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Methods were developed to estimate the potential impacts and natural resource damages resulting from oil spills using probabilistic modeling techniques. The oil fates model uses wind data, current data, and transport and weathering algorithms to calculate mass balance of fuel components in various environmental compartments (water surface, shoreline, water column, atmosphere, sediments, etc.), oil pathway over time (trajectory), surface distribution, shoreline oiling, and concentrations of the fuel components in water and sediments. Exposure of aquatic habitats and organisms to whole oil and toxic components is estimated in the biological model, followed by estimation of resulting acute mortality and ecological losses. Natural resource damages are based on estimated costs to restore equivalent resources and/or ecological services, using Habitat Equivalency Analysis (HEA) and Resource Equivalency Analysis (REA) methods. Oil spill modeling was performed for two spill sites in central San Francisco Bay, three spill sizes (20th, 50th, and 95th percentile volumes from tankers and larger freight vessels, based on an analysis of likely spill volumes given a spill has occurred) and four oil types (gasoline, diesel, heavy fuel oil, and crude oil). The scenarios were run in stochastic mode to determine the frequency distribution, mean and standard deviation of fates, impacts, and damages. This work is significant as it demonstrates a statistically quantifiable method for estimating potential impacts and financial consequences that may be used in ecological risk assessment and cost-benefit analyses. The statistically-defined spill volumes and consequences provide an objective measure of the magnitude, range and variability of impacts to wildlife, aquatic organisms and shorelines for potential spills of four oil/fuel types, each having distinct environmental fates and effects.

Natural resource damage assessment (NRDA) models for oil spills have been under development since 1984. Generally applicable (simplified) versions with built-in data sets are included in US government regulations for NRDAs in US waters. The most recent version of these models is SIMAP (Spill Impact Model Application Package), which contains oil fates and effects models that may be applied to any spill event and location in marine or freshwater environments. It is often not cost-effective or even possible to quantify spill impacts using field data collections. Modeling allows quantification of spill impacts using as much site-specific data as available, either as input or as validation of model results. SIMAP was used for the North Cape oil spill in Rhode Island (USA) in January 1996, for injury quantification in the first and largest NRDA case to be performed under the 1996 Oil Pollution Act NRDA regulations. The case was successfully settled in 1999. This paper, which contains a description of the model and application to the North Cape spill, delineates and demonstrates the approach.