Petroleum Hydrocarbons (PHCs)
“With the overwhelming number of potential unique markers present in oil and refined products, in depth knowledge of these components allows the trained environmental forensics expert to distinguish between even very similar sources.”Court Sandau
Petroleum Hydrocarbons (PHCs) is the name given to a very broad range of chemicals that comprise oil and products refined from oil, such as gasoline and diesel. This mixture of chemicals can also be described by common chemical characteristics such as boiling point ranges or size of the molecules. These characteristics are very important for determining the value of a crude oil. For example, a light crude contains mainly small, valuable chemicals, whereas a heavy bitumen contains mainly larger, less valuable chemicals, which must be refined more thoroughly to add value.
The chemical characteristics of PHCs are also important when assessing potential impacts of spilled material. However, this poses a problem, as it is simply not practical to measure each and every chemical associated with PHCs.
Certain groups of chemicals within PHCs present specific toxicological hazards, for example benzene and toluene, and the polycyclic aromatic hydrocarbons (PAHs), and these chemicals are individually measured in a PHC mixture.
The remaining PHCs are often grouped together with a specific health guideline for the group or groups. The nature of how this is carried out is not globally unified, and the details can be rather complicated. Measurement methods vary between countries, between provinces and states within a country, and even tailored for a specific location, depending on the nature of the PHCs.
The simplest method is called total petroleum hydrocarbons (TPH), which is an estimated value for the total of PHCs. However, TPH does not recognize the range of toxicity within PHCs.
In Canada, most provinces apply the guidance from the Canadian Council of Ministers of the Environment (CCME) for PHCs. The CCME method recognizes that the range of PHCs presents a scale of toxicological hazards; the lighter PHCs present a higher hazard than the heavier PHCs. Thus, the CCME guidance for PHCs splits the analysis of PHCs into 4 fractions, generally based on molecular size. Each fraction has a specific guideline for toxicological hazards, so that overall the method can account for the different hazards associated with, for example, a light crude oil and a heavy bitumen oil.
In the US, TPHs are often separated into specific classes such as gasoline range organics or diesel range organics. As with the Canadian method, this attempts to account for the different hazards associated with different types of PHC product. However, some US states require the separation of different classes of chemicals called aliphatics and aromatics.
PHCs also act like a fingerprint of the oil product and are commonly used in environmental forensics investigations. Specific groups of chemicals can be used to provide a forensic determination of where a spill may have originated, the age of the spill and ultimately linking the spill to a responsible party. The ‘fingerprint’ of PHCs is comprised of a number of highly specific characteristics, these are produced by the following range of mechanisms:
- The nature of the living material that originally died and decayed, millennia ago,
- The geological mechanisms of heat, pressure and chemical catalysis that converted the organic material to produce oil,
- Upgrading and refining of oil,
- Environmental weathering, such as evaporation and microbiological alteration.
Different types of hydrocarbon sources can be identified by examining the general composition of the PHCs. At its simplest level this can be done by analyzing the GC chromatogram, or extracted ion chromatograms and comparing this to examples of different product types. We have used this type of analysis to distinguish many different types of impacts, such as gasoline and diesel components of a mixed impacted site as part of a source allocation project.
A more thorough and more precise analysis can be completed when analyzing specific groups of chemicals within the PHCs. Many of these are called biomarkers. These are chemicals that retain some general structure from the biology. Comprehensive analysis of biomarkers presents a key method of determining the origin of a spill or to characterize oil products. Other important chemical groups such as PAHs and naphthenic acids can also help distinguish very similar oil products. With the overwhelming number of potential unique markers present in oil and refined products, in depth knowledge of these components allows the trained environmental forensics expert to distinguish between even very similar sources.