The public prosecution department is taking legal action against gas and oil company NAM for injecting polluted water into an empty underground gas chamber in the far north of the Netherlands.

NAM, a joint venture between Shell and ExxonMobil, is suspected of mixing various types of waste water together without having the proper permits and of committing an environmental crime, the public prosecutor said on Wednesday.

One of the elements in the water is likely to be mercury, found in waste water produced at a NAM gas refinery in the North Sea.

The water, from at least four sources, has been deposited in the Borgsweer gas field by NAM for the past 12 years.

The department said it currently had no indications that this way of discharging waste poses a danger to people and the environment, or increases the risk of earthquakes.

Nevertheless, the investigation is continuing and the prosecutor hopes to take the case to court next year.