Quantifying the effects of sunlight on the fate of oil spilled at sea

Danielle Haas Freeman, PhD., 2024
Collin P. Ward, Advisor

Oil spilled at sea is transformed by sunlight-driven photochemical reactions. The transformed oil has different properties and behavior in the environment compared to the fresh oil, resulting in different fates and effects. My work in this thesis was to put numbers on these changes, with the goal of better predicting where oil goes and how it behaves in diverse spill scenarios. First, I focused on how sunlight generates water-soluble compounds from oil, which can lead to the dissolution of oil-derived compounds in seawater (photo-dissolution; Chapter 2). To find out whether photo-dissolution could be an important fate process during an oil spill, I used a combination of experiments and photochemical rate modeling to calculate photo-dissolution rates for the 2010 Deepwater Horizon spill (DwH) in the Gulf of Mexico (GoM). I found that photo-dissolution likely converted ~10% of the floating surface oil to dissolved organic carbon during DwH, a fraction similar in magnitude to other well-recognized fate processes. Moving beyond DwH, I evaluated the sensitivity of oil photo-dissolution and photochemically-altered oil physical properties to temperature. I found that if a spill like DwH had occurred in 5°C water rather than the exceptionally warm 30°C water of the GoM, 7x less oil could have dissolved via photo-dissolution and the viscosity of the remaining insoluble oil could have been 16x higher, resulting in lower entrainment of oil into the water column as small droplets (Chapter 3). The net result is that more oil would stay at the sea surface in a cold-water spill. Finally, I determined photo-dissolution rates for diverse oil products beyond the light crude that spilled during DwH (Chapter 4). I found that oil photo-reactivity could be predicted from oil chemical composition. I also found that photo-dissolution likely affects oil mass balance in spills of light oils forming thin slicks but not in spills of light or heavy oils forming thick slicks. Overall, this work advances our understanding of how oil changes in the environment upon sunlight exposure. This information can be applied to better predict, evaluate, and mitigate the effects of oil spilled at sea on marine ecosystems, including humans.