Key factors influencing metal concentrations in sediments along Western European Rivers: a long-term monitoring study (1945-2018)
Since the 1960s, a large amount of heterogeneous data in terms of sampling, analytical protocols and matrices has been acquired to survey river sediment quality, especially concerning regulatory metals such as Cd, Cr, Cu, Hg, Ni, Pb, and Zn. This article proposes a first attempt to decipher spatial and temporal trends of metal contamination in Western Europe, i.e. along seven major continental rivers (from France, Belgium, Germany, and the Netherlands). A large dataset (>12000 samples) from various sediment matrices (bed and flood deposits – BFD, suspended particulate matter – SPM, dated sediment cores – DSC) was set up based on monitoring and
scientific research from the 1960s to the 2010s. This work investigates the impact of analytical (sediment matrix, sampling, fractionation, extraction), location and time factors (closely related to geology and anthropogenic activities) on metal concentration
trends. Statistical analyses of these parameters highlight crossed-interactions in space and time between the sediment matrix (metal concentrations in SPM and DSC > BFD), and the extraction procedure (also related to river lithology). This original
intercomparison highlights major spatio-temporal trends common to several rivers: (i) an increase of metal concentrations in downstream direction towards the main urban industrial areas (e.g. Paris-Rouen corridor on the Seine River, Bonn-Duisburg corridor
on the Rhine River), (ii) a long-term influence of former mining areas located in crystalline zones, releasing heavily contaminated sediments for decades (Saint-Etienne Basin in the Upper Loire River, Middle Meuse section), (iii) a temporal decrease of metal concentrations since the 1970s (except for Cr and Ni, rather stable over time). The global improvement of sediment quality in the most recent years reflects a decisive role of environment policies, such as more efficient wastewater treatments, local applications of the Water Framework Directive, and urban industrial changes in the river valleys since the 1990s.