Degradation of Deepwater Horizon Oil Buried in Beach, Shelf and Slope Sediments of the Northeastern Gulf of Mexico

• Other Authors: Wells, Wm. Brian (William Brian) (author)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Marine biology
• Online Access: http://purl.flvc.org/fsu/fd/2019_Spring_Wells_fsu_0071E_15167

Polycyclic Aromatic Hydrocarbons (PAH) buried in sandy beaches of the northeastern Gulf of Mexico after the Deepwater Horizon accident posed a potential risk to environmental and human health. Therefore, concentrations and decay rates of the EPA’s 16 priority PAHs were determined in the supratidal area of the sandy beach at Pensacola Beach, Florida from 30 June 2010 through 16 June 2011. The results revealed no concentrations exceeding NOAA guidelines and decay to background concentrations within a year. The relatively rapid decay of the buried PAHs was facilitated by tidal pumping that maintained aerobic conditions within the beach sediment. Submerged-oil-mats (SOMs) that settled onto the seafloor in the northeast Gulf of Mexico were buried in the surface layers of the permeable inner shelf sands, which raised the question whether this embedded oil is preserved under anoxic conditions in the shelf sands despite their high permeability, and whether the sand layer covering the buried oil can effectively prevent release of potentially harmful petroleum hydrocarbons from the sediment. A set of laboratory flume experiments demonstrated that advective pore water flows, generated when bottom currents interact with the ripple topography of the sand bed, transports oxygen to and releases PAHs from the embedded oil to the water column. This process allows rapid aerobic decomposition of oil buried in the surface layers of permeable shelf sediment but also enhances the release of potentially harmful substances from this oil. Large phytoplankton blooms were associated with the Deepwater Horizon oil spill, and significant amounts of oil particles and algal cells settled in the form of marine snow onto northeastern Gulf of Mexico inner shelf and slope sediments. This raised the question how the metabolism of these sediments responded to this fossil and modern organic matter input and whether the combined input would affect the decomposition of the settled oil particles. Measurements of dissolved inorganic carbon, oxygen and nitrogen fluxes across the sediment-water interface at 5 to 20 m water depth and incubation of sediments retrieved from 310 m and 1000 m depth revealed how the seafloor at the different water depths responded to the experimental deposition of realistic amounts of phytoplankton, weathered oil particles, and a mixture of phytoplankton and oil particles. The permeable sand sediments and warmer temperatures of the inner shelf in general produced stronger absolute responses by the benthic microbial community, but the relative increases in sediment metabolism were higher in the muddy slope sediments. The results of this thesis research underline the role of oxygen, temperature and sediment composition for the decomposition of crude oil compounds that were deposited on shore, shelf and slope environments in the Gulf of Mexico. The findings emphasize the role of the transport mechanisms that facilitate aerobic microbial breakdown of the petroleum hydrocarbons. This research provides information to coastal managers and decision makers that can help when designing response plans to future oil spills in the Gulf of Mexico. === A Dissertation submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Spring Semester 2019. === March 29, 2019. === Includes bibliographical references. === Markus Huettel, Professor Directing Dissertation; Bill Cooper, University Representative; Jeffrey P. Chanton, Committee Member; Eric Chassignet, Committee Member; Ian MacDonald, Committee Member.