The Lynetteholm cyanide discharge in Øresund breached the project’s environmental permit in 2024, with four of ten samples above the 10 µg/L cap and a peak at 17 µg/L; Copenhagen authorities were informed nine months after the first exceedance, intensifying debate over the artificial-island project during the Copenhagen municipal election campaign.
What the exceedances show
Four of the ten water samples taken in 2024 from water pumped out of the Lynetteholm construction site exceeded the cyanide limit of 10 µg/L, with the highest reading at 17 µg/L in April 2024.
The measurements relate to water discharged through a dedicated outflow during the island’s infill phase. The exceedances occurred over several months before levels fell again later in the year.
Delayed notification and the duty to inform
Under the project’s environmental permit, the supervising authority must be notified immediately when limits are exceeded. Instead, the City of Copenhagen was informed nine months after the first breach.
By & Havn attributes the delay to a reading error—mixing up micrograms and milligrams—and says procedures have since been reviewed. The municipality calls the delay “not good,” but does not assess a broader risk from the specific incident.
Marine impact: localised risk, not a bathing ban
Marine scientists consulted by Danish media say the exceedances could have affected small organisms close to the outflow, such as fish larvae, because thresholds are set to protect marine life.
At the same time, the Øresund is a large body of water and concentrations dilute quickly, so experts do not expect mass fish deaths or health risks for bathers. The episode nonetheless underscores the need for robust monitoring and timely reporting.
Election backdrop: Lynetteholm in the capital’s campaign
The Lynetteholm project is a central theme in Copenhagen’s local race ahead of 18 November 2025. The latest exceedances and the late reporting have prompted political criticism and exchanges between mayoral contenders.
The incident feeds into a broader campaign question: whether the island’s promised flood protection and housing benefits outweigh the environmental risks and governance lapses highlighted by the case.
How the discharge from Lynetteholm works
The island is being built by enclosing a perimeter with stone and sand embankments to form a basin, then filling it with soil and dredged seabed material. As infill rises, entrained seawater is pumped out to Øresund, and some water is pressed through the embankment.
The outflow water is subject to regular sampling for pollutants, including cyanide. By & Havn says its monitoring is extensive and that no elevated cyanide has been detected in 2025; independent advisers (e.g., DHI) support ongoing monitoring of water chemistry and ecological indicators.
What to watch next
Copenhagen’s technical and environmental administration says the 2024 exceedances were temporary and limited in scope, but stresses the importance of compliance and prompt alerts.
With construction continuing for years, scrutiny will focus on sampling frequency, reporting discipline, and cumulative impacts in the Øresund—a shared Danish–Swedish waterway central to the region’s ecology and economy. The outcome may shape how Nordic cities balance coastal protection, urban growth, and marine conservation in large-scale infrastructure projects.
