From Dredging to Restoration: Enhancing Coastal Ecosystems with Reused Sediments

Port areas, located at the land-sea interface, are crucial coastal socio-ecosystems exposed to local pollution and terrigenous inputs from watersheds. Silting and sedimentation in ports pose complex management challenges. It is essential to understand port environments, which requires regular monitoring of water and sediment quality. In-depth knowledge, especially under varying hydroclimatic conditions, helps better understand and manage the effects of dredging operations, including the potential reuse of sediments based on their quality.

Special attention has been given to data acquisition and comparing dredging operations at two Mediterranean observatories: the Ghar El Melah lagoon (GEM) in Tunisia, an UNESCO-IHP Ecohydrology Demonstration Site through the Observatory of MEdjerda LIttoral (OMELI) and the Gulf of Aigues-Mortes (GAM) (France), studied by the Human-Environment Observatory Mediterranean Coastline (OHM-LM). Research at these sites focuses on knowing, understanding, and better managing coastal ecosystems. Extensive fieldwork, including spatio-temporal monitoring during various cycles and dredging operations, has been conducted between 2019 and 2023 to determine chemical contamination and characterisation of water and sediments. The resulting database, analyzed with chemical tracers, helps identify major contamination sources, understand contaminant dynamics and distribution and sediment proprieties. A crucial aspect of this work involves the precise characterization of sediments. detailed characterizations of dredged sediments (composition, classification, distribution, ...) open up opportunities for valorization.

At the GEM lagoon site, sampling campaigns revealed heterogeneous trace metal contamination in the water column. High arsenic concentrations, exceeding environmental quality standards (EQS), were systematically observed. The northeast zone showed very high copper levels (7 times the EQS), and most water samples had high zinc concentrations. To assess contamination profiles and sediment quality for potential reuse in beach nourishment, sediment toxicity was evaluated using ERL (Effects Range Low) and ERM (Effects Range Median) calculations. Very high cadmium, copper, lead, and zinc levels were found in sediments near ports, likely due to active navigation and high hydrodynamic processes. These spatio-temporal data coupled with physical and mineralogical characterizations will allow for the selection of the most suitable sediments for dune cordon replenishment.

In the GAM case, monitoring of chemical contamination in several ports was done before, during and after dredging. Dredging increased chemical concentrations in the water, resuspending trace metals (As, Cu, Zn, Ni) and organotin compounds from sediments. Sediments in concrete formulation was explored. Concrete formulated with 15 or 30% sediments showed properties like the control. Finally, the potential release of contaminants was studied through dynamic leaching tests. While surface leaching followed by diffusion for certain metals was observed, the released quantities remained very low and all below environmental quality standards. These concretes were tested in maritime structures.

This approach aims to promote the rational reuse of dredged materials in maritime construction, fostering sustainable and equitable management practices. This includes circular economy principles and nature-based solution, such as beach nourishment, contributing to ecosystem restauration. These efforts are crucial for the sustainable management of port ecosystems, balancing economic development and environmental preservation. The results guide port managers’ decisions, considering meteorology, port usage changes, occupancy and traffic.