Author(s): Yuling Han 1, T. Prabhakar Clement 2,*
The Deepwater Horizon (DWH) accident, which resulted in one of the largest marine oil spills in U.S. history, released about 5.0 million barrels of crude oil into the Gulf of Mexico (GOM) . Michel et al.  estimated that the DWH oil spill contaminated over 1,700 kilometers of GOM shoreline and impacted several amenity beaches, marshes and other ecologically sensitive coastal ecosystems. Along the Alabama shoreline, the spill impacted about 50 kilometers of sandy beaches located in between Orange Beach to Fort Morgan. These amenity beaches are major tourist attractions, and they support the local economy of several coastal towns. Therefore, there is considerable interest in understanding the post oil spill recovery levels, and also assessing the changes in background levels in the aftermath of the DWH oil spill. Field studies have shown that the DWH spill has substantially increased the background oil levels of several GOM beaches [3-5]. For example, based on multiple field datasets, Clement et al.  estimated that the background oil levels in Alabama's beaches have increased by at least thousand fold. The average historic background level for Alabama's beaches prior to the DWH oil spill event was estimated to be 2 g/km/year (about 2 to 4 tar balls/km/year; they are highly weathered tar balls with each weighing about 0.5 to 1 g). The levels estimated for Alabama beaches based on a field survey completed on January, 2016 ranged from 2,400 to 31,000 g/km/year . Some of the beaches were heavily contaminated; for example, from a kilometer long beach in Fort Morgan they recovered 233 fragments of oil residues, weighting about 1,310 grams, within an hour . The size of each residue ranged from 0.5 cm to 7 cm and the weight ranged from 0.5 to 50 g. Previous studies have also shown that the DWH oil spill residues found along GOM beaches have partially weathered crude oil that contain various types of toxic polycyclic aromatic hydrocarbons [5-8].
Historically, most GOM beaches are often contaminated by different types of petroleum residues (residues that are not of DWH origin) that could have originated from natural oil seeps, accidental releases from oil rigs and ship oil, or anthropogenic waste oil dumping activities [9,10]. Therefore, in order to assess the impacts of the DWH oil spill, one needs to identify and differentiate DWH residues from other petroleum residues. The DWH oil spill residues are often referred to as "tar balls," a term that normally refers to rubbery, black, highly-weathered masses of oil, which could be found along some GOM beaches. However, DWH oil residues have several physical and chemical characteristics that are different from these traditional black tar balls .
When crude oil is discharged into the ocean, various weathering processes that are driven by winds and waves break the floating oil into smaller patches, which are then transported by ocean currents. During this process, almost all the lighter components in the crude oil rapidly evaporate . The unevaporated portion...