French researchers have found that underground geological formations can act as historical archives for heavy-metal pollution.
What did the Paris environment look like hundreds of years ago? For Emmanuel Dumont the answer might be found in a maze of aqueducts built in the Middle Ages under Belleville, a neighbourhood in northeastern Paris. “We’re in the presence of a construction that has not changed for centuries,” he says. “And the water is still running.” A hydrogeologist at the Centre for Risk, Environment, Mobility and Development, Dumont believes the sediments on the aqueduct walls hold the clues.
Dissolved limestone builds up over time into stalactite-like geologic formations called speleothems. Each successive layer traps traces of elements carried by water from the surface through the soil, taking a rock-solid snapshot of a given moment. Scientists have long used speleothems as proxies for atmospheric carbon dioxide or sulphur from volcanic eruptions. But formations were only recently found to also retain evidence of human activity, like coal burning in Italy and pre-Roman lead mining in Great Britain.
The Parisian team is the first to use urban speleothems to measure levels of heavy-metal pollution. In samples from the walls under Belleville, they have identified aluminium, copper, cadmium, magnesium and lead going back 300 years. Their analyses show that lead levels were high in the 18th century, then dropped before picking back up in the mid-20th century.
These findings have surprised historians who study Paris’s pollution. The lead might have come from local artisans who used the soft metal to craft caskets and other funeral accessories. Another source could be city waste: In the 18th century, a garbage dump in nearby Montfaucon provided fertiliser for the surrounding farming activities.
Paris’s underground could hold many other similar speleothems. “We can keep going further back, beyond the Middle Ages,” said Edwige Pons-Branchu, who leads the project. “There are even traces of Roman aqueducts underneath Paris. If we could find a sample from that, it would just be too beautiful.”