Long after the Deepwater Horizon oil spill, the marshy shores of the Gulf of Mexico were still feeling the effects of the disaster. Swamp grass trapped the oil, which suffocated the plants, and the soil continued to break up faster than before the spill, causing the shoreline to recede faster than it would otherwise, a new study shows.
After the explosion in April 2010, the Deepwater Horizon drilling rig pumped almost 800 million liters of oil into the sea. The disaster killed dozens of people and countless marine life. And oil and its byproducts have been disastrous for the Persian Gulf ecosystem, both underwater and along the shore.
But the oil also caused structural damage to the shoreline, killing marsh plants important for holding soil in place, researchers reported Jan. 25 in Environmental Pollution . This makes the coast more vulnerable to tropical storms, which may increase due to climate change.
“If plants are compromised in any way, shape or form, you lose a lot of land,” says Giovanna McClenachan, an ecologist at Nicholls State University in Thibodeau, Louisiana.
McClenachan was working on her Ph.D. at Louisiana State University in Baton Rouge when the disaster occurred. She and her supervisor, coastal ecologist Eugene Turner, quickly established research sites on the marshy coast of southern Louisiana. Three times a year for the next eight years, they tested the strength of the soil with a straight spade, 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 the spill over a much longer 23-year period.
Field tests showed that immediately after the Deepwater Horizon disaster, the concentration of some of the most volatile components of oil, called aromatics, in the marsh soil 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 were still more than 10 times higher than before the leak.
Soil strength also halved after the spill. Before the spill, the average strength of the top 30 centimeters of soil was 26.9 kilopascals, which measures pressure in physics. Soil strength dropped to 11.5 kilopascals at its lowest point in 2011. While the strength began to recover at a rate of 5 percent per year, it still did not fully recover until 2018, the last year of the field study, when it rose again to 16.4 kilopascals.
This is partly due to the severe storms that have passed in the years since the spill. McClenachan says the initial oil spill killed many plants on the marshy bank. After they died, the soil held by the marsh grass roots was loosened and washed away. But the oil stayed in the water and got further into the swamp, where it killed more plants.
“The strength of the soil hasn’t recovered because there’s still oil in the marsh, and it’s causing these really severe erosion events during storms that weren’t there 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 sinking by an average of 0.8 meters per year prior to the spill due to a combination of natural shifting marsh levels and anthropogenic factors such as sea level rise. But that loss has since increased to nearly 1.7 meters per year on average, although in the 12 months after Hurricane Isaac hit Louisiana in 2012, the marsh receded by about 2.5 meters.
Some studies have questioned whether the oil spill contributed to shoreline erosion, rather than storms over 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, McClenachan’s satellite analysis shows. That means it’s not an either/or question; rather, the lingering effects of the oil spill made the coastline 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, said Scott Zengel, an ecologist at Research Planning Inc., a private research consultancy in Tallahassee, Fla., that often analyzes the impact of the Deepwater Horizon disaster.
“This supports the idea that there was indeed an erosion effect,” he says, adding that the length of the study adds to previous research showing that oil has played a role in changes to the marsh.
These changes can be mitigated to some extent. Zengel’s work shows that practices such as marsh grass transplants can help reduce the rate of shoreline erosion. According to him, “plants are one of the key factors” to strengthen soil retention.