By Srivindhya Kolluru
Deep beneath the boreal forest of the Athabasca region in Alberta, Canada, lie some of the largest reserves of low-grade petroleum. Known as bitumen, this thick mixture can be extracted and refined into oil.
While it is lucrative for oil and gas companies to process bitumen, doing so releases a broad class of chemicals known as polycyclic aromatic compounds (PACs) into the atmosphere. Most PACs are harmful to the environment and human health, but researchers haven’t studied them long enough to draw conclusions about their long-term effects.
In light of this, Dr. Cassandra Rauert and her team at Environment and Climate Change Canada set out in 2015 to monitor PAC levels to establish their historical trends in the Athabasca oil sands region (AOSR). Ideally, monitoring PACs from the past several decades would enable researchers to study how PACs interact with their environment and the communities that reside in the AOSR, as opposed to data that only goes back several years.
Instead of using conventional methods, Rauert and her team used tree cores — which uptake and trap certain volatile chemicals through their foliage — to measure the concentrations of atmospheric PACs over time.
The results of their work were published in a Science of the Total Environment study on June 5.
Rauert collected samples of Jack pine trees, which are common in the region, at 18 sites ranging from one to 110 kilometres (km) from the AOSR mining site, both in Alberta and the neighboring province of Saskatchewan.
Once the samples arrived at the team’s lab in Ontario, the researchers used ring sizes to group sections of the cores into five-year intervals, from 1940 to 2015, to graph historical concentrations of PACs at each site.
The researchers extracted the chemicals from the wood samples using a series of solvents and analyzed them using a gas chromatograph (GC) coupled to a mass spectrometer (MS). The GC vaporizes the sample which pushes it through the MS. There, the sample is ionized and the MS generates a spectrum of peaks according to its structural and chemical components. These spectra are matched up against existing libraries of these compounds.
The team detected 44 PACs in the samples which elicit different chemical effects based on their structural properties.
One of the authors, Dr. Tom Harner, who is also a scientist at Environment and Climate Change Canada, called the technique forensic monitoring. The method is akin to using fingerprints to solve a crime, in which evidence left behind can help solve a puzzle.
The results were consistent with previous studies of PACs in the region: the highest levels of PACs were found within 40 km of the perimeter of a mining site of the AOSR. Still, the measured concentrations of PACs at each site were below levels set by provincial guidelines.
These findings could indicate which communities are the most exposed to atmospheric PACs, including people from First Nations that have resided in the AOSR for centuries.
Harner noted that sampling tree cores for PACs is a cheaper and faster alternative to conventional methods that might require expensive instrumentation or that need to be deployed for weeks at a time.
In addition, unlike previous methods that used sediment or peat cores which uptake water-soluble and -insoluble PACs, the tree cores measure concentrations of atmospheric PACs alone, which are insoluble. This would depict a more accurate figure for atmospheric PAC concentrations in a region.
Examining tree cores allows researchers to go back in time, well before there was interest in measuring historical trends of PACs.
“We only have data from 2011 onward with these passive air samplers,” said Harner. “However, the tree cores provide an opportunity to go back beyond 2011 because we can go decades back and see what the levels of these compounds were 30, 40, 50 years ago and how they changed over time.”
Dr. Leah Chibwe, a postdoctoral fellow at the University of Toronto, has spent a majority of her scientific career probing the environmental effects of PACs. “It’s definitely a very interesting study, especially since there aren’t many long-term studies in the oil sands,” said Chibwe, who was not involved in this research.
In future studies, the team hopes to increase their sample size for more conclusive results.
“We might not know enough yet about the types of toxicities that these chemicals can elicit and at what levels,” said Harner. “But this information is helping to build that bigger picture of what the results mean overall.”
Srivindhya Kolluru graduated from the University of Toronto in June 2020 with a degree in biological chemistry. She previously held editor positions at The Varsity, Canada’s largest student newspaper. See her work at www.srivindhya.com and follow her on Twitter @vindhya_kolluru.
This story was produced as part of NASW's David Perlman Summer Mentoring Program, which was launched in 2020 by our Education Committee. Kolluru was mentored by Anne McGovern.
