Harbour sediments are very often contaminated. The contaminants may end in the harbour sediment by a number of routes, e.g. via industrial waste water, shipping, spillage during transhipment or via polluted water from rivers and streams. The contamination of dredged material is the result of current activities as well as poor environmental management in the past. Some contaminants are not at all or only poorly broken down. If the substances are only poorly soluble in water, they may attach firmly to sediment particles. Dredging and dispersion of polluted harbour sludges/sediment may contribute to the pollution of coastal waters.
The Water Framework Directive (WFD)
The main objective of the Water Framework Directive (WFD) is to obtain a good status of water bodies within 15 years. The details above make it clear that in order to achieve the goals of the WFD, it is necessary to look at sediments. The legislation within the EU will lead to improvement in water quality, leading in turn to a corresponding improvement in the dredged material. Regulation will prevent the dredging and deposit of polluted dredged material in the aquatic ecosystem.
Most often, the environmental regulations of the site in question require a documentation of the environmental impact of dredging and reclamation of harbour sludges/sediments. The costs of this documentation can be reduced by the use of numerical modelling of the spreading and deposition of sludges/sediment in the environment. In most cases, modelling is also applied to predict the biological and chemical impact of various dredging and reclamation operation scenarios.
Environmental modelling
The dredged sediment will be transported and diluted in the recipient. Furthermore, sedimentation and resuspension may occur. Simulation of these processes can be carried out by the DHI MIKE software which is a hydrodynamic model. The contaminants bound to the sediment will partly dissolve in the water during the transport and may degrade or transform, e.g. via biodegradation, oxidation and photolysis. The modelling of these processes can be made by formulating the fate processes in an ECOLAB template, which can be linked to the hydrodynamic models, thus simulating the transport, dilutions, sedimentation, dissolution and transformation reactions in one simulation. As a result of these simulations, the temporal and spatial variations of the concentration of the contaminants can be simulated.
The key output of the calculations is a comparison of the predicted environmental concentrations with the threshold concentrations. On the basis of this, the area of potential impact is identified, e.g. an evaluation of whether the health of ecosystems is at risk. Also, the most problematic contaminants are identified and the calculations can be used to define strategies for dredging, e.g. time and location for a minimum environmental impact, maximum amounts and concentrations of contaminants in the sediment to be dredged.
DHI has a decade of experience of environmental risk assessment of dredging and reclamation of harbour sediments and has participated in a number of studies on dredging in order to carry out an environmental impact assessment of the dredging of contaminated harbour sludge close to vulnerable areas important for e.g. fishery. For further information and references see the attached link.