Combination of geochemical tracers for reconstructing contaminant trajectories in an industrialized and anthropized watershed
Abstract
The Orne River, a tributary of the Moselle River, is a small watercourse that flows in northeastern France. Since the 1870s, the Orne watershed was highly impacted by industrial activities. For the purpose of steel-making facilities, most of these industrial activities like iron mining, coke production, and blast furnaces were installed in the vicinity of the Orne River. In response to the industrial needs, some sections of the Orne River were redirected; channels and banks were created as well as the building of a few small dams. These structures created artificial water reservoirs as well as the promotion of suspended particulate matter settling. Dams along the Orne were built in the last century and are expensive to operate and maintain. According to the Directive 2000/60/EC of the European Parliament and the council, the removal of engineered structures such as dams is recommended to restore the natural functions of rivers. In the Orne River, the main dams were opened in 2019 and dam removals were organized. However, the dam removal is canceled and the structures will be used as bridges to link the two sides of the River. The opening dam action can caused the remobilization and spreading of contaminants stored in the river bed. These contaminants are the results of the former industrial activities that left behind dozens of tons of waste such as sludges stored in the River banks for decades with high levels of Zn, Pb, and Fe contents. The objectives of this study are to identify the sources of particulate Zn and Pb introduced into the Orne River, to evaluate if these sources can contribute to Zn and Pb into the Orne River during flood events, and to quantify their respective contributions if it is possible. The objectives were realized by sampling the potential sources (soils, urban samples, and steel-making wastes) as well as the suspended particulate matters (SPM). Characterization was done for these samples using X-ray diffraction (XRD), chemical composition using ICP-OES and ICP-MS, and isotopic fingerprints (MC-ICP-MS Neptune Plus). For source identification, soil samples were collected on distinct geomorphological areas characterized by different soil types and land uses. They all display detrital minerals assigned to the geological background. Urban dusts and steel-making residues display specific mineral phases (sulfates and iron oxides, respectively). Element compositions present strong discrepancies between the distinct sets of samples. SMWs are particularly enriched in iron, Zn, and Pb. For each of the three sets of samples (soils, urban, industrial), variations of geochemical fingerprint were observed. For soils, the relatively large variations of Zn and Pb isotopic compositions were attributed to distinct land use and the contribution of atmospheric deposition. For industrial samples, the variations were more intense and may be attributed either to distinct industrial processes in the production of pig iron or to distinct furnace-fume treatment modes. The three sets of samples (urban, industrial, and detrital) could be distinguished based on Zn and Pb contents and isotopes. This study not only highlighted the sources that released particulate Zn and Pb into the Orne River system, it also demonstrated that urban particles are well constrained in terms of Zn and Pb isotopic signatures, and those isotopic signatures could be extrapolated to other case studies. During the flood event, SPM showed distinct variations in Zn and Pb contents and their isotopic composition as a function of the variation of water discharges. Overall, high values of Zn and Pb are recorded at low water flows, whereas, low ones are recorded at high water flows. It’s worth mentioning that these two elements are usually increased at the beginning of each flood event. Both elements, as well as their isotopic compositions, displayed significant variations according to the water flow stages. Throughout the flood, a notable mixture of distinct sources emerged, encompassing contributions from soil erosion, urban inputs, and waste from the iron and steel industry. Using a model based on Monte Carlo simulation, the results of Zn and Pb contribution showed (1) the dominance of soil erosion at high water levels, (2) the high contribution of urban inputs at low water flows,
and (3) the high contribution of steel-making wastes at the beginning of each water rise. The inverse model adopted provided a realistic picture of the different sources of particles during flood events.
Keywords: Orne River, steel-making wastes, geochemical fingerprints, Zn and Pb isotopes, soils.
Composition du jury :