A typology of anthropogenic deposits as a tool for modeling urban subsoil geochemistry: example of the Ile de Nantes (France)
Résumé
Purpose: Urban soils may be contaminated not only by industrial activities but also by the materials contained in the anthropogenic deposits. In order to anticipate the management of excavated soils linked to urban redevelopment, their quality and quantities need to be assessed. Urban soils and subsoils
contain nevertheless a large variety of anthropogenic deposits and present important geochemical heterogeneities. The paper aims to develop various geochemical types of made grounds relevant for a 3D urban subsurface model.
Materials and methods: To build a 3D urban subsurface model, using geological tools, the challenge is to define a stratigraphic pile both geologically and geochemically relevant. In this frame, we defined a typology of made grounds based on an iterative approach. We used (a) published classifications,
(b) a dataset of the study area (Ile de Nantes) comprising the borehole descriptions (2400), and (c) results of analyzed samples (more than 1800), gathered in a georeferenced database. The layers (8400) found in the borehole descriptions were interpreted according to the a priori made grounds typology and to the underneath natural layers. The relevance of the made ground typology proposed was verified by a statistical approach on the representative samples and analyses selected in the database.
Results and discussion: Three types of made grounds have been retained. They show contrasted geochemistry. The made grounds that are comparable to natural subsoils correspond in the case study to alluvial type materials. They show similarity with a typology and geochemistry of alluvial materials. The
questionable man-made grounds, containing potentially contaminated anthropogenic materials such as bottom ash or slag, are characterized statistically by higher contaminant levels than other types of made grounds (e.g., PAH, Zn, Pb, Cu,...). The various man-made grounds, corresponding to the remaining
materials and containing for instance demolition materials, are characterized statistically by higher levels of PCBs. The typology has been used in a 3D representation of the Ile de Nantes subsoils, which served as a decision aid tool for the developer.
Conclusions: The knowledge on urban subsoil geochemistry may help defining redevelopment projects, by adapting soil use to subsoil quality. In this frame, the development of a geochemically relevant made ground typology taking into account their intrinsic potential of contamination appears useful. A proper description of the intrinsic components of the made grounds is essential. It is indispensable to use some rigorously defined and internationally agreed terms.