Mapping the soil pollution risk at territorial scale for urban planning: examples from French cities

The municipalities often lack a global knowledge of their soil quality, including soil pollution. Mapping soil geochemical quality is carried out in some cities using the URGE protocol. This approach is however not systematically applied due to financial reasons, but also due to strategies focusing on investigations at the scale of redevelopment project or at site scale, in a case by case approach. A global information may thus be missing for urban planning. The presentation deals with a methodology to map the soil pollution risk at territorial scale, based on historical and current pollution pressure.

The methodology takes into account information on historical and current industrial activities and agricultural activities, but also on anthropogenic deposits. The pressure linked to industrial sites uses a database that correlates activities and contaminants. This industrial pressure is considered higher than the agricultural one, because industrial activities generate more point-source contaminations, and agricultural activities generate more diffuse contaminations. The obtained maps are compared with neighbourhood scale studies. Two metropolis (Nantes and Rennes, France) serve as pilot cases.

The results show that a large part of urban soils of the 2 pilot metropolis are concerned by potential soil pollution. The technical services of the metropolitan areas were surprised by the large footprint of the anthropic activities. Some intend to use the results in the planning documents to alert on the potential presence of pollutants in soils. In this frame, they choose a precautionary approach taking into account the maximum potential extent of the former industrial sites. Some others would like to use this information to nuance the soil multifunctionnality, in order to take into account the soil quality to reach the no net land take objective. Another application concerns the mapping of the desealing potential of soils, where the potential soil pollution risk is one of the environmental constraints taken into account. The comparison with the local scale studies shows the satisfying approximation of the methodology. For former industrial sites and anthropogenic deposits, a detailed knowledge of their extent allows a better precision of the map.  

The progress concerns currently the sources of contamination. Further developments are needed to integrate the diffusion of contaminants linked to atmospheric deposition in particular. They could also consider potential pollution plume in groundwater. The pollution potential map should also be crossed with the potential natural anomalies linked to the geological setting.