Auburn University scientists documented submerged oil mats and surface residual balls (also known as tar balls) on Alabama’s sandy beach systems and analyzed the physical and chemical evolution of compounds matching the characteristics of Macondo oil.
←Typical mousse, SOM, and SRB samples recovered from Alabama’s beaches. (Image provided courtesy of P. Clement from Yin, et al., study)
They found that tar balls became relatively more resistant to physical degradation as they decreased in size, and the rate of polycyclic aromatic hydrocarbon (PAH) weathering slowed or ceased when oil became buried in nearshore sediment. They published their findings in the January 2015 issue of the Marine Pollution Bulletin (Fate of Deepwater Horizon oil in Alabama’s beach system: Understanding physical evolution processes based on observational data) and the March 2015 issue of Science of the Total Environment (Long-term monitoring data to describe the fate of polycyclic aromatic hydrocarbons in Deepwater Horizon oil submerged off Alabama’s beaches).
Immediately following the Deepwater Horizon incident, Auburn University researchers started surveying Alabama beaches. Twenty field surveys of oil mats and tar balls, conducted between May 2010 and August 2014, documented beach conditions before oil arrived, during cleanup efforts, and after cleanup ended. Both of these studies draw on the samples collected during these surveys.
The study published in Marine Pollution Bulletin described the physical observations of oiled samples. Scientists found an increase in tar balls activity after beach oiling first occurred followed by a decreasing trend. They determined that all sticky, fragile, brownish colored tar balls collected from Alabama’s beaches had chemical fingerprints matching the characteristics of Macondo oil. Over time, the tar balls exhibited similar transport behavior as shell fragments, suggesting that nearshore sediment transport processes controlled their movement. The tar balls were also more resistant to physical degradation as they decreased in size, suggesting that smaller pieces may persist in the beach environment. The authors explained that that this smaller size could also increase the effective weathering rates and enhance ecological recovery.
The study published in the Science of the Total Environment focused on the chemical fate and evolution of PAHs trapped in oil spill residues. Laboratory analyses and experiments revealed that evaporation and other open ocean weathering processes likely removed the majority of PAHs (primarily the low molecular weight compounds), depleting nearly all of the light PAHs and a portion of the heavier PAHs. Comparisons of ocean-weathered oil to shoreline-weathered oil samples over the four year period indicated that once open ocean weathered oil was trapped within the nearshore sediment system, PAH weathering rates slowed considerably. The concentration of several heavy PAHs in shoreline-weathered samples remained almost constant over the four year period, suggesting the potential of continuing ecological risks. These results indicate that evaporation and other physical weathering processes such as photo-oxidation, dissolution, and biochemical reactions played a significant role in the depletion of PAHs in natural systems.
Prabhakar Clement, one of the studies’ authors, provided some insight into their findings. “This research has helped me understand how nature efficiently deals with complex environmental problems, but the problem is not over yet.” He continued, “the Alabama beaches still have residual Deepwater Horizon oil, though those levels have declined over the past four years. It is unlikely that these beaches will return to pre-spill conditions any time soon, and perhaps they might reach a new normal background level. The primary concern is the fate of certain toxic petroleum compounds, namely PAHs, trapped in buried oil residues.” He explained that further studies will help quantify the new background level, explain why weathering rates decreased in submerged oil, and may also uncover the long-term environmental effects of persisting partially-weathered oil in beach ecosystems.
The studies’ authors are Joel S. Hayworth, T. Prabhakar Clement, Gerald F. John, and Fang Yin (Marine Pollution Bulletin, 2015, 90, 95 – 105) and Fang Yin, Gerald F. John, Joel S. Hayworth, and T. Prabhakar Clement (Science of the Total Environment, 2015, 508, 46 – 56).
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Auburn University Department of Civil Engineering for their project Analysis of Hydrocarbons (PAHS) in Sediment Samples. Other funding sources included the City of Orange Beach, the National Science Foundation (NSFMRI #G00006697), and the Auburn University Samuel Ginn College of Engineering.