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Unique oil:sand aggregates, termed surface residue balls (SRBs), were formed on coastal headland beaches along the northern Gulf of Mexico as emulsified MC252 crude oil mixed with sand following the Deepwater Horizon spill event. The objective of this study is to assess the biodegradation potential of crude oil components in these aggregates using multiple lines of evidence on a heavily-impacted coastal headland beach in Louisiana, USA. SRBs were sampled over a 19-month period on the supratidal beach environment with reasonable control over and knowledge of the residence time of the aggregates on the beach surface. Polycyclic aromatic hydrocarbons (PAHs) and alkane concentration ratios were measured including PAH/C30-hopane, C2/C3 phenanthrenes, C2/C3 dibenzothiophenes and alkane/C30-hopane and demonstrated that biodegradation was occurring in SRBs in the supratidal. These biodegradation reactions occurred over time frames relevant to the coastal processes moving SRBs off the beach. In contrast, submerged oil mat samples from the intertidal did not demonstrate chemical changes consistent with biodegradation. Review and analysis of additional biogeochemical parameters suggested the existence of a moisture and nutrient-limited biodegradation regime on the supratidal beach environment. At this location, SRBs possess moisture contents <2% and molar C:N ratios from 131–323, well outside of optimal values for biodegradation in the literature. Despite these limitations, biodegradation of PAHs and alkanes proceeded at relevant rates (2–8 year−1) due in part to the presence of degrading populations, i.e., Mycobacterium sp., adapted to these conditions. For submerged oil mat samples in the intertidal, an oxygen and salinity-impacted regime is proposed that severely limits biodegradation of alkanes and PAHs in this environment. These results support the hypothesis that SRBs deposited at different locations on the beach have different biogeochemical characteristics (e.g., moisture, salinity, terminal electron acceptors, nutrient, and oil composition) due, in part, to their location on the landscape.

Buried and surface MC252 oil from the intertidal on a coastal headland beach was sampled using a randomized block methodology. Weathering indices contrasting alkylated 3- (phenanthrenes) and 4-ring (chrysene) PAH concentrations were computed based on analyses of these samples. Buried oil was detected at a frequency of 18% and, of those samples, 29% had indices indistinguishable from oil sampled near the wellhead indicating that PAH weathering was not occurring. These samples with persistent PAHs were associated with one or more conditions in the beach profile: located at or near the depth of the water table, nearest to the shoreline, or within a thicker oil mat deposit. Surface oil samples, consisting of oil:sand aggregates recently washed in by the surf, had a higher percentage (81%) of oil with weathering indices indicating persistence. These observations, coupled with measurements of biogeochemical analyses on the beach from other studies, suggest that insufficient oxygen in the nearshore environment of these coastal headlands creates conditions for PAH persistence over time frames of years to decades.

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