Decadal stability and forensic reliability of petroleum biomarkers in tropical coastal environments: Validating laboratory kinetics against 10-year field residues.

Petroleum contamination in tropical coastal zones poses a critical threat to groundwater quality due to the long-term sequestration of heavy oil non-aqueous phase liquids (NAPLs) within heterogeneous media. This study quantifies the chemical evolution and longevity of sequestered heavy fuel oils (Teh Shiang Taipei and Tzimini) using a high-resolution 48-week laboratory simulation at 25 °C. Gas chromatography-mass spectrometry (GC-MS) revealed rapid progression to an advanced weathering stage, where near-complete depletion of highly mobile n-alkanes (<C11) caused the hydraulic immobilization of oil residues within sediment pore spaces. Diagnostic ratios of pentacyclic terpanes (Ts/Tm and C29H/C30H) remained virtually invariant throughout the simulation. Crucially, these ratios precisely matched authentic field residues, including 10-year archived samples from the 2008 Morning Sun incident and dynamically weathered residues from the 2017 Green Island spill. These findings establish C30-hopane as a highly recalcitrant, conservative internal standard for quantifying NAPL mass transfer in complex coastal systems. Furthermore, the systematic depletion of alkylated polycyclic aromatic hydrocarbons (APAHs) marks a kinetic shift from initial physical sequestration to chemical and microbial oxidation. This oxidative transition governs the leaching of water-soluble, toxic oxygenated PAHs (OPAHs) into underlying coastal aquifers. Ultimately, our results provide a robust, tiered forensic framework to calibrate groundwater transport models and predict the environmental persistence of legacy petroleum sources at vulnerable maritime-terrestrial interfaces.