Substance unknown to science identified in tap water

After eluding chemists for 40 years, a mystery compound found in drinking water has finally been identified.

The compound – called chloronitramide anion – is previously unknown to science.

While it’s important to research the toxicity of the compound, independent experts say it’s no reason to avoid your tap water.

The compound forms, according to a paper in Science, as a by-product of common disinfectants called chloramines.

The mystery molecule has a very simple structure, by chemical standards: Cl-N-NO₂^–.

The researchers identified it in 40 samples of tap water from the USA, at an average concentration of 23 micrograms, or 0.00023 grams, per litre.

“This compound will be present in any chloramine system,” lead author Associate Professor Julian Fairey, a civil engineer at the University of Arkansas, USA, tells Cosmos.

“However, it could also form in systems that use chlorine or chlorine dioxide in which ammonia-nitrogen or other nitrogen-containing compounds are present.”

The compound was first detected more than 40 years ago, as an unidentified signal from an analysis using UV light.

Fairey began trying to unmask it 10 years ago. The tiny molecular size of the compound made it hard to pin down.

“It does seem like a long time coming!” he says.

“However, chloramine chemistry is complicated and until our recent work, the formation pathway of this mysterious compound was not well understood.”

The researchers were able to identify it by making it from compounds used to disinfect water, and performing a number of analytical techniques to show it was the same thing as the mystery water feature.

Because no-one knew the compound’s identity, it’s not yet clear if it’s dangerous to human health. But computer modelling suggests it could be problematic.

“We plan to collaborate with toxicity experts to help assess the public health relevance of this compound,” says Fairey.

“It’s widely acknowledged that we don’t know the precise chemical identity of all the substances that form as byproducts from drinking water disinfection,” Professor Stuart Khan, a civil engineer at the University of Sydney, tells Cosmos.

Khan, who was not involved with the study, says that it would be useful to have occurrence and toxicity data for the compound.

“It’s not surprising – and it’s potentially very helpful – to see the science chugging away at gradually identifying more and more of the components,” he says.

“I agree that a toxicological investigation of this would be useful now that we know its identity, but I am not overly worried about my tap water,” says Professor Oliver Jones, a chemist at RMIT University, who also wasn’t involved with the research.

“The compound in question is not newly discovered, just newly defined.”

Fairey says that agencies need to develop analytical techniques for finding Cl-N-NO₂^– in drinking water, so it can be monitored.

“We also need to advance formation methods for this compound to generate sufficient quantities for health effects studies,” he says.

Khan adds that disinfecting drinking water “saves millions of illnesses and deaths”.

“The very low levels of toxicity that have been reported for disinfected drinking water are many orders of magnitude safer than a situation we would have without disinfection,” he says.

“I used to work in a lab in the US that had a sign on the wall that read ‘drinking water disinfection has allowed millions of people to live long enough to worry about disinfection byproducts’.

“That sign was, and remains, the truth!”