Enhancing mercury pollution modeling in tropical rivers affected by artisanal gold mining: development of a speciation module and application to the Lom River

Abstract
Mercury contamination in tropical river systems is a major environmental concern, particularly in areas affected by artisanal and small-scale gold mining. To address the limitations of existing water quality models in representing mercury speciation processes, this study presents the development of a dedicated mercury speciation module. The module explicitly simulates key processes controlling mercury fate in rivers, including phase partitioning, chemical and photochemical transformations and the influence of environmental drivers such as pH, temperature, dissolved organic carbon, suspended particulate matter and dissolved oxygen. The module was applied to the Lom River (Eastern-Cameroon), heavily impacted by gold mining. Simulation results reveal strong spatial and seasonal variability in dissolved HgII concentrations, with peaks downstream of mining areas. Concentrations can exceed international water quality standards (up to 4.23 times the EU 2020/2184 Directive for drinking purposes and 60.4 times the Environmental Quality Standard during high flow) immediately downstream of mining discharges. Seasonal variations show transition from particulate-dominated mercury during the dry season to dissolved-dominated forms in the rainy season, governed by a flow-dependent partition coefficient. Speciation results indicate that HgII remains the dominant specie (> 99.6 %), while Hg⁰ and MeHg are produced in trace amounts and their formation is controlled by environmental drivers. The module reproduces observed patterns with improved accuracy. This work highlights the need for process-based modeling approaches to accurately simulate mercury fate. The developed module enhanced the simulation of mercury processes in a tropical river impacted by gold mining.