What is Gasoline?
Gasoline is a complex chemical mixture of refined petroleum produced from crude oil. Gasoline is primarily composed of carbon and hydrogen atoms, typically ranging from four-carbon compounds (C4; butane) to twelve-carbon compounds (C12; dodecane). This makes it a lighter petroleum product, produced from the earlier cuts of fractional distillation when crude oil is refined.
Gasoline contains high concentrations of benzene, toluene, ethylbenzene and xylenes (BTEX), petroleum hydrocarbons Fraction 1 and Fraction 2 (PHC F1, F2), and contains various proprietary additives by different suppliers for engine performance.
Gasoline does not contain significant amounts of polycyclic aromatic hydrocarbons (PAHs), which is often used to assess risk and is used as the remedial benchmark for spills and releases of petroleum products in the environment. However, gasoline contains naphthalene, a 2-ring PAH, and sometimes alkylated-naphthalenes, which are naphthalenes with one- or two- carbon groups attached, such a 1-methylnaphthalene. The ratios of naphthalene and 1- and 2-methylnaphthalene can be used as one of many tools to differentiate gasoline from diesel, which has much higher concentrations of naphthalenes.
Where is gasoline found in the environment?
Most often environmental investigations relating to gasoline will be as a result of releases from storage tanks and pipework, particularly at sites of old gas stations where underground storage tanks and pipes can leak. The design of underground storage tanks has been updated over the years to include more stringent safety standards to prevent leaks. These sites are often located in urban and residential areas. In addition, spills relating to transport, for example via tanker trucks may be an emergency response action relating to diesel.
Gasoline impacts from gas stations make this an urban contaminant with moderate mobility in the subsurface, with the possibility of reaching the groundwater table. Gasoline is a light non-aqueous phase liquid (LNAPL), meaning it will not mix with water and will float on top.
- Polycyclic Aromatic Hydrocarbons (PAHs)
- Ignitable Liquid Residue (ILR)
- Petroleum Hydrocarbons (TPH/PHC F1 to F4)
- Crude Oil
- Petroleum Biomarkers
- Benzene, Toluene, Ethylbenzenes, Xylenes (BTEX)
- Natural Toluene
- Volatile Organic Compounds (VOCs)
- Legal Sampling
- Chain of Custody
- Study Design
- Data Analysis and Visualization
- Data Wrangling
- Multivariate Statistical Analysis
- Principal Component (PCA); Hierarchical Cluster (HCA)
- Science Communication
- Data Science/Big Data
- Multidimensional Gas Chromatography (GC×GC)
- Source Apportionment
- Chemical Fingerprinting
- Diagnostic Ratios
- Fate and Transport
- Arsonous Wildfires
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Environmental Forensics Relating to Gasoline
Simple identification of gasoline in environmental samples can be conducted using standard BTEX analysis in conjunction with Total Petroleum Hydrocarbon (TPH) or Petroleum Hydrocarbon (PHC) analysis. These tests can even be used to determine if the gasoline is fresh or weathered to some degree.
Often, the simple identification of gasoline is not enough for an environmental forensics investigation. In most instances, a spill may have occurred where the party or parties responsible is in doubt, for example, where there are two gas stations located close together. Liability may also be dependent on when the spill occurred. In both scenarios, a more detailed chemical analysis of soil and groundwater is required.
Diagnostic chemicals are present in gasoline, such as lead-containing compounds, alcohols, and ethers, which can be analysed using fingerprint patterns as well as statistical analysis. Chemistry Matters has also used patterns of various compounds found in gasoline, such as alkylated benzenes, alkylated naphthalenes, and indanes, to distinguish sources of gasoline (gas station identification). These diagnostic chemicals can help differentiate sources of gasoline, ages of gasoline, and are particularly relevant if suspected source samples are available for analysis. These chemicals can be analyzed in product samples, soil samples, and groundwater samples to potentially identify sources of gasoline. Specialised analytical techniques, such as comprehensive two-dimensional gas chromatography (GC×GC) and compound specific isotope analysis (CSIA), are also powerful tools used to fingerprint gasoline samples in order to determine the source. An example of using GC×GC for a contaminated site with multiple gas stations in close proximity is shown in Figure 1, where two different patterns of contaminants are clearly visible. Further data processing provides a comprehensive fingerprint of each sample.
Fate and Transport
When found on surface water or impervious hard surfaces, gasoline can lose a substantial amount of mass quickly, leaving behind the less volatile and less soluble chemicals since it can ‘weather’ quickly. However, gasoline can accumulate on ground water and migrate as a free-phase on top of the groundwater as LNAPL, which is a common issue due to the storage of gasoline in underground storage tanks for vehicle fuel.
General weathering principles can be followed for any hydrocarbon mixture, and gasoline is no exception. For releases exposed to the air, the release can change composition quickly. In the subsurface, that change will be slower and affected most by volatility, biodegradation, and water solubility. However, these weathering mechanisms generally affect the composition of gasoline in the same manner, and thus, weathering can be observed to follow a fairly standard series of chemical changes.
Ratios of certain chemicals, such as BTEX and other ratio compounds, can even be cautiously used to estimate the timeframe of a release if it occurred within the subsurface rather than on the surface and migrated underground. This can be done if sufficient samples are acquired and submitted for the requisite analytical processes and the nature of the subsurface is understood.
Chemistry Matters has experience in litigation cases relating to gasoline and other petroleum hydrocarbons. Chemistry Matters personnel have extensive experience in the collection, analysis, identification, and chemical fingerprinting of gasoline and related petroleum hydrocarbon mixtures for the purpose of litigation proceedings. These include environmental forensics investigations examining the source and timing as well as source apportioning multiple sources of gasoline or other petroleum hydrocarbons. Chemistry Matters has also been involved in forensic chemistry cases examining fire debris samples as part of arson, wildfire, and criminal investigations, which includes the analysis of gasoline as an ignitable liquid residue as well as the identification of gasoline sources used as accelerants to start fires.