Unexpected spatial impact of treatment plant discharges induced by episodic hydrodynamic events: Modelling Lagrangian transport of fine particles by Northern Current intrusions in the bays of Marseille (France).

Bertrand Millet,Ivane Pairaud,Christel Pinazo,Rémi Pagès,Daniela Banaru,Pierre Guiart,João Miguel Dias

doi:10.1371/journal.pone.0195257

Abstract

Our study highlights the Lagrangian transport of solid particles discharged at the Marseille Wastewater Treatment Plant (WWTP), located at Cortiou on the southern coastline. We focused on episodic situations characterized by a coastal circulation pattern induced by intrusion events of the Northern Current (NC) on the continental shelf, associated with SE wind regimes. We computed, using MARS3D-RHOMA and ICHTHYOP models, the particle trajectories from a patch of 5.104 passive and conservative fine particles released at the WWTP outlet, during 2 chosen representative periods of intrusion of the NC in June 2008 and in October 2011, associated with S-SE and E-SE winds, respectively. Unexpected results highlighted that the amount of particles reaching the vulnerable shorelines of both northern and southern bays accounted for 21.2% and 46.3% of the WWTP initial patch, in June 2008 and October 2011, respectively. Finally, a conceptual diagram is proposed to highlight the mechanisms of dispersion within the bays of Marseille of the fine particles released at the WWTP outlet that have long been underestimated.

Highlights

Large coastal cities are a significant source of marine pollution in the Mediterranean Sea
We considered the 2 sets of results computed by the RHOMA model corresponding to the 2 selected study periods, considered as representative of strong intrusions of the Northern Current (NC) on the shelf associated with S-SE and E-SE wind conditions
Our study takes into account the specific hydrodynamic context which is the only one allowing the transport of particles from the WWTP effluent towards the inner bays of Marseille: a period of intrusion of the NC on the continental shelf associated with a SE wind regime

Summary

Introduction

Large coastal cities are a significant source of marine pollution in the Mediterranean Sea. Metal and organic pollutants are discharged into the marine environment through the sewage system and from the surrounding catchments. Metal and organic pollutants are discharged into the marine environment through the sewage system and from the surrounding catchments They are released as diffusive inputs or during flood events. The city of Marseille has one of the largest Wastewater Treatment Plants (WWTP) in Europe, serving 1.7 million inhabitants and using both physical and biological treatment processes.

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Conclusion