Marnie Winter remembers seeing the oil spread through the waters of Barataria Bay in southeastern Louisiana. It was 2010, just a few weeks after an explosion on the Deepwater Horizon oil rig in the Gulf of Mexico began the largest marine oil spill in US history. About 4 million barrels of oil were poured into the sea, affecting more than 1,300 miles of shoreline from Texas to Florida.

“It was scary,” says Winter, who’s the assistant director of the environmental department at Jefferson parish, one of the three parishes surrounding Barataria Bay. She and other local officials began working with the Coast Guard to keep the oil from reaching the shore, but despite their efforts, the oil began coating the Louisiana marshlands. “It was no surprise that the vegetation would die,” Winter says.

Those plants and their roots, however, had been keeping the land from eroding. After the oil spill killed the plants, the marshlands lost a lot of land — especially in the shoreline areas that were heavily coated, according to a new study published last week in the journal Geophysical Research Letters. That’s bad news for an area that’s already badly affected by land erosion. “It’s a big deal,” says Edward Overton, a professor emeritus of environmental sciences at Louisiana State University, Baton Rouge, who was not involved in the study. “The land that’s lost basically is lost. There’s almost no way to get it back.”

The Louisiana wetlands are important for a variety of reasons. They’re home to many bird, mammal, and fish species. They purify water by filtering pollution and trapping sediment. They absorb carbon from the atmosphere and function as storm buffers, protecting people who live inland from flooding during storms. Finally, fishermen depend on wetlands to harvest oysters, shrimp, crabs, and fish.

These fragile ecosystems have been threatened by erosion for decades, and the deck is heavily stacked against them. Barataria Bay is located on the western side of the Mississippi River Delta. Once upon a time, the Mississippi River drained through these marshes, leaving its sediment in the delta — which replenished the land lost to the waves. But over the last century, people built levees in the Mississippi, which prevents the river from spreading sediment. That means the marshes just keep receding. There are other threats, too — for instance, the thousands of miles of canals that were dredged for oil and gas exploration. These canals allow saltwater to penetrate into the wetlands and kill freshwater vegetation. As if all that weren’t enough, this area is also prone to hurricanes and sea levels are rising — two more forces for erosion.

The oil spill increased the losses in Barataria Bay, according to the study done by the US Geological Survey with NASA. It’s not alone in oil-related erosion, either. Other studies have shown that the Deepwater Horizon oil spill caused land loss along the coasts of Louisiana, Alabama, and Mississippi. This latest research adds to that body of knowledge, using very valuable data from even before the oil spill occurred. “We just happened to have the perfect dataset,” says study co-author Cathleen Jones at NASA's Jet Propulsion Laboratory.

The researchers used shoreline images collected by a NASA radar instrument that can see through clouds and take highly detailed images of our planet during night and day. They analyzed measurements from June 2009 to October 2012. Before the spill, isolated places scattered throughout Barataria Bay lost land to the sea. In the first year after the Deepwater Horizon spill, areas that had been most exposed to the oil were heavily eaten away: the erosion was more widespread. The length of shoreline that lost 13 to 26 feet of land quadrupled the first year after the spill. In the second year after, from 2011 to 2012, the higher land loss rates extended even farther — to areas that hadn’t been as heavily coated.

“Erosion is faster in the heavier oil sites than in the lighter ones,” says Eugene Turner a professor at the department of oceanography and coastal sciences at Louisiana State University, who was not involved in the research. “It’s a before and after comparison.”

Restoration projects that include restoring sediments and planting new vegetation have been going on since the 1990s. But even with those efforts, erosion is continuing. And with a leveed Mississippi river that cuts off the natural flux of sediments, there’s little to do to solve the situation, says Jones at JPL. “So in this area, when erosion happens, you’re just very unlikely to reform any land that’s eroded, especially at this rate,” she says.

The land loss is fairly easy to notice, too, according to Overton at Louisiana State University, Baton Rouge. He says that in the marshes affected by the oil spill, it’s fairly common to spot plastic poles stuck 10 feet in the open water. Those poles were put by researchers during the Deepwater Horizon crisis to mark places where research was taking place. When those poles were placed six years ago, however, they’d been placed on land.