Before the Heat Peaks, Algae Warnings Reach Busan’s Water Source

A hotter summer outlook, heavier early-season rain and algae warnings on the Nakdonggang River are turning seasonal weather into a test of public health, watershed management and drinking-water trust.

Harmful cyanobacteria at Mulgeum-Maeri, a lower Nakdonggang River monitoring point linked to the region’s drinking-water supply, rose from 2,418 cells per milliliter on June 1 to 8,458 a week later, bringing the river close to the 10,000-cells threshold for a higher algae alert before the hottest weeks of summer had arrived.

The reading triggered this year’s first attention-level algae alert at Mulgeum-Maeri and Chilseo, after both sites exceeded the 1,000-cells-per-milliliter standard in consecutive measurements. It came as the Korea Meteorological Administration projected a warmer-than-normal summer, with June and July each carrying a 60 percent chance of above-normal temperatures and early-summer rainfall leaning normal to above normal during Jangma, Korea’s summer rainy season.

The numbers put two seasonal warnings into the same lower-river channel. Heat can raise river temperatures and speed cyanobacterial growth. Jangma rain can move phosphorus and nitrogen from fields, livestock areas, roads and tributaries into the basin. Behind the Nakdonggang River’s weirs, slower flow can leave that nutrient-rich water in place long enough for a reading measured in cells per milliliter to become a drinking-water concern.

The river is already showing the season’s first warning

Mulgeum-Maeri had not crossed into Korea’s caution-level algae alert as of June 8, but the direction of the data was hard to treat as routine. The harmful cyanobacteria count there more than tripled in a week, from 2,418 cells per milliliter on June 1 to 8,458 on June 8. At Chilseo, another lower Nakdonggang River point, the count climbed from 4,877 to 7,280 cells per milliliter over the same period.

Korea’s algae-alert system for drinking-water source areas sets the first attention-level warning at more than 1,000 harmful cyanobacteria cells per milliliter in two consecutive measurements. The caution level begins at 10,000 cells per milliliter, and the major-bloom level at 1 million. Mulgeum-Maeri remained below the next formal threshold, but the increase placed the lower river near the line where early-season monitoring begins to carry operational weight.

The timing also follows a longer pattern. Busan’s own water authorities have said algae warnings at the Mulgeum and Maeri sites lasted 194 days last year, after warning periods of 196 days in 2022, 146 days in 2023 and 160 days in 2024. The latest alert, then, did not arrive in an unfamiliar river. It arrived in a lower Nakdonggang system where summer algae has become a recurring management condition rather than an exceptional event.

KMA’s three-month outlook adds the weather side of the equation. The agency puts the probability of above-normal temperatures at 60 percent for both June and July and 50 percent for August. For rainfall, June and July each carry a 40 percent chance of above-normal precipitation and another 40 percent chance of near-normal precipitation. The forecast does not point to a dry heat pattern alone. It points to a warmer early summer with enough rain to keep runoff in the picture.

That combination is difficult for the Nakdonggang River because heavy rain and algae do not move in simple opposition. A Jangma downpour can raise flow and break up a surface bloom, but it can also wash fertilizer, manure, road runoff and sewage-related nutrients into tributaries before the next hot, sunny stretch. If that water then slows behind the river’s weirs, the bloom has both new material and more time.

The first alert of the season does not mean treated tap water is unsafe. It means the lower river has entered the weeks when weather forecasts, watershed runoff and water-treatment operations begin to move on the same calendar.

The heat problem begins at night

The weather-warning system itself is changing before the season reaches its peak.

KMA is adding a severe heatwave warning above the existing advisory and warning system and introducing a tropical-night advisory for the first time. The new nighttime alert is designed for the hours after sunset, when high humidity and warm air can keep the body from cooling before the next day begins.

That shift is especially relevant in a coastal city where the afternoon maximum does not always tell the full heat story. Busan may avoid some of the highest inland temperatures, but warm nights, dense housing, older residents and uneven access to cooling can allow exposure to build over several days. A resident who spends a hot day indoors without reliable cooling, or returns from outdoor work to a poorly ventilated room, may not recover simply because the sun has gone down.

Busan’s recent health data points in the same direction. The city recorded 171 heat-related illness cases and one death last year, up from 129 cases and one death a year earlier. This year, its heat-related illness emergency room surveillance system began operating on May 15 and will run through Sept. 30, with 35 emergency medical institutions reporting cases through the national disease-control network.

The surveillance system captures heat after it has reached the body. Weather alerts come earlier; emergency-room data comes later. The distance between the two is where public-health measures have to work — cooling shelters, welfare checks, workplace precautions, outreach to older residents and support for households that ration air conditioning because of cost.

The hardest places to reach are often private or mobile: small apartments with weak ventilation, older homes, construction sites, delivery routes, markets, late-night workplaces and rooms where a tropical night becomes more than an uncomfortable forecast. The same summer being measured in algae cells at Mulgeum-Maeri will also be measured in emergency rooms.

Jangma rain can reload the river

KMA’s summer outlook does not point to heat alone. For June and July, the agency gives above-normal rainfall a 40 percent probability and near-normal rainfall another 40 percent, leaving a wetter-than-dry early summer as the more likely shape of the season. In the Nakdonggang River basin, the forecast matters less as a rainfall total than as a pathway for what rain can move.

