Synergistic impacts on a multi-purpose transboundary aquatic ecosystem resulting from cyanide and mercury in artisanal gold mining tailings

BACKGROUND AND OBJECTIVES: The transition from mercury amalgamation to cyanide-based extraction in artisanal and small-scale gold mining has reshaped contaminant behavior in mining-impacted watersheds. This study integrates simultaneous measurements of cyanide, nitrogen species, sulfate, bicarbonate, and mercury to evaluate their coupled dynamics along the extraction–tailings–river continuum in the Portovelo–Zaruma district (southern Ecuador). The objectives of this study were to: i) quantify free and total cyanide and dissolved mercury in leaching tanks, tailings ponds, and river sites; ii) characterize nitrogen, sulfate, and bicarbonate formation associated with cyanide degradation and ore mineralogy; and iii) assess how these processes influence downstream mercury mobility and environmental vulnerability.METHODS: Sampling included feed-ore mineralogy; leaching-tank solutions (n = 5); tailings-pond solutions (n = 5); and river sites (n = 12, each with three cross-sectional replicates). Free, total cyanide and mercury were measured during processing. Levels of mercury, nitrate, sulfate, bicarbonate, chemical oxygen demand, conductivity, and total dissolved were investigated in tailings solutions and river waters.FINDINGS: Free cyanide decreased from 517 ± 135 milligram per liters in leaching tanks to ~182 milligram per liters gram per tailings ponds (~80 percent reduction) and to 99.9 percent attenuation). Total cyanide declined from 695 ± 159 milligram per liters in leaching-tank effluents to <0.004 milligram per liters in river water. Mineralogy (25–35 percent pyrite; 10–15 percent calcite/dolomite) contributed sulfate up to 500 milligram per liters and bicarbonate up to 250 milligram per liters in tailings, both increasing downstream. nitrate rose from 0.49 to 2.54 milligram per liters, and chemical oxygen demand from 40–50 to 200–240 milligram per liters. Dissolved mercury remained <0.001 milligram per liters in river water, though tailings solids contained 0.50 ± 0.10 milligrams per kilogram mercury. These patterns indicate cyanide degradation, nitrogen enrichment, mineral-derived anion release, and associated shifts in mercury behavior.CONCLUSION: The Puyango River functions as a reactive geochemical corridor where cyanide attenuation, nutrient enrichment, organic loading, and mineral-driven sulfate–bicarbonate release converge with legacy mercury to shape water quality and ecological vulnerability. Effective management requires integrated monitoring of cyanide, nitrogen, and mercury pathways supported by sediment, biota, and microbial assessments.