Rethinking soil contamination assessment for urban brownfield management: from total hazard to bioavailability

Hazard identification at contaminated sites usually rely on total concentrations of potentially toxic elements (PTE), but it is largely admitted that only the bioavailable fraction can trigger adverse effects. Assessing total PTE amounts is thus a worst-case scenario, that overestimates environmental risks and often leads to costly remediation actions. This study presents a practical and cost-effective methodology for assessing soil contamination by PTE, taking into account their bioavailability. To address this issue, we performed a three-tiered hazard identification considering: (1) total hazard (total PTE content), (2) environmentally available hazard (CaCl₂ leachable fraction), and (3) environmentally bioavailable hazard (accumulation in plant leaves). Additionally, an Integrated Hazard Excess (IHE) index was computed, for aggregating raw contaminant levels into a single integrated hazard value. The methodology was applied to a former industrial site in Saint-Étienne, France. Soil and plant samples were analyzed for As, Cd, Cr, Cu, Ni, Pb, and Zn. Mapping of IHE values provided a clear and holistic view of contamination across the site. Results showed that while total PTE levels were very high, the mobile and bioavailable fractions were overall within the range of local background observed in urban parks used for recreational activities. This indicated that actual environmental risk was significantly lower than total concentrations suggested. Thus, distinguishing total, available, and phytoavailable PTE amounts provided a more accurate and ecologically meaningful assessment of soil hazard. This approach could help decision-making and support the adoption of less disruptive and more sustainable remediation strategies, avoiding unnecessary excavation and landfill operations.