Using a new dispersed oil model to support ecological risk assessment

Abstract

A new model is being used to support dispersant Ecological Risk Assessment (ERA) workshops. User-driven output includes trajectory maps for both chemically dispersed and undispersed oil, and concentration isopleths reported by depth and over time. To help make toxicological sense of the output, oil concentration isopleths were nominally fixed at concentrations and exposure times of concern developed by consensus during past ERA workshops. Two No. 6 fuel spill scenarios, each with alternative outcomes (not dispersed vs 80% dispersed) were developed, one in open ocean water (10,000 bbls spill), and the other in an estuary (2000 bbls spill). Plume epicenter maximum dispersed oil concentrations peaked in the range of 10-20 ppm but decreased within 24 hours to 1-2 ppm or less. Average concentrations in the most contaminated portions of the dispersion area never exceeded 3 ppm in either scenario. Plankton in a small (< 25%) fraction of the open ocean plume were at moderate risk at 24 hours. These effects must be compared to those of the non-dispersion alternative, which could impact wildlife and shorelines.