The impact of wildfires on the abundance of polycyclic aromatic hydrocarbons (PAH) in soils of Northwestern Canada

The intensifying wildfire regimes under climate change, as expressed, for example, in the recent fire seasons in the boreal zones, call for an improved understanding of the impacts of forest fires on air, water and soil quality. One group of compounds released during wildfires are the polycyclic aromatic hydrocarbons (PAHs), of which many are considered toxic for organisms, including human health.

This study investigated 13 sites along a transect across five boreal forest and tundra biomes for the abundance and composition of 16 USEPA listed PAHs in soil organic and mineral horizons. Accelerated solvent extraction in a combination with organic solvents (MeOH:DCM and n-hexane:DCM)  was used for PAHs extraction  and subsequently analyzing them using an Agilent 7890A gas chromatograph coupled to an Agilent 5975C mass spectrometer. We tested how far distance to the nearest fire event (<1 km up to 73 km), years since the fire event (1989 to a recent burn that extinguished few weeks before sampling in August 2022) and fire intensities calculated using MODIS Thermal Anomalies/Fire Location Collection 6.1 (MCD14DL) and LANDSAT based dNBR indices affected PAHs concentrations and composition across sites and in organic and mineral horizons, respectively.

Our results revealed significantly higher PAHs concentrations in freshly burnt sites, with litter samples showing values up to 17181 ng/g, surpassing the strict regulatory thresholds set by Canadian government. Phenanthrene was the only PAH significantly more abundant in organic compared to mineral horizons. At older fire sites (>15 years), total PAH concentrations declined significantly (range: 0.7 to 1806.7 ng/g) in comparison to recent fire sites, likely due to degradation and wash-out processes. Litter horizons generally exhibited higher PAH levels than organic and mineral horizons, with high molecular weight (HMW) PAHs dominating (~30% LMW vs. ~70% high molecular weight). Apparently, the distance to the fire source had no significant effect on PAHs abundance. However, fire intensity, as indicated by fire radiative power (FRP) and dNBR, correlated with PAHs levels in the litter horizon, suggesting that temperature and combustion conditions are critical determinants of PAHs formation and persistence. Diagnostic PAHs ratios also confirmed the predominance of pyrogenic sources. These initial findings highlight the post-fire loss of PAHs via degradation and wash-out, reducing soil toxicity over time. More research might focus on a high-resolution soil and water monitoring shortly after wildfires to better understand how far degradation and wash-out dominates PAHs loss that could shift burning residues from soil to water biomes.