Has anyone ever heard of naturally occurring toluene?
By: Anna Durante, Hydrogeologist
As we all know, toluene is a highly mobile, water-insoluble, toxic petroleum hydrocarbon compound that can be harmful to the environment/ public, so it’s pretty obvious why toluene can be problematic in even small concentrations (and explains why it has very strict guidelines). However, it is an organic chemical compound and specific ecological niches support the accumulation of biogenic toluene and therefore, can confound remedial investigations of petroleum impacts.
Common orientation for biogenic toluene:
- Northern Environment;
- Rich organic environment, like peat and wetlands;
- Limited nutrients;
- Low oxygen/anaerobic; and
- Limited decomposition pathway present.
So, How do we identify toluene’s orientation?
- The best way to identify if toluene is biogenic is by looking at the chromatography for the sample. Specifically, at the cymene values – biogenic toluene will have higher or differing concentrations, while petrogenic toluene has fixed concentrations for cymene in PHCs (ie. distinct fingerprint). Pyrene, cymene, benzaldehyde, etc. are common constituents you can see on the chromatogram to know if it’s biogenic, which gives biogenic toluene a highly specific chemical pattern;
- If you plot toluene ratio vs. cymene ratio, there is a clear diagnostic ratio between petroleum and biogenic toluene (biogenic toluene has a higher ratio); and
- If you look at the rest of the BTEX compounds (benzene, ethylbenzene, xylene) in an environmental setting, these concentrations will be very low or are not common at all in biogenic toluene. In PHCs, that is not normally the case.
So, how do we test for biogenic toluene?
- The determination of the origin of toluene that is acceptable by regulatory bodies requires a scientifically credible demonstration using a forensic analytical approach, and forensic data interpretation;
- Application of arson analysis methodology and forensic data interpretation methods are used to distinguish if toluene is biogenic or petrogenic. Arson analysis are conducted for crime scene investigators to provide legal proof for the presence of petroleum distillates, if present, in relation to arson investigations. The same method can be applied in the environmental field for the determination of toluene; and
- A minimum of 10-20 samples would need to be collected in order to carry out this analysis, and high summer (July/ August) is the best time to collect these samples.
So, next time you are testing for PHCs in a wetland environment, and you see higher toluene values and low/ ND benzene, ethylbenzene, and xylenes values, consider paying close attention to the chromatography and plotting ratios for toluene vs. cymene to help determine the orientation of toluene before initiating a remediation plan.
Source: AGAT Tech Talks 2017, “Using an Effective Method for Differentiation, Where Does Toluene Come From: Petrogenic Families and Biogenic Loners?” By Dr. Richards, Chemistry Matters.
Note: Chemistry Matters is in the process of publishing a paper related to this topic, and is expected to be out Summer 2017. A summary of the presentation is linked here.