Lead and Zinc status evolution during pedogenesis in a 100 years old Technosol

International audienceIn the frame of the transposition of the European directive on waste in French law, a national technical committee has been implemented in France since 2009 to set up a methodology of management of excavated lands. The reuse of excavated lands in a lot of city planning and landscaping configurations is designed for long-term uses. Their reuse and deposition on site should lead to the formation of new soils, generally called Technosols. Although contaminated, these materials present a pedologic interest since they fulfill several functions such as biomass production, storage-filtration and transformation of nutriments, trace elements and water, or formation of ecological niches. However, the long-term behavior of these materials is difficult to study since the pedological processes exceed the human timescale. Nevertheless, early pedological transformations could sometimes be observed in Technosols. The study of these processes needs to focus on “old” Technosols, i.e. on materials that have been settled for hundreds of years. The first industrial activities started in Europe in the early 19st century. At this period, the earthworks carried out for factory building led to the backfill of wide areas. These materials are still in place in some old brownfields. Their investigation could highlight several early pedological processes, as well as assessing the status of some pollutants, especially by studying soil profiles where the deeper horizons are considered less evolved than the upper horizons. For this study, a ~2m depth profile was characterized in a 100 years-old Technosol developed on backfills that contain significant concentrations of lead (Pb) and zinc (Zn). The results indicate that in the studied Technosol, pedogenesis does not induce a dramatic increase of Pb and Zn solubility since efficient scavengers are concomitantly formed in the system. However, the weathering processes during soil evolution influence Pb and Zn speciation. Primary Zn and Pb bearing minerals have been identified as sulphides. However, these phases have evolved at the surface of this Technosol to form sulphates, not stable in conditions of pH observed here. During this evolution, various proportions of Pb and Zn have been trapped in Fe (hydr)oxides (ferrihydrite) neoformed at the surface of the profile since the initial deposit of the backfills. The input of organic matter at the surface is slightly visible after 100 years of evolution. Technosols are highly heterogenous and widely differ from one place to another because of the variety of their constitutive man-made materials. In this case, 100 years of soil evolution has led to the precipitation of secondary minerals at the surface of the profile. This behavior is similar to pedogenic processes occurring in natural soils, but other specific evolutions due to large thermodynamic disequilibrium can be highlighted, in particular sulphide oxidation and sulphate precipitation