High levels of a chemical found in oil and gas waste have been identified in shellfish living downstream from past fracking waste disposal sites.

Freshwater mussels filter water to feed. As they do so, they accumulate substances in their hard shells, effectively providing a record of any contaminants entering the water.

Researchers studying these creatures in Pennsylvania found strontium in their shells, a discovery they say demonstrates the long-lasting effects fracking can have on surrounding environments.

After scientists found that wastewater was contaminating waterways with toxic and naturally-occurring radioactive elements, the Pennsylvania authorities called on the industry to begin recycling its waste in 2011.

However, the mussels analysed by a Pennsylvania State University team still bore evidence of the three-year fracking boom that preceded those restrictions, when 2.9 billion litres of wastewater were dumped in the state’s rivers.

These contaminants did not vanish from the shells after 2011, suggesting strontium and various toxic contaminants were lingering in the environment.

“Freshwater pollution is a major concern for both ecological and human health,” said Professor David Gillikin, a geologist at Union College and co-author on the study. ”Developing ways to retroactively document this pollution is important to shed light on what’s happening in our streams.”

Publishing their findings in the journal Environmental Science and Technology, Prof Gillikin and his fellow authors suggested that animals like freshwater mussels could be used to monitor levels of contaminants and potential leakages.

Anti-fracking campaigners protest outside of Royal Courts of Justice for activists in Preston prison for obstructing Cuadrilla activities

“Like tree rings, you can count back the seasons and the years in their shell and get a good idea of the quality and chemical composition of the water during specific periods of time,” said his colleague Professor Nathaniel Warner, an environmental engineer at Pennsylvania State University.

The team now intend to investigate the soft tissues of these creatures to determine whether they contain harmful chemicals that could be passed on to fish and mammals that feed on them.

With fracking accounting for two thirds of US gas production and half of its oil production, the scientists warned that the industry must be careful in its management of these substances.

“The wells are getting bigger, and they’re using more water, and they’re producing more wastewater, and that water has got to go somewhere,” said Prof Warner. ”Making the proper choices about how to manage that water is going to be pretty vital.”

Fracking operations recently begun for the first time in seven years in the UK, as the oil and gas company Cuadrilla announced it had begun drilling despite local protests.