Jangma can briefly weaken a bloom by raising flow and disturbing algae near the surface. The same rain can also load the river for what comes next. When downpours cross fields, livestock areas, roads, construction sites and older drainage networks, they can carry phosphorus, nitrogen, manure, soil, organic matter and sewage-related nutrients into smaller streams before that water reaches the Nakdonggang River’s main channel.

The government’s own Nakdonggang water-quality plan treats that runoff as part of the algae problem. The plan calls for a 30 percent cut in total phosphorus emissions by 2030 and targets sewage, urban stormwater, livestock manure and farm nutrients before they reach the main river. It also points to a basic weakness in the basin: much of the material that feeds algae does not enter through one obvious discharge point, but through dispersed routes that become harder to control when rain falls hard.

A downpour may make the lower Nakdonggang look cleaner for a short period as flow increases, but the water moving downstream can carry a new nutrient pulse. If cooler weather and stronger flow follow, that pulse may pass through. If heat and sun return while water slows behind weirs, the river can shift from dilution to bloom conditions within the same seasonal cycle.

The storm may fall upstream, the runoff may enter through a tributary, and the nutrients may not become a drinking-water concern until harmful cyanobacteria counts rise downstream at monitoring points such as Mulgeum-Maeri. By then, the rain itself has already passed, leaving the treatment system to deal with what the basin delivered.

The bloom is fed upstream and held in place

The government’s Nakdonggang plan begins with phosphorus, not heat.

In its 2026 plan for the basin, the environment ministry set a target to cut total phosphorus emissions by 30 percent by 2030 while tightening controls on sewage, urban stormwater, livestock manure and farm nutrients before they reach the main river. The target amounts to an official acknowledgment that the material feeding algae is not produced at Mulgeum-Maeri and is not produced by hot weather alone.

Phosphorus and nitrogen enter the river through routes that are difficult to see from the lower basin. Some move through wastewater systems and tributaries. Some are washed from fields after fertilizer or liquid manure has been applied. Some arrive with road runoff, construction-site drainage or older urban sewer networks during heavy rain. By the time harmful cyanobacteria counts rise near the intake system, the chemistry of the bloom has often been assembled much farther upstream.

The second condition is time. A 2019 analysis by the environment ministry and the National Institute of Environmental Research found a clear difference between rivers where weirs had been opened more widely and the Nakdonggang River, where openings were more limited. On the Geumgang and Yeongsangang rivers, harmful cyanobacteria levels during the summer fell 95 percent and 97 percent, respectively, compared with the pre-opening average. On the Nakdonggang River’s eight weirs, average bloom levels rose 32 percent over the same comparison period.

The figures do not settle every argument over the Four Major Rivers Project. They do make one point difficult to avoid: algae season is shaped by how long warm, nutrient-rich water remains in place. The weirs did not create the phosphorus and nitrogen moving through the basin, but they changed the river’s hydraulics. In slow sections of the river, residence time becomes part of the water-quality problem.

Recent modeling research on the Nakdong River reaches a similar conclusion. Studies of harmful cyanobacteria under weir-operation and climate scenarios have found that reducing hydraulic retention time through weir operation can help offset some bloom pressure created by warmer conditions. The finding does not make weir opening a single solution. Nutrient loads still have to fall. But it shows why a cleaner basin and a faster-moving river cannot be treated as separate questions.

Mulgeum-Maeri records what has already passed through the basin: fertilizer management upstream, livestock-waste treatment, sewage capacity during storms, tributary conditions, weir operation and summer heat. None of those factors alone explains every bloom. Together, they decide whether a warning remains a seasonal reading or becomes a larger burden on intake and treatment facilities.

At the tap, the watershed arrives

The lower Nakdonggang River becomes a public issue when algae readings move from the water-quality table to the intake system.

An alert at Mulgeum-Maeri does not mean treated tap water is unsafe. It means the source water has entered the part of summer when treatment plants have to watch more closely for harmful cyanobacteria, algal toxins and the taste-and-odor compounds that can follow blooms. Raw water and treated water are not the same thing. The first carries the condition of the river; the second reflects monitoring, filtration and treatment.

When algae increases in the lower river, operators can adjust treatment processes, increase testing and use measures such as activated carbon and ozone to control odor compounds and reduce toxin risk before water reaches households. Those steps are the reason an algae alert should not be read as a tap-water scare.

Treatment starts after the river has already delivered the problem. It cannot decide how much phosphorus is washed from fields during Jangma rain, how livestock manure is stored upstream, how sewage systems handle stormwater, how tributaries carry runoff or how long water remains behind weirs during hot weeks. By the time the water reaches the lower river, many of those choices have already been made outside the city.

Drinking-water confidence therefore depends on more than the performance of treatment plants. It depends on whether the basin reduces the nutrients that feed blooms, whether wastewater and livestock-waste controls improve before heavy rain, whether industrial discharge is watched before it enters the main river, and whether weir operation leaves less warm, nutrient-rich water sitting through the hottest stretches of summer.

The government’s latest Nakdonggang plan reflects that wider map. It does not focus only on downstream treatment; it sets targets for reducing total phosphorus, tightening sewage and runoff controls, managing livestock manure and farm nutrients, and strengthening treatment for industrial wastewater entering the main river. Those measures sit far upstream, but their results are read downstream at places such as Mulgeum-Maeri.

The lower Nakdonggang River is where those choices arrive. In a hotter summer, with heavier early-season rain possible and algae already being measured near the intake system, the city’s water question no longer begins at the treatment plant. It begins upstream, before the river reaches Busan.