A French collaboration involving the LSCE has studied an atypical sediment archive: the material accumulated between May 2015 and March 2016 in a settling basin in the Orléans sewerage system. Using original markers, they were able to link the content of these sediments to rainfall events during this period.
Sediment cores taken from the bottom of lakes and rivers can be used to ‘read’ past changes in climate and ecosystems. Similarly, sediments accumulated in sewers can be seen as archives of the urban environment in our time, marked by the imprint of Man (Anthropocene). This is the guiding idea behind the Golden Spike research project, carried out in 2017-2018 in Orléans, with the support of Orléans Métropole.
The scientists took advantage of the renovation work on the Orléans ‘sand chamber’. This structure is used to settle particles carried by wastewater and rainwater upstream of treatment plants. Between two sediment-cleaning operations, a 1.5 m long core was taken, corresponding to almost a year’s accumulation of matter.
It shows clear changes, with fine, organic sediments at the base (the oldest), followed by coarse, mineral sediments and fine, mineral sediments at the top (the most recent). To understand the origin of these different compartments, the researchers used original markers: faecal tracers (bile acids), radioelements and glass microbeads, which are used in the composition of white road marking paints to increase their reflectivity (pedestrian crossings, traffic lines).
Fecal markers were very abundant in organic sediments, indicating sewage input. Conversely, glass microbeads were mainly found in mineral sediments, indicating an input of rainwater.
The researchers then compared their sequence of sediments with changes in water flows in the network and with rainfall events. In the absence of rainfall, the material deposited in the sand chamber is mainly wastewater, loaded with organic matter. Conversely, during heavy rainfall, coarse mineral particles from the washing of pavements and pavements predominate.
According to the study, these atypical archives respond very directly to meteorological events affecting the urban environment. In particular, rainfall disrupts the regular sedimentation of wastewater, carrying away and depositing highly distinct materials, in the same way as floods in lakes.
The rest of the work will be devoted to the deposits accumulated over more than thirty years in the sand chamber and saved before the renovation work. Other markers – medicines in particular – will enable us to trace certain elements of the daily lives of urban populations.
The collaboration brings together the Institut des Sciences de la Terre d’Orléans (CNRS-Université d’Orléans-BRGM), the ‘Environnements, dynamiques et territoires de la montagne’ laboratory (CNRS-Université Savoie Mont-Blanc), the LSCE (CEA-CNRS-UVSQ) and the Bureau de recherches géologiques et minières.
Jacob, J., Thibault, A., Simonneau, A., Sabatier, P., Le Milbeau, C., Gautret, P., Ardito, L., Morio, C., 2020. High-resolution sedimentary record of anthropogenic deposits accumulated in a sewer decantation tank. Anthropocene 30. doi: 10.1016/j.ancene.2020.100238 – insu-02507329
See also the photo report in “Le Journal du CNRS” : « The astonishing memory of wastewater ».
