Modelling potential impacts of effective dispersant use on aquatic biota

Oil spill transport and fate modelling was used to evaluate maximum potential water column hydrocarbon concentrations with dispersant use. Oil spill modelling was performed with SIMAP, and used wind data, current data, and transport and weathering algorithms to calculate the mass of oil components in various environmental compartments; oil pathway over time; surface oil distribution; and concentrations of the oil components in water and sediments. Simulations assuming no dispersant use were compared to simulations where dispersant was applied after 8 or 16 hours of weathering for 2 wind conditions. With 2.5 m/s winds and no dispersant, the concentration plume was relatively small and short-lived. In 7.5 m/s winds, natural dispersion was considerable, and the application of dispersant at 80 per cent efficiency during the same wind conditions increased the volume affected by a factor of 4-6. In 2.5 m/s winds, dispersant use at 80 per cent efficiencies decreased the water volume affected proportionately. Results indicated that the highest water column impacts occurred when chemical dispersant was applied under light wind conditions where dilution was relatively slow. For the oil volume examined and assuming no dispersant use, wildlife impacts would occur on the scale of 100s km2, whereas water column effects with dispersant use in a worst case scenario would occur on the scale of 1 km2 in the upper mixed layer. An exception to the support of dispersant use would be if sensitive water column biota are present in the area of the slick. The conclusions applied to unconfined water bodies that were at least 10 metres deep. 18 refs., 4 tabs., 5 figs