The Deepwater Horizon oil spill ruined long-term coastal stability

Long after the Deepwater Horizon oil spill, the marshy shores of the Gulf of Mexico were still feeling the effects of the disaster. Marsh grass held back plant-choking oil and soil continued to crumble at a faster rate than before the spill, causing the shoreline to recede faster than usual, a new study shows.

After an April 2010 explosion, the Deepwater Horizon oil rig pumped almost 800 million liters of oil into the sea (SN: 02/12/20). The disaster killed dozens of people and countless marine life. And the oil and its by-products have been disastrous to the Gulf ecosystem, both underwater and along the coast (SN: 4/3/15).

But the oil was also causing structural damage to the coast by killing the marsh plants crucial for holding the soil, researchers report Jan. 25 environmental pollution. This makes the coast more vulnerable to tropical storms, which may increase in intensity due to climate change.

“If the plants are affected in any way, shape or form, you’re going to lose a lot of land,” says Giovanna McClenachan, an ecologist at Nicholls State University in Thibodaux, La.

McClenachan was working towards her Ph.D. at Louisiana State University in Baton Rouge when the disaster happened. She and her supervisor, coastal ecologist Eugene Turner, quickly set up research plots on the swampy coast of south Louisiana. For the next eight years, they conducted soil strength tests three times a year using a shearing shovel, a common tool farmers use to test soil strength, and analyzed the amount of oil it contained.

They also examined satellite imagery from 1998 to 2021 to analyze what swamp vegetation looked like before, during and after the spill over a much longer 23-year period.

The field test found that immediately after the Deepwater Horizon disaster, the oil concentration of some of the most volatile oil components, called aromatics, in the marsh bottom jumped from an average of 23.9 nanograms per gram of sediment before the spill to 17,152 nanograms per gram of sediment in 2011. By 2018, average levels had dropped to 247 nanograms per gram of sediment — but still more than 10 times higher than before the spill.

The soil strength also decreased by half after the burial. Before the burial, the average strength of the top 30 centimeters of the ground was 26.9 kilopascals, which is a measure of pressure in physics. Soil strength fell to a low of 11.5 kilopascals in 2011. While strength then began to recover at a rate of 5 percent per year, it had not fully recovered by 2018, the last year the field study was in. It was again increased to 16.4 kilopascals.

This is partly due to the strong storms that have occurred in the years since the burial. McClenachan says the initial oil spill killed many plants on what was then the swamp shore. As these died, the soil held back by the swampgrass roots was loosened and washed away. But the oil stayed in the water and was pushed further into the swamp, killing more plants.

“The soil strength hasn’t recovered because there’s still oil in the sump, and that’s causing these really strong erosion events during storms that didn’t exist before the oil spill,” McClenachan says.

Analysis of the satellite imagery showed that the rate of swamp loss doubled after the burial. The shoreline along the study area was receding at an average rate of 0.8 meters per year prior to the spill, due to a combination of natural shifts in marsh levels and man-made factors such as sea level rise. But that loss thereafter increased to an average of nearly 5 feet per year, although the swamp receded about 8 feet in the 12 months after Hurricane Isaac hit Louisiana in 2012.

Some research has questioned whether it is the oil spill that is affecting shoreline erosion, rather than the storms of the past decade. But Hurricane Katrina, which hit the Louisiana coast in 2005 years before the oil spill, didn’t cause nearly the same magnitude of coastal damage as much weaker post-spill storms, McClenachan’s satellite analysis shows. This suggests that it’s not an either/or question; Rather, the continued effects of the oil spill made the shoreline more vulnerable to storm damage.

The new study is unique in that it also shows the effects of the spill on the stability of the soil itself, says Scott Zengel, an environmental scientist at Research Planning Inc., a private research consultancy in Tallahassee, Fla. that often studies the effects of the Deepwater Horizon Catastrophe.

“This supports the idea that there really was an erosion effect,” he says, adding that the length of the study complements previous research showing that oil played a role in the changes in the swamp.

These changes can be mitigated to some extent. Zengel’s work shows that techniques such as replanting marsh grass can help reduce the rate of shoreline erosion. For promoting soil retention, he says, “it really shows that plants are one of the key factors.”