What is Diesel?
A distillate of oil, diesel is a mixture of hydrocarbons with a boiling point range from around 200 to 350°C. In reality, it is more accurate to describe diesel with vapor pressure rather than boiling point, but the distinction is similar. For the compounds that diesel contains, the diesel mixture generally equates to a range from nC10 to nC22, where n signifies straight chain alkane (n=normal) and the number represents the number of carbons in the chain. Therefore, diesel contains all the compounds between decane (10) and docosane (22). All of the hydrocarbons from the oil source within this range are contained within this diesel cut, thus making diesel a large, complex mixture of hydrocarbons.
Diesel is most commonly used as a fuel for vehicles and home heating, but also finds use for its solvent properties and is used in drilling mud in the oil and gas industry. These uses tell us what to expect for types of diesel impacts in the environment.
The addition of fatty acid methyl esters or FAME, into diesel produces a biodiesel product that is still suitable for standard diesel combustion engines. FAME is derived from esterification of fatty acids, themselves most often coming from vegetable oils. The presence of FAME in diesel is readily observed using relatively standard analytical techniques and can be a useful distinction for specific investigations as it is an age-related marker, and also may be attributable to a particular source if multiple sources are suspected.
Diesel contains only low concentrations of benzene, toluene, ethylbenzene and xylenes (BTEX), which are more concentrated in a lighter distillation cut used for gasoline. However, even though they are only present in low concentrations in diesel, groundwater plumes of BTEX can originate from diesel.
Diesel contains polycyclic aromatic hydrocarbons (PAHs), a chemical class often used to assess risk and is used as the remedial benchmark for diesel spills and releases in the environment. PAHs can thus be used as source indicators and for fate and transport studies for diesel or mixed contamination sites where diesel is a suspected impact.
Where do we find diesel in the environment?
Most often environmental investigations relating to diesel will fall into two categories:
- Releases from storage tanks and pipework,
- Impacts relating to drilling waste.
In addition, spills relating to transport, for example via tanker trucks may be an emergency response action relating to diesel.
Thus, diesel impacts at gas stations, as well as heating fuel tanks make this a distinctly urban contaminant with moderate mobility in the subsurface.
- 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
- Soil gas sampling
- Surface casing vent sampling
- Arsonous Wildfires
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Environmental Forensics relating to diesel
Simple identification of diesel in environmental samples can be conducted using standard total petroleum hydrocarbon (TPH) or Petroleum Hydrocarbon (PHC) chromatogram analysis. This can even be used to determine if the diesel is fresh or weathered to some degree.
More detailed analysis of diesel uses the range of chemicals within the distillation cut. Diagnostic chemicals are present in diesel. These include the familiar isoprenoids as well as sesquiterpanes and diamondoids, which can be analysed using fingerprint patterns as well as statistical analysis. These diagnostic chemicals can help differentiate different sources of diesel and are particularly relevant if example suspect source samples are available for analysis.
Patterns of alkylated benzene, alkylated naphthalene and alkylated indane congeners provide a way to distinguish sources of diesel, and we have adapted this kind of analysis from a suite of chemistry that we apply for arson analysis. As with normal biomarker analyses, these can be used to answer typical forensic questions such as differentiating sources.
Of caution, the composition of diesel can vary over time and between different producers, because the refinery where the diesel is produced can receive a number of different crude oil sources. This means that a specific source, “gas station A”, may not be represented by a static diesel signature, and we wouldn’t want to attribute a pattern with “gas station B”. Knowing the timescale of the release can be very important for forming that connection between source samples and environmental samples. Each tank at different gas stations represent a unique source of diesel that has been purchased from various suppliers and filled at different rates. These unique sources can be chemically fingerprinted and can be distinguishable using comprehensive analytical techniques such as GC×GC analysis.
We have used these forensic lines of evidence to investigate mixed plumes of diesel and gasoline that overlap from different sources to be able to determine the source of chemicals of concern.
Fate and Transport
When found on surface water or impervious hard surfaces, diesel can lose a substantial mass quickly, leaving behind the less volatile chemicals since it can degrade quickly. This degradation is generically referred to as ‘weathering’. However, diesel can accumulate on ground water and migrate as a free-phase on top of the groundwater level, which is a common issue due to the storage of diesel in underground storage tanks for vehicle fuel and home heating fuel.
General weathering principles can be followed for any hydrocarbon mixture, and diesel 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 affect the composition of diesel in the same manner following the physical-chemical properties of the compounds, and thus, weathering can be observed will follow a standard series of changes.
Ratios of certain chemicals can even be 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.
CMI have experience in litigation matters relating to diesel and other petroleum hydrocarbons. CMI personnel have extensive experience in the collection, analysis, identification, chemical fingerprinting of diesel 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 diesel or other petroleum hydrocarbons.
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