Fate of Methane from the Nord Stream Pipeline Leaks

In September 2022, three major subsea leaks occurred in the Nord Stream pipelines, releasing an estimated 225 kt of natural gas into the Baltic Sea. To explain its behavior in the water column, we developed a combined model to simulate the near-field buoyant bubble plumes and the far-field advection, dispersion, volatilization, and biodegradation of the dissolved methane fraction. According to these simulations, 94.9% of the leaked methane was emitted to the atmosphere immediately above the leaks by ascending gas bubble plumes. The remaining 5.1% of the methane (11 kt) initially dissolved, leading to concentrations of up to 625,000 nM or 5 orders of magnitude above the local Baltic Sea natural background. The modeling suggests that within 35 days, 71.4% of the dissolved methane volatilized and 26.0% biodegraded, and the maximum water column concentration decreased to 70 nM. We find that up to 409 km3 of seawater experienced concentrations greater than 10 times above the natural background. The modeling shows that more than 10 marine protected areas in the Baltic Sea were exposed to elevated dissolved methane concentrations.