Fate of mercury from artisanal and small-scale gold mining in tropical rivers: Hydrological and biogeochemical controls. A critical review

Abstract

Despite numerous existing publications, the fate of mercury (Hg) in ASGM-affected tropical aquatic systems remains uncertain. This is driven by an incomplete understanding of the factors controlling Hg distribution and transport in tropical ecosystems. The review of existing Hg research into ASGM indicated that hydrology exerts major control on the final sink of Hg in tropical rivers. Total Hg concentrations in river water increase during high discharge events due to increased erosion and Hg export from soils. Geochemical composition and grain-size distribution strongly control Hg concentrations and distribution in sediment and soils. This review highlights the absence of standardized protocols for investigating ASGM-related Hg pollution, and that the applied research methodologies are often unsuitable for adequately assessing the processes involved. We underline the need to consider an extensive suite of biogeochemical, physical and hydrological information when investigating the final sink and risks of ASGM-related Hg. Standardized and accurate research methodologies, applicable to the challenges presented by these tropical environments, would greatly ease not only data comparability and synthesis between studies, but also help to differentiate between anthropogenic and natural Hg. Establishing background values in tropical regions is also imperative for accurate quantifications of Hg enrichment levels in ASGM-affected sites.