Four Years of Chemical Measurements from the Deepwater Horizon Oil Spill Define the Deep Sea Sediment footprint and Subsequent Recovery

Chemical data acquired during and after the DWHOS showed that several mechanisms were responsible for transport of oil from the water column to the sediments in the deep sea off the continental shelf. Three primary pathways were identified:Sorption onto and sinking of drilling mud particles during “Top Kill” response activity, highly scattered deposition of residuesfrom in situ burns, and deposition of oil combined with microbial organic matter from diffuse oil plumes (“marine snow”). Data collected during 2010, 2011 and 2014 were used to define the oil footprint and estimate time to recovery. More than 1200 stations were sampled. Of these, 27 stations were visited all three years, providing a time series from which recovery rates were calculated using the loss of total polycyclic aromatic hydrocarbons (TPAH) over time fit to first order kinetics. Results showed that the footprint of the oil was limited to the area around the wellhead and in patches to the southwest. Mostsamples had returned to background levels by 2015, with some exceptions close to the wellhead. Deposition to the northeast (DeSoto Canyon) was minor as evidenced by the absence of oil in sediments in that area. Samples with the longest recovery times were within 2 nautical miles of the wellhead, and often contained drilling mud, as shown by olefin signatures on the GC/FID chromatogram. Detailed chemistry data evaluation and chemical fingerprinting provided evidence that oil was being degraded in situ.