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.