After the relevant results at the regional and departmental scale , we also monitored smaller watershed by collecting samples in Parisian sewers , to allow for a more precise description of the viral dynamics and allow for the detection of clusters . Viral concentrations in samples collected from the sewer system of the city of Paris are plotted in Fig . 3 . Results showed that the median of data taken from the sewer visually correlated with the city trends , emphasizing the possibility to refine the virus spreading in the area . Monitoring of individual sampling points showed some specific dynamics before the start of the second wave , demonstrating the possibility to detect local cluster using sewer monitoring ( lower panel ) . For example , an important concentration was locally detected during week 22 ( October 14 , 2020 ) , at the beginning of the second wave , probably due to local clusters . Same trend could be easily observed before the second lockdown .