Hydrocarbon Forensics: Cracking the Code

Forensic chemists help solve some of the most complex source questions in environmental science. Hydrocarbon contamination is one of them. It can stem from fuel releases, crude oil spills, natural seeps, residues from manufactured gas plants (MGPs), background contamination from air deposition and plant-produced compounds. To identify where that contamination came from, we use chemical characteristics to distinguish one source from another and to understand how the material has changed over time.

That matters because source identification shapes what happens next. It informs remediation strategy, supports equitable cost sharing and helps protect environmental and human health.

Fuel releases such as gasoline, diesel and jet fuel are frequently encountered in hydrocarbon forensics. These refined products have distinct chemical compositions, which makes them identifiable when the right markers are analyzed. Gasoline is typically rich in lighter alkanes, from C4 to C12, along with benzene, toluene, ethylbenzene and total xylenes (BTEX). Substituted monoaromatics, alkylates and naphthenic compounds can help characterize a specific refinery formulation. Additives such as organoleads, methyl tert-butyl ether (MTBE) and ethanol can also provide timing clues because their use peaked in different production periods. Polycyclic aromatic hydrocarbons (PAHs) are usually more informative in diesel and jet fuel than in gasoline, while alkylated PAHs (APAHs) are especially useful in heavier fuels, where homolog totals can be plotted as histograms for source profiling.

Crude oil releases are more complex because crude oil is a variable mixture that changes from one source to another. We look at the distribution of alkanes, including both n-alkanes and branched compounds such as pristane and phytane, to understand source and weathering. APAHs are often present at higher loadings than parent PAHs, and petrogenic APAH profiles can be distinguished from thermogenic residues. Biomarkers such as steranes and hopanes are especially valuable because they resist degradation and can help differentiate oils and fuels produced from different geologic formations. We also use PAH diagnostic ratios to distinguish crude oils from one another and to separate petroleum from non-petroleum hydrocarbon sources.

Manufactured gas plant residues were produced from coal, oil or other feedstocks, leaving behind complex mixtures with distinctive forensic signatures. Coal tar residues are rich in PAHs, particularly heavier PAHs and APAHs, and compounds such as benzo[a]pyrene and chrysene can point to MGP impacts. Diagnostic ratios of PAHs and APAHs help distinguish coal tar from oil residues. Phenolic compounds are also more common in MGP residues than in many other hydrocarbon sources, which makes them important markers during source identification. Cyanides and sulfur compounds can add another line of evidence when we need to separate MGP residues from other contamination types.

Background hydrocarbon contamination can complicate the picture. Air deposition can introduce combustion byproducts, including PAHs from vehicle emissions and industrial activity. Those profiles are often broader and more mixed than point-source releases. Plant-produced hydrocarbons, such as terpenes and waxes, can also appear in environmental samples, but their structures are distinguishable from petroleum hydrocarbons. A sound forensic interpretation has to account for these background contributions before assigning a site-related source.

Analytical techniques allow us to identify and quantify hydrocarbons from different sources with progressively greater resolution. High-resolution gas chromatography with flame ionization detection is often the first step, or Tier 1, because it allows rapid comparison to reference chromatograms and a first view of hydrocarbon range and degradation. Whole oil analysis can capture profiles from C4 to C44. Where extraction is needed, saturated hydrocarbon analysis can quantify C8 to C40 alkanes and selected biomarkers. Gas chromatography-mass spectrometry (GC-MS) then separates and identifies individual compounds, which helps us select targeted volatile organic compound, PAH/APAH and biomarker analyses. High-performance liquid chromatography (HPLC) is useful for nonvolatile and thermally labile compounds, including certain PAHs and phenolic compounds. Stable isotope analysis adds another line of evidence by using carbon and hydrogen isotopic ratios to distinguish biogenic and petrogenic sources.

The code is cracked when forensic chemists combine these chemical markers and analytical techniques to distinguish between different types of hydrocarbon contamination. By understanding the signatures of fuel releases, crude oil spills, MGP residues and background hydrocarbons, we can identify the source more accurately and connect that answer to site decisions. Increasingly, clients rely on this approach to support effective remediation, equitable cost sharing and protection of environmental and human health.

The best place to start is with the source question itself. Once that question is clear, the analytical program can be built to answer it with precision.