Impact of tailings on environments: Example of elemental transfers from spoil tips in the North of the France

The North of the France was the scene of an active past coal mining activity, whose wastes form about 300 spoil tips. Although these materials are considered as sterile, spoil tips are colonized by pioneer species, which contribute to the weathering of parent geological material and the formation of a neo-soil. Due to their composition and weathering processes, spoil tips – and neo-soils on its - can be at the origin of elemental transfers in the environment (pollutants as trace elements, sulfur, organic compounds. These transfers must be strongly understood and quantified to manage spoil tips and minimize their impacts. To qualify the spoil tip/environment interactions, we have characterized the parent geological material and alteration processes, we have studied the neo-soil and its role in elemental transfers, and we have determined transfers, through field measurements and laboratory leaching tests. We selected one spoil tip made of black schists (quartz, clays, pyrites, oxides, coal residues), partially covered by a forest at the origin of a neo-soil, and surrounded by ponds. The spoil tip shows an unexpected neo-soil including three distinct horizons, corresponding to different degrees of parent material weathering (variation of mineralogy and oxidation state) and of organic matter incorporation. The mineralogical characterization (XRD, SEM, TEM) shows a S-rich alteration front at the schist surface, coupled to mineralogical transformations, and the formation of jarosite and Fe-oxides. Ponds at the bottom of this spoil tip present various chemical and biological characteristics. Especially, the heavy metal concentrations measured in pond water are very variable from one pond to another. These analyses coupled to lixiviation test from each neo-soil horizon allow quantifying elemental transfers from black schist spoil tip and informing about the extent of the area of spoil tip influence.