Methylmercury in Water Bodies Adjacent to Coal-fired Thermal Power Plants: A Global Critical Review of Pathways, Evidence and Uncertainties

Abstract

Coal-fired thermal power plants remain one of the largest anthropogenic sources of mercury, yet the extent to which they raise methylmercury, the bioaccumulative and neurotoxic form, in nearby lakes, reservoirs, rivers, wetlands and coastal waters is contested. Atmospheric deposition, ash disposal, flue-gas desulphurisation wastewater and seawater desulphurisation discharges each deliver mercury to receiving waters, but methylmercury is formed in situ by anaerobic microorganisms, and its concentration in biota depends on local biogeochemistry and food-web structure as much as on inorganic mercury loading. This critical narrative review synthesises peer-reviewed field, experimental, isotopic and modelling evidence, together with authoritative institutional sources, published from 1990 to July 2026, to evaluate whether proximity to coal-fired plants predicts methylmercury in adjacent water bodies across North America, Europe, Australia, southern Africa, South Asia, East Asia and South America. Literature was identified through multidisciplinary scholarly indexes and citation searching and was appraised for design, analytical adequacy, source attribution and representativeness. The evidence is strongest for near-field enrichment of inorganic mercury in precipitation, soils and dated lake sediments, supported by stable-isotope fingerprints and historical sediment records that track plant commissioning and retrofitting. Evidence that such enrichment translates into higher methylmercury in water and fish is inconsistent. Several studies report no distance gradient or even lower fish mercury near plants, a pattern plausibly explained by plume reduction of oxidised mercury, co-emitted selenium, sulphur-mediated effects on methylation and ecosystem-specific methylation potential. Ash spills and ash-amended sediments can stimulate methylation, whereas outcomes of regulated wastewater and seawater desulphurisation discharges are site-dependent. Major gaps include the scarcity of methylmercury measurements in waters near plants in India, South-East Asia and much of Africa, the dominance of total mercury rather than methylmercury as the reported endpoint, and few before-and-after designs around plant retirement or pollution-control retrofitting. Source-resolved monitoring that pairs compound-specific isotopes with methylation rate measurements and food-web indicators is needed to determine where plant-level controls will most reduce exposure. The review concludes that coal-fired plants are a well-established regional and global driver of aquatic methylmercury, but that their local signature is conditional and cannot be assumed from proximity alone.