Trace element behavior and partition versus urbanization gradient in an urban river (Orge River, France)
Résumé
In order to assess the spatial (i.e., multi-site) and temporal (i.e., seasonal time scale) dynamics of trace elements (TE) along the Orge River (France), the geochemistry of both suspended particulate matter (SPM) and dissolved phase (<0.45 μm) was investigated during a whole hydrological year. TE concentrations in the different compartments were discriminated using principal component analysis (PCA), which allows to highlight both correlated or opposite behaviors between major, minor and trace elements. The Orge River watershed presents a highly contrasted land use pattern, with, upstream, mainly forests and agricultural lands and, downstream, a highly dense urban area (up to 8000 inh. km−2), in the suburbs of Paris Megacity. TE concentrations, particularly urban ones (Pb, Zn, Cu, and Sb) drastically increased in both dissolved and particulate compartment due to urbanization in the Orge River catchment. This enrichment was found to be associated with a spatial increase in the dissolved contribution of TE (20–30% for Zn), which may lead to their increasing bioavailability and toxicity. Urban runoff and sewage releases were found to have a strong influence on the river geochemistry and PCA allows identifying the limits of urbanization influence. According to statistical analyses, the dissolved data are more representative of urban processes, i.e., spatial trends, while the particulate compartment data mainly described seasonal variability, i.e., the temporal trends. Finally, this study highlights the necessity of following both dissolved and particulate concentrations of TE to characterize accurately the contamination trends and their possible implications on urban river systems. As the present European guidelines are only fixed for dissolved species, we show here that the SPM contamination must also be monitored to better understand the geochemical processes involved and the degree of contamination of the river.