Critical review of leachate pathways, groundwater contamination, and health risks in tropical urban aquifers

Abstract

Rapid, under-serviced urbanization across the tropical belt has left many cities dependent on open dumps. These dumps and poorly engineered landfills often sit above shallow, heavily used aquifers. This review uses the Doddabidarakallu municipal solid waste facility in Bengaluru, India, as an illustrative anchor case. It sits in a peri-urban, fractured Precambrian basement-rock setting. We synthesize evidence on several fronts. First, how leachate from such sites is generated and mobilized under tropical hydro climatic conditions. Second, how it degrades groundwater quality, as measured by water quality indices (WQI), heavy-metal indices, and pesticide co-contamination. Third, how multivariate statistical techniques and isotopic tracers apportion contamination sources. Fourth, how spatial and seasonal dynamics shape exposure. Fifth, how emerging contaminants—per- and polyfluoroalkyl substances (PFAS) and micro plastics—add a further, largely unquantified layer of risk. Finally, how human health risk is estimated and could, in principle, be prioritized for mitigation. City-scale evidence for Bengaluru makes Doddabidarakallu a plausible and policy-relevant anchor case. Isotopes confirm leachate-infiltration fractions of 1–33% of local groundwater. Nitrate-driven health risk varies by season and affects up to two-thirds of adults post-monsoon. Municipal groundwater dependence is heavy, and leachate contamination is documented at the nearby Mavallipura landfill. Even so, no published site-specific hydro geochemical or health-risk dataset for Doddabidarakallu itself was identified. This review makes that gap explicit rather than assuming it away. The broader evidence base is strong for engineered, temperate landfill settings. It is also strong for tropical open-dump case studies from South Asia, West Africa, and Southeast Asia. But it is fragmented across disconnected indices and receptor models. It is thin for Latin America and Central Asia. And it is largely silent on tropical-specific PFAS occurrence and formal contamination-prioritization frameworks. We synthesize findings across more than one hundred studies. We highlight methodological inconsistencies, particularly around seasonal directionality and receptor-model disagreement. We identify concrete research gaps. These include site-specific characterization of Doddabidarakallu itself, the near-total absence of tropical PFAS data, and the lack of any Pareto-style or multi-criteria prioritization framework built for tropical landfill-groundwater management.