Impact of wind-driven circulation on contaminant dispersion in a semi-enclosed bay

Mediterranean semi-enclosed bays are often exposed to high levels of contaminants originating from anthropogenic activities in the bay. To assess their fate and impact on the environment, it is essential to investigate coastal circulation regimes which may play an important role in the dispersion of contaminants across the bay and beyond. In this study, a high resolution coupled hydrodynamic - passive tracer model was combined with ADCP observations, to identify major circulation patterns and associated dissolved contaminant dispersion pathways in the contaminated semi-enclosed bay of Toulon (South of France, NW Mediterranean Sea). Two dominant circulation patterns and two derived ones could be identified, driven by winds (Mistral and easterly winds) and offshore water intrusions. Medium to strong Mistral events (> 6 m s) with a WNW direction cause a bi-layer pattern with surface waters flowing out of the bay and marine waters entering at depth. Less frequently, west Mistral winds of medium to strong strength (> 6 m s) may generate an anticyclonic circulation. During easterly wind conditions (> 6 m s), an inward flow can be observed which is sometimes reinforced by offshore water intrusions, probably from the local boundary current, the Northern Current (NC). Furthermore, dissolved contaminant dispersion pathways were simulated under typical wind forcing conditions with three point sources of copper (Cu) that were identified based on surface Cu observations. While most of the WNW Mistral wind events transport dissolved copper plumes across and out of the bay, contaminant dispersion can remain confined to the bay under certain west mistral conditions. Conversely, during easterly wind events, contaminants are exiting the bay as a narrow vein along the Saint-Mandrier peninsula, before probably converging on the NC offshore. Accordingly, this study demonstrates the important impact of hydrodynamic-driven processes on the dispersion of contaminants within a semi-enclosed bay.