Deepwater Horizon oil spill ruined long-term shoreline stability

Long after the Deepwater Horizon oil spill, the swampy shores of the Gulf of Mexico were still feeling the effects of the disaster. Swamp grass trapped the smothering oil from the plants and the soil continued to crumble at a faster rate than before the spill, causing the shoreline to recede faster than it otherwise would, according to a new study.

Following an explosion in April 2010, the Deepwater Horizon platform pumped nearly 800 million liters of oil in the sea (SN: 02/12/20). The disaster killed dozens of humans and countless marine life. And the oil and its by-products were catastrophic for the Gulf ecosystemboth underwater and along the shore (SN: 03/04/15).

But the oil also caused structural damage to shoreline by killing marsh plants crucial for holding soil in place, 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 compromised 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, Louisiana.

McClenachan was working on her doctorate. 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 southern Louisiana. Three times a year for the next eight years, they performed soil strength tests with a weather vane, a tool commonly used by farmers to test soil strength, and analyzed how much oil it contained. .

They also looked at satellite images from 1998 to 2021 to analyze what the marsh vegetation looked like before, during and after the spill over a much longer period of 23 years.

The field test found that immediately after the Deepwater Horizon disaster, the oil concentration of some of the most volatile components of oil, called aromatics, in the marsh soil rose 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 were still more than 10 times higher than before the spill. spill.

The soil resistance also halved after the spill. Prior to the spill, the average resistance of the top 30 centimeters of soil was 26.9 kilopascals, which is a measure of pressure in physics. Ground resistance fell to a low point of 11.5 kilopascals in 2011. While resistance then began to recover at a rate of 5% per year, it was still not fully recovered in 2018, the last year of the field study, when it had risen to 16.4 kilopascals.

This is partly due to the heavy storms that occurred in the years following the spill. McClenachan says the initial oil spill killed a lot of plants on what was then the shoreline of the marsh. When these died, the soil held by the roots of the marsh grass was loosened and washed away. But the oil stayed in the water and was pushed further into the swamp, where it killed more plants.

“The ground strength hasn’t recovered because there’s still oil in the swamp, and that’s causing these very strong erosion events during storms that didn’t happen before the oil spill,” McClenachan says. .

Analysis of satellite images showed that the rate of marsh loss doubled after the spill. The shoreline along the study area was already retreating an average of 0.8 meters per year before the spill due to a combination of natural shifting marsh levels and human-induced factors such as sea level rise. But that loss increased to nearly 1.7 meters on average per year thereafter, although in the 12 months after Hurricane Isaac hit Louisiana in 2012, the swamp receded d about 2.5 meters.

Some research has questioned whether it was the oil spill that affected 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, did not cause nearly the same level of shoreline loss as much weaker storms after the oil spill. according to McClenachan’s satellite analysis. This suggests that it is not an either/or question; rather, the lasting effects of the oil spill made the coastline more vulnerable to storm damage.

The new study is unique in that it also shows the effect 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, Florida, which often analyzes the impacts of the Deepwater Horizon disaster.

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

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 bank erosion. To boost soil retention, he says, “it really shows that plants are one of the key factors.”