Origin and degradation of hydrocarbons in mangrove sediments (Rio de Janeiro, Brazil) contaminated by an oil spill

Compositional alterations of oil (MF380) spilled in mangrove sediments was studied over 4 years with an emphasis on weathering of PAH. Sediment cores were sampled yearly in three mangrove forests in Guanabara Bay, Rio de Janeiro, two of which were contaminated by the spill in January 2000 and one was used as a reference area. Hopanes and steranes (46 compounds) were successfully applied to identify the presence of the oil in the most contaminated mangrove (Suruí). However extensive alterations under these tropical climate conditions hampered, in some cases, relating high concentrations of petrogenic PAH to the spill. Accentuated weathering of C34-17α,21β(H)-tetrakishomohopane 22S, C34-17α,21β(H)-tetrakishomohopane 22R, C35-17α,21β(H)-pentakishomohopane 22S and C35-17α,21β(H)-pentakishomohopane 22R was observed while the ratio 17α,21β(H)-30-norhopane/C30-17α,21β(H)-hopane in the samples varied from 0.42 in 2000 to 0.32 in 2004 (compared to 0.43 in the original oil) demonstrating the intensity of biodegradation processes. Fluoranthene/pyrene and ∑3–6 rings PAH/Σ5-alkylated PAH series were identified as the most robust PAH diagnostic ratios to distinguish samples with petrogenic contamination. Vertical migration of the oil in the sediment column and a decrease by 70% in total PAH were observed during the study period. Relative decay constants were obtained for total and individual PAH after normalization to the C30-17α,21β(H)-hopane and by adjusting the data to first order kinetics. PAH decay rates decreased with increasing degree of alkylation. The highly eutrophic conditions in the bay and the sandy character of sediments contributed to the observed degradation in surface sediments as well as in deeper sediment layers.