Oil soaked "sand patties" continue to wash ashore years after the Deepwater Horizon oil spill.

 The study shows that the amount of oil deposited on the seafloor might have been underestimated by other scientists.

The research at the Woods Hole Oceanographic Institution and Bigelow Laboratory for Ocean Sciences could also help scientists understand how oil degrades over time.

The Gulf oil spill was one of the worst oil spills in the history of the U.S.: the accident killed 11 people and poured 4.9 million barrels of oil in the Gulf. Research has shown that even after four years of the incident, several species of birds, animals and fishes are facing health problems due to oil exposure.

A new research shows that oil still lingers in the Gulf and oil-soaked "sand patties" continue to wash ashore. These patties persist in the salt marshes in the Gulf of Mexico.

"We were looking at two questions: how could we identify the oil on shore, now four years after the spill, and how the oil from the spill was weathering over time," explained Christoph Aeppli, lead author of the study.

Researchers used two-dimensional gas chromatography (GCxGC) to see how certain biomarkers have degraded since the spill. Biomarkers are like human fingerprints and help scientists indentify where a particular sample of oil has come from. Oil has several compounds which can be degraded by bacteria or by exposure sunlight. Researchers call the study "oil spill forensics."

"We found that some biomarkers-homohopanes and triaromoatic steroids (TAS), specifically - degraded within a few years following the Deepwater Horizon spill," said Chris Reddy, a scientist at Woods Hole Oceanographic Institution and co-author of the paper, according to a news release. "These biomarkers are not as resilient as once thought and they may provide a future window into determining how much, and how quickly, these oil components may linger in the environment when exposed to air, sunlight, and the elements."

The study was based on oil-soaked "sand patties," which were collected along the Gulf shore over a 28-month period. Essentially, scientists found what can be called as the signature of DWH oil. This knowledge is helping us improve our oil spill forensics. It is providing a foundation for better, longer-term identification techniques that account for exposure of oil to wind, waves, sunlight, and microbial degradation over long times," added Aeppli.

The study is published in the journal Environmental Science & Technology.