Regional risk assessment Part II: identification of diffuse soil contamination hotspots.

Abstract

Diffuse soil contamination represents a widespread environmental issue, resulting from multiple low-intensity and spatially dispersed sources, including atmospheric deposition, agricultural practices, traffic emissions, and legacy industrial activities. Unlike point-source pollution, diffuse contamination lacks a clear spatial origin, requiring methodologies that can integrate measured data with environmental processes. In this case concentration-based mapping alone is often insufficient to support risk-informed management decisions. This study presents a regional-scale environmental risk assessment framework for diffuse soil contamination, building on the methodological advances introduced in the companion paper (Part I) and extending them to spatially distributed pressures. The assessment is expanded to a multi-contaminant context, including both inorganic and organic compounds, and contaminant concentrations are evaluated against land-use-specific regulatory threshold values. Hazard maps are normalised according to the established criterion whereby soils are considered hazardous when concentrations exceed threshold values by one order of magnitude, enabling consistent comparison and aggregation across contaminants. Exposure is modelled through four exposure pathways (groundwater, surface water, direct contact, and air dispersion) and receptor vulnerability is further articulated for groundwater resources, surface waters, protected areas and human health. The resulting framework supports a transition from a predominantly regional human-health-oriented assessment towards a more comprehensive environmental risk assessment, producing spatially explicit prioritisation maps for individual contaminants and aggregated diffuse-contamination risk. Application to the Toulouse Metropolitan Area (France), demonstrates the ability to move beyond concentration exceedances by identifying areas of higher management priority where diffuse pressures, transport processes and receptor vulnerability converge. Implemented within a GIS-based Model Builder environment, the methodology is reproducible and transferable to other regions where soil data are available but site-specific information is limited, offering a scientifically grounded basis for supporting decision-makers in land-use planning, monitoring prioritization and early-stage risk-based management actions, in line with the new EU Soil Monitoring and Resilience Directive.