Landfill Leachate from Nigeria Damages DNA in Plants, Fish and Mice and Raises Cancer Risk

A dark, foul-smelling liquid seeping from one of Nigeria’s busiest waste dumps has been shown to break chromosomes, damage blood cells and scar internal organs across three very different living systems, and its metal content may carry a lifetime cancer risk above internationally accepted thresholds. The findings come from a team led by Adekunle A. Bakare of the University of Ibadan and the National University of Lesotho, working with colleagues in Nigeria and Lesotho, in a study published in the journal Discover Toxicology. The researchers took raw leachate from the Lapite dumpsite in Moniya, Ibadan, a roughly 20-hectare municipal waste site that has operated since 1998 without any subsurface drainage system to capture its effluent, and subjected it to a battery of chemical analyses and biological tests spanning plant, fish and mammalian models.

The Lapite site receives a daily mixture of domestic, commercial, medical, agricultural, pharmaceutical and industrial waste, and its leachate flows directly into surrounding water bodies and percolates into the soil, where underground water can become contaminated. Because the dumpsite sits within the city and is worked by cart pushers, truck drivers, food vendors and scavengers who live and earn their living on it, the potential for occupational and residential exposure is substantial. Previous studies had already documented toxic substances in the soil, streams, groundwater and edible crops around the site, but no one had systematically tested the leachate’s effects on living organisms or calculated what its contents might mean for human health.

Chemically, the leachate was alarming. The team measured metals at concentrations ranging from 0.004 to 10.69 milligrams per litre and detected polycyclic aromatic hydrocarbons and phthalate esters at levels between 1.5 times ten to the minus four and 4.32 times ten to the minus two micrograms per litre, all exceeding United States Environmental Protection Agency limits. Alkalinity, chloride, nitrate, sulphate, ammonia, lead, copper, iron, cobalt, arsenic and chromium were all above standards set by Nigerian and international regulators. The leachate contained four phthalates, including bis(2-ethylhexyl) phthalate and di-n-butyl phthalate, both well known as endocrine disruptors, and five PAH congeners flagged as priority pollutants by the USEPA, including benzo(b)fluoranthene and benzo(g,h,i)perylene. A computed Leachate Pollution Index of 7.506, with lead as the largest contributor, exceeded the international benchmark of 7.378 and the value of 5 that signals polluted conditions and a high likelihood of groundwater contamination.

The first biological test used the root tips of the onion Allium cepa, a classic plant model for detecting chromosomal damage. Onion bulbs grown in leachate dilutions from 1 to 40 percent showed a concentration-dependent collapse in root growth, with root length falling to as little as 11.96 percent of controls and a half-maximal effective concentration of just 2.15 percent. The mitotic index, which measures the proportion of cells actively dividing, dropped from 8.33 to 1.25 percent as concentration rose, while the frequency of chromosomal aberrations climbed from 26.73 to 112 percent. Microscopy revealed sticky, vagrant and disoriented chromosomes, anaphase bridges, disturbed spindles and binucleated cells. The pattern of damage pointed the authors toward spindle inhibition rather than direct chromosome breakage: the leachate’s constituents appear to sabotage the machinery that pulls copied chromosomes apart, arresting cell division. Correlation analysis strengthened the case, with the analysed metals and organics showing a strong positive relationship with root growth inhibition and aberration frequency, and a strong negative relationship with the mitotic index.

The aquatic model was the African mud catfish Clarias gariepinus, a commercially important species across West Africa. In a 96-hour acute toxicity test, mortality rose with concentration and reached 100 percent at the 50 and 100 percent dilutions, yielding a median lethal concentration of 17.78 percent. For the longer micronucleus experiment, juvenile fish were exposed to 5 to 25 percent leachate for up to 21 days. Their peripheral red blood cells accumulated micronuclei, small DNA fragments left outside the main nucleus after cell division, along with nuclear buds, binucleated, apoptotic and necrotic cells, all increasing with both concentration and exposure duration. Correlation coefficients between exposure time and nuclear damage ranged from 0.96 to 0.99, indicating an almost linear relationship between how long the fish swam in contaminated water and how badly their genomes fared.

Under the microscope, the fish tissues told a parallel story of systemic injury. Livers showed severe glycogenic and lipid-filled cytoplasm, degeneration of liver cells and fat infiltrating the hepatic tissue, while gills displayed fusion of filaments, inflamed gill arches and haemorrhagic, necrotic lamellae. Gill damage of this kind impairs respiration and ion regulation, and hepatic steatosis reflects a general failure of lipid metabolism under xenobiotic assault. Because fish gills and livers are in direct, continuous contact with the water they inhabit, these lesions offer a realistic preview of what chronic leachate exposure does to aquatic life in streams receiving dumpsite runoff.

