Heavy metals in unsaturated urban soils: Experimental insights and stochastic forecasting

The toxicity and long-term persistence of heavy metals pose a major threat across environmental compartments. Heavy metal contamination of soils poses long-term risks to groundwater resources, particularly in urban environments where rainfall-driven infiltration under unsaturated conditions can promote contaminant migration toward underlying aquifers. The aim of this study is to assess how uncertainty in soil hydraulic and sorption properties affects predictions of heavy metals migration from urban soils to groundwater. To this aim, we cast our work in a stochastic framework that integrates column scale drainage experiments and batch sorption tests to estimate key parameters governing water flow and heavy metal sorption dynamic, along with their associated uncertainty. Monte Carlo simulations are then performed to mimic the rainfall-induced drainage of heavy metals from metal-enriched soils through sewage sludge application. We consider two representative urban soil types and five heavy metals commonly detected in urban environments, i.e. lead (Pb), copper (Cu), chromium (Cr), nickel (Ni) and zinc (Zn). Our results indicate that Cr is highly mobile, while Ni, Zn, Cu exhibit consistently low mobility across both soils. Lead displays a strong soil-dependent behavior, transitioning from high to low mobility depending on the substrate. Outputs of a rigorous global sensitivity analysis reveal that, for highly mobile metals, flow and transport parameters have significant influence on metals migrations, reflecting the strong interplay of unsaturated flow and reactive transport processes. On the other hand, for low mobile heavy metals, parameters defining the initial contamination conditions and soil water content dominate the temporal evolution of metals retention in the soil. These findings indicate that parameter uncertainty and flow-transport processes in the vadose zone should be explicitly considered in groundwater contamination risk assessment protocols for urban environments.