Do storm event samples bias the comparison between sewer deposits contribution

Previous researches demonstrated the occurrence of unique in-sewer sediment, the organic layer, on the Marais site in Paris capable of explaining the entire wet weather sewer production of suspended particles. Other studies on other sites, Clichy in Paris and Ecully in Lyon, demonstrated a comparable wet weather sewer deposits contribution (SDC) to effluents pollution between this site and others having no similar organic sediment casting therefore doubts on the implication of the organic layer to the outlet discharges pollution. So, an in-depth comparative investigation of the different sites9 means SDC was carried out to confirm or infirm the major role of this layer vis-a-vis sewer production. The size and characteristics of the events9 sample used to calculate the SDC were analyzed to find whether a statistical bias may have masked a difference that would be more coherent with field observations. After homogenizing these elements, the organic layer regained some of its previously alleged participation in sewer contribution (a maximum of 36% of the total SDC) but another unknown source was still dominant. This suggests that sewer production during wet weather is a result of multiple sediment erosion: the organic layer and another major source not yet identified.