From surface to aquifer: a global synthesis of landfill leachate composition and its impact on groundwater systems.

Municipal solid waste (MSW) landfilling remains a major source of groundwater contamination risk because infiltrating water generates chemically complex leachate capable of migrating through the vadose zone into underlying aquifers. This review provides a global synthesis of evidence on landfill leachate composition, migration pathways, groundwater impacts, and containment performance. The study was reported in accordance with PRISMA 2020 and synthesizes evidence from 422 studies across 78 countries published between 1980 and 2025. The findings show that landfill leachate commonly contains diverse contaminant groups, including aromatic hydrocarbons, halogenated aliphatics, phthalates, phenols, ammoniacal nitrogen, heavy metals, and emerging contaminants such as per- and polyfluoroalkyl substances (PFASs) and microplastics. Contamination risk is amplified where high rainfall, shallow groundwater, permeable subsurface conditions, and inadequate containment occur together, with particularly elevated vulnerability identified in parts of South Asia, Sub-Saharan Africa, and Latin America. The synthesis further indicates that unlined or weakly controlled disposal sites remain associated with the greatest groundwater impacts, whereas engineered systems with composite liners and leachate collection infrastructure generally reduce contaminant migration, although performance declines with age and depends strongly on construction quality, maintenance, and long-term monitoring. An exploratory multivariate model based on a review-derived synthetic dataset suggests that rainfall, containment integrity, landfill age, and aquifer permeability are jointly important controls on groundwater contamination severity (R2 = 0.78, p < 0.01). The review highlights the need for harmonized monitoring protocols, long-term post-closure surveillance, stronger assessment of emerging contaminants, and climate-responsive landfill design. Advancing groundwater protection will require integrated engineering, hydrogeological planning, and regulatory systems capable of sustaining containment performance over the full landfill life cycle.