Chemical characterization of naturally weathered oil residues in arid terrestrial environment in Al-Alamein, Egypt.

The main objective of this study was to investigate compositional changes in a range of source- and weathering-dependent molecular parameters in oil residues in the arid terrestrial environment of Al-Alamein, Egypt. The results of aromatic hydrocarbon analysis demonstrated the stability of several aromatic hydrocarbon ratios in oil residues over a wide range of weathering and hydrocarbon concentrations. The ratios of C2-dibenzothiophenes/C2-phenanthrenes (C2-DBTs/C2-PHENs), C3-dibenzothiophenes/C3-phenanthrenes (C3-DBTs/C3-PHENs), C2-chrysenes/C1-chrysenes (C2-CHRYs/C1-CHRYs), and C2-dibenzothiophenes/C1-dibenzothiophenes (C2-DBTs/C1-DBTs) were nearly constant in oil residues of varying degrees of weathering. Biomarker ratios of hopanes and steranes were useful for source identification even for severely weathered oil residues. The data confirmed previous observations that C29 20S/(20S + 20R) and C29 betabeta/(betabeta+ alphaalpha) sterane epimer ratios, and C29/C30, C30 alphabeta(alphabeta + 3alpha), C31 22S/(22S + 22R) hopane ratios correlate well even after extensive weathering of spilled oils, although some exceptions were encountered for an extensively weathered surface sample. The data provided clear evidence of correlation between samples of the weathered oil residues and the possible source oil. Moreover, the results of biomarker compositions were in good agreement with weathering classification based on alkanes and aromatic hydrocarbons. In general, samples with lowest pregnane indices (PI) and tricyclic terpane indices (TriTI) showed the lowest concentrations of n-alkanes and alkylated polycyclic aromatic hydrocarbon (PAH) homologues, the highest weathering ratios (WRs), the highest values of unresolved complex mixture (UCM)/total resolved peak (TRP) ratios, and the highest ratios of C2 and C3-alkylated CHRYs to their counterparts in the PHEN and DBT series.