Heavy metal distribution in MSW-derived soils and migration within the subjacent soil strata of landfills

Landfills are significant sources of environmental contamination, yet heavy metals migration from landfill matrix into subjacent soil strata and the subsequent downward percolation have been underexplored. Municipal Solid Waste (MSW), subjacent soil, leachate, and groundwater samples were collected from six boreholes (≤65 m depth) in both the unlined and lined landfills. The unlined landfill exhibited higher contamination levels in MSW-derived soils compared to the lined landfill, with Pb and As exceeding EPA Regional screening levels (RSLs) by 4.15 and 44 times, respectively. In the unlined landfill, the heavy metals accumulated 3 m above the liner reflects the downward migration trend and the contaminant interception effectiveness of liner. Within subjacent soil strata (30-53 m depth) beneath the unlined landfill, the migration of different heavy metals was retarded by 66-97 %. High Electrical Conductivity leachate alters adsorption capacity and pore structure of soil, thereby significantly influencing transport and enrichment of heavy metals in strata. The risk assessment identified Cr and Zn as the primary contributors to the pollution index (PI) of MSW-derived soils, whereas Cd dominated the potential ecological risk index (RI) calculated based on Chinese Risk screening values (RSVs). The decision among in-situ closure, excavation and screening for resource recovery, and complete incineration should be based on a comprehensive evaluation of economic viability and environmental risk.