Composition of weathered oil residues: changes with depth and toxicological implications

Residues from oil spills are deposited in coastal environments and can persist for decades in the environment. Following an oil spill, analysis of homogenized oil residues scraped off rocks and debris from coastlines is common practice. This analysis of bulk oil samples may not accurately reflect the environmental availability of the oil components present in the residue. Weathering processes such as evaporation, dissolution, photodegradation, and biodegradation can lead to the formation of a visually distinct skin on the oil surface, an observation we term differential weathering of an oil residue. This distinct surface layer may lead to physical shielding of the bulk oil below resulting in differences in the chemical composition between the surface and bottom layers of a residue, which may have toxicological implications. Here we assess the implications of traditional bulk analysis methods by sampling oil in vertically sliced sections to examine the chemical differences between surface and deeper layers of a simulated heavy fuel oil residue. We used gas chromatography with flame ionization detection and mass spectrometry to quantify polycyclic aromatic hydrocarbon (PAH) and total petroleum hydrocarbon (TPH) concentrations in each layer. Surface layers exhibited more extensive weathering compared to bottom layers in a more viscous oil residue. Lower viscosity oil residues did not exhibit this differential weathering. Fourier transform infrared spectroscopy and elemental analysis was used to explore oxidation with depth in oil samples. A toxicity assay using water accommodated fractions from oil residues was used to examine changes in the lethality of weathered oil with depth. Oxidation was observed in all irradiated oil samples with greater oxidation observed in surface layers. Oxidation did not translate into differences in toxicity to the marine algae Isochrysis galbana. Future work will explore differential weathering over a broader range of oil viscosities and include expanded toxicity assays at more environmentally relevant concentrations.