GC-MS Tracks Buried Oil's Decades-Long Breakdown

Oil spills near tropical coastlines can seriously threaten the groundwater people rely on, mainly because heavy oils tend to get trapped underground and stick around for a long time in the mixed-up layers of soil and rock. In this study, researchers from National Taiwan Ocean University’s Institute of Earth Sciences used gas chromatography-mass spectrometry (GC-MS) in their running of a detailed lab experiment over 48 weeks at room temperature (25°C) to see how two types of heavy fuel oil (Teh Shiang Taipei and Tzimini) break down and change chemically over time once they're trapped underground. A paper based on this work was published in the journal Marine Pollution Bulletin.¹

Why Does Underground Oil, Especially in Hot Tropical Climates, End Up Trapped and Persisting for Decades Instead of Fully Breaking Down?

Once oil ends up underground, what happens to it next depends on a handful of natural processes working on it over time. Some of it evaporates, some dissolves into water, sunlight breaks some of it down, and microbes gradually eat away at the rest.²In tropical regions, intense sun and heat speed up all these breakdown processes. In several major oil spills, such as the 1989 Exxon Valdez disaster in Alaska or the 2010 Deepwater Horizon disaster in the Gulf of Mexico, a lot of the oil's lightest, most watery, easily-spreading components evaporated or dissolved away fairly quickly. Once those lighter parts were gone, what remained behind became noticeably thicker and heavier.³

As the oil loses those lighter components, it gets thick and heavy enough to essentially lodge itself in the small gaps between sediment particles, getting physically stuck in place. This turns it into a lingering, long-term pollution source that can stay buried in the ground for decades.^4,5

How Does Buried Heavy Fuel Oil Chemically Change Over Time in Coastal Sediment, and Can That Process Be Used to Predict How It Will Contaminate Groundwater?

The laboratory analysis showed the oil aged and broke down quickly, reaching an advanced stage of weathering. The lightest, most mobile compounds (the ones that would normally seep and spread the furthest) were almost completely gone early on, which caused the remaining oil to essentially get "stuck" in place within the tiny pore spaces of the sediment instead of continuing to migrate. Meanwhile, certain more stable chemical markers in the oil (ratios used to fingerprint it) barely changed at all over the course of the experiment. What's notable is that these same stable ratios matched up almost exactly with real-world oil samples collected from actual spills: oil that had sat in the ground for 10 years after the 2008 Morning Sun incident, and oil that had weathered naturally after the 2017 Green Island spill. This is noteworthy because it means the compound C30-hopane barely breaks down at all and can be used as a reliable "ruler" (a fixed reference point) for measuring how much of the oil has moved or been lost over time in real coastal environments.¹

The study also found that certain aromatic compounds in the oil steadily disappeared over time, signaling a shift in how the oil was breaking down: it moved from simply being physically trapped in the sediment to actually being chemically and biologically broken apart (largely by microbes). This breakdown process matters because it's what allows toxic, oxidized byproducts of the oil (ones that dissolve easily in water) to leach out and potentially contaminate the groundwater below.¹

“Ultimately,” write the authors of the paper,¹ “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.”

Read More on Similar Topics
Fingerprinting Crude Oil with SPME-GC–MS: Insights from Ken Furton

Reliable Separation and Efficient Group-type Quantitation of Total Petroleum Hydrocarbons

References

1. Chang, Y. J.; Lin, C. T. Decadal Stability and Forensic Reliability of Petroleum Biomarkers in Tropical Coastal Environments: Validating Laboratory Kinetics Against 10-year Field Residues. Mar Pollut Bull. 2026, 233 (Pt 2), 120267. DOI: 10.1016/j.marpolbul.2026.120267
2. Tarr, M. A.; Zito, P.; Overton, E. B. et al. Weathering of Oil Spilled in the Marine Environment. Oceanography2016, 29 (3), 126-135. DOI: 10.5670/oceanog.2016.77
3. Aeppli, C.; Carmichael, C. A.; Nelson, R. K. et al. Oil Weathering After the Deepwater Horizon Disaster Led to the Formation of Oxygenated Residues. Environ. Sci. Technol.2012, 46 (16), 8799-8807. DOI: 10.1021/es3015138
4. Barakat, A. O.; Mostafa, A. R.; Qian, Y. R. et al. Application of Petroleum Hydrocarbon Chemical Fingerprinting in Oil Spill Investigations—Gulf of Suez, Egypt. Spill Sci. Technol. Bull. 2002, 7 (5–6), 229-239. DOI: 10.1016/S1353-2561(02)00039-7
5. Arekhi, M.; Terry, L. G.; John, G. F. et al. Environmental Fate of Petroleum Biomarkers in Deepwater Horizon Oil Spill Residues Over the Past 10 Years. Sci. Total Environ.2021, 791, 148056. DOI: 10.1016/j.scitotenv.2021.148056