A simple protocol for estimating the acute toxicity of unresolved polar compounds from field-weathered oils

Abstract

Crude oil spilled at sea is chemically altered through environmental processes such as dissolution, biodegradation, and photodegradation. Transformation of hydrocarbons to oxygenated species increases water-solubility. Metabolites and oxidation products largely remain uncharacterized by common analytical methods but may be more bioavailable to aquatic organisms. Studies have shown that unresolved (i.e. unidentified) polar compounds (‘UPCs’) may constitute > 90% of the water-accommodated fraction (WAF) of heavily weathered crude oils, but still there is a paucity of information characterizing their toxicological significance in relation to other oil-derived toxicants. In this study, low-energy WAFs (no droplets) were generated from two field-weathered oils (collected during the 2010 Deepwater Horizon incident) and their polar fractions were isolated through fractionation. To allow establishment of thresholds for acute toxicity (LC₅₀) of the dissolved and polar fraction of field collected oils, we concentrated both WAFs and polar fractions to beyond field-documented concentrations, and the acute toxicity of both to the marine copepod Acartia tonsa was measured and compared to the toxicity of the native WAF (non-concentrated). The difference in toxic units (TUs) between the total of the mixture and of identified compounds of known toxicity (polycyclic aromatic hydrocarbons [PAHs] and alkyl phenols) in both WAF and polar fractions was used to estimate the contribution of the UPC to overall toxicity. This approach identified that UPC had a similar contribution to toxicity as identified compounds within the WAFs of the field-weathered oils. This signifies the relative importance of polar compounds when assessing environmental impacts of spilled and weathered oil.

Keywords:

• Chemical fractionation
• ecotoxicity
• oil weathering
• unresolved complex mixture
• acute toxicity
• toxic unit
• Deepwater Horizon
• Macondo

Acknowledgments

The authors acknowledge Kjersti Almås, Kristin Bonaunet, Inger Steinsvik, and Marianne U. Rønsberg for their valued assistance in the laboratory.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was conducted as part of the natural resource damage assessment for the Deepwater Horizon incident, with funding from Gulf Coast Recovery Office (GCRO) as part of the Deepwater Horizon Natural Resource Damage Assessment (DWH NRDA). Additional funding to prepare this research for publication was provided by the American Petroleum Institute (API).