisory is raised when the chlorophyll-A concentration exceeds 15 milligrams per cubic meter and the number of blue-green algal cells is over 500 cells per milliliter based on two sets of test.

This is the fourth advisory for Paldang Lake in the last five years. The lake, artificially constructed in 1973 to run Paldang Dam, serves as one of the capital’s main water sources. Over 244 million tons of water is stored there.

The low rainfalls and high water temperatures amid the heat wave have contributed to the algal bloom reoccurrence, the ministry said.

The advisory was also raised for the Hangang River as of Tuesday afternoon, 18 days after Seoul Metropolitan Government had lifted the alert. The river is linked to Paldang Lake.

In late June, the city issued a warning for the entire downstream section of the river for the first time in 15 years. The alert lasted for a month.

As part of its efforts to alleviate the green tide, the city said it would temporarily open Singok weir, a small dam, in the mouth of the river in Gyeonggi Province, which has recently been pointed to as a culprit of the green tide by civic groups.

While the weir was constructed to control the water flow and prevent floods and back flow of sea water, activists have called for it to be torn down, claiming that it actually slows down the natural water flow, thereby causing ecological problems.

Seoul City also urged the Ministry of Land, Infrastructure and Transport on Tuesday to contemplate whether to demolish the weir. The ministry has the authority to make the decision as it owns Singok weir.

Based on a long-term study of the weir’s efficacy, Seoul City concluded earlier this year that it is economically more beneficial to tear down the small dam. The city sent the study report to the Environment Ministry and Land Ministry in May, but no responses had been received, city officials said.

The Land Ministry has reportedly been reluctant to pull down Singok weir, citing the benefits of keeping the water level steady.

If no rain is forecast by next week, Seoul Metropolitan Government will temporarily open the small dam and monitor the ecological differences for three days, authorities said.

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