An international team of researchers from Denmark and Germany has determined that live mines, or unexploded munitions, sitting on the floor of the Baltic Sea — deposited there at the conclusion of World War II — release more contaminants when cleared via controlled detonations using weaker explosions.

These live mines, which still threaten to spew geysers or release contaminants into the water, are typically detonated with experts conducting controlled explosions to clear the underwater munitions. However, concerns have grown over the potential environmental impacts from these controlled explosions, with the researchers suggesting that contamination produced by detonation depends on the blast type, with weaker explosions reportedly leaving behind more potentially toxic residues.

Source: Uwe Wichert

With a reported 385,000 metric tons of unexploded munitions, including 40,000 tons of chemical munitions, deposited in the Baltic Sea following World War II, the weapons continue to pose a threat with the potential of exploding and forcing jet plumes of water and sediment upward, sending shock waves through the ocean and possibly punching holes into ships’ hulls. Further, the metal shells of the mines corrode in seawater and leak toxic explosive compounds — like TNT — into the environment over time. Such munitions have historically been cleared using controlled explosions, leading experts to wonder which type of blast — weak or strong blasts — leaked more toxic residue into the environment.

To make this determination, the researchers in conjunction with the Royal Danish Navy measured the explosive residues present near underwater mines following the controlled mine detonations using two different intensities.

Once the team identified two intact and two corroded World War II mines near a busy shipping route off the coast of Denmark, divers from the Navy collected ocean water and ocean floor sediment in the areas surrounding the mines. Using mass spectrometry, the team measured the levels of TNT present in the sample, and unsurprisingly, found that levels of chemical contamination were higher near the corroded mines than the intact mines.

Further, the team then used both a low-powered detonation and a high-powered detonation to destroy leaking mines and analyze the levels of TNT released from both types of blasts. According to the team’s findings, sediment collected from the site of the weaker explosion featured up to 100 million times more TNT than before detonation and only 250 times more TNT following the stronger blast. Likewise, the TNT levels in the water following the weaker blast were significantly higher than the TNT levels in the water following the stronger blast.

An article detailing the team’s findings, "Ecotoxicological Risk of World War Relic Munitions in the Sea after Low- and High-Order Blast-in-Place Operations," appears in the ACS journal Environmental Science & Technology.