Globally-significant arsenic release by wildfires in a mining-impacted boreal landscape

Metal mining and smelting activities are one of the largest anthropogenic sources of arsenic pollution to the environment, with pervasive consequences to human and environmental health. Several decades of metal processing activities near Yellowknife, NT, Canada have resulted in widespread accumulation of arsenic in biomass, soils, and sediments, exceeding environmental and human health limits. The landscape surrounding Yellowknife is frequently disturbed by wildfire, most recently in 2023, when 2500 km2 burned. While wildfire-mediated release of stored arsenic around Yellowknife likely represents an incipient threat to human and ecosystem health, a quantification of the potential magnitude of arsenic remobilization from wildfires is absent. Here we combine publicly available soil and biomass arsenic concentrations and land cover datasets with the current best estimates of pyrogenic arsenic speciation and release in upland and wetland ecosystems to estimate the potential range of arsenic remobilization due to wildfires in the region surrounding Yellowknife from 1972 to 2023. Since 1972, wildfires have potentially led to the release of 141–562 Mg of arsenic, with 61–381 Mg emitted to the atmosphere and 39–109 Mg mobilized as water-soluble species. The large range in potential atmospheric emissions was due to the range in peat emission efficiency (5%–84%) that resulted in more arsenic being released from wetlands than the uplands. In 2023 alone, our estimated atmospheric release from just four wildfires was between 15%–59% of global annual arsenic wildfire emissions and likely represented between 2 and 9% of total global arsenic emissions from all natural sources. Given that climate change has and will continue to increase both annual area burned and soil burn severity, we emphasize that future increased wildfire activity closer to Yellowknife will place legacy soil arsenic stores at risk of an even larger catastrophic and unprecedented release, especially as wetlands become drier.