Technical Bulletin: Biogenic Toluene
Does your Site show one or more of the following characteristics?
Analytical results showing anomalous toluene exceedances in delineation samples that were not found in Source samples.
Background samples with suspiciously high toluene concentrations.
Generally high toluene concentrations in the absence of benzene, ethylbenzene, xylenes, and trace level petroleum hydrocarbons.
Located within an area of highly organic soils and/or decomposing organic matter.
If so, biogenic toluene may be of concern. This bulletin aims to assist in identifying and justifying the potential presence of biogenic toluene on your Sites.
What is Toluene?
Figure 1: Chemical representation of benzene, toluene, ethylbenzene and ortho-, meta- and para- substituted xylene, BTEX.
Toluene is an aromatic hydrocarbon constituent present in several raw and refined petroleum products such as crude oil, gasoline, and diesel. It is the mono-substituted methylated derivative of benzene (Figure 1). Toluene is highly mobile, insoluble in water, carcinogenic, and can be harmful to wildlife and the environment. For these reasons, health-based guidelines have been established for toluene concentrations in soil and water in most Canadian jurisdictions including the Alberta Tier 1.
Most commonly, the presence of toluene in the environment is believed to be the result of petrogenic sources due to releases of toluene-containing hydrocarbon products. However, petrogenic sources are not the only toluene contributors in the environment. Under certain ecological conditions, toluene can form under a biogenic pathway. Specifically, from anaerobic microbial activity causing the oxidation-reduction transformation of biogenic organic compounds (BOCs) such as phenylalanine, phenylacetate and terpenes commonly found in peat soils within fen and bog wetlands (Figure 2).
Figure 2: Potential biogenic pathways of toluene formation. Path A illustrates the formation of toluene, via a phenylacetate intermediate, from phenylalanine (an essential amino acid found in plant and animal products). Path B shows toluene formation from
Determining the Source
The difficulty arises when attempting to characterize the source of toluene on a site as biogenic versus petrogenic. There are no chemical differences between biogenic vs petrogenic toluene and so careful consideration of the site and interpretation of data is required in order to determine the source accurately.
The following factors must be considered when determining toluene source in soil:
Concentrations of BTEX
Laboratory analytical identifying the presence of detectable concentrations of benzene, toluene, ethylbenzene, xylenes and hydrocarbons of the fractions 1 to 4 (F1 to F4) suggests the presence of petrogenic toluene. Biogenic toluene is often found in the absence of benzene, ethylbenzene, xylenes, and petroleum hydrocarbons (PHCs) F1 to F4.
Site Considerations
Biogenic toluene is most commonly found in rich organic environments such as peat, wetlands, and forested areas. The biogenic pathways discussed in Figure 2 are most efficient under anaerobic conditions. Therefore, biogenic toluene is often found within deeper subsoil depths.
Background and Source Sample Characterization
Based on the laboratory analytical for background samples and the product source sample; is toluene detectable in the site background samples but non-detectable in the product source? Although alone, this is not a definitive indicator of biogenic toluene, in consideration with other factors, can help to determine the nature of the toluene.
Sample Moisture Content and Saturation Percentage
Although not a conclusive tool to determine the toluene source, moisture content can be used indirectly to confirm the presence of highly organic soils such as peat that are often associated with biogenic toluene. Highly organic soils typically have a moisture content greater than 60% whereas moisture content in mineral soils is often below 40%. Consideration must be given to factors such as precipitation on site. Similarly, highly organic soils will often have a saturation percentage greater than 100%.
GC-MS/GC-FID Chromatogram Interpretation
Although the toluene signal alone on a gas chromatography (GC) chromatogram does not provide evidence of the source (petrogenic vs biogenic), the chromatogram can provide support of the presence of other biogenic compounds or petrogenic impacts. Interpretation of the GC chromatogram is one of the best lines of evidence to support the presence of BOCs found in peat and other soil types.
Additional Laboratory Analytical Testing for Biogenic Biomarkers
In situations where the toluene source is inconclusive after consideration of the factors above, additional laboratory analytical can be performed to determine the presence of precursor compounds of the biogenic toluene formation pathways. Laboratories will most often analyze for the presence of pinene, pyrene, cymene, etc. Interpretation of the biomarker compounds can assist in toluene source determination (Figure 3).
Figure 3: GC chromatograms of two samples containing toluene. Figure 3a shows a sample containing biogenic toluene; figure 3b, a sample containing petrogenic toluene. The speciation of cymene can be used to determine toluene source. Samples containing biogenic toluene will most often contain para-cymene with trace concentrations of meta- and ortho-cymene. Samples containing petrogenic toluene often contain para-, meta- and ortho-substituted cymene with p-cymene consisting of 20-60% of the total cymene within the sample (Chemistry Matters Inc, 2017).
Site Example:
During the remediation of a client’s transformer oil release, laboratory analytical indicated the presence of a single toluene exceedance in one of the confirmatory samples, SL09:1.2. Toluene in detectable concentrations was not found in any other sample at the site during remediation.
The site was situated in a heavily forested area with highly organic soils. Additionally, detectable toluene was not identified in the source sample and was only found in the deepest confirmatory sample at 1.2 metres below grade (mbg):
Interpretation of the GC chromatograms for SL09:1.2 showed peak signal patterns that were comparable to those seen for peat samples.
Based on these factors, we were able to justify the elevated toluene for sample SL09:1.2 as biogenic and no further remediation was required. (Ridgeline, 2021).
References
Chemistry Matters Inc (2017). Where does toluene come from? Petrogenic families and biogenic loners.
Richards, PL; Sandau, CD (2017). Forensic Source Attribution for Toluene in Environmental Samples. Environmental Toxicology and Chemistry – Volume 37, Number 3, pp. 729-737.
Ridgeline Canada Inc (2021). Release Remediation Report.
