Modelling the sensitivity to various factors of shipborne pollutant discharges

Most of the marine pollution attributable to ship actions is associated with the illicit discharge of oily residues or ballast water, in what is commonly termed operational pollution. In the particular case of ballast water, careless disposal can lead to severe ecological damage, including the irreversible establishment of communities of invasive species. In its 2004 Convention on ballast water management, the International Maritime Organisation defined the measures to be followed to prevent these environmental issues, including the exchange with mid-oceanic water at a minimum distance from the shore. This paper uses two numerical models to analyse the effects of several factors on the dilution of a discharge from a ship moving in the Spanish Mediterranean waters, in order to determine under which conditions the environmental impact of the discharge will be minimum. The results highlight the dual role played by a freshwater plume, which may contribute either to decrease or to enhance the onshoreward transport of the discharged substance, and the relevance of the ship's speed in determining the dilution rates.