The nation is on high alert over fast-spreading algal blooms on major rivers and lakes, threatening to affect public health and drinking water.
The government attributes the algae to hot weather and the long drought. Activists blame the government's massive development of four rivers that they claim slowed water flows and damaged the eco-system.
Rapid increases in algae are typically caused by nutrient enrichment, high temperatures and still water conditions.
Blue-green algae, or cyanobacteria, are single-celled organisms, toxic to humans and other animals. Water blooms can use up the dissolved oxygen in the water, killing other organisms.
Seoul and other local governments are trying to minimize possible damage from the algal tides, which began to occur in the upper streams of the Han River in late June and have since been spreading through rivers and lakes.
Algae warnings have been issued in the northeastern parts of the river, which is the source of drinking water for some 22 million people in Seoul and surrounding areas.
Blue-green scum has also been discovered in other major rivers including Nakdong, Geum and Yeongsan.
The Seoul government has secured 12 tons of yellow soil to contain algal blooms. The city tested water samples taken from five locations including Amsa-dong, Guui-dong and Pungnap-dong and found high levels of chlorophyll a, an indicator for algal biomass, and blue-green algae.
The test also shows a high level of geosmin, an organic compound with a distinct earthy flavor, which causes odour in water.
The concentration level of geosmin was 33.3 parts per trillion (ppt) to 41.6 ppt, which is more than 20 ppt, the standard level for drinkable water.
The city is currently using activated carbon in the water treatment to get rid of unpleasant smells but concerns are increasing due to the spreading green algae tide in the river.
The Ministry of Environment said that toxins caused by blue-green algae have not been found in Seoul and the surrounding areas.
"The ministry is monitoring the situation and the water treatment system to minimize inconveniences," the ministry said in the statement.
The environment authority said low precipitation and hot temperatures caused the algae blooms.
When the excessive level of algae cells is detected in the second test scheduled on Wednesday, the warning could be issued as early as Friday, according to a city official.
"The situation is severe in the upper stream of the river. If it does not rain over the weekend, the warning will be issued in Seoul," the city official said
An algae warning is issued when the concentration level of chlorophyll a in water is 15 milligram per cubic meter or above and 500 blue-green algae cells per milliliter are detected.
A higher level of warning is issued when the level of chlorophyll a records over 25 milligram per cubic meter and more than 5,000 algae cells are found.
The tests are conducted twice within two weeks before issuing the warning.
The government has already issued the algae warning around the upper reaches of the Han River and Paldang Dam, the main sources of drinking water for Seoul citizens.
Rivers in North and South Chungcheong provinces, west Korea, are also becoming discolored by algae.
The National Institute of Environmental Research said Tuesday that the chlorophyll a level in the Yeongsan River marked 71.2 milligram per cubic meter and 3,725 blue-algae cells were detected.
"The Yeongsan River is not used for drinking water but for farming. The algae blooms could be spreading throughout larger areas," an official from the institute said.
Environment groups claim that microcystis, a carcinogenic alga, was detected in the Nakdong River, water sources for people in the Gyeongsang provinces and major cities.
The alga is known to cause liver diseases including liver cancer. The authorities have not officially announced any discoveries of toxins.
Concerns over red tides are also rising in southern coastal areas.
An algae warning has been issued in the seas off the coastal cities of Tong Young, Geoje in South Gyeongsang Province, for first time in four years.
Red tides kill fish and cause respiratory irritation in humans.
The National Fisheries Research and Development Institute tested the seawater around the areas and detected 1,500 cochlodinium, organism causing red tides, per milliliter of saltwater, exceeding 300 cells per milliliter, the level when a warning is issued.
On Monday 86,000 fish were killed on a fish farm in Yeosu, South Jeolla Province. It is known that sea water affected by red algae blooms was pumped into the aquarium on land and killed the farmed fish.
As the scorching hot temperature is continuing the algae warning will be spreading throughout southern seas.
The institute cited this summer's high level of sunlight and higher-than-average water temperatures in the ocean as the causes of the red tides.