Continuous high-resolution glider observations of methane following the Nord Stream leaks

After detonations on the 26th September, methane gas leaked at four locations from the damaged Nord Stream pipelines into the Bornholm Basin of the Baltic Sea and into the atmosphere. A permanent Ocean Glider Observatory is located about 20 km east of the leaks, providing continuous data from prior, during and after the incident. We responded quickly by deploying an additional glider with a methane sensor close to the northern leak sites. The glider sampled the methane concentrations for the following three months. During the first week of deployment, we observe an saturation of the methane sensor in the downstream direction of the leaks, indicating methane levels above 1 μM. Outside of the projected downstream advection zone, the methane concentrations are about 100 times lower initially (~10 nM), but quickly increasing as the methane polluted area grows by advection and mixing. A week later the methane is spread along our observation trajectory. About four weeks after the onset of the leaks, the methane levels dropped significantly below 1 μM and after 6 weeks, they stabilize at the initially observed background levels in most places, but occasional patches of increased concentrations, especially in direct vicinity of the leaks remain. Using an advection-dilution scheme in combination with a high resolution Baltic Sea analysis model, we predict the temporal and spatial evolution of methane levels in different basins of the Baltic Sea. Comparing the modelled methane levels with our observations, we estimate rates of atmospheric outgassing and bacterial breakdown.