Zhengzhen Zhou and Laodong Guo from the University of Southern Mississippi used a fluorescence excitation emission matrix (EEM) technique to analyse water samples. They found that while there is no evidence of oil on the surface of the water, there are still oil components in deeper waters.

“Nature has done an amazing job of degrading the oil on the surface,” Guo told environmentalresearchweb. “Microbial and photo-degradation have been very effective and rapid. But we found that anomalous dissolved organic matter (DOM) with high optical yields still resides in deep waters even 15 months after the oil spill in October 2011, showing a persistent influence of oil in deep waters.”

The researchers identified three main oil components, each with different maximum fluorescence intensities. These three components represent different degradation states of oil and can be used as a sensitive index to track the fate, transport and transformation of oil in the water column.

Guo and Zhou found that the oil fluorescence signatures derived from EEM spectra were weak in samples collected during October 2010 and October 2011, indicating effective dilution, degradation and transformation of oil in the water column.

“But oil is not meant to dissolve in water,” Guo points out. “The weak oil fluorescence signatures observed after the oil spill likely also resulted from the application of vast quantity of dispersants during the oil spill and thus the interactions of oil with dispersants in the water column. Another reason for the weak signal include possible sorption effect during sample filtration processes since oil can be readily removed on filters and sorbs on the bottle wall during sample processing.”

While large studies have been carried out to investigate the impacts of oil on ecosystems, and other work has reported the extent and transport of oil, methane and dispersants in the Gulf of Mexico since the Deepwater Horizon spill, this is the first published study on the characterization of oil from seawater samples using fluorescence excitation emission matrix (EEM) techniques coupled with parallel factor (PARAFAC) analysis.

“The fate and degradation pathways of oil from the Deepwater Horizon spill remain poorly understood,” said Guo. “How oil interacts with natural organic matter is largely unknown. We hope our research goes some way towards explaining this.”

Zhou and Guo published their study in Environmental Research Letters (ERL).