Weathered oil is a term that is often used without communicating the degree of weathering and components that still remain. The subjective use of vague and often inaccurate weathering terms (i.e., lightly, moderately, heavily, or extensively weathered) often leads to misperceptions or misinformation about potential persistence and potential toxicity of oil residues observed in the environment. A semi-quantitative approach is needed for defining the stages of weathering, without adjectives, for oil released into the environment. MC252 oil from the Deepwater Horizon accident was evaluated to develop such an approach. The weathering of MC252 oil started immediately after release; during its 1,500 meter ascent to the sea surface most of the water soluble components partitioned into the water column. Once on the sea surface, the oil continued to weather via evaporation, dissolution, photo-degradation, and biodegradation. After reaching the shoreline, weathering continued and biodegradation became more pronounced. Extensively weathered and biodegraded MC252 oil samples have been collected on the shoreline and in some cases show a complete loss of resolved n-alkanes and near complete loss of PAH components, while still maintaining identifiable MC252 characteristics. At the same time that compounds have been lost due to degradation, others (e.g., some oxygenated compounds) have been formed as part of the weathering processes. The hydrocarbon chemistry data associated with more than 800 weathered MC252 oil samples were evaluated and quantitative metrics were developed to aid in defining the weathering stages. Sample GC/FID chromatograms were digitized and multiple chromatographic features (e.g., peak height to UCM height) were captured quantitatively. Together, the chemistry and chromatograms were analyzed to define seven (7) stages of weathering providing researchers with an objective means of discussing the weathering for oil after its release to the environment.

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