Distribution of organic pollutants and natural organic matter in urban storm water sediments as a function of grain size.

The sealing of surfaces in urban areas makes storm water management compulsory. Contaminated particles carried from urban surfaces are deposited in infiltration ponds. This gives rise to a highly organic (11% DW) contaminated sedimentary layer (Zn:1.2 mg/g, Cd:15 mg/kg) that could threaten groundwater quality. During infiltration, particle arrangement impacts infiltrating water and sediment exchanges. In this context, understanding particle arrangement and leachable components is essential. This study investigates Organic Matter (OM) not only as a pollutant but also as a substrate and a structuring element. The leachable fraction was collected and grain size fractionation was performed. OM of sediments and isolated fractions were characterized by measuring organic carbon content, isolating aromatic hydrocarbons, saturated hydrocarbons and polar compounds after dichloromethane extraction, and by gas chromatography-mass spectrometry (GC-MS) molecular analyses. The organic compounds observed were petroleum byproducts (steranes and terpanes, unresolved complex mixture (UCM) and polycyclic aromatic hydrocarbons (PAH)), but plant and bacteria biomarkers were also found (phytol and derivatives, sterols). Leachable OM consisted of 6% of sediment OM (associated with particles >0.45 microm). This leachable OM is easily extractable by dichloromethane (96%) and contains fewer macromolecules than other fractions. Isolated grain size fractions showed dissimilarities (total organic carbon from 3.5 mg/g to 88.6 mg/g, extraction rate from 24 to 96%, aromatic hydrocarbon distribution) and similarities (proportions of aromatic and saturated hydrocarbons and polar compounds, molecular distribution of saturated hydrocarbons and polar compounds). The results suggest that organic macromolecules take part in the aggregation of sediments and prevent fine particles (<10 microm) from being leached. On the other hand, leachable particles (20 microm grain size mode) could carry low molecular weight organic molecules. The physical structure of the sediments and the leaching of particles containing contaminants are considerably affected by the presence of OM.