What is biogenic toluene?
Toluene is a petroleum hydrocarbon that is common in crude and refined products. It is often detected and reported along with benzene, ethylbenzene and xylene isomers. A word of caution about toluene as well, it is highly ubiquitous and is found in tape, pen ink, and adhesives which commonly cause incidental contamination of environmental samples.
Toluene can also be formed naturally, as part of a decomposition pathway within specific ecosite types. The presence of natural toluene in these ecosystems was problematic when petroleum releases were also in these areas. Since toluene has environmental guidelines that need to be met and would be present in both the natural ecosystem as well as from the release, environmental practitioners required a method to distinguish and quantify natural toluene. CMI produced a peer-review scientific paper discussing the formation of toluene in natural environments, and the forensic approach to definitively determine the source of toluene as either petrogenic or biogenic (Richards and Sandau 2017).
Toluene from a petrogenic source will be present with other alkylated benzenes, such as ethylbenzene and xylenes, and many other more complex alkylated benzenes. Toluene from a biogenic source will not be present with these other alkylated benzenes, except for para-cymene, which is formed as part of the same biological decomposition process from terpenes, in particular alpha-pinene, a major chemical of pine resins. Benzaldehyde may also be present, which is the decomposition product of toluene. This pathway, as well as a possible pathway from phenylalanine, is presented in Figure 1.
Where do we find biogenic toluene in the environment?
We have detected biogenic toluene in air, water and soil samples, including peat samples. This is because toluene has a high volatility and high water solubility for an organic chemical, meaning detectible concentrations can readily be found in all matrices.
The detection of toluene in uncontaminated samples is most obvious in ecosites that are anaerobic, acidic, highly organic and contain a low mineral content. We have observed highest concentrations of toluene in treed poor bog (type i1). Toluene was also detected in treed poor fen (type j1), however at much lower concentrations. Based on most likely formation mechanisms, the detection of toluene in a j1 fen, is most likely due to migration from adjacent i1 bog. Figure 2 shows typical concentration ranges of toluene from sampling locations in treed poor bog and treed poor fen.
Detection of toluene in peat and water samples from northern Alberta shows a strong spike in July. This is indicative of a biological response to seasonality and an indicator for a biogenic source. A petrogenic source would be either unresponsive to seasonality, or even show reduced concentrations in summer due to dilution from melt-water and increased rates of loss by weathering mechanisms. Figure 3 shows a typical profile of toluene concentrations from sampling locations in northern Alberta, with a clear strong increase in early summer.
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