Ship waste as a major source of chronic oil contamination on the northeastern Brazilian coast enlightened by forensic geochemistry

This work investigated the occurrence, characteristics, and potential sources of petrogenic oil residues contaminating the beaches of Ceará, Northeastern Brazil, following the massive 2019 oil spill, using forensic geochemistry and testing correlations with known events. Field surveys were conducted in 2021 and 2022 to collect oil residues from beaches over ~488 km of Brazilian coastline, yielding 19 and 13 oil samples, respectively. Ten spilled oil samples from the 2019, 2022.1, and 2022.2 events were also used. Hydrocarbons were assessed by GC-FID and high-temperature GC-FID, and saturated and aromatic petroleum biomarkers by GC–MS. Most oil residues collected in 2021 and 2022 (~80%) showed high abundances of long-chain n -alkanes ( n C 31 to n C 49 ) and moderate to high unresolved complex mixture (UCM), indicating waxy and heavy petrogenic material. Diagnostic ratios based on terpanes, steranes, and tetracyclic polyprenoids, along with principal coordinates and hierarchical clustering analyses, revealed that only three 2021 and three 2022 oil residue samples (~20%) were geochemically correlated with the 2019 and 2022.1 events, respectively. The other petroleum residues showed general differences, suggesting multiple sources. PAHs and alkylcyclohexanes were mostly not detected in the oil residues, but the triaromatic steranes were detected, consistent with heavy petroleum residues. These findings may suggest that the stranded oil residues originate from waste ship-oil sludges (WSOSs) or other heavy oil residues generated during tanker operations in the South Atlantic Ocean. Such residues are probably discharged at sea and subsequently transported to the Brazilian coast by oceanic currents, contributing to chronic oil contamination along northeastern Brazilian beaches.