Characterization of oil components from the Deepwater Horizon oil spill in the Gulf of Mexico using fluorescence EEM and PARAFAC techniques

Abstract

Three-dimensional (3D) fluorescence spectroscopy and parallel factor analysis (PARAFAC) were used to characterize oil components and to examine their fate and transformation processes during the unprecedented Deepwater Horizon oil spill in the Gulf of Mexico. Water samples were collected during two cruises in May and June 2010 soon after the spill began. Fluorescence excitation-emission matrix (EEM) spectra of seawater samples strongly resemble those of crude oil from the Macondo well and weathered oil collected from Gulf surface waters. This indicates the influence of oil on marine dissolved organic matter (DOM) which yielded elevated dissolved organic carbon (DOC) concentrations, higher UV–vis absorbance, and higher optically inferred molecular weight DOM in the water column. Two major types of DOM were found in the water column: one with low abundance but high optical activity and the other with high mass concentration yet low optical activity. Higher specific UV absorbance but lower spectral slope values observed for deep-water samples indicates the presence of less degraded or fresher crude oil in deep waters. Results of PARAFAC modeling revealed three oil-related components corresponding to crude and weathered oil in the water column: the most prominent one (namely C1) centered on Ex/Em 226/340nm and the other two components (C2 and C6) centered on 236/360 and 252/311nm, respectively. In addition, there is another DOM component (C3), partially representing chemically dispersed oil. Interestingly, the oil component ratios, an intensive property, varied with time and the degradation status of oil. We hypothesize that C2 (236/360nm) is mostly a degraded product while C6 (252/311nm) is a crude oil component with lower molecular weight, but at the same time, could be derived from oil degradation. Changes in fluorescence component ratios can thus be used as a compelling index to track the fate and transport of oil in marine environments.