Anthropogenic sources of toluene are generally associated with mixtures of different volatile and/or semi-volatile compounds, most notably benzene, ethylbenzene and xylenes (BTEX). In fact, over 60% of the water-soluble portion of gasoline fuel is made up of the BTEX compounds. As a component of petroleum crude it is identified as a contaminant from oil spills, disposal of petroleum-related products, emissions of gasoline and diesel fuels, or other combustion processes. 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 produced naturally within pristine environments, particularly in oxygen-deficient regions of water bodies, and in soils with high organic content. Studies point to anaerobic microbial activity as the source, specifically the redox transformation of naturally occurring organic matter decomposition by-products such as terpenes, phenolic acids and amino acids.
There is no chemical difference between biogenic (natural) and petrogenic (petroleum based) toluene. To determine the origin of toluene in a sample it is necessary to compile “multiple lines of evidence”. This process involves an in-depth review of existing analytical data supplemented with additional data from analyses specifically tailored for the determination of biogenic toluene. A summary of the approach taken by Bureau Veritas is outlined below.
Bureau Veritas has developed a staged approach for processing samples suspected to be impacted by biogenic toluene. The process ensures all samples are processed in a systematic and standardized manner across the Bureau Veritas network. Procedures are routinely reviewed and updated to ensure they continue to meet client and regulatory requirements.
Stage 1 – Initial Testing
The process starts with the initial (routine) analysis of the test samples. More data for review is always better, but the tests outlined below represent the recommended minimum.
Hydrocarbon Analysis (BTEX, CCME F1-F4 or VPH/EPH): The presence of a mixture of BTEX parameters as well as hydrocarbons that span several fractions is a good indicator of a petrogenic impact. In contrast, a sample containing primarily toluene with little or no other associated hydrocarbons, or hydrocarbons isolated to specific fractions may be an indicator of biogenic toluene;
Moisture Content: The soil’s moisture is used as an indicator of organic content. Highly organic soils, such as peat, are often associated with biogenic toluene. Typical mineral soils have moisture content values below 40%. Highly organic soils typically have a moisture content higher than 50%, up to as high as 95%.
Stage 2 – Secondary Data Review and Supplemental Analysis
If the Stage 1 results suggest a sample may contain biogenic toluene, and the concentration associated with that sample equals or exceeds that of an applicable regulatory limit, Bureau Veritas would recommend proceeding with a Biogenic Toluene analysis. This analysis is comprised of the following actions:
Secondary Data Review: The Stage 1 data is reviewed to draw out lines of evidence to support the hypothesis that the toluene is biogenic. Particular attention will be given to the analytical data outlined above along with associated chromatograms. Chromatograms are a critical tool in determining the source of the impact. Petroleum products have characteristic chromatographic peak patterns that are easily distinguishable. Many organic materials such as peat typically present isolated clusters of peaks within specific carbon ranges.
Biogenic Biomarkers: This is a supplementary GC/MS analysis used to detect the presence of a specific class of compounds known as monoterpenes. Most of these are associated with the biological production of toluene, and others are typically associated with petroleum-based products. The analysis and subsequent evaluation of the data will provide critical insight into the compound’s origin. No additional sample is required for this analysis as it can be performed on the previously submitted BTEX/F1 sample. As the analysis is qualitative, hold time is not a concern.
Upon completion of Stage 2, Bureau Veritas will issue a report providing a conclusion, along with the data used to support that conclusion. On the rare occasion that the finding is inconclusive, we can offer a Stage 3 option.
Stage 3 – Compound Specific Isotope Analysis (CSIA)
CSIA analysis is subcontracted to a partner laboratory and provides a ratio between the two stable isotopes of carbon: 12C (the predominant form) and the heavier 13C. This ratio has been observed to differ significantly between biogenic and petrogenic sources. It is important to note that the CSIA analysis should be considered as another “line of evidence” for use in data interpretation.
Water samples are collected within 40mL vials (zero-headspace) preserved with sodium bisulphate. Soil samples are collected in 40mL vials preserved with methanol and a soil jar is also required for moisture determination.
For samples collected for Stage 1 analysis, the standard hold time is 40 days for field preserved toluene in soil and 14 days for water based on the CCME guidance manual (2016).