Utilizing polycyclic aromatic hydrocarbons as an indicator of post-wildfire contamination and as a tracer for source apportionment in a large watershed in central British Columbia, Canada

The Nechako River Basin in central British Columbia is a large (52,000 km2), regulated basin that supports sockeye and chinook salmon and the Nechako white sturgeon. These species have been experiencing population declines and one potential cause of this decline is excess sediment, which clogs their spawning habitat, reducing juvenile success. The basin is under pressure from a dammed reservoir, industrial forestry and agriculture, the Mountain Pine Beetle epidemic, and large wildfires in 2018 and 2021, each of which influence the river's sediment regime. This research aims to identify the connectivity between upland burned areas and adjacent waterways after the 2018 wildfires by determining spatial and temporal contamination by polycyclic aromatic hydrocarbons (PAHs) in the Nechako River and its tributaries. PAHs are produced during the combustion of organic matter and may be toxic to aquatic organisms and humans. Source soil samples were collected in 2018 from burned and unburned sites, the former being re-sampled in 2020 and 2021, while suspended sediment samples were collected from 2018-2021 in three tributaries and three mainstem sites. Samples were analysed for PAHs and other properties such as loss-on-ignition, particle size, and colour. Results from the fall 2018 source samples show a significant difference in PAHs between unburned and burned soils, and concentrations in burned areas in 2020 remain elevated compared to unburned soils. Elevated concentrations of PAHs were detected in tributaries in the first year after the fire however, the highest values were found at the most downstream site on the Nechako mainstem, located in a town with some industry, suggesting an additional PAH source. The highest concentrations were found in samples taken during the spring snowmelt period in 2019 but are well below sediment quality guidelines for total PAHs. This study also explored the use of PAHs as a sediment tracer to determine if burned areas were a more significant contributor of fine sediment than unburned areas. Initial modeling shows that burned organic material can be a significant source of sediment to the river, but that unburned material is still the dominant source, likely due to the lack of observed overland erosion in the basin and significant contributions from unburned banks.