Monitoring the consequences of dam flushing operations on suspended particulate matter and associated contaminants in the Rhône River downstream of Lake Geneva

International audienceThe increasing need on energy and fluvial transport due to industrialization during the last century resulted in the construction of dams along rivers. To improve the production of electricity and avoid problems due to sedimentation, dam flushing operations are periodically organized. The Rhône Sediment Observatory investigated the impacts of such operations on suspended particulate matter (SPM) dynamics (resuspension and fluxes) and quality (physicochemical characteristics and contamination) during a flushing operation performed in June 2012 on 3 major dams on the Rhône River downstream the Lake Geneva (Verbois, Chancy-Pougny and Génissiat dams). The concentrations of several contaminants (hydrophobic organic contaminants, trace metal elements, radionuclides) as well as particulate organic carbon (POC) and particle size distribution were measured on SPM samples collected during this event at a permanent monitoring station (150 km downstream of the last flushed dam). Data obtained at this station from 2011 to 2016 were also used to compare the SPM and contaminant concentrations and fluxes during the 2012 dam flushing operations with those during flood events and baseflow regime. For the same water discharge, mean SPM concentrations during flushing were on average 6–8 times higher than during flood events recorded from 2011 to 2016. While of short duration (19 days), the flushing operations led to the massive resuspension of SPM and contributed to a third of the mean annual SPM flux. The SPM contamination was generally lower during flushing than during baseflow or flood, probably due to the fact that SPM during flushing operations is only issued from resuspended sediment, with no autochtonous particles nor recently eroded soil. The only exception are polycyclic aromatic hydrocarbons and bis(2-ethylhexyl)phthalate with higher concentrations during flushing, which must be issued from the resuspension of legacy-contaminated sediments stored behind the dams before the implementation of emission regulations. The observed increase of organically bound tritium is related to the resuspension of fine tritiated particles from watchmaking activities of the past century. During flushing, the variations of POC and contaminant concentrations are mostly driven by particle size