Improvement of excavated materials management by using urban geochemical baseline data – Nantes case study (France)
Résumé
In France, the reuse of excavated materials is conditioned by their geochemical compatibility with the receiving site, referring to the notion of urban geochemical baseline. To avoid a case by case approach, a more general characterization of urban geochemical baseline has been tested in several quarter of the city of Nantes to anticipate the management of excavated materials from redevelopment projects. The challenges to be faced were numerous, including: 1) No systematic characterization of urban soils and subsoil geochemistry was available, only pollution diagnostics were available; 2) The excavation of materials occurring on several meters of depth, a 3D approach was necessary. We focus here on the reflection carried out on the potential use of geochemical baseline to drive the management of excavated materials.
The methodology consisted in gathering pollution diagnostics results into a georeferenced database, cleaning the data from point source pollutions, assigning the samples to the typology of materials defined on the quarters (including a typology of made-ground according to their intrinsic potential of pollution), calculating statistical distributions of the various geochemical parameters per type of materials, and verifying the coherence. Statistical anlysis using box plots were also carried out. The urban geochemical baseline was supposed to correspond to the 90th percentile of the data set available.
A single urban geochemical baseline value would have been very welcome operationally. Nevertheless, it appeared inconsistent with the heterogeneity of the materials encountered in the studied urban areas. In particular, the questionable made-grounds, containing industrial residues, show a poor geochemical quality. They also strongly contrast with some clean sandy anthropogenic deposits. To face this problem of heterogeneity, we considered several geochemical baselines, one per typology of materials.
Nevertheless, using the 90th percentile as reference value conducts to consider that 10 % of the excavated materials are not acceptable for reuse, whereas their geochemical quality correspond to usual values. The notion of level of anomaly appeared operationally more attractive to us. In this frame, we considered the upper whisker. The postulate that the upper whisker is higher than the 90th percentile, verified for the natural geochemical baseline, was not verified for some parameters (Pb, Zn, PAH, …). This could be related to a more important diffuse pollution by these contaminants on the studied areas. Considering this, we propose to consider the highest value between the 90th percentile and the upper whisker to define geochemical compatibility levels.
The geochemical compatibility levels defined, integrating the notion of anomaly levels, appears more appropriate to manage excavated materials than geochemical baseline values. The statistical approach should however be completed by a spatial approach, especially if the data used were acquired for other purposes (eg., pollution diagnostics) than the calculation of geochemical reference values.