Urban sediments, the memory of a city

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Who would have thought that the sediments piled up in the city sewers would one day have such a documentary value for researchers and public decision-makers? In Orléans, Jérémy Jacob, a CNRS researcher, followed this track in 2014, going to tap into the urban lowlands, exhuming the sludge and particles accumulated in the city’s sanitation networks. For four years, accompanied by a team of researchers (read the benchmarks), he carried out a vast coring operation, carried out on the banks of the Loire, in the city center. Precisely, in the “sand chamber”, where the domestic wastewater and rainwater end up. Surely he would find in this matter the archived translation of our daily newspapers, more exhaustive than all databases and scientific collections assembled. A way, in short, of ” read the memory of the city ».

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The researcher, who has long studied sediments at the bottom of lakes, tracing climate change and its impact on vegetation “Over periods ranging from tens of millions to more recent, over the last ten thousand years”, wanted to move on. By approaching contemporary reality, he had the feeling of connecting his ” life social and family » to his scientific research, meaningful.

To establish his project in Orleans, he was inspired by the work of counterparts Swiss and Australian, who had launched before him “Epidemiological studies based on the presence or concentration of molecules – especially drugs – in wastewater, with the idea of ​​observing the health of populations on a daily basis”. And, prior to this coring operation, he measured, by positioning himself upstream of a treatment plant in the Center-Val de Loire, “The levels of drug and medication use, their variability on a weekly scale”, which he was able to compare with data from other population basins or other urban contexts.

Long-term information

The study of sludge and particles, which representatives of the sewerage networks see as ” a problem, forcing them to regular cleaning operations “, constitutes ” a gold mine “ of information which the researcher echoed in an article published in the journal Anthropocene. Providing information ” deferred ”, the examination of sediments provides a long-term perspective, while that of wastewater provides day-to-day data, “Much like 24 hour news channels”, he compares.

In this urban environment, where ” a large number of individuals, who consume very differently “, sewers are full of molecular traces characteristic of their lifestyles. Here are found ” excreted markers ” of rare diversity: ” This ranges from what we eat – from meat, wheat, sugar cane or brassicasterol (a molecule characteristic of cabbage) – to textile hemp, nicotine, caffeine, traces of medication or drugs , up to molecules that come from the road, oils and oils from cars ”, lists this geologist by training.

Nothing escapes this unalterable “filter”. Our urban basements store what consumer society generates with ” the explosion of plastics and plastics, a specificity of the XXe and XXIe centuries ”. While these materials have recently appeared, diversity is not new. The study of septic tanks in medieval cities reveals the same accumulation of sediment ” from such a wide variety of compounds and materials. “

By looking for “tracers” – such as microbeads of white paint from pavements or organic matter residues – they were able to determine precisely what each of the superimposed layers of sediment reveals. ” In Orleans, which has a unitary sanitation system, mixing wastewater and rainwater, it is above all the latter that will wash the sidewalks and the pavement and which will bring coarse particles into the sandbox “, add the researcher. On the other hand, in calm periods, without rain, it accumulates in this last sand chamber, ” organic matter, which comes mainly from waste water from showers, toilets, sinks. ”

Useful measures for society

This research has had a very concrete impact. Thanks to these measures, operators can identify the accumulated materials and, if necessary, anticipate or adapt their treatment. ” Depending on weather events, we can therefore rather have mineral or organic matter, with varying levels of drugs and excess. So when the city of Orléans has to clean up its settling tanks, it will be able to direct the sludge towards the right reprocessing process. “

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The monitoring of the daily evolution of the life of the city in the sewers is also of interest for public policies. For Jérémy Jacob, there are not many indicators ” as integrative, as spatial information as the sewers to know how the city is doing. For example, in such a neighborhood, we can sound the alarm because the population ingests more antidepressants than the average. ” The sewer survey was even proposed at the height of the coronavirus crisis to assess the proportion of the population infected with the disease at the level of a city or a district.

Health issues are at the heart of this approach. In a future publication, Jérémy Jacob wishes to highlight ” drug measurements in the sediment to see how it stores the health information it gets. “

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Many laboratories involved

For this study, Jérémy Jacob, CNRS researcher and geologist by training, has partnered with other laboratories, such as the Orléans Earth Sciences Institute, the Edytem laboratory in Chambéry, the Orléans Institute of Organic and Analytical Chemistry, l ‘Métis mixed research unit (Environmental environments, transfers and interactions in hydrosystems and soils) in Paris.

Coring operations in Orleans were carried out by the C2FN (National Coring and Drilling Center).

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