Landfill Leachate Poses a Growing Global Threat to Groundwater, Major Review Finds

Beneath thousands of the world’s landfills, a slow-moving chemical plume is seeping toward the water that billions of people drink. A sweeping new synthesis of more than four decades of research confirms that landfill leachate—the toxic liquid generated when rainwater percolates through decomposing municipal solid waste—is one of the most pervasive and underappreciated threats to global groundwater systems. The review, published in Environmental Monitoring and Assessment, analyzed 422 studies spanning 78 countries between 1980 and 2025, offering the most comprehensive picture yet of what leachate contains, how it travels through the subsurface, and which engineering defenses actually hold the line.

The scale of the analysis is itself remarkable. Following the PRISMA 2020 reporting framework, which standardizes how systematic reviews identify and screen evidence, the researchers led by Kunle Adefarati Ibrahim of Chengdu University of Technology compiled findings from nearly half a century of landfill investigations across every inhabited continent. This breadth matters because landfill science has historically been fragmented: individual case studies of single sites in Nigeria, Brazil, China, or Norway could reveal local contamination patterns, but nobody had systematically stitched those threads into a global fabric. The new synthesis does exactly that, and the resulting picture is sobering.

What emerges first is a portrait of leachate as one of the most chemically complex liquids humans routinely generate. The review documents a diverse arsenal of contaminant groups flowing from decomposing waste: aromatic hydrocarbons such as the BTEX compounds—benzene, toluene, ethylbenzene, and xylenes—alongside halogenated aliphatics, phthalates, phenols, ammoniacal nitrogen, and an array of heavy metals. These are the legacy pollutants, well characterized for decades. But the synthesis also highlights a newer and arguably more troubling class: emerging contaminants, including per- and polyfluoroalkyl substances, or PFASs, and microplastics. PFASs, sometimes called forever chemicals because of their extraordinary resistance to degradation, have been detected in landfill leachate worldwide, and recent studies cited in the review show that these compounds can transform and interact with liner materials in ways that are still poorly understood.

The chemistry of leachate evolves over the life of a landfill. In young sites, biological decomposition proceeds rapidly, producing leachate rich in easily degradable organic matter, reflected in high measurements of biological oxygen demand and chemical oxygen demand. As landfills mature, the organic load declines but ammoniacal nitrogen can persist at elevated concentrations for decades, posing a long-term threat to aquatic ecosystems and drinking water supplies. Studies of aged landfills, including work at Norway’s granitic-region sites and at the Kupferberg landfill in Namibia, demonstrate that landfill age, closure status, and even season leave detectable fingerprints on groundwater chemistry. A landfill does not stop being a contamination source the day it stops accepting waste.

How does this chemical cocktail reach aquifers? The review traces the migration pathway in detail. Infiltrating precipitation percolates through the waste mass, dissolving and mobilizing contaminants as it descends. The liquid then passes through the vadose zone—the unsaturated layer of soil and rock between the landfill base and the water table—where some attenuation may occur through filtration, adsorption, and microbial degradation. But where the vadose zone is thin, permeable, or absent, contaminants move essentially unimpeded into the saturated zone, where they form plumes that spread with groundwater flow. Techniques such as borehole hydrochemical and hydrodynamic profiling and total dissolved solids profiling have allowed researchers to map these plumes directly, revealing contamination extending far beyond landfill boundaries.

The global risk map that emerges from the synthesis is deeply unequal. Contamination risk is amplified where several vulnerability factors converge: high rainfall that maximizes leachate generation, shallow groundwater tables that shorten the travel path, permeable subsurface conditions that accelerate transport, and inadequate containment that offers little resistance. The review identifies particularly elevated vulnerability in parts of South Asia, Sub-Saharan Africa, and Latin America—regions where rapid urbanization, growing waste volumes, and limited sanitation infrastructure combine with hydrogeological settings that offer little natural protection. In many of these areas, open dumpsites rather than engineered landfills remain the dominant disposal method, and unlined sites are consistently associated with the greatest groundwater impacts documented in the literature.

Engineering, however, can make a measurable difference. The synthesis indicates that engineered systems equipped with composite liners and leachate collection infrastructure generally reduce contaminant migration compared with unlined or weakly controlled disposal sites. Composite liners typically combine a low-permeability geomembrane with compacted clay, creating a layered barrier that slows the escape of leachate to a trickle. Leachate collection systems then remove the accumulated liquid for treatment, preventing the buildup of hydraulic head that would otherwise drive contaminants through defects. Performance studies cited in the review confirm that low-permeability liners can minimize groundwater contamination when properly designed and constructed. But the review adds an important caveat: liner performance declines with age and depends strongly on construction quality, maintenance, and long-term monitoring. A liner is not a permanent solution but a component whose integrity must be verified across decades.

To quantify these interacting factors, the researchers built an exploratory multivariate model based on a review-derived synthetic dataset, producing a Groundwater Contamination Index analysis. The model suggests that rainfall, containment integrity, landfill age, and aquifer permeability are jointly important controls on contamination severity, together explaining a substantial proportion of the variance in outcomes, with a coefficient of determination of 0.78 and a p-value below 0.01. While the authors describe the model as exploratory—derived from synthesized literature data rather than primary measurements—the result reinforces a central message of the synthesis: groundwater contamination from landfills is not governed by any single factor but by the confluence of climate, engineering, time, and geology. A well-lined landfill in a wet climate with permeable soils may face different risks than an unlined site in a dry region atop impermeable clay, and effective regulation must account for all of these variables simultaneously.

The implications extend into a changing climate. The review highlights the need for climate-responsive landfill design, a recognition that intensifying rainfall and extreme weather events can alter leachate generation rates and contaminant transport in ways that existing infrastructure was never designed to withstand. Heavy precipitation increases infiltration, raising leachate volumes and hydraulic pressures on liners, while floods can inundate disposal sites and mobilize contaminants directly. As climate patterns shift, landfills engineered for the hydrology of the past century may face conditions their designers never anticipated, making adaptive design and monitoring increasingly urgent.

Ultimately, the review’s authors argue that protecting the world’s groundwater will require more than better liners. They call for harmonized monitoring protocols so that data from different countries and decades can be meaningfully compared, long-term post-closure surveillance that continues for decades after landfills stop operating, stronger assessment of emerging contaminants such as PFASs and microplastics whose behavior in the subsurface remains incompletely understood, and regulatory systems capable of sustaining containment performance over the full landfill life cycle. Groundwater supplies drinking water to a large share of the global population, and unlike surface water pollution, which is visible and often recoverable, aquifer contamination can persist for generations and is extraordinarily expensive to remediate. The synthesis makes clear that the cheapest time to prevent it is before it happens—through integrated engineering, careful hydrogeological planning, and regulation that treats every landfill not as a static pile of waste but as a decades-long chemical experiment whose consequences unfold slowly beneath the surface.

Subject of Research: Landfill leachate composition and its impact on global groundwater contamination

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

Article References: Ibrahim, K. A., Xia, Q., Li, Y., Ukwuoma, C. C., & Akeredolu, R. J. (2026). From surface to aquifer: a global synthesis of landfill leachate composition and its impact on groundwater systems. Environmental Monitoring and Assessment, 198(10), Article 1083. https://doi.org/10.1007/s10661-026-15871-4

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15871-4

Keywords: landfill leachate, groundwater contamination, PFAS, microplastics, heavy metals, municipal solid waste, hydrogeology, landfill liners, emerging contaminants, waste management, ammoniacal nitrogen, climate change