The decanters used in sewage networks allow the sedimentation of rather coarse suspended solids, which are therefore not very favourable a priori for trapping organic contaminants. However, these decanters are located close to excretion areas, and above all upstream of wastewater treatment plants. This makes them ideal for trapping a wide range of organic molecules present in raw wastewater. Among these, tracers such as pharmaceuticals and illicit drugs have been used for some twenty years to recalculate usage in the wastewater collection zone. In this work, we used an epidemiological approach based on changes in the levels of various contaminants in sediments accumulated in a decanter draining a combined sewer system (wastewater and stormwater). The aim was to reconstruct changes in the historical consumption of around twenty illicit drugs and pharmaceutical products. Thanks to a thick accumulation of sediment, the decanter investigated, the Chambre à Sable d’Orléans, opened up a time window of potentially 70 years.
The sedimentary archive shows very significant vertical variations in the organo-mineral assemblages, with two main types of facies. The first, which is rather fine and organic, comes from sewage inputs. The second, coarser and less organic, comes from rainwater. This major difference between the two facies has a major impact on the affinity of organic contaminants for these solid matrices. The main result of this work was to show that the vertical evolution of contaminant levels was impacted more by the speciation of the molecules and their relationship with the facies than by consumption levels in the collection area. Thus, despite a common source for all these molecules, cationic compounds are distributed across both types of facies, while neutral and anionic compounds are almost exclusively present on organic facies. In order to be able to recalculate historical consumption of medicines and drugs, a first type of standardisation will have to be used (using organic carbon) for certain compounds. However, this will not be sufficient for all the molecules under consideration. So there is still work to be done to identify more precisely the factors controlling the distribution of cationic compounds in these sediments.
