1228 of acidic mine waters that can reach pH ~ 5, and reinforces that moderate acidity can coexist with relevant concentrations of metals and metalloids. The most critical finding is the behavior of sulfate: the scenario with variable hydroclimate does not necessarily increase the average sulfate level compared to the stationary scenario, but it does generate extreme pulses (up to 3.95 g/L) and an annual variation range that exceeds that of the average precipitation scenario by several orders of magnitude. This implies that mining closure decisions based on average conditions or smoothed series run the risk of underestimating the magnitude and frequency of high-load episodes that often coincide with dry-wet transitions, where the “first flush” mobilizes accumulated soluble salts and water concentrated by evaporation. Consequently, infrastructure control and design (drainage, containment, and treatment) must be evaluated against extreme conditions, consistent with what is proposed in the literature on acid drainage under prolonged droughts and more intense storms. The comparison between scenarios also suggests that, under steady-state conditions, pH and sulfate evolve in a coupled manner (high negative correlation), while under hydrological pulses they partially decouple. This decoupling has direct implications for monitoring: pH alone may not capture increases in sulfate load during transient events, so operational control should incorporate complementary indicators such as sulfate and selected metals, especially in periods following prolonged droughts. Finally, the trajectories of iron and arsenic (Figure 2) show gradual changes and less variability than sulfate, reinforcing that the risk under changing hydrological regimes is expressed mainly as “pulses” of highly mobile solutes associated with oxidation and redissolution of salts, rather than as a slow and uniform change in all species. Consequently, it is concluded that the explicit integration of climate scenarios into acid drainage modeling is a necessary advance over conventional practices based on stationary climate. The evidence presented demonstrates that omitting such variability can lead to underestimations of the actual range of hydrogeochemical behavior of the system. ACKNOWLEDGEMENTS The authors would like to thank Anddes Asociados for their support and assistance with this work, under the framework of the LixiviAnddes Project. REFERENCES Allison, J. D., Brown, D. S., & Novo-Gradac, K. J. (1991). MINTEQA2/PRODEFA2, A Geochemical Assessment Model for Environmental Systems: Version 3.0 User’s Manual (EPA/600/3-91/021). U.S. Environmental Protection Agency. American Psychological Association (2009). Publication Manual of the American Psychological Association (6th ed.). Washington, DC: Author. Anawar, H. M. (2013). Impact of climate change on acid mine drainage generation and contaminant transport in water ecosystems of semi‑arid and arid mining areas. Physics and Chemistry of the Earth, Parts A/B/C, 58-60, 13-21. A. Yekta, P. Salinas, S. Hajirezaie, M. Amooie, C. Pain, M. Jackson, C. Jacquemyn, and M. Soltanian (2021) . Reactive transport modeling in heterogeneous porous media with dynamic mesh optimization. Computational Geosciences, 25(1):357–372. Appelo, C. A. J., & Postma, D. (2005). Geochemistry, groundwater and pollution (2nd ed.). CRC Press.
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