Long-term fate of contaminants in landfill-recovered soil: Evidence from leaching experiments and numerical modelling

Landfill mining generates substantial quantities of landfill-recovered soil-like material (LRSLM), which is reused in geotechnical applications. However, elevated salts and heavy metals pose long-term leaching risks, and despite existing leaching studies, realistic long-term contaminant transport modelling remains unexplored. This study addresses this gap by integrating laboratory leaching experiments with numerical modelling to evaluate contaminant migration from LRSLM under different reuse scenarios. LRSLM extracted from two waste dumpsites in New Delhi was examined for physicochemical properties and leaching behaviour. The material was predominantly sand-sized (45-50%) with 22.5% moisture and 10% organic content. Acid digestion revealed elevated metal concentrations, with Cr (265.9 ± 26.1 mg/kg) and Cu (387.6 ± 64.3 mg/kg) being 15-20 times higher, and Ni, Pb, and Zn approximately 4-6 times higher in LRSLM than background soils. Batch leaching showed high sulphate and chloride concentrations (4563-2631 mg/kg), whereas column leaching tests exhibited declining concentrations with increasing L/S ratios. Numerical modelling indicated that desorption follows the Freundlich isotherm (R² = 78-98%), with Ni and Zn showing both instantaneous and kinetic behaviour, while Cu and Cr were dominated by kinetic release. Monte Carlo simulations over 50 years predicted that heavy metals remain primarily within 2.5-3.5 m, while chloride migrates rapidly, reaching groundwater within 20 years for embankment reuse and 1 year when placed in deep pits. A compacted clay liner substantially delayed chloride breakthrough, demonstrating the effectiveness of engineered barriers. This integrated experimental numerical framework provides a robust basis for developing safe guidelines for LRSLM reuse.