Spatial and Temporal Extent of a Subsurface Hydrocarbon Intrusion Following the Deepwater Horizon Blowout

The Deepwater Horizon (DWH) oil spill in the Gulf of Mexico (GoM) released an estimated 4.9 million barrels of oil between April 20, 2010 and July 15, 2010. An estimated 36% of the oil formed a neutrally buoyant intrusion, containing both dissolved compounds and oil microdroplets, between 1000 and 1300 m depth. This study used geographic information systems software, and data from water samples that were collected as part of the National Resource Damage Assessment (NRDA), to determine that an area of at least 1,600 km2 was exposed to DWH oil. Toxic BTEX (benzene, toluene, ethylbenzene, and xylenes) compounds and polycyclic aromatic hydrocarbons (PAHs) reached concentrations 950 and 50 times higher than maximum background concentrations, respectively. BTEX and n-alkane concentrations above pre-2010 values were present through late August, more than a month after the wellhead was capped. This study is the first to examine the DWH intrusion over such a large temporal and spatial extent. We further estimated that an area between 500 and 1000 km2 may have been exposed to harmful PAH concentrations, based on studies of PAH toxicity and U.S. Environmental Protection Agency (EPA) guidelines. We also found evidence of aggregation and deposition of oil near the DWH wellhead, as well as an area of 400 km2 where the intrusion may have impinged on the seafloor. While relative rates of dilution, degradation, and deposition in the intrusion are unknown, we have shown evidence that supports two previously proposed processes that may have deposited DWH oil from this deep intrusion onto sediments, where toxic compounds could be resuspended and continue to be bioavailable to benthic organisms