Combining flux monitoring and data reconstruction to establish annual budgets of suspended particulate matter, mercury and PCB in the Rhône River from Lake Geneva to the Mediterranean Sea.

Long term and high resolution data on water discharge, suspended particulate matter (SPM) and contaminant concentrations in rivers are required for a better understanding of particulate transfers from the continental areas to the seas. The aim of this study was to provide a novel estimation of annual fluxes of SPM and related pollutants in the Rhône River from Lake Geneva to the Mediterranean Sea by combining high frequency or time-integrative monitoring and novel relations between SPM concentration (Cs) and water discharge (Q). At six stations of the Rhône Sediment Observatory (OSR), SPM fluxes were calculated over the 2000-2016 period by combining observational data and Cs-Q relations. Monthly average concentrations of mercury (Hg) and PCB 180 were obtained by analysis of SPM samples collected in time integrative particle traps between 2011 and 2016. These pollutants were selected because of the well documented contamination of the Rhône watershed by these substances. Inter-annual fluxes at the Rhône River outlet averaged 6.6 Mt. yr for SPM, 572 kg yr for Hg and 14 kg yr for PCB 180. The Isère and Durance tributaries were found to be the main contributors of SPM fluxes. Annual SPM budgets were not balanced, suggesting deposition, remobilization of bottom sediments and/or contributions from non-monitored tributaries. The SPM sampled at the outlet was more contaminated than the combined SPM inputs from the monitored tributaries, suggesting that intermediate sources of contamination were not captured in the budget.