Assessing pollution in sediment and water before, during and after sediment dredging in a Mediterranean harbor

Marinas are semi-enclosed environments with low hydrodynamic energy, high sedimentation rates, and reduced oxygen levels, making them prone to contaminant accumulation. In the Mediterranean, where tidal amplitude is low, these conditions lead to infrequent dredging and prolonged pollutant buildup. This study assessed the impact of sediment dredging on trace metal elements (TMEs) and organotin compounds in the water column of a Mediterranean marina. A 21-month monitoring campaign was conducted, covering pre-dredging (12 months), dredging (4 months), and post-dredging (5 months) phases. Such long-term, high-frequency monitoring of the water column is rarely performed, particularly in marinas, where environmental assessments typically focus on sediments. The results revealed significant changes during dredging, including increased pH and oxygen levels, and frequent exceedances of turbidity thresholds set by French regulations. TME concentrations in the water column generally decreased during and after dredging, especially for arsenic (As), nickel (Ni), lead (Pb), and lithium (Li), while organotin levels remained stable. Aluminum (Al) concentrations stayed high throughout, and cadmium (Cd) and copper (Cu) showed temporary reductions that were not sustained post-dredging. In sediments, TME levels decreased, but organotin concentrations rose, particularly in technical zones. This study highlights the need to revise current dredging regulations by expanding monitoring beyond turbidity to include resuspended contaminants and hydrodynamic conditions. The use of predictive tools, adaptive management strategies, and collaborative approaches among scientists, port authorities, and regulators is essential to improve environmental risk assessments and promote sustainable dredging practices.