Deepwater Horizon oil spill ruined long-term shore stability

Long after the Deepwater Horizon oil spill, the swampy shores of the Gulf of Mexico were still feeling the effects of the disaster. Marsh grasses retained the oil that lubricated the plants, and the soil continued to erode at a faster rate than before the spill, causing shorelines to retreat more quickly, a new study shows.

After an explosion in April 2010, the Deepwater Horizon rig Pumped around 800 million liters of oil In the ocean (Sn: 2/12/20, The disaster killed dozens of humans and untold numbers of marine life. and oil and its by-products were devastating to the Gulf ecosystemboth under water and along the shore (Sn: 4/3/15,

but oil also caused structural damage to the shoreline by killing marsh plants Important for keeping soil in place, researchers report Jan. 25 environmental pollution, This is making the coast more vulnerable to tropical storms which may increase in intensity due to climate change.

“If plants are compromised in any way, shape, or form, you’ll lose a lot of land,” says ecologist Giovanna McClenachan of Nicholls State University in Thibodaux, La.

McClenachan received his Ph.D. Was working on When disaster struck at Louisiana State University in Baton Rouge. She and her supervisor, coastal ecologist Eugene Turner, quickly established research plots on the marshy coast of south Louisiana. Three times a year for the next eight years, he tested the strength of the soil with a shear vane, a common tool farmers use to test the strength of the soil, and analyzed the amount of oil it contained.

They also examined satellite images from 1998 to 2021 to analyze what marsh vegetation looked like before, during, and after that 23-year-long period.

Field tests showed that immediately after the Deepwater Horizon disaster, oil concentrations of some of the most volatile components of oil, called aromatics, in marsh soil increased from an average of 23.9 nanograms per gram of sediment before the spill to 17,152 nanograms per gram . Gram of sediment in 2011. By 2018, the average level had dropped to 247 nanograms per gram of sediment – ​​but was still 10 times higher than before the spill.

After spilling, the strength of the soil also remained half. Before spillage, the top 30 cm of soil had an average strength of 26.9 kilopascals, which measures pressure in physics. In 2011 the soil strength had decreased to 11.5 kilopascals. While the strength began to recover at a rate of 5 percent per year, it did not fully recover until 2018, the final year of the field study, when it climbed back to 16.4 kilopascals.

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

“The soil strength hasn’t recovered because there’s still oil in the swamp, and it’s causing really strong erosion events during those storms that weren’t happening before the oil spill,” McClenachan says.

Analysis of satellite images showed that the marsh loss rate doubled after the spill. The shoreline along the study area was already receding at an average rate of 0.8 m per year due to a combination of human-caused factors such as natural shifting of marsh levels and sea level rise. But this loss has since increased to an average of about 1.7 meters per year, although the marsh retreated about 2.5 meters in the 12 months following Hurricane Isaac in Louisiana in 2012.

Some research has questioned whether this oil spill is affecting shoreline erosion over the past decade, rather than storms. But Hurricane Katrina, which slammed into the Louisiana coast in 2005, several years before the oil spill, did not cause the same level of damage as much weaker storms after the oil spill, McClenachan’s satellite analysis shows. This shows that it is not an either/or question; Rather, the lasting effects of the oil spill made the shoreline more vulnerable to storm damage.

The new study is unique in that it also shows the impact of the spill on soil stability, says Scott Zengel, an environmental scientist at Research Planning Inc., a private research consultancy in Tallahassee, Fla., that frequently analyzes impacts. . Deepwater Horizon disaster.

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

These changes can be minimized to some extent. Zengel’s work suggests that techniques such as replanting marsh grasses Can help reduce the rate of shoreline erosion, In promoting soil retention, he says, “it really points to plants as one of the key factors.”