BP Deepwater Horizon oil sped erosion in worst-coated marsh, study says

(from NOLA \ Mark Schleifstein \ Sept. 28, 2016)

Patches of marsh grass that had a 90 percent or greater coating of oil from the BP Deepwater Horizon disaster suffered dramatic erosion rates during the two years after the 2010 well blowout, according to a new study. These marshes “didn’t recover; they’re now gone, having been converted to mudflats,” one of the lead scientists said.

But surprisingly, marshes with less than 90 percent oil coating did not erode as quickly as the scientists had expected. They might even recover, given enough time.

The study was published Tuesday (Sept. 27) in Nature Scientific Reports. It was conducted by a team of scientists that included researchers at Duke University and the University of Louisiana at Lafayette.

The report outlines a period during and after the 87-day disaster when oil not only killed marsh grass outright in some spots but also affected grass roots in other locations, making those areas more likely to die. As that material decomposed, the soil that the grasses and their roots had been holding together was more likely to wash away, resulting in higher erosion rates in the second year after the spill. In the third year after the spill, the erosion rates returned to pre-disaster levels.

The researchers used data gathered at 103 sites during the federal Natural Resource Damage Assessment. They found that in locations where oil covered more than 90 percent of plant stems, the grass died, followed by rapid erosion at rates of 4.6 feet to 5.25 feet per year greater than in un-oiled areas.

Wetland locations in Louisiana made up the largest portion of the study’s test sites and had the highest erosion rates, the scientists said. That comes as no surprise to state officials, who repeatedly complained in the two years after the disaster about the rapid erosion of wetlands at heavily oiled locations such as Bay Jimmy in northern Barataria Bay.

“Marshes that experienced elevated erosion due to high levels of oiling didn’t recover; they’re now gone, having been converted to mudflats in the shallow underwater environment of the Gulf,” said Brian Silliman, the Rachel Carson associate professor of marine conservation biology at Duke’s Nicholas School of the Environment.

In an interview Wednesday, Silliman warned that the rapid erosion resulted in deeper areas in several locations that will be difficult to restore.

“If they’re just going to go planting grasses back in areas where the accelerated erosion occurred, it would take a tremendous amount of money and material to alter the physical stresses in that area,” he said, in part because of the pre-existing rapid rate of erosion.

He said the state and federal trustees who will be directing restoration should not focus on the exact area where damage occurred, but instead in areas more likely to survive erosion, “where you get a bigger bang for the buck.”

Oil from the Deepwater Horizon rig explosion was monitored across 435 miles of wetlands along the coast of the Gulf of Mexico, including well over 200 miles in Louisiana. The oil was concentrated along the outer edge of the marsh, “often visible as a black belt along the shoreline” that was about 15 to 50 feet wide.

The worst oiled areas are estimated to have included between 37 and 62 linear miles of saltwater marshes across the Gulf Coast.

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