Evolution of the Concentrations of Polycyclic Aromatic Hydrocarbons in Burnt Woodland Soils

Abstract

Little is known of the fate of polycyclic aromatic hydrocarbons (PAHs) in soils under burnt woodland. It is not clear what the behavior of the overlying wood ash layer will be along months. In this study, the levels of eight representative PAHs in the 1-5 cm layer of a periurban woodland soil that had undergone wildfire were compared with those measured in nearby and distant unburnt periurban woodland soils and in a distant unburnt rural woodland soil, and the levels at the burnt site were monitored during some 10 months. The analytical method optimized for the purpose afforded recoveries of 74-111% (depending on PAH) and repeatabilities (RSDs) better than 9%, with limits of detection ranging from 1 to 7 microg/kg. PAH levels in the 1-5 cm layer of the burnt periurban soil were very similar to those of distant unburnt periurban soil (188 vs 173 microg/kg), about seven times the 26 microg/kg measured in unburnt rural soil, which furthermore contained no detectable quantities of the highest molecular weight PAHs typical of traffic and other urban sources, as the periurban soils did. At the burnt site, PAH levels fell along the months (the total PAH level from 188 to 119 microg/kg), apparently as the result of rainfall and the prevention of further input from the atmosphere by the overlying layer of wood ash, which had a very high PAH adsorption capacity (1169 microg/kg) and did not itself appear to act as a source of PAHs. PAH transport may have been assisted by increased mobilization of PAHs associated with dissolvable organic matter due to an increase in soil pH due to alkaline ash components.

Effect of a compost mulch on seed germination and plant growth in a burnt forest soil from NW Spain

“…Although a peak in their concentration was observed in the soil heated at 200ºC, in coincidence with the peak for phytotoxicity, the concentrations found were too low (under 1 mg kg -1 in all cases) to be the cause of phytotoxicity, since phytotoxic effects of PAHs have only been reported in soil concentrations ranging from 25 to >100 mg kg -1 (Efroymson et al, 1997). Besides, the PAHs concentrations found here are similar to values found by other authors in woodland burnt soils in Spain (García-Falcón et al, 2006), so it cannot be said that…”

“…As several authors have indicated the apparition of aromatic compounds in soil during forest fires (García-Falcón et al, 2006), the presence of polycyclic aromatic hydrocarbons (PAHs) was also determined, in order to test if they may be responsible for phytotoxicity.…”

Vertical distribution of polycyclic aromatic hydrocarbons (PAHs) in Hunpu wastewater-irrigated area in northeast China under different land use patterns

“…Simpson et al (1996) noted that the relationship between total PAHs and organic carbon is significant only for highly contaminated sites where the total PAH concentration was in excess of 2,000 μg/kg, and the high concentration had only occurred in the 2-5 cm layer of the HU4 section in the study area Besides the SOC, ionic strength , pH (Laor et al 1998), and colloids can also influence the binding of PAHs in soils. The PAHs downward transportation may have been assisted by increased mobilization of PAHs associated with dissolvable organic matter due to an increase in soil pH (Gabcia-Falcón et al 2006). This view is supported by the fact that pH values increase from 4 to 5 with depth.…”

Sorption of PAHs to Colloid Dispersions of Humic Substances in Water

“…Determination of PAHs in different media has been extensively reported in the literature (García-Falcón et al, 2004a, b;Rey-Salgueiro et al, 2004;García-Falcón et al, 2005;García-Falcón et al, 2006). The quantity of PAHs adsorbed was determined by difference between the initial Sorption kinetics of PAHs to HSs were monitored to know whether the time necessary for equilibrium or constant sorption could be lower than 24 h. Kinetics were measured by duplicate in water at a low (58 mg L -1 ) and a high (464 mg L -1 ) concentration of HSs.…”