Explosions to dispose of WWII munitions could contaminate ocean

World War II ended decades ago, but active mines lurking on the ocean floor still pose a threat, potentially spewing unexpected geysers or releasing contaminants into the water. Experts carry out controlled explosions to clear underwater munitions, but concerns have arisen about the environmental impacts of these explosions.

Now, the results published in Environmental science and technology show that the contamination produced by detonation depends on the type of explosion, with weaker explosions leaving behind more potentially toxic residue.

After World War II, according to research estimates, up to 385,000 tons of unexploded ordnance, including 40,000 tons of chemical munitions, were dumped into the Baltic Sea. These abandoned weapons remain dangerous: they have the potential to project plumes of water and sediment upward, send shock waves across the ocean, and punch holes in ship hulls.

Additionally, the metal shells of mines can corrode in seawater, releasing potentially toxic explosive compounds, such as TNT, into the environment over time. Technicians typically clean historic munitions with controlled explosions, but scientists debate whether low or high explosions are better.

While smaller explosions minimize shock waves and physical damage, Edmund Maser and his colleagues suspect that these weaker explosions release more toxic residue than powerful explosions. To check whether this is true, the team wanted to measure explosive residue near underwater mines after controlled detonations of two different intensities.

Researchers, working closely with the Royal Danish Navy, have identified World War II mines near a busy shipping route off the Danish coast for the first time, choosing the sites of two intact devices and of two corroded devices. Navy divers collected seawater and sediment from the ocean floor around the mines, and researchers then used mass spectrometry to measure the samples’ TNT levels. As the researchers expected, chemical contamination was higher near corroded mines than near intact ones.

Then, using either a low-power detonation or a high-power detonation, the team destroyed the leaking mines and assessed the TNT released by the explosions. The sediments contained up to 100 million times more TNT after the weakest explosion and only 250 times more TNT after the strongest explosion. Similarly, TNT levels in the water after the weaker explosion far exceeded those around the strongest explosion.

Researchers say the pollution released by the low-power explosion meets or exceeds levels previously reported to be toxic to microalgae, sea urchins and fish. Due to potential threats to nearby marine life, researchers are encouraging less invasive methods to remediate submerged World War II relics, such as robotic techniques to open and remove explosive contents from abandoned mines, to avoid Unwanted explosions and contamination.