Photochemical Oxidation of Oil Reduced the Effectiveness of Aerial Dispersants Applied in Response to the Deepwater Horizon Spill

Abstract

Chemical dispersants are one of many tools used to mitigate the overall environmental impact of oil spills. In principle, dispersants break up floating oil into small droplets that disperse into the water column where they are subject to multiple fate and transport processes. The effectiveness of dispersants typically decreases as oil weathers in the environment. This decrease in effectiveness is often attributed to evaporation and emulsification, with the contribution of photochemical weathering assumed to be negligible. Here, we aim to test this assumption using Macondo well oil released during the Deepwater Horizon spill as a case study. Our results indicate that the effects of photochemical weathering on Deepwater Horizon oil properties and dispersant effectiveness can greatly outweigh the effects of evaporative weathering. The decrease in dispersant effectiveness after light exposure was principally driven by the decreased solubility of photo-oxidized crude oil residues in the solvent system that comprises COREXIT EC9500A. Kinetic modeling combined with geospatial analysis demonstrated that a considerable fraction of aerial applications targeting Deepwater Horizon surface oil had low dispersant effectiveness. Collectively, the results of this study challenge the paradigm that photochemical weathering has a negligible impact on the effectiveness of oil spill response and provide critical insights into the "window of opportunity" to apply chemical dispersants in response to oil spills in sunlit waters.

Influence of Climatic Parameters on Changes in the Density and Viscosity of Diluted Bitumen after a Spill

“…However, its influence on the changes in the physical properties of Heidrun was limited, since solar radiation decreased over the length of the experiments conducted in both seasons. The results for Heidrun are substantiated by the work of others, where photo-chemical weathering had a major influence on the rates of changes in the physical properties of a conventional crude oil spilled in water [10].…”

“…The warmer air temperature in summer caused evaporation of volatile chemicals (<C10), resulting in thicker more viscous oil slicks. A recent lab study [10] reported that both evaporation and photo-chemical oxidation result in increases in the density and viscosity of oil, but the increases were predominately higher for oil exposed to solar radiation. However, AWB contains condensate as the diluent, a liquid product of natural gas extraction.…”

“…The most important of these physical processes affecting oil spills on the water surface is evaporation [8], which occurs in the first few days after a spill and accounts for a 5 to 75% loss of the mass of the spilled oil, depending on the oil type and environmental conditions [9]. The chemical process of photo-oxidation has emerged to be an equally important process to evaporation; a recent lab study has shown that exposure of oil to solar radiation can cause photo-chemical weathering of oil, causing significant changes in the chemical composition of light crude oil within a few days [10]. In addition, based on photo-oxidation of oil under ultraviolet irradiation conducted in the lab, Appeli et al [11] reported that the partial photo-oxidation of oil on the water surface led to 50% of the weathered saturates and aromatics being transformed to oxygenated hydrocarbons within 10 days during the Deepwater Horizon oil spill.…”

Oxygen Isotopes (δ18O) Trace Photochemical Hydrocarbon Oxidation at the Sea Surface

“…Collectively, δ 18 O is a novel approach to track the oxidative fate of hydrocarbons released to aquatic systems, which may improve our capacity to respond effectively to future accidents (Ward, Armstrong, et al, 2018). In addition to improved sensitivity, δ 18 O may provide a means to disentangle the relative importance of pathways that can simultaneously weather hydrocarbons in surface waters, such as evaporation, photochemical oxidation, and biological oxidation.…”

“…Second, the kinetic isotopic fractionation of dissolved O 2 photochemically incorporated into hydrocarbons ( 18 ε oxidation = −9‰) appears to be independent of OC physical and chemical properties, raising the possibility that a variety of OC types (e.g., crude oil and DOC) share a common step in the photochemical oxidation mechanism. Collectively, δ 18 O is a novel approach to track the oxidative fate of hydrocarbons released to aquatic systems, which may improve our capacity to respond effectively to future accidents (Ward, Armstrong, et al, 2018).…”