Toluene is often identified as a contaminant in a variety of environmental media. It is a naturally occurring component of petroleum crude and, therefore, identified as a contaminant from oil spills, disposal of petroleum related product, emissions of gasoline and diesel fuels or other combustion processes.
Anthropogenic sources of toluene impact are generally associated with mixtures of different volatile and/or semi-volatile compounds, most notably alongside benzene, ethylbenzene and xylenes (BTEX). In fact, over 60% of the water soluble portion of gasoline fuel is made up of the BTEX compounds. Furthermore, toluene has been widely used as a key component of various industrial and commercial products including solvents, adhesives, cleaning and degreasing products, and pesticides.
However, anthropogenic sources are not the only contributors of toluene. Numerous studies have shown that toluene can be naturally found in pristine environments, particularly in oxygen deficient regions of surface water bodies and soils with high organic content. Studies point to anaerobic microbial activity as the source, specifically the redox transformation of naturally occurring organics such as phenyalanine, phenyllactate, phenylacetate and phenylpyruvate to toluene.
The difficulty arises when attempting to compare quantified concentrations of toluene in water or soil samples to applicable criteria. Often times, the concentrations of naturally occurring (biogenic) toluene vary from just above to less than 10x RDL (0.02 to 0.2 ug/g in soils and 0.2 to 2 ug/L in water). Furthermore, there is no chemical difference between biogenic and petrogenic toluene. The key lies in the interpretation of the data and any evidence that would suggest the presence of either biogenic material or petrogenic impact in the sample.
Bureau Veritas Laboratories has developed a staged approach for addressing situations where biogenic toluene is suspected. Since each project is different, sufficient lines of evidence may be drawn from existing analytical data in support of conclusions that address project objectives. However, subsequent, more detailed, analyses may also be necessary.
Stage 1 – Review and Interpretation of Routine Petroleum Hydrocarbon (PHC) Data
The value of routine analytical data and the evidence it presents is often overlooked. The list below outlines key lines of evidence towards confirming presence of biogenic material or petrogenic impact.
- Quantitative results (BTEX, C6-C10 and C10-C50): The presence of a mixture of quantified BTEX parameters as well as hydrocarbons that span several fractions is a good indicator of petrogenic impact. In contrast, quantified low level concentrations of toluene without any other hydrocarbons detected may be an indicator of biogenic related toluene;
- Moisture Content for soil samples may also be used as a line of evidence to confirm high organic content soils, such as peat, that are often associated with biogenic toluene. Typical mineral soils have moisture content values below 40%. High organic content soils such as peat and bog materials have moisture content that can be much higher than 50%;
- Chromatogram Interpretation: A review of the chromatographic profiles in the C6-C10 and C10-C50 ranges, including comparison to petroleum standards such as common fuels and crude oils, provides a good line of evidence to support the presence of either biogenic material or petrogenic impact. Petroleum products have characteristic chromatographic peak patterns that are easily distinguishable. Biogenic material such as peat present a random pattern of peaks within specific carbon ranges.
Stage 2 – Biomarkers and Open Scan Analysis
In situations where initial data review and interpretation are inconclusive or additional lines of evidence are required, Bureau Veritas Laboratories has several testing options.
- Biogenic Biomarkers: This is a GC/MS analysis that targets specific biomarker compounds:
The compounds listed in the first two columns above are indicators of biogenic material. The presence of all three cymene isomers is an indication of petrogenic impact. however, The presence of p-cymene only suggests biogenic material.
α-Pinene α-Terpinene p-Cymene β-Pinene Limonene o-Cymene Camphene γ-Terpinene m-Cymene (+)-3-Carene α-terpinolène
- Open Scan GC/MS Analysis can be applied to both volatile and semi-volatile compounds and is used to identify other types of compounds present in a sample that may support the presence of biogenic toluene. The presence of petroleum-related compounds such as n-alkanes or aromatics is a good line of evidence to suggest petrogenic impact. Identifying other biogenic compounds such as terpenes and sterols would support the presence of biogenic material.
Stage 3 – Compound Specific Stable Isotope Analysis (CSIA)
For projects where the data remains inconclusive, CSIA analysis of the 13C/12C ratio in the measured toluene may be appropriate. This test provides an indication of the ratio between the two stable isotopes of carbon: 12C (the predominant form) and 13C. This isotopic ratio may be an indicator of the source and formation of toluene, and will differ significantly between biogenic and petrogenic sources.
The reported δ13C values for petrogenic toluene typically fall in the rage of -22 to -30‰. Biogenic toluene has δ13C values well below -30‰.
To ensure that there is adequate sample to run multiple analyses without having to revisit the site, it is recommended that samples be collected in duplicate (at a minimum). Toluene is part of the volatile suite of compounds. Samples are collected in 40mL vials and preserved with methanol for soil samples and with sodium bisulphate (zero-headspace) for water samples. For soil samples, an additional jar of non-preserved soil is needed to determine the percent moisture.
The standard hold time for toluene in soil is 40 days based on the CCME guidance manual (2016), or 14 days (with the option of extending to 40 days) based on the Analytical Protocols referenced in Ontario Regulation 153/04. In water samples, the standard hold time for toluene is 14 days. Although forensics analyses are largely qualitative in nature and are not necessarily covered under regulatory frameworks, all efforts should be made to analyze samples within the hold times listed above.