The Ministry of Climate, Energy and Environment directed Seoul and Han River Basin agencies to begin limited dredging and temporary weir adjustments upstream of the Jamsil marathon course after late-September heat sustained cyanobacteria mats along slow-moving reaches. Marathon organizers said the race will stay on the published riverside route, but water stations will use bottled supplies and misting crews will avoid pumping river water—a precaution while microcystin spot tests run daily through race week.
What triggered the order
Automated buoys near Banpo recorded chlorophyll-a above seasonal thresholds for nine consecutive days, with visible scum collecting along eddies below Olympic Bridge. Ministry monitors did not find toxin levels above drinking limits at intakes, but recreational exposure guidelines prompted action before tens of thousands of runners inhale aerosols during the October event. The order mirrors broader 2026 policy to manage blooms seasonally rather than wait for public complaints, including temporary weir openings to increase flow when reservoirs run warm and stagnant.
Dredging scope
Contractors will remove nutrient-rich sediment from two pockets near Seogang and Jamwon, totaling roughly 42,000 cubic meters—far smaller than Four Rivers-era volumes. Work must follow autumn “environmental windows” that minimize fish spawning disruption; estuary studies recommend November-heavy schedules, but ministry officials granted an exception with silt curtains and turbidity caps so marathon week stays clear. Dredged spoil will truck to licensed drying beds; residents along Nodeul-ro already received odor advisories.
Weir and runoff controls
Operators will lift gates at Jamsil and Hapjeong weirs during cool nights to flush algal cells toward the estuary, paired with inspections of livestock manure storage in Gyeonggi tributaries. Research on the South Han River links longer residence time behind weirs to Microcystis dominance when phosphorus loads rise; ministry task forces formed this year after pledging seasonal bloom management in the national water master plan. Compost windrows near Paldang remain a focus—runoff after Chuseok rains can spike nutrients even when dredging removes legacy sediment.
Marathon and resident impacts
Race directors said elite and mass-start routes are unchanged, but pacers will carry advisories for athletes with asthma to shorten riverside segments if scum reappears. Bicycle commuters may see path detours when barges moor; city parks posted bilingual signs warning against pet drinking near green film. Long-term, the ministry’s Four Rivers re-naturalization roadmap could remove or reconfigure weirs, but this month’s fix is operational: move water, scrape hot spots, and prove toxins stay below recreational thresholds before the starting gun.
Environmental groups asked whether dredging alone treats symptoms; ministry briefings acknowledged internal phosphorus loading but argued marathon timing justified a surgical cut rather than waiting for spring budgets. Fishermen upstream reported temporary murk but supported flow releases that oxygenate pools holding carp. If chlorophyll-a fails to drop by race morning, organizers prepared a contingency to shift 5K fun-run segments inland—unlikely, officials said, given forecast cooling and the barges already on station.