Researchers at Florida State University and the Georgia Institute of Technology analyzed degradation processes of oil that was deposited along Gulf of Mexico beaches following Deepwater Horizon. They found that small millimeter-size oil particles and thin oil films that coated sand grains disappeared within a year, facilitated by the large surface-to-volume ratio of the small particles and films that allowed space for microbial colonization and biodegradation. In contrast, the degradation of golf-ball sized sediment-oil-agglomerates (SOAs or tarballs) with a smaller surface-to-volume ratio is a lengthier process.

Using a novel in-situ experimental setup, the researchers followed the degradation of buried SOAs for three years and, based on decay rates, estimated that the SOA decomposition would take about 30 years. The degradation of the same SOAs kept in a dark lab environment would take approximately 100 years, highlighting the key role of the beach environment and its microbial community in the oil degradation process.

The researchers published their findings in two studies, one in Marine Pollution Bulletin: Degradation of Deepwater Horizon oil buried in a Florida beach influenced by tidal pumping and one in Scientific Reports: Decomposition of sediment-oil-agglomerates in a Gulf of Mexico sandy beach.

Oil associated with Deepwater Horizon reached the Florida panhandle sandy beaches of the Florida panhandle on June 22, 2010. Waves generated by the distant passage of Hurricane Alex, deposited oil mousse high onto the beaches and strong winds blew an oily sea spray across the beach, coating the sands with oil.

Mixing of oil and sand in the swash zone produced large SOAs that were buried in the beach. The deposition of oil continued, and by the end of July, sands in the upper 70 cm of the beach were stained brown and veined by dark compacted layers of SOAs. Questions arose about the length of time that this oil would persist in Florida beaches.

To assess the degradation of the oil particles and oil films coating the sands, the teams of Markus Huettel and Joel Kostka quantified concentration changes of aliphatic and aromatic oil components; assessed microbial communities’ abundance, composition, and succession; and determined the transport of oxygen and carbon dioxide from June 2010-July 2011. To assess oil degradation in buried SOAs, the team conducted an in-situ experiment from October 2010-December 2013 using golf-ball size standardized SOAs that were embedded in the beach. They compared oil decomposition in buried SOAs to laboratory-incubated SOA material to determine the beach environment’s contribution to oil degradation.

Study author Markus Huettel explained the method for their in-situ experiment, “We combined and homogenized Deepwater Horizon SOAs that we collected at Pensacola Beach one week after the oil came to shore and filled the resulting SOA material in 100 golf-ball-size stainless steel teaballs. Five pairs of such standardized SOAs were attached to a vertical PVC pipe and buried in the beach, positioned at 10, 20, 30, 40 and 50 cm sediment depth, respectively. The ten arrays were removed from the beach one at a time over a period of 3 years. Using this method, we could follow the degradation of the SOAs at different sediment depths over time.”

Huettel emphasized the role of beaches as biocatalytical filters at the land-ocean interface, “Microbial degradation activities typically are most efficient when oxygen and warm temperatures are present, and this was supported by the tidal groundwater table oscillations in the beach. When the ebb tide sets in, the groundwater level in the beach drops, drawing air into the highly permeable beach sand. This ‘beach inhaling’ carries oxygen and heat into the sand, boosting the biodegradation activities within the beach. The rising groundwater table of the following flood acts like a piston pump, pushing air enriched in carbon dioxide out of the beach and moisture from deeper sands into the upper drier beach layers. This ‘beach exhaling’ is beneficial for the decomposition processes in the beach as gases resulting from the oil decomposition can reduce aerobic microbial degradation processes, and microbes need moisture to ‘drink.’ The beach, breathing in tidal rhythm, thus has similarities to an organism that aerobically ‘digests’ the buried oil, inhaling oxygen and exhaling carbon dioxide. After most oil had been decomposed, the microbial community of the beach reversed to a community typical to an unpolluted beach environment.”

Data for the study published in Marine Pollution Bulletin are archived at the National Center for Biotechnology Information (NCBI) under BioProject ID PRJNA294056 and publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at DOI 10.7266/N7765CV9, DOI 10.7266/N7XW4HBZ, DOI 10.7266/N7BZ64J8, DOI 10.7266/N73J3BGD, DOI 10.7266/N7PZ56VV, DOI 10.7266/N7PG1Q83, DOI 10.7266/N7T72FZZ, DOI 10.7266/N78C9TSB, and DOI 10.7266/N7MG7N1S.

Data for the study published in Scientific Reports are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at DOI 10.7266/n7-wjj4-dq16, DOI 10.7266/n7-jjcn-y650, DOI 10.7266/n7-r0ca-f740, DOI 10.7266/n7-kzth-6056, DOI 10.7266/n7-jgbx-p395, and DOI 10.7266/n7-kavs-t279.

The Marine Pollution Bulletin study’s authors are Markus Huettel, Will A. Overholt, Joel E. Kostka, Christopher Hagan, John Kaba, Wm. Brian Wells, and Stacia Dudley.

The Scientific Reports study’s authors are Ioana Bociu, Boryoung Shin, Wm. Brian Wells, Joel E. Kostka, Konstantinos T. Konstantinidis, and Markus Huettel.