The 2010 BP Deepwater Horizon oil-well blowout caused dramatic, widespread shoreline loss along Louisiana’s coast, says a new study by the U.S. Geological Survey and the National Aeronautics and Space Administration. The study revealed that oil-coated wetland margins in Barataria Bay, the site of the heaviest Deepwater Horizon contamination, suffered greater land loss than the bay’s non-oiled shorelines exposed to just normal wave action.

A hurricane that struck about six months after the study was completed provided further measurement of the damage caused by oiling: The oiled shorelines suffered much greater loss than marshes that had not been oiled. “There was broadly dispersed erosion due to oiling from the Deepwater Horizon spill and more severe, but localized, erosion from Hurricane Isaac,” said Amina Rangoonwala, USGS geophysicist and lead author of the study.

“The most startling thing was that the pattern of shoreline loss changed dramatically,” says Cathleen Jones, a NASA member of the research team. In a normal year, areas exposed to incoming waves erode the most, she says. But the oil had coated all the shores and all the shorelines were eroded, not just the shores exposed to waves, she notes.

Oiled marshland is vulnerable because the marsh vegetation’s roots stabilize the soil. “Damage from petroleum was visible on the aboveground portion of the plants, but there was also an unseen effect on the roots,” says Chuck Perrodin, spokesman for the Louisiana Coastal Protection and Restoration Authority. “The roots were reduced and weakened due to the stress placed on the plants by the oiling. The weakened plants were easily eroded by Hurricane Isaac.”