Polycyclic Aromatic Hydrocarbon Contents in Soil Organic Matter Fractions Along an Elevation Gradient in the French Alps

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous persistent organic pollutants that accumulate in soils because of their high affinity for soil organic matter (SOM). As these pollutants are toxic to humans and the environment, a better understanding of their fate in the environment is required. This study aimed to assess the PAH distribution within soils according to different soil fractions: the free particulate organic matter (fPOM), the occluded particulate organic matter (oPOM) and the mineral-associated organic matter (MaOM). PAH contents were measured in bulk soils and SOM fractions of alpine soils along an elevation gradient in the French Alps (Lautaret) from 1920 m to 2840 m a.s.l. A specific PAH distribution was identified, with highest PAH contents in the oPOM, followed by the fPOM, then the MaOM. Organic matter (OM) contents of each fraction can partly explain this distribution, but results of nuclear magnetic resonance (NMR) spectroscopy on fPOM and oPOM also highlighted a correlation between the PAH contents and the degree of decomposition of SOM. This indicates that the PAH distribution may be linked to the formation and transformation of fractions: (i) PAHs in the fPOM correspond to relatively recent deposits and mainly reflect the background contamination, (ii) in the oPOM are the PAHs that resist biodegradation during the transformation of fPOM into oPOM and accumulate in the oPOM; this accumulation may be further enhanced by the formation of aggregates. Finally, (iii) in the MaOM, the lower PAH contents can be explained by the different formation pathway of this fraction and its high degree of decomposition. As the PAH distribution may have an impact on their dynamics in soils, it should be taken into consideration in future research.