Understanding the Fates and Chemical Compositions of Weathered Oil in Coastal Marshes since the 2010 Deepwater Horizon Oil Spill

Coastal marshes were heavily impacted by the Deepwater Horizon (DWH) oil spill in 2010, with approximately 90% of shoreline impacts occurring in Louisiana's coastal wetlands. Spilled crude oils impact an environment through four major mechanisms: ecosystem exposure to reactive and toxic aromatic compounds; covering and smothering that hinders normal plant and animal physiology; depletion of dissolved oxygen; and disruption of the aquatic food web. Crude oil's ability to cause environmental harm depends upon its composition, which is a very complex mixture of many thousands of reduced carbon compounds made from the degradation of plant material deposited deep underground. This study reviews the results from the chemical characterization of petroleum hydrocarbons, at various weathering stages, in >2000 marsh surface sediments and select sediment cores samples collected from various sampling locations in Terrebonne Bay, Grand isle, and northern Barataria Bay from 2010 to 2018. The sediment samples were analyzed for target saturated alkanes, polycyclic aromatic compounds, and the forensic biomarker (hopane and sterane) compounds. The chemical characterization of the compositional changes of target compounds in DWH oil, from its pre-stranding stage just offshore in the Louisiana Bight, through stranding on marshy shorelines and through its degradation and weathering over eight years has given insights into the complexity of oil residues and potential for impacts in these varying environmental conditions. Stranded oil initially had two prominent fates: settling on surface sediment/soils of the marshes, and subsurface deposition primarily by means of settling into fiddler crab burrows. Both initial fates affected shorelines and 10–20 meters inward. Over time, surface oil residues were spread beyond initially impacted areas by Tropical Storm Isaac in 2012 and other weather events, and oil residues were quickly degraded. Subsurface stranded oil was degraded much more slowly under anaerobic conditions and some was re-released as fairly fresh oil during the coastal erosions caused by DWH surface oiling damage to the marsh plants. However, these re-releases were relatively slow and were quickly aerobically degraded once the stranded oil reached marsh surfaces. There was also evidence of anaerobic degradation of heavily weathered surface oil residues during the 2015 to 2018 timeframe. This eight-year study establishes a very complex narrative between the physical and chemical properties of stranded oil and its interactions with coastal marsh environments.