Understanding the fate of polycyclic aromatic hydrocarbons at a forest fire site using a conceptual model based on field monitoring.

Forest fires are a well-known source of polycyclic aromatic hydrocarbons (PAHs). After forest fires, residual ash above a soil layer can be highly contaminated with PAHs. However, little is known about the fate of these contaminants, particularly about their susceptibility to be transferred deeper into underlying soil or downstream during rainfall events. In this study, meteorological conditions, organic carbon (OC) content, and the 16 US-EPA priority PAHs in unburnt control soil, burnt soil, and ash were monitored for 16 months after a forest fire. Whereas the ash was significantly contaminated with PAHs, the levels of PAHs in the underlying burnt soil were similar to those of the control soil. In the ash bed, the levels of PAHs normalized by OC also decreased. Neither PAHs nor OC lost from the ash bed was substantially transferred to the underlying soil. Instead, significant amounts of PAHs in the ash bed were likely removed by surface runoff. Light PAHs were dominantly emitted from the forest fire, but they showed higher decreasing rates with total precipitation. These findings were explained by a conceptual model for the fate of PAHs, involving four distinct processes related to precipitation and two states of the ash bed.