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Dissolving oil in a sunlit sea

I’m excited to share the results of a research project I’ve been working on in grad school for a couple of years now, about what happens to oil after it is spilled at sea (its “environmental fate”). I’ve been interested in the fate of oil from the 2010 Deepwater Horizon spill in the Gulf of Mexico for a while now, and my work with Dr. Collin Ward at WHOI has focused on this puzzle. The results of our latest project, published in Science Advances in Feb 2022, gives us another puzzle piece. Most exciting, however, is that it also provides data that will inform our understanding of future oil spills. Here’s the story:

Deepwater Horizon

In April of 2010, the Deepwater Horizon oil rig, located off the coast of Louisiana, experienced a catastrophic fire. Eleven people working on the rig died in the explosion. Unseen, about a mile below this human tragedy, the Macondo oil well connected to the Deepwater Horizon rig (by a long, long pipe) ruptured, releasing barrel upon barrel of oil and natural gas into the Gulf of Mexico. The location of the oil release, a mile deep, underwater, at high pressure, meant that emergency responders didn’t have a preexisting blueprint for how to the stop the spill. So, for eighty-seven days, the ill-fated, aptly-named Macondo oil continued to flow into the Gulf, with about half of it staying underwater (the “plume”) and half of it floating up to the sea surface and forming miles of oil slicks. By the time the well was plugged, 5 million barrels of oil had been released into the Gulf. (Side note: it is sometimes erroneously reported that the Deepwater Horizon spill was the largest in marine history. This is not true. It was the largest in US history. Regardless, it was a lot of oil.)

Oil fate and sunlight

What happened to all the oil? Twelve years later, scientists are still puzzling out the answer. Some pieces of the puzzle are well recognized. For example, we know that some of the oil evaporated off of the sea surface into the atmosphere. We know that a lot of the underwater plume oil was eaten by microbes (biodegradation).

Our project gives a size and a shape to another piece of the puzzle: the transformation of crude oil by sunlight-initiated chemical reactions into new compounds that dissolved in seawater, acting as a removal process from those floating surface oil slicks.

How and why would this happen? You might know that oil and water don’t mix. This is why floating oil slicks form on the sea surface in the first place. The rainbow oil sheens you might see in a puddle near a gas station or on a city street are another example of this phenomenon; you can also pour some vegetable oil into a cup of water in your kitchen as a demonstration to yourself. But sunlight exposure can spark chemical reactions that change the qualities of oil at a molecular level. Some of the new compounds generated by this sunlight-driven chemistry (“photochemistry”) do mix with water easily, like sugar dissolving into hot tea. We call this phenomenon “photo-dissolution” (“photo” means “light”).

Scientists have known for a long time that this process can affect oil, but no one knew how much it could affect oil. That’s where our project comes in. We set out to quantify how fast oil photo-dissolution happens, and to use our results to calculate the impact that the process likely had on the Deepwater Horizon spill, and could have on future spills.

To do this, we couldn’t just shine any old light on the oil sample for any amount of time and throw it in water to see how much dissolved. We had to study how the oil was changed by different colors of light in the UV (high-energy light invisible to the human eye) and visible (lower-energy light that makes up our rainbow, from violet to red) range for different amounts of total light exposure. We used light-emitting diode (LED)-based photo-reactors that were custom-built in our lab to irradiate the oil under these different conditions, then exposed the oil to seawater and measured how much oil dissolved. We took our experimental data on the rates of photo-dissolution in the lab and combined it with information about the amount of oil present on the sea surface during the Deepwater Horizon spill and the solar irradiance, and we used the results to calculate the fraction of oil which likely dissolved into seawater via this process. We also took our experimental data and plugged it into a bunch of different hypothetical spill scenarios happening at different times of year, at different latitudes, etc., to explore how important this process might be in other, future spills that happen under different conditions.

What did we find out?

Our experimental results showed that photo-dissolution happens fast—fast enough that, according to our calculations, it could have removed almost 10% of the surface oil during the Deepwater Horizon spill. While 10% might seem small, we have to keep in mind that the oil fate puzzle is split up into many small puzzle pieces, and 10% is similar in magnitude to the fractions of surface oil removed by other well-recognized fate processes, such as evaporation.

We also found out that sunlight-driven reactions can be important under other conditions besides Deepwater Horizon—for example, we found that these reactions could be important during the summer in the Arctic, where a lot of people are concerned about a potential oil spill due to higher ship traffic in an increasingly ice-free summer sea.

Why does it matter?

We don’t know yet whether the net effect of oil photo-dissolution is good or bad after a spill—it could have some positive and some negative effects at the same time. For example, if photo-dissolution is removing a substantial amount of oil from the sea surface, that could mean that less oil is making it to sensitive coastal areas, which is a good outcome. On the other hand, we have to consider the impacts of the dissolved compounds on marine life. There isn’t a lot of data on the toxicity of these compounds to marine life, so we don’t know yet whether it’s a problem that these compounds are ending up in seawater. This is an important next step for research on this topic! Our hope is that incorporation of the photo-dissolution puzzle piece into future experimental research and oil spill modeling will give spill responders improved spill forecasts so that cleanups are more effective and damage assessments are more accurate.