The explosions that tore holes in the Nord Stream’s undersea gas pipelines lifted long-buried toxins to levels high enough to threaten marine life for more than a month, according to a site analysis.
Last September, a series of four explosions ruptured the Nord Stream 1 and 2 gas pipelines – which connect Russia to Germany – near the Danish island of Bornholm in the Baltic Sea, releasing more than 100,000 tons of methane in the sea and the atmosphere. It is unclear who was behind the explosions, but the most recent speculation is that a pro-Ukrainian group was responsible.
Beneath the dramatic sight of bubbles spewing from the sea, the explosions also blasted sediment from the seafloor, sending it back into the water column. A team of environmental scientists, led by Hans Sanderson of Aarhus University in Denmark, feared the consequences were being overlooked, especially as the blasts occurred near a historic dumping ground for the chemical warfare, including mustard and arsenic agents from World War II. They rushed to practice how these chemicals could affect marine life.
Researchers used decades of sediment monitoring data in the busy shipping area of the Bornholm Basin and hydrological models of sediment transport to determine the impact of the explosions, each of which is estimated to have been equivalent in power to the ‘blast. of 500 kilograms of TNT. Their models showed that the explosions, which occurred about 70 meters deep, threw up a total of 250,000 tonnes of sediment, which reached up to 30 meters below sea level. Sanderson and his colleagues have took known concentrations of various contaminants in the sediment, including the biocide tributyltin (TBT), heavy metals and chemical warfare agents, to see what was happening.
They determined how much of each contaminant would become bioavailable and calculated a sediment toxicity threshold. More than 5.8 milligrams of resuspended sediment per liter of seawater was predicted to be harmful to marine life. The team found contaminants in the sediment, including lead and TBT – an endocrine disruptor used to protect ship hulls – accounted for most of the toxicity. Lead and TBT alone were responsible for 75% of the toxicity.
The sediment thrown up by the Nord Stream 1 explosion contained contaminants that exceeded the safety threshold for 15 days, at depths between 95 and 53 meters. For the Nord Stream 2 gas pipeline, the threshold was crossed for 34 days, at depths between 78 and 42 meters. In total, the explosions contaminated 11 cubic kilometers of seawater for more than a month. The work, which is peer-reviewed in a journal, has been published as a preprint1.
“It’s an impressive modeling exercise,” says Rodney Forster, a marine scientist at the University of Hull, UK. And the teams’ calculations of the dispersion of the sediment plume clearly show that the sediment did not reach the surface of the sea. “This explains why, apart from the bubble plume, no visible suspended sediment plume can be seen on satellite images after the event,” adds Forster.
The impact of contaminants on marine life will not be known with certainty for months or even years. But Sanderson is concerned about some animals in the area. The area is a known spawning site for cod. “There are high concentrations for a long time in a large area,” he says of the contaminated sediment. “That could potentially have quite a big impact” on the cod stock. And the presence of TBT is “not good news for these organisms”, he says. The explosions occurred at the end of the cod spawning season and the eggs are floating at depths reached by the churning sediment. Sanderson speculates that the physical weight of the sediment could be another problem for the eggs, weighing them down so they no longer float at the optimum depth to thrive before they hatch.
Other potentially affected animals include the harbor porpoise, of which it is estimated that there are only 500 individuals left in the Baltic Sea. The loss of just one would affect this population. It is possible that the explosions damaged the porpoises’ hearing and thus their ability to communicate.
Sanderson says the team’s research shows that sediment content needs to be addressed alongside any activity that agitates the seabed, such as installing pipelines or wind turbines, or fishing. “There are also environmental impacts of conflict and these also need to be clarified,” he adds.