How The Mighty Mississippi Saved Shores From BP Deepwater Oil Spill

Researchers say the river's surge kept Deepwater Horizon gunk out at sea.

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Most of us have by now seen the happy, sunny commercials BP has sprinkled across the television landscape in the past few months, welcoming Americans back to the beaches in Florida, Louisiana, and other southern coasts affected by the Deepwater Horizon oil spill in the spring of 2010.

The ads sponsored by BP feature state and local officials who are obviously relieved that predictions of oil contamination along the coastlines never reached the levels some had initially predicted. They show lovely beaches, and remind viewers that the BP oil spill could have been much, much worse.

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Now, a study from researchers at the University of Pennsylvania published in PLoS ONE may help explain why much of the crude oil from the Deepwater Horizon oil spill never made it to land: The Mississippi River pushed back--and kept much of the oil offshore.

"We noticed that there was a big disconnect between the forecasts of where the oil was going to be the next day and where the oil actually was the next day," said Douglas Jerolmack, one of three geoscientists who used publicly available data sets to look at whether the force of the Mississippi River emptying into the Gulf of Mexico could have plausibly countered some of the effects of the oil spill.

The first place researchers looked was outside the usual scope of ocean circulation models that generally don't take "secondary" forces like major rivers emptying into the oceans into account when they predict where things (like oil) will travel in the ocean. They found that the Mississippi River had, in fact, protected the coastline.

"That maybe shouldn't be a surprise, because these computer models were not generated to forecast the movement of oil," he said. "They were generated to forecast the movement of water."

As the researchers looked through the data, what they found was a very fortunate combination of natural forces that protected the coastline. While ocean circulation models were predicting oil slick migration in the Gulf of Mexico--and potentially even along the eastern U.S. coastline--they weren't looking at the effects of secondary eddy slopes or Mississippi River hydrodynamics.

Instead, the data showed that, under conditions of relatively high river discharge and weak winds, a freshwater mound could form around the Mississippi River Delta and push back against the incoming water, keeping the oil from reaching the coastline.

Jerolmack, an assistant professor in the University of Pennsylvania's department of earth and environmental sciences, explained that the water surface containing the oil essentially tilted "downhill" due to the natural forces of the river--in effect, pushing the oil back out to sea.

Jerolmack, along with lead author Federico Falcini (who was a postdoctoral investigator in his lab at the time) and a third researcher from Italy, pulled data from the Colorado Center for Astrodynamics Research and other satellite data to compare water surface movements. They were able to separate out the effects of the freshwater forces from the Mississippi.

"We recognized that there was a very persistent mound, a bump or a bulge, in the elevation of the sea surface in the vicinity of the Mississippi Delta," he said. "The model was able to predict the speed at which the oil moved away from this freshwater mound and how long it took for the oil to move away from the mound."

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The researchers also studied what might happen if an oil spill happened during hurricane season, which generally begins around the first of June each year. High winds could counter the natural forces of the freshwater mounds from the Mississippi River emptying into the gulf.

The researchers have developed a new model for the physical mechanism and novel effect of the Mississippi River if, heaven forbid, there's another oil spill in the Gulf of Mexico and science is asked to predict where the oil might migrate. Their work will be added to more complex ocean models and improve forecasts of slick migration for future spills.

For long, continuous oil spills like Deepwater--which will remain a possibility as long as oil drilling occurs in the deep waters of the Gulf of Mexico--researchers will need every tool they can find to predict daily oil slick migration. If science can predict where the oil will go, resources can then be deployed to the coastal areas most likely to be affected.

So, perhaps, BP should thank the Mississippi River in its new round of TV ads welcoming us back to the beaches--because the effects of the Deepwater Horizon oil spill could have been a whole lot worse otherwise.

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