The mammalian arm of the study exposed mice to 2.5 to 50 percent leachate in their drinking water for 5, 10 or 20 days. Bone marrow analysis revealed concentration-dependent increases in micronucleated polychromatic and normochromatic erythrocytes, alongside a declining ratio of young to mature red blood cells, a sign that the leachate was accelerating cell turnover and shortening red cell lifespan as bone marrow proliferation was disturbed. Other abnormalities, including Cabot’s rings, teardrop cells, burr cells, helmet cells and oversized or undersized erythrocytes, accumulated with longer exposure, with pooled abnormality counts rising from 89.81 at five days to 114.38 at twenty days. Mouse livers showed necrosis, cytoplasmic vacuolation, portal inflammation, congestion and haemorrhage, with severity generally increasing with concentration. Body weight gain was negatively correlated with leachate exposure, while micronucleus frequencies correlated positively, mirroring the plant and fish results.

The authors attribute the genetic and tissue damage to the leachate’s toxic metals and organic pollutants acting in concert. Metal ions can bind to nuclear proteins and DNA, forming cross-links that alter DNA structure and trigger cell cycle disruption, apoptosis or carcinogenesis, while oxidative stress and reactive oxygen species amplify the injury. PAHs, phthalates and related compounds are established drivers of genomic and epigenetic instability through DNA adduct formation, strand breaks and inhibition of repair pathways. Lead, cadmium and arsenic appear on the World Health Organization’s list of ten chemicals of major public health concern, and the WHO attributes to arsenic circulatory problems, skin destruction and neurotoxic effects, to cadmium kidney damage, bone demineralization and links to neurodegenerative disease, and to lead impaired cognition, anaemia, reduced fertility and risks of stillbirth and miscarriage.

The human health risk assessment used USEPA protocols to estimate exposure through ingestion, dermal contact and inhalation for both adults and children. Hazard quotients followed the order ingestion greater than dermal greater than inhalation, and children registered higher values than adults at every pathway, likely reflecting hand-to-mouth habits that make it easy to consume contaminated soil and water. Hazard indices for individual metals stayed below one, the threshold for non-carcinogenic concern, in both age groups. The cancer picture was far less reassuring. Total lifetime cancer risk values for lead reached 1.63 times ten to the minus four for adults and 1.53 times ten to the minus three for children, both above the 1 times ten to the minus four threshold that defines a high risk of developing cancer, with children again more vulnerable. The authors note that vegetables were being harvested on the dumpsite during sampling, maize was growing on its margins, and goats, dogs and human scavengers foraged across it, all potential routes for biomagnification into the human food chain.

The study has limits the authors acknowledge: because the leachate is a complex mixture, the individual contribution of each chemical cannot be separated due to collinearity, and the precise molecular mechanisms of the damage remain to be worked out. Even so, the convergence of chromosomal damage in plants, micronuclei in fish blood and mouse marrow, and pathological lesions in liver and gill tissue across three kingdoms of life makes a compelling case that uncontrolled dumpsite leachate is not merely an odour problem but an active genotoxic hazard. The team calls for regular monitoring of the site, regulation of raw leachate discharge, and a shift from open dumping toward waste reduction, recovery, recycling and reuse. For the communities living and working around Lapite, and for the millions of people in rapidly urbanising cities across the developing world who live beside similar dumps, the message is stark: the liquid that drains from our waste carries the capacity to reach into the genome itself.

Subject of Research: Cytogenotoxicity and human health risks of municipal solid waste landfill leachate

Article Title: Genetic and systemic disruption by a municipal solid waste leachate in aquatic and terrestrial models, and human cancer risk assessment of the leachate’s metallic constituents

Article References: Bakare, A. A., Balogun, K. B., Ogunbona, M. A., Onyia, E. C., Coker, M. M., Pitso, K. M., Bakare, O. C., Maduna, L. D., Fagbenro, O. S., Magama, S., Taole, M. M., Adeniji, A. O., Phalatsi, M. S., Gbadebo, A. M., Chatanga, P., & Alabi, O. A. (2025). Genetic and systemic disruption by a municipal solid waste leachate in aquatic and terrestrial models, and human cancer risk assessment of the leachate’s metallic constituents. Discover Toxicology, 2(1), Article 22. https://doi.org/10.1007/s44339-025-00044-8

Image Credits: AI Generated

DOI: 10.1007/s44339-025-00044-8

Keywords: landfill leachate, genotoxicity, micronucleus assay, Allium cepa, Clarias gariepinus, heavy metals, polycyclic aromatic hydrocarbons, phthalates, cancer risk assessment, municipal solid waste, Nigeria, histopathology