Modeling Impacts and Tradeoffs of Dispersant Use on Oil Spills

Successful application of dispersants can reduce oil-spill impacts to wildlife and shoreline habitats, with the tradeoff of dispersed oil potentially causing impacts to water column organisms. Oil-spill fate and transport modeling was used to evaluate the maximum potential water column hydrocarbon concentrations and impacts of oil spills with dispersant use in offshore waters. The model estimated expected concentrations in the surface mixed layer for the largest potential volume of oil that could be dispersed in U.S. waters at any one location and time, that dispersed by a single sortie of a C-130 aircraft assuming a 20:1 oil to dispersant ratio and 80% efficiency (378.5 m3 of light Arabian crude oil). For this oil volume and no dispersant, wildlife impacts would occur on the scale of 100 s km2, whereas upper (<20 m) water column effects with 80% dispersion in one contiguous area would occur on the scale of 1 km2. The model results for these offshore scenarios show that the tradeoff of decreasing wildlife impacts with dispersant use at the expense of possibly increasing water column impacts is supportive of dispersant use. The exceptions would be when sensitive water column biota are present in surface water under the slick and in shallow (<10 m) confined water bodies where dilution would be slower.