A data-driven framework for defining stages of oil weathering

Oil weathering is often described subjectively after a spill. Adjectives like "moderate" and "severe" help define the extent of oil loss but fail to communicate quantitatively and reproducibly the degree of weathering. The use of subjective weathering terms often leads to misperceptions about persistence and toxicity of oil residues in the environment. The weathering of MC252 oil from the Deepwater Horizon spill started immediately after release during the 1500-m ascent to the sea surface and continued as it was transported on the surface and reached the shoreline. Weathering processes included evaporation, dissolution, photo-degradation, and biodegradation, among others. With extensive sample collection and detailed chemistry and source fingerprinting analyses, the Deepwater Horizon data provide a unique opportunity to evaluate weathering processes semi-quantitatively. An objective method of defining the degree of oil weathering is developed based on the detailed chemical results for over 700 MC252 oil samples from the environment.