Mercury transfer and transformation from mine soil to river sediments: the potential role of amorphous iron oxides in methylation processes in southern Burkina Faso

Since the early 2000s, artisanal and small-scale gold mining (ASGM) has rapidly expanded in Burkina Faso. Mercury (Hg) is widely used to extract gold and its release through burning amalgams has led to soil contamination near mining sites. However, the fate and speciation of Hg in soils remains poorly understood, especially the reactivity or methylation potential of soil particles eroded into rivers. In this study, Hg contamination levels and speciation were assessed in water, soil, and sediments from five ASGM districts along the Mouhoun River. Surface waters near riverside mining sites showed high levels of particulate Hg (11-239 ng L^-1), while more arid sites showed Hg contamination localised to ore-washing ponds. Mercury thermodesorption and selective extraction analysis revealed that in soils collected in the vicinity of amalgam burning sites, around 10% of total Hg (THg) was elemental (Hg0), with most remaining Hg bound in the divalent state to amorphous iron oxides (∼60% THg) and organic matter (∼30% THg). In river sediments, Hg bound to amorphous iron was halved, while methyl Hg (MeHg) levels increased fivefold (0.7 ± 0.2 ng g^-1) suggesting that iron reduction in sediments promotes MeHg production and accumulation. These results highlight the potential risks of Hg exposure for local communities and the need for regional Hg management in ASGM areas.