In 2020, wildfires burned 10.3 million acres across the United States and produced smoke that traveled as far as northern Europe. While wildfires introduce many obvious challenges and sometimes tragic consequences, they also present less obvious concerns such as polycyclic aromatic hydrocarbon (PAH) compounds released into the air and via ash emissions. Those working to characterize and/or remediate former industrial sites with residual PAHs should be aware that PAH particulates can be transported as components of wildfire smoke and ash and require chemical forensics and statistical analysis to distinguish between legacy impacts and wildfire sources.

PAH and former industrial sites

Any soil scorched by wildfires or encountered by associated smoke and ash is subject to PAH deposition, a reality that complicates soil characterization at former industrial sites where anthropogenic PAH contamination is also a concern. With the rising intensity and magnitude of wildfires, an understanding of potential PAH sources, fate, and transport, and how to distinguish types of PAH soil contamination will become increasingly important at former industrial sites undergoing characterization, risk assessment, remediation, rezoning, or redevelopment.

Many PAHs have toxic, mutagenic, and/or carcinogenic properties that can affect human health (ATSDR, IRIS), and similarly, many PAHs are harmful to the health of wildlife, causing damage to growth, longevity, and reproductive ability.  As a result of these human-health and ecological impacts, some chemicals within the group are under the purview of various regulatory frameworks including the Emergency Planning and Community Right-to-Know Act (EPCRA; 40 CFR Subpart J), Resource Conservation and Recovery Act (RCRA; 40 CFR 261.33), Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Toxic Substances Control Act (TSCA), Clean Air Act (CAA), and Clean Water Act (CWA). In conjunction with human health-based risk evaluations, assessment of ecological receptors for a site is imperative when wildfire-sourced PAHs may be present, as ecological screening criteria for PAHs can be lower (e.g. for sediment) than those for human health, and thus more easily exceeded by wildfire PAH contribution.

Determining PAH compound origins

Investigating the source of PAHs when characterizing soil at former industrial sites can help identify potentially responsible parties and the degree to which those parties are responsible for remediation. The specific chemical signature of produced PAHs is dependent upon conditions such as temperature, oxygen, and process duration. As a result, certain PAH molecules are indicative of the processes by which they can be generated, and can therefore be used as indicators of a contamination source. The fraction of low molecular weight (LMW) versus high molecular weight (HMW) PAHs present is another characteristic that helps identify the contamination source. Human-induced combustion processes, such as the burning of coal, petroleum, trash, and other organic matter mostly produce HMW molecules, while combustion associated with wildfires predominantly produces LMW chemicals. The molecular weight of PAHs also directly affects contamination persistence, with LMW PAHs being more water-soluble, prone to biodegradation, volatile, and mobile than HMW PAHs (Igwo-Ezikpe et al., 2010; Kanaly and Harayama, 2000; Ravindra, Sokhi, and Grieken, 2007). Examining PAH distribution and characteristics using forensic techniques, such as the presence of key marker compounds, compound ratios, and Principle Component Analysis (PCA), can ultimately help delineate PAH sources at contaminated sites and provide information for remedial planning, particularly in areas that may be affected by wildfire activity.  

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Contact us for additional information about how wildfires affect PAH composition in soil or how we can assist with PAH characterization and remediation at your